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Saturated x-ray lasers at 196 and 73 Å pumped by a picosecond traveling-wave excitation
Abstract
Traveling-wave irradiation with a laser pulse of duration ∼1 picosecond has been shown to achieve saturated operation of Ne- and Ni-like x-ray lasers. Gain at 196 Å was confirmed by observation of both forward and backward x-ray laser beams from a germanium plasma under ideal and nonideal traveling-wave conditions at a small signal gain of >40 cm-1. Saturation was observed for targets >4 mm long consistent with a model of laser amplification along the plasma medium and with the output of a detailed ray-tracing post-processor coupled to a hydrodynamic and atomic physics code. Ni-like samarium targets, pumped under ideal traveling-wave conditions exhibited a small signal gain of ∼19 cm-1 at 73 Å with saturation observed for targets 8 mm long.
... There are several schemes proposed and examined in producing laser plasma conditions for X-ray lasing both at shorter wavelengths and efficiently. Plasma based recombination lasers [1] collisionally pumped [2,3] and transient collisionally pumped, using picosecond CPA pulses, X-ray lasers [4], using a capillary discharge [5], a free electron laser [6], optical field ionisation of a gas cell [7] are examples of such schemes. Among various pumping techniques for the X-ray lasers the collisional pumping of different materials in the Ne-like ionisation state between the 3p-3s energy levels have shown a more stable and higher output. ...
... In this paper we propose that the possibility of mixing between these two energy levels, especially when the 2p-hole transition is in saturation mode, could be a reason for the smallness of the gain coefficient and output intensity for the inner-shell transitions of the Ne-like ions. Experimental data from the last decade have shown that more ASE systems in the soft-X-ray region of the electromagnetic spectrum perform well at the saturated gain [3,4,14]. Consequently the saturation effects on the outputs of these laser lines can be a very important factor. ...
... The output ratio, I ' 4 =I s4 , is plotted as a function of the 2p-hole gain ratio, G 0 3 =G 3 , for the two extreme smallsignal gain-length product of 10 and 40. Increasing gain-length product results in an increase in the output level. ...
In most collisionally pumped X-ray lasers the lasing transitions considered are a result of collisional excitation of a Ne-like ground state 2p electron into a 3p excited level. However, there are suggestions of producing plasma conditions for collisional pumping of the inner-shell 2s electrons into highly excited 3s levels, with lasing arising as a 2p electron fills the 2s hole. Simulations on various Ne-like ions such as germanium, krypton, and yttrium, using collisional pumping, to get gain on the inner-shell transitions at 62, 50.2, and 42Å, respectively, have shown gains on the inner-shell transitions up to 30cm-1. It has been suggested that the large Doppler linewidth associated with shorter wavelengths is responsible for the smallness of the small-signal gain in the inner-shell transitions relative to than that predicted for the more usual 2p-hole transitions. However, experimental investigations of this 2s-hole inner-shell laser line, using collisional pumping technique, were unable to register any output. In this paper we report the result of calculations of the gain and the total spontaneous emission rate for the 2s-hole and the 2p-hole X-ray laser lines using a coupled four level model. It is shown that the small-signal gain of the 2s-hole inner-shell transition decreases with increasing pumping rates of the 2p-hole upper and lower levels. The output characteristics of the Ne-like inner-shell transition is simulated using this four-level model and the effects of saturation of the 2p-hole line on the 2s-hole transition is studied showing that the saturation of the former may have a severe effect on the output of the later.
... The highest SXRL energy obtained is determined to be ∼1.6 ± 0.5 µJ with respect to the solid angle of the spectrometer, the reflectivity of the gold grating and the fused silica substrate, and the transmission value of the carbon filters. The determined SXRL energy is in agreement with the modeling in [11]. By changing the delay in the Mach-Zehnder interferometer setup, we studied the dependence of the SXRL output on the delay in the double pulse. ...
... The small time delay and its narrow window are caused by fast changes in the hydrodynamics of the preplasma [17]. The optimum of 90 ps is close to the result of [11], in which an optimal delay of 130 ps was obtained. The effect of the double-pulse energy ratio was studied with a total pump energy of ∼100 J and a double-pulse delay of ∼80 ps. ...
... The application of double-pulse single-beam non-normalincidence pumping proves to be a simple and efficient method for the generation of short-wavelength soft-x-ray lasers. Nilike samarium soft-x-ray lasing at 7.36 nm was achieved at a low total pump energy threshold of 36 J, a value predicted by simulations in [17], and half the total pump energy applied in [11]. This confirms the suitability of the applied pumping scheme for the photon energy region close to 200 eV, with an efficiency of ∼4 × 10 −8 . ...
We demonstrated a 7.36 nm Ni-like samarium soft-x-ray laser, pumped by 36 J of a neodymium:glass chirped-pulse amplification laser. Double-pulse single-beam non-normal-incidence pumping was applied for efficient soft-x-ray laser generation. In this case, the applied technique included a single-optic focusing geometry for large beam diameters, a single-pass grating compressor, traveling-wave tuning capability, and an optimized high-energy laser double pulse. This scheme has the potential for even shorter-wavelength soft-x-ray laser pumping.
... As a consequence of saturation, G0 decreases from g0 according to G0 g0∕1 I∕I sat , and as a result of Eq. (2), the group velocity speeds up to reach c. In both small-signal and saturation regimes, the numerical values of v g obtained from Eqs. (1) and (2) are similar [16], and we will use Eq. (2) throughout the remainder of this contribution. ...
... So far we have assumed that the small-signal gain remains unchanged during amplification, whereas in order to include the velocity mismatch, one has to introduce the time-dependent local small-signal gain gz; t. We adopt the assumption of Strati [16] that the gain has a step increase to its peak value, followed by an exponential decay of 1∕e lifetime τ, i.e., gz; t gz; 0 exp−tz mis ∕τ, where tz mis ≥ 0 is the delay time of the ASE signal arriving at position z after the excitation has already passed. In order to examine the delay at position z L , it is essential to calculate the detailed local gain Gz 0 → z L , the group velocity of the signal v g z 0 → z L , and the local amplified signal intensity Iz 0 → z L from z 0 to z z L in advance, because those three are governed by the radiative transfer function and coupled together contribute to the value of tz L mis . ...
Efficient amplification of coherent short-wavelength pulses along a plasma gain column requires traveling-wave excitation with the sweep velocity matched to the signal group velocity. Through simulations incorporating the gain dynamics of the system, we show that the group velocity is not constant but increases monotonically along the line focus due to strong saturation. We demonstrate a line-focusing configuration that results in traveling wave excitation with the sweep velocity well matched to the spatially varying group velocity. Moreover, we show through numerical simulations that the improved velocity matching yields a significant improvement in signal amplification.
... It is worth noting that the beam trajectories in this exponential plasma density profile are symmetrical with regards to the turning point, as can easily be shown by Eq. (14). Therefore the beam deflection angle at the exit from the gain zone is equal to the initial angle. ...
... To calculate the gain-length product, we integrate the gain along the trajectories defined in Subsection 2.C. Such a task is not solvable analytically (the numerically calculated gain profiles in Ref. 11 are not analytical), but it is easy to integrate the gain numerically as gl ϭ ⌬z ͚g(x͓z͔), where ⌬z is the step along the target and g(z) is the gain in dependence on the trajectory calculated analytically in Eq. (14). Furthermore, we account for the gain saturation that plays an important role after several millimeters transit through the amplifying medium. ...
Transient soft-x-ray lasers are generated from a solid target irradiated by two intense pump laser pulses. Amplification is achieved in the plasma column thus produced. Knowledge of the beam propagation is vital for the intensity and quality of the x-ray laser output. In this paper, x-ray laser beam propagation in transient plasmas is studied both analytically and numerically. General one-dimensional formulas are developed for beams in electron density gradient media, including the exponential profile that describes the plasma created from a solid target. The gradient is predicted to limit the amplification length within the maximum gain to <2.6 mm in standard experiments. The result given by the analytical model is confirmed by numerical ray tracing of x-ray laser beams within an amplifying medium as it is defined by the full numerical simulation results of the ehybrid code.
... X-ray lasers are a class of lasers in which gain has been demonstrated over various discrete wavelengths ranging from 3.56nm to 46.9nm. Because of the very short-duration and highenergy excitation pulses required to generate these lasers [1] [2], photo excitation method [3], Electron collisional pumping method (ECP), charge transfer technique, electron collisional recombination process and dielectronic recombination pumping are examples of X-ray pumping procedures which using picoseconds chirped pulse amplification (CPA) pulses [4]- [6]. Globally it's often observed that carbon is abundant element in astrophysical sits having the atmosphere. ...
... To improve the efficiency of the soft X-ray lasing action, shorter pulse pumping using a mode of operation termed 'transient collisional excitation' [259,260] helped to push the development of shorter pulse laser drivers, with collaborative experiments being performed through the Laserlab-Europe network [261] . Grazing incidence pumping in which the turning point in the plasma could be optimally tuned to overlap with where the conditions for a desired soft X-ray lasing transition were optimized, enabled high transient collisional excitation gain [262] . A number of the UK X-ray Laser Consortium's results were world firsts, with many confirming results from other groups, enabling numerous refinements to be delivered. ...
The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years.
... To achieve TWTS, particular arrangements of compressor gratings have been designed for accurate dispersion control. The traveling-wave geometry has also been proposed for several other applications, including THz generation [15], ultrafast electron diffraction [16], traveling-wave excitation of x-ray lasers [17][18][19], and plasma based Raman amplification and compression [20,21]. This paper is organized as follows. ...
We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electron spectroscopy. This can enable ultrafast time-resolved measurements of the dynamics of relativistic electrons in the presence of extremely intense light fields, either in vacuum or in plasma, such as in laser wakefield accelerators. We demonstrate, with test-particle simulation and analysis, the capability of this technique for measurements of various high field phenomena: radiation reaction of electrons due to scattering, dephasing of a laser wakefield accelerator, and acceleration of electrons in multiple buckets by a laser wakefield.
... A spatiotemporal pulse-front tilt of a laser pulse means that the arrival time varies in the direction perpendicular to the pulse propagation. 20 It has many applications of velocity matching, such as for large-area THz-pulse generation, 21 traveling-wave excitation of X-ray lasers, 22 ultrafast electron diffraction, 23 and a FEL-type x-ray source via Thompson scattering. 24 In difference to the flying focus scheme, 18 in which the ionizations of neutrals are necessary due to the geometry of collinear collision, SRNBS utilizes a geometry of tilted colli- sion so that a preformed plasma can be used, thereby allevi- ating the instabilities that might be introduced by the ionization process. ...
A new type of Raman plasma amplifier has been proposed using a stimulated Raman near-backscattering scheme with a pulse-front-tilted pump. In order for the seed to overlap spatiotemporally with the pump, the propagation angle of the pump with respect to the seed path is set at twice the angle of the pulse-front tilt. The length of the pump passing each point of a plasma is dependent on the angle of the tilt. As a result, the tilt angle can be tuned to control the plasma temperature as well as the Raman spontaneous radiation growth length, which can be optimized for the seed amplification and significantly suppress the precursors growing from the plasma noise.
... [1][2][3][4]) and experimental (see e.g. [5][6][7][8]) studies on the lasing properties of the soft x-ray lines between 2p 5 3p levels with angular momenta J 0, 1, 2 = and 2p 5 3s levels with J=1 of highly charged Ne-like ions. These x-ray lasers are pumped by electron collisions in hot and dense plasmas which, in most cases, are generated by the interaction of short and intense laser pulses with solid or gaseous targets. ...
A new asymmetry parameter characterizing the differences between the polarized π and σ gain components of the soft-x-ray J = 0-1 lasing line of neon-like ions is calculated in the case of Ge²²⁺ assuming an electron distribution which is a weighted sum of an isotropic Maxwellian and a monoenergetic beam. Using a quasi steady-state collisional-radiative model, we determine in the weak amplification regime the relative populations of the upper M = 0 and lower magnetic sublevels of the lasing line as a function of electron density from 10²⁰ to cm⁻³. This model includes inelastic and elastic collisional transitions, as well as spontaneous radiative decay between all the 337 M-sublevels arising from the 75 lowest-lying Ge²²⁺ J-levels. The computations were performed for a temperature of the Maxwellian component between and K, a kinetic energy E 0 and a fraction f of the beam component in the ranges and , respectively. The basic atomic data, such as level energies, radiative decay probabilities and inelastic collision strengths, were calculated with the flexible atomic code. However, some modifications of this code were made to get the collision strengths for transitions between M-sublevels due to impact with isotropic electrons as well as due to impact with an electron beam in the case of de-excitation. We find that the newly introduced asymmetry parameter may become significant under certain conditions of electron distribution corresponding to relatively low ( K) and E 0 (3-6 keV). The results reported here may be useful in the evaluation of the polarization degree of the J = 0-1 x-ray laser output from a germanium plasma in the presence of fast directional electrons.
... X-ray laser pumping methods are electron collisional excitation, photo-excitation, charge transfer, electron collisional recombination, and di-electronic recombination pumping using picosecond chirped pulse amplification pulses [4]. However, the electron collisional excitation pumping of the inner shell and outer shell of highly ionization states have shown a more stable higher output [5,6]. ...
Energy levels of 249 excited levels in nickel-like erbium are calculated using the 3s²3p⁶3d¹⁰ as a ground state and the single electron excited states from n = 3 to n = 4, 5 orbitals, calculations have been performed using FAC code (Gu. Astrophys. J. 582, 1241 (2003). doi:10.1086/344745). The populations are calculated over electron densities from 10²⁰ to 10²³ cm⁻³ and electron temperatures 1/2, 3/4 of the ionization potential of Ni-like Er. The gain coefficients of the transitions are calculated.
... For a laser medium to be studied, usually experimental measurements for the ASE intensity I ASE and linewidth Dk ASE versus medium excitation length l AMP are arranged and the plots of I ASE versus l AMP are commonly fitted to well known formulations, given initially by Shaklee and Leheny (1971) and later by Linford et al. (1974), respectively for evaluation of the optical gain coefficients. The introduced formulations although were further modified (Svelto et al. 1998;Dal Negro et al. 2004;Strati and Tallents 2001;Tallents 2003;Janulewicz et al. 1995) during the past years, but for obtaining more accurate results, in particular for X-ray lasers, it was found that it is required to develop several sophisticated mathematical approaches (Sasaki et al. 1989;Pert 1983;Cauble et al. 1996;Holden et al. 1994;Shlyaptsev et al. 1993) to explain the ASE behavior in different X-ray laser media. ...
A theoretical study for the spectral linewidth behavior of the Se X-ray laser at 206.4 Å has been made to obtain the intrinsic linewidth. For a Se target of 6.3 cm in length it is shown that the amplified spontaneous emission (ASE) initiates at the threshold length of z
th
= 0.13 cm. The method gives an excellent agreement with the measurements, leading to 44 mÅ intrinsic linewidth for Se X-ray lasers at this wavelength. The calculation is also extended for another Se X-ray transition at 209.6 Å. We further confirm that the calculated linewidth follows a Voigt profile and its sensitivity to the collision broadening is examined. For the approach the geometrically dependent gain coefficient (GDGC) model is used. The results of the deduced gain parameters obtained from the experiment related to the target of 6.3 cm in length is used to calculate the gain profile explaining gain coefficients for samples of different excitation lengths such as 1.12 and 2.24 cm in length corresponding to the first report on the Se X-ray laser. The plot of gain coefficient versus target length for different measurements confirms that the GDGC model is able to unify Se X-ray lasers. Finally, a summary of the past reported analyses for different types of self terminating lasers will be given, where it further verifies the validity of the GDGC model to be used in different types of laser media.
... After ionization, in the equilibrium plasma, ions having specific number of electrons such as 2 (helium-like), 10 (neon-like), 28 (nickel-like) and 46 (palladium-like) were relatively stable and survived in a wide range of temperature and density [2] [3]. X-ray lasers pumping methods are electron collisional excitation, photo excitation, charge transfer, electron collisional recombination and de-electronic recombination pumping using a picosecond chirped pulse amplification (CPA) pulse [4], a capillary discharge [5] [6] and a free electron laser. However, the electron collisional excitation pumping of the inner shell and outer shell of highly ionization states have shown a more stable and a higher output [7] [8]. ...
Energy levels, transition probabilities, oscillator strengths, and collision strengths have been cal-culated for transitions in Ne-like Ge. The data refer to a 241 fine-structure levels belonging to the configurations 1s 2 2s 2 2p 5 nl, 1s 2 2s 1 2p 6 nl (n = 3, 4, 5, 6; l = s, p, d, f, g and h), which have been cal-culated by the fully relativistic flexible atomic code (FAC). These data are used in the determina-tion of the reduced population for the 241 fine structure levels and gain coefficients over a wide range of electron densities (from 2 × 10 +20 to 4 × 10 +22 cm −3) and at various electron plasma tem-peratures (650, 850, 1050, 1250, 1450, 1650, 1850) eV by using the MATLAB R2013a Computer program for solving simultaneous coupled rate equations. The reduced population for the 241 fine structure levels the gain coefficients for those transitions with positive population inversion fac-tor are determined and plotted against the electron density.
... Some examples of hydrodynamic codes include MEDUSA [4], and LASNEX [5]. Plasma transient collisionally pumped, using picosecond Chirped pulse amplification (CPA), X-ray lasers [6], using a capillary discharge [7], a free electron laser [8], optical field ionization of a gas cell [9] are also examples of such schemes. Among various pumping techniques for the X-ray lasers, the collisional pumping of different materials in the Ne-like ionization state between the 3p-3s energy levels has shown a more stable and higher output. ...
Energy levels, oscillator strengths, and transition probabilities, for the 1s 2 2s 2 2p 6 , 2p 5 3l (l = 0, 1, 2), 2p 5 4l (l = 0, 1, 2, 3) states in Ni XIX and Cu XX are calculated using COWAN code. The Correlation and relativistic effects are considered. The calculations are compared with other results in the literature. A good agreement is found. We also report on some unpublished energy values.
... Since the mid 1980s with the Ne-like [1][2][3][4] ions and later in the 1990s with the Ni-like [5][6][7][8][9][10][11][12] ions, short-wavelength lasers in the extreme UV and soft X-ray domains are demonstrated by means of amplified spontaneous emission (ASE) across a hot and dense plasma. Such table-top systems are commonly referred to as ''X-ray plasma laser'' (XPL), regardless the effective spectral range is mostly in the XUV. ...
... 4p 1 P 1 line of nickel-like lanthanum can be achieved by using a total pump energy of only 4 J. While the experimental realization can be expected to require larger pump energy to compensate for factors not considered in the model, for example, processes that decrease absorption at high irradiation intensities [19], imperfections in the plasma column uniformity and target oxide layer, its magnitude is likely to remain in the range that allows for a high-repetition-rate SXRL operation. A gain with a peak value of approximately 90 cm À1 and a duration of approximately 5 ps is computed to result from the irradiation of a solid lanthanum target with a sequence of a 3:3 Â 10 12 W cm À2 prepulse, 210 ps FWHM in duration followed by traveling-wave excitation with a 2 Â 10 14 W cm À2 , 3-ps FWHM duration pulse impinging at a grazing incidence angle of 35 . ...
We have demonstrated the efficient generation of sub-9-nm-wavelength picosecond laser pulses of microjoule energy at 1-Hz repetition rate with a tabletop laser. Gain-saturated lasing was obtained at =8.85 nm in nickel-like lanthanum ions excited by collisional electron-impact excitation in a precreated plasma column heated by a picosecond optical laser pulse of 4-J energy. Furthermore, isoelectronic scaling along the lanthanide series resulted in lasing at wavelengths as short as =7.36 nm. Simulations show that the collisionally broadened atomic transitions in these dense plasmas can support the amplification of subpicosecond soft-x-ray laser pulses.
This document is part of Subvolume B ‘Laser Systems’, Part 2 of Volume 1 ‘Laser Physics and Applications’ of Landolt-Börnstein Group VIII ‘Advanced Materials and Technologies’. It contains: 7.1.1 Introduction 7.1.2 Principles of X-ray lasers 7.1.2.1 Active medium 7.1.2.1.1 Pump energy absorption 7.1.2.1.2 Population inversion and gain 7.1.2.1.3 Pump power requirements for soft X-ray lasers in plasmas 7.1.2.1.4 Kinetics of the active medium -- working regimes 7.1.2.1.4.1 Steady-state approximation 7.1.2.1.4.2 Quasi-steady-state approximation 7.1.2.1.4.3 Non-stationary or transient approximation 7.1.2.1.5 Medium size and output geometry -- refraction 7.1.2.1.5.1 Refraction 7.1.2.2 Excitation mechanisms 7.1.2.2.1 Electron collisional excitation 7.1.2.2.1.1 Ne-like scheme 7.1.2.2.1.2 Ni-like schemes 7.1.2.2.2 Recombination X-ray lasers 7.1.2.2.3 Inner-shell photoionization (ISPI) 7.1.2.2.4 Photoresonant pumping 7.1.2.2.5 Other excitation schemes 7.1.2.2.5.1 Charge-transfer schemes 7.1.3 Output characteristics 7.1.3.1 Output intensity 7.1.3.2 Output energy and conversion efficiency 7.1.3.3 Saturation 7.1.3.4 Wavelength 7.1.3.5 Spectral linewidth 7.1.3.6 Pulse duration 7.1.3.7 Coherence 7.1.3.7.1 Transverse (spatial) coherence 7.1.3.7.2 Longitudinal (temporal) coherence 7.1.4 Practical X-ray laser schemes 7.1.4.1 Collisionally pumped X-ray lasers 7.1.4.1.1 Quasi-steady state (QSS) scheme 7.1.4.1.2 Low-energy prepulse pumping 7.1.4.1.3 Multi-pulse pumping 7.1.4.1.4 Transient excitation scheme 7.1.4.1.4.1 Traveling-wave pumping 7.1.4.1.4.2 Grazing incidence pumping (GRIP) 7.1.4.1.4.3 XUV master oscillator--power amplifier (XMOPA) 7.1.4.1.5 Fast capillary discharge 7.1.4.1.6 Hybrid X-ray lasers 7.1.4.1.7 Dense gases 7.1.4.1.8 Review of realized collisional X-ray lasers 7.1.4.2 Recombination-pumped X-ray lasers 7.1.4.3 Optical-field ionization (OFI) X-ray lasers 7.1.4.3.1 Optical-field ionization as a plasma source 7.1.4.3.2 Propagation issues of OFI 7.1.4.3.3 OFI with linearly polarized pumping pulse 7.1.4.3.4 OFI with circularly polarized pumping pulse 7.1.4.4 Inner-shell photoionization X-ray lasers 7.1.5 Applications 7.1.5.1 Diagnostics with X-ray lasers 7.1.5.2 Interferometry 7.1.5.3 Reflectometry 7.1.5.4 Excitation of nonlinear processes 7.1.6 Conclusion References for 7.1 Landolt-Börnstein home Volume VIII/ 1B2 Index
Large output variations have been observed in recent samarium (Sm, Z=62) laser experiments under nominally identical pump condition. In this work, we focus on the spatio-temporal mismatch between the traveling-wave excitation (TWE) and the x-ray pulse group velocity. Through simulations incorporating the gain dynamics of the system we show that the group velocity is not constant but monotonously increases along the line focus due to gain saturation. We demonstrate a line-focusing configuration that results in TWE with the sweep velocity well matched to the spatially varying group velocity. A novel line-focusing configuration with adapted TWE velocity has been implemented into the existing 10-TW Nd:glass CPA laser system.
A code series for transient collisional excitation (TCE) was newly developed based on the previous code series for QSS scheme, the TCE Ne-like Ge 19.6nm X-ray laser experiment done by RAL in 2000 was modeled to test our code series, a relatively good fit is obtained which provides convenience for our study of TCE scheme.
Strongly amplified X-ray laser observed in the recent transient collisional excitation (TCE) experiments pumped by a single short pulse suggests that with the help of low level prepulse producing lowly ionized preplasma it is also possible to obtain high gain in the TCE scheme. The TCE Ne-like Ge 19.6 nm X-ray laser pumped by low level prepulse were studid with our code series. Time evolution of plasma status shows that the position of peak gain drifts outward and the gain region is widened as time goes on, which is similar to the QSS scheme.
This paper presented the results of experimental studies on transient soft X-ray laser using a picosecond pulsed laser facility. A rather intensive Ni-like Ag X-ray laser at 13.9 nor with output energy of about 5-10 nJ was obtained by irradiating the solid flat targets with a several hundred picosecond long laser pulse in combination with an 1 ps ultra-short laser pulse.
Theoretical study of Ne-like X-ray laser may provide insight into the Ni-like X-ray laser. A series of TCE (transient collisional excitation) Ne-like Ge X-ray laser experiments driven by 2ω1ω pulses were designed and the Ne-like Ge 19.6 nm X-ray laser produced in this way was modeled with the newly developed code series. The plasma status in the gain region was presented and the characteristics of the X-ray laser compared to that of the X-ray laser driven by the 1ω1ω driving scheme was given.
The transient collisional excited Ni-like Ag 13.9 nm X-ray laser was simulated. Driven by two picosecond short pulses preceded by a 330 ps long prepulse, different short pulses of 1 ps, 2 ps and 3 ps were considered and for each case the angular characteristics of the output X-ray laser are presented in this paper. Detailed analysis shows that the (330 ps, 1 ps, 1 ps) pumping combinations delayed by about 500 ps can extract higher gains near critical surface and it works best. To get the effect of the first short pulse, X-ray lasers driven by only one short pulse following the same long prepulse was simulated and optimization was made. By comparing the optimized case of the two pumping scheme, the advantages of the two short pulses pumping scheme was indicated.
The propagation of a Ni-like Ag x-ray laser operating at 13.9 nm is numerically simulated. A two-dimensional ray tracing code is used to investigate the refraction and saturation effects of the x-ray laser beam as a postprocessor of a one-dimensional hydrodynamic code. The x-ray laser beam trajectory in a plasma medium and the spatially integrated laser output intensity as functions of the plasma length are calculated. Numerical results show that the refraction effect plays an important role in the beam propagation and amplification process.
We calculate the atomic structure, energy levels, oscillator strengths, transition probabilities, and collision strengths for Kr XXVII. The data refer to the 157 fine-structure levels belonging to the configurations (1s2) 2s2 2p6, 2s22p53l, 2s12p63l, 2s22p54l, 2s12p64l, 2s22p55l, and 2s12p65l, where l = s, p, d, f, and the calculations are performed using the fully relativistic atomic structure program FAC. We use the obtained data to calculate the level populations and gain coefficients employing the MATLAB R2012a computer program for solving simultaneously the coupled rate equations. Finally, we determine the 157 fine-structure population levels and gain coefficients for those transitions with a positive inversion factor and plot the electron density in wide range from 1019 to 1023.
The results of an investigation into X-ray lasing in Ni-like Sm, pumped
by a frequency doubled pre-pulse and a fundamental short pulse, are
presented. Strong lasing was observed across the
4d3/2-4p3/2 transition at 7.3 nm. A weaker laser
line of wavelength 6.9 nm was also present from the
4d3/2-4p1/2 transition. The XRL output was seen to
be sensitive to both delay between the pre-pulse and the heating pulse
and also to the energy in the heating pulse. The results are compared to
those from a similar experiment employing pre-pulses at the fundamental
wavelength and the output of the XRL is shown to be enhanced by at least
two orders of magnitude for the same target lengths. An application for
an XRL pumped by Taranis is discussed.
We report on two-dimensional near-field imaging experiments of the 11.9‐nm Sn x-ray laser that were performed with a set of Mo∕Y multilayer mirrors having reflectivities of up to ∼45% at normal and at 45° incidence. Second-moment analysis of the x-ray laser emission was used to determine values of the x-ray beam propagation factor M2 for a range of irradiation parameters. The results reveal a reduction of M2 with increasing prepulse amplitude. The spatial size of the output is a factor of ∼2 smaller than previously measured for the 14.7‐nm Pd x-ray laser, while the distance of the x-ray emission with respect to the target surface remains roughly the same.
Results of numerical simulations on a Ni-like silver x-ray laser pumped by a single picosecond laser pulse are presented. Since the mechanisms responsible for the significant reduction in the pump energy are not well understood, the results of theoretical simulations with emphasis on the plasma kinetics and dynamics in a Ni-like Ag x-ray laser are presented and referred to the experimental data. Special attention has been paid to the influence of the pump pulse shape and length on the gain and its duration. It was found that a low-level pulse pedestal being an integral part of the leading edge of the pump pulse is very beneficial to the pump energy reduction. The thermal cooling process has been identified as the mechanism strongly contributing to gain termination if a low-energy single-profile laser pulse with the width of a few picoseconds is used in the pump process.
We present within this paper a series of experiments, which yield new
observations to further our understanding of the transient collisional
x-ray laser medium. We use the recently developed technique of
picosecond x-ray laser interferometry to probe the plasma conditions in
which the x-ray laser is generated and propagates. This yields two
dimensional electron density maps of the plasma taken at different times
relative to the peak of the 600ps plasma-forming beam. In another
experimental campaign, the output of the x-ray laser plasma column is
imaged with a spherical multilayer mirror onto a CCD camera to give a
two-dimensional intensity map of the x-ray laser output. Near-field
imaging gives insights into refraction, output intensity and spatial
mode structure. Combining these images with the density maps gives an
indication of the electron density at which the x-ray laser is being
emitted at (yielding insights into the effect of density gradients on
beam propagation). Experimental observations coupled with simulations
predict that most effective coupling of laser pump energy occurs when
the duration of the main heating pulse is comparable to the gain
lifetime (~10ps for Ni-like schemes). This can increase the output
intensity by more than an order of magnitude relative to the case were
the same pumping energy is delivered within a shorter heating pulse
duration (< 3ps). We have also conducted an experiment in which the
output of the x-ray laser was imaged onto the entrance slit of a high
temporal resolution streak camera. This effectively takes a
one-dimensional slice of the x-ray laser spatial profile and sweeps it
in time. Under some conditions we observe rapid movement of the x-ray
laser (~ 3um/ps) towards the target surface.
We examine the development of soft X-ray lasers using collisionally pumped Ni-like ions to identify the possibilites for further improvements in the pumping efficiency as we move to shorter wavelengths. The underlying requirements of the pumping system are reviewed. Silver and samarium are examined by detailed simulation in the light of this analysis. It is found that there is significant potential for further reductions in the pump demands for silver using long pre-pulse/mainpulse delays and grazing incidence pumping. Samarium however offers less possibilty of improvement as recombination in the expanding plasma plume inhibits the use of long delays. We find that grazing incidence pumping at an appropriate angle can lead to significant reduction in pump energy.
The results of experimental studies on transient Ni-like Ag soft X-ray laser with the picosecond pulsed laser facility at
the National Laboratory of High Power Laser and Physics (NLHPLP), China, has been reported in this paper. An somewhat intense
Ni-like Ag X-ray laser beam at 13.9 nm with output energy 5~10 nJ was obtained from the solid flat targets under the joint
irradiation of a long pulse laser beam of several hundred picosecond duration and another 1ps ultra-short pulsed laser.
We have used XUV lasers to make absolute measurements of the photoabsorption coefficient of Al at energies just below that of the L3 absorption edge at 72.7 eV. Transmission measurements at photon energies of 53.7 and 63.3 eV have been made using Ne-like Ni and Ge XUV lasers. The XUV laser output was recorded in first and second orders using a flat-field spectrometer. Al foils with steps of various thicknesses were placed over the first order diffracted signal, while the second order diffraction was used to monitor the beam profile at each position. The transmission data agree extremely well with the original measurements at these wavelengths made by Henke and co-workers (Henke B L, Gullikson E M and Davis J C 1993 At. Data Nucl. Data Tables 54 181), but are in conflict with subsequent measurements which are currently in common use (Gullikson E M, Denham P, Mrowka S and Underwood J H 1994 Phys. Rev. B 49 16 283). The exact values of the absorption coefficients in this region of the spectrum have significant implications for the diagnosis of the energy and intensity output of XUV lasers.
The development of soft x-ray lasers collisionally pumped by a relatively long prepulse and short main pulse has markedly improved the effectiveness of these devices. As a result it is possible to envisage their use in a well equipped university laboratory. In this note we break down the various elements of the interaction and pumping into their simple components in order to see how relatively simple analytic concepts can be used to identify the best mode of operation. However detailed simulation is still required to identify the actual experimental operating conditions. The scaling results are compared with simulation, confirming the value of this analysis.
The temporal history of the x-ray laser output has been measured in dependence of the emission angle of the x-ray laser with respect to the target plane. The results show both the exit time (the time at which the x-ray laser pulse is leaving the plasma) and the pulse duration to depend on the angle of emission. Simulations of the gain in Ni-like Pd combined with ray tracing through the plasma were used to model the dependence of the time of emission on the deflection angle of the individual rays within the laser emission cone. The simulations are in reasonable agreement with the measurements.
We have observed strong amplification with a gain of 14.5 cm−1 for the 4d→4p transition of the Ni-like La ions at a wavelength of 8.8 nm pumped by a chirped pulse amplification Nd:glass laser with an energy of 18 J. In this experiment, the laser pulses consist of a prepulse and a main pulse with durations of 200 ps and 7 ps, separated by 250 ps. The keV x-ray spectroscopy and theoretical calculation indicate that the plasma temperature increased effectively by reducing the prepulse intensity, and that the substantial gain is generated in a small overcritical region heated by the electron heat transport.
A femtosecond laser driven collisional Ne-like Ti x-ray laser at 32.6 nm is numerically investigated using a hydrodynamic code coupled with an atomic data package for a 100-{}m-thick Ti planar target irradiated by a single or double prepulse followed by an intense femtosecond laser pulse. By using an optimized drive pulse configuration, a gain of 40{\mathrm{cm}}^{-{}1} can be generated from a 5\mathrm{mm}\ifmmode\times\else\texttimes\fi{}50\mu line focus using only about 1 J pump energy.
We have demonstrated the generation of a highly coherent x-ray laser at 13.9 nm by an oscillator-amplifier configuration with two targets. In the oscillator-amplifier configuration, a seed x-ray laser beam from one target is injected into a plasma amplifier at another target. When the propagation of the x-ray laser beam is not affected by refraction in the amplifier, a highly coherent x-ray laser beam with small divergence of 0.2 mrad is generated. The observed divergence is close to the diffraction limited value, and this beam has full spatial coherence.
Soft x-ray lasers operating in the super- 100 Å regime and using grazing incidence pumping methods are now established as efficient sources of radiation in this waveband. The concepts underlying this approach are to separate the ionization and excitation phases of the laser, and to match the pumping density of the latter to the optimal for gain generation. It is therefore of considerable interest to examine whether these ideas can be successfully applied to sub-100 Å lasers. Three problems arise: first the adverse scaling of ionization with temperature for high atomic number ions, second the strong thermal conduction at these temperatures leads to a large hot zone upstream of the absorption, and third the optimum pumping density is greater than the critical density of 1 μm wavelength, solid state pump lasers. Using analytic models and simulation we identify a strategy to overcome these problems using a pre-pulse of a mixed harmonic and fundamental radiation of Nd-glass laser radiation followed by the main pumping pulse of the fundamental normally incident. Due to the large upstream thermal zone and the high ionization temperature, we find that the energy required in the pre-pulse is much (∼3 times) larger than that in the main, and that the energy needed consequently scales rapidly with the atomic number and therefore decreasing x-ray wavelength. Systems generating output energies of a few tens of μJ are examined at wavelengths between 50 and 70 Å.
We review the status of research on soft X‐ray lasers generated from hot and dense plasmas and explain their potential as high‐brightness sources for applications in different fields of science. © 2007 American Institute of Physics
We have applied R-matrix Floquet theory to study multiphoton ionization of Ar7+ in the combined field of an x-ray laser and either the fundamental or the second harmonic of a Nd:YAG laser for a variety of intensities. Our interest is focused on the photon energy region between 52.5 and 54 eV, where absorption of two x-ray photons and emission of one (frequency-doubled) Nd:YAG photon resonantly excites the 7s state. We demonstrate that the ionization spectra are strongly influenced by resonances involving the absorption or emission of photons due to both laser fields. We compare two scenarios for resonantly enhancing the excitation of the 7s state: either due to a resonance reached after absorption of a single x-ray photon or after absorption of two x-ray photons. Finally, we investigate whether the present results can assist in developing experiments on four-wave mixing using x-ray and visible-light photons in Ar7+.
Three decades ago, large ICF lasers that occupied entire buildings were used as the energy sources to drive the first X-ray lasers. Today, X-ray lasers are tabletop, spatially coherent, high-repetition rate lasers that enable many of the standard optical techniques such as interferometry to be extended to the soft X-ray regime between wavelengths of 10 and 50 nm. Over the past decade, X-ray laser performance has been improved by the use of the grazing incidence geometry, diode-pumped solid-state lasers, and seeding techniques. The dominant X-ray laser schemes are the monopole collisional excitation lasers either driven by chirped pulse amplification (CPA) laser systems or capillary discharge. The CPA systems drive lasing in neon-like or nickel-like ions, typically in the 10–30 nm range, while the capillary system works best for neon-like argon at 46.9 nm. Most researchers use nickel-like ion lasers near 14 nm because they are well matched to the Mo:Si multilayer mirrors that have peak reflectivity near 13 nm and are used in many applications. The past decade has seen the birth of the X-ray free electron laser (XFEL) that can reach wavelengths down to 0.1 nm and the inner-shell Ne laser at 1.46 nm.
Recent high-temporal-resolution nickellike x-ray laser experiments have yielded important insights into the output characteristics of picosecond-pumped x-ray lasers. However, current experimental observations do not fully explain the plasma dynamics, which is critical to gain generation within the x-ray laser medium. A numerical study of the nickellike silver x-ray laser has therefore been undertaken to complement our experimental results in an attempt to further our understanding of the processes at work in yielding the observed x-ray laser output. High gain coefficients existing with picosecond lifetimes are predicted, which is consistent with the short x-ray laser durations experimentally observed. The late onset of the continuum emission relative to the temporal peak of the x-ray laser output is explained as a sign of high electron density evolution near the target surface.
In this manuscript an optimized pumping geometry for transient collisionally excited soft x-ray lasers is presented. In contrast to the standard scheme, where a nanosecond pre-pulse under normal incidence is assumed to provide the optimal plasma preparation and a picosecond pulse under grazing incidence performs the final heating-and excitation process, two picosecond pulses of equal duration under non-normal incidence are applied. Both pulses are produced in the front-end of the CPA pump laser. They are focused collinearly onto the target with the same spherical mirror under non-normal incidence, optimized for efficient traveling wave excitation for the main-pulse. X-ray lasing was achieved on Ni-like palladium (14.7 nm) at less than 500 mJ total pump energy on the target. This proves that this configuration is at least as favorable as the standard scheme, providing much simpler and more reliable operation. Moreover using this concept x-ray lasing was realized on Ni-like Samarium (7.3 nm)
The multi-Terawatt laser system, terawatt apparatus for relativistic and nonlinear interdisciplinary science, has been recently installed in the Centre for Plasma Physics at the Queen's University of Belfast. The system will support a wide ranging science program, which will include laser-driven particle acceleration, X-ray lasers, and high energy density physics experiments. Here we present an overview of the laser system as well as the results of preliminary investigations on ion acceleration and X-ray lasers, mainly carried out as performance tests for the new apparatus. We also discuss some possible experiments that exploit the flexibility of the system in delivering pump-probe capability.
Atomic processes affecting the population kinetics of Ni-like Ag are investigated using a collisional radiative model that includes the Cu-, Ni-, and Co-like ions. The atomic processes, which determine the steady-state ion abundance and detailed level populations, are investigated to determine the population inversion for the X-ray laser application, with several different sets of configurations. The dielectronic processes are considered by including doubly excited and inner-shell-excited states explicitly which allows one to identify the dominant channel of the dielectronic recombination from Co-like to Ni-like, and Ni-like to Cu-like ion. Further, the resonant excitation from ground state to low excited states of Ni-like ion in high-density plasmas (ne>1020/cm3) is also studied.
The output of amplified spontaneous emission (ASE) lasers such as X-ray lasers operated without mirrors is calculated exactly for Gaussian and Lorentzian small signal gain profiles by a simple Taylor series expansion. The accuracy of the ‘Linford’ formula commonly used as an approximation for the output of ASE lasers is evaluated by comparison to our exact solutions. The Linford formula is accurate to better than 10% for intensities produced by a Gaussian gain profile, but requires multiplication by a correction factor of at gain length product greater than 5 for Lorentzian gain profiles.
An extended ray-tracing study of the main pumping pulse (MP) focusing optics was made in order to develop a transient collisionally excited (TCE) x-ray laser (XRL) set-up for sub 10 nm wavelength by using non-normal incidence pumping. The study takes into account limitations determined by the large beam dimensions of the high energy MP. A setup using two off-the-shelf spherical mirrors is proposed, corresponding to a possible realization at the PHELIX laser facility.
At the Prague asterix laser system (PALS) of the Academy of Sciences of the Czech Republic the 1-TW asterix iodine laser is used as a pump source for soft X-ray laser experiments. The prepulse technique was applied which is known to enhance the X-ray laser output at the J=0–1 transition dramatically. Since Zn slab targets were used the laser wavelength was 21.2 nm. A prepulse beam having 1.6 J was preceding the main pulse by 10 ns. The main and the prepulse beam are focused by two different optical systems separately. Implementing a half-cavity set-up for double-pass amplification using a Mo/Si multilayer mirror the X-ray laser output was 10 times stronger than at single pass amplification in a 3-cm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse duration was at least 120 ps longer than the round trip time in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing 4 mJ energy what is proved to be enough for applications. In this contribution the X-ray laser features like divergence in two dimensions, the beam quality (symmetry), the pointing angle and the time-integrated output energy are investigated over more than 100 shots. To characterize the stability of the X-ray laser the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots in a series — achieved during one single day — the corresponding values are given and detailed chi-squared tests characterize the Zn X-ray laser as a robust tool suitable for applications. At PALS soft X-ray laser beam time can be reserved for external research groups.
At the Prague Asterix Laser System Center (PALS) the Asterix iodine laser delivering up to 700 J/0.5 ns is used as a pump source for X-ray laser experiments and applications. The prepulse technique was applied which is known to improve the neon-like X-ray laser output at the transition dramatically. Since Zn slab targets were used the operating wavelength was 21.2 nm. A prepulse having up to 20 J precedes the main pulse by 10 ns. The main beam and the prepulse beam are focussed by two different optical systems separately and their foci are superimposed at the target surface. By implementing a half-cavity set-up for double-pass amplification using a Mo/Si multilayer mirror – which can be used for more than 100 shots – the X-ray laser output was more than 10 times stronger than at the single pass in a 30 mm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse duration was at least 150 ps longer than the round trip time ( ps) in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing approx. 4 mJ energy which has been proved to be enough for future applications. In this contribution, the X-ray laser features like divergence in two dimensions, the beam quality (symmetry), the pointing angle and the integrated intensity giving an estimation of the output energy are investigated over 110 shots. To characterize the stability of the X-ray laser the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots in a series – what was achievable during one day – the corresponding values are given, and a statistical analysis carrying out a chi-squared test characterize the Zn X-ray laser as a robust tool suitable for applications. In the future it is planned to allocate X-ray laser beam time to external research groups.
Soft x-ray lasers in the 3.5–50 nm wavelength range have been developed in many laboratories. The shortest wavelengths and highest output irradiances have been produced using plasmas created by optical lasers as the lasing medium. The optical laser is focused into a line on a solid target and the x-ray laser action occurs by amplification along the line with sufficiently high gain that mirrors are not needed. Population inversions are produced by free-electron collisions exciting bound electrons into metastable levels in neon- and nickel-like ions. This topical review presents a summary of the atomic, plasma and propagation physics of x-ray lasers created in this way.
X-ray laser output at 23.1 nm and the intensity of resonance line and continuum emission
between 0.6 and 1.8 nm emitted from a nickel plasma are simulated using a fluid and atomic
physics program. The simulations are undertaken for the conditions of a recent experiment
using a Nd:glass laser with a ~1.2 ps pulse at 7 × 1015 W cm−2
irradiance pumping a plasma pre-formed by a 280 ps duration pulse at
2 × 1013 W cm−2
with peak-to-peak pulse separation usually set at 300 ps. The simulated duration of x-ray
lasing (~12 ps) agrees with the measured laser duration and the temporal output of continuum and
resonance lines is in agreement with measurements made using a streak camera.
We report a time- and space-resolved investigation of the J = 0–1 and J = 2–1 lasers in neonlike germanium at 19.6, 23.2, and 23.6 nm. Germanium slabs were irradiated by the Asterix IV iodine laser at an intensity of approximately 2.0 × 10¹³ W cm⁻² without and with a prepulse, which was 5.23 ns before the main pulse. The position of the lasing region was measured as a function of the prepulse intensity. It was found that lasing on the J = 0–1 transition at 19.6 nm occurs earlier in time and closer to the target surface than on the two J = 2–1 transisitions at 23.2 and 23.6 nm. The position for the 23.6-nm laser is the farthest from the target surface. A larger prepulse shifted all lasers farther from the target surface. Numerical simulations showed good qualitative agreement with experimental results when a prepulse was applied. For the case without a prepulse, calculations indicated the importance of beam refraction in modifying the effective gain of the soft-x-ray laser beam.
We report a systematic study of double pulse pumping of the Ni-like Sm x-ray laser at 73 Å, currently the shortest wavelength saturated x-ray laser. It is found that the Sm x-ray laser output can change by orders of magnitude when the intensity ratio of the pumping pulses and their relative delay are varied. Optimum pumping conditions are found and interpreted in terms of a simple model. © 1999 American Institute of Physics.
A discussion on the roll-off in the emitted intensity in transient collisional excitation TCE schemes for x-ray lasers is presented. The high gain in TCE lasers results in a group velocity significantly below the light velocity for the amplified pulse. This effect is proven to be the main reason that the measured gain coefficients fall considerably short of theoretical predictions. Is is outlined how a more effective excitation can be achieved when using the TCE scheme. A recent topic of high interest in the field of x-ray lasers is the successful application of the transient collisional excita-tion TCE scheme 1,2, which exploits the possibility of creating short-lived large population inversions 3. Predic-tions of gain coefficients exceeding 100 cm 1 have been made for both Ni-like 3,4 and Ne-like ions 1,5, and for pump energies well in the range of table-top laser systems. These properties make the TCE scheme very attractive. However, a common characteristic of the experimental results obtained so far has been a gain coefficient far below that theoretically predicted and a roll-off in the emitted in-tensity of the x-ray laser source versus the length of the target 1,2,6,5,7. This roll-off generally appears at a target length of around 3 mm, well before it is generally assumed to become effective, taking into account the lifetime of the inversion and the propagation velocity of the amplified pulse. Proposals to overcome the problem by using traveling-wave excitation at the speed of light, c, have been made and em-ployed, but without any improvement or pronounced differ-ence with respect to instantaneous excitation 6,5. Many possible causes have been put forward to explain the unexpectedly small gain coefficient and the anomalous intensity roll-off, including refraction, reduced effective am-plification path due to short gain lifetime, and collisional line broadening 1,2,6,5,7. Nevertheless, all of these effects seem to be far from fully explaining the experimental results. In this Brief Report, we propose a different reason to account for the experimental observations. It is demonstrated that the low group velocity caused by the high gain coeffi-cient quite naturally explains all observations of small gain and output intensity roll-off, and indicated how more effec-tive excitation can be performed when using this particular scheme. First, a short review of the main results obtained so far is given. The first experimental demonstration of the TCE scheme was given by Nickles et al. 1 in 1997. Lasing from the 3p-3s(J0 –1) transition in Ne-like Ti at 32.6 nm was demonstrated, with a gain coefficient g(19 1.4) cm 1 , for an emitted-pulse duration of about 15 ps. The output intensity showed pronounced roll-off starting from a target length of 3 mm. Instantaneous excitation was used. Warwick et al. 6 applied the TCE scheme to the 3 p-3s(J0 –1) transition in Ne-like Ge at 19.6 nm. Lasing with a gain g30 cm 1 for an emitted-pulse dura-tion of about 15 ps was demonstrated. Roll-off in the output intensity starts at a target length of about 5 mm. Traveling-wave excitation was used with v ex c. Kalachnikov et al. 5 improved the performances for the 3p-3s(J0 –1) transition of Ne-like Ti at 32.63 nm. A gain g(461.4) cm 1 , for an emitted-pulse duration of about 10 ps, was measured, with roll-off in the emitting in-tensity starting at a target length of 3 mm. Comparison be-tween traveling-wave excitation at v ex 2.5c and v ex c showed no significant difference. In 1998, Dunn et al. 2 extended the TCE scheme to Ni-like ions. Lasing from the 4d-4 p transition in Pd, at 14.7 nm, was demonstrated, with a gain g35.3 cm 1 . Roll-off in the emitted intensity is evident for target lengths larger than 3 mm. Interestingly, higher gain (46 cm 1) could be measured for lengths shorter than 1.5 mm. Again, Dunn et al. 7 proved lasing by using the TCE scheme and low-Z-number Ni-like elements, viz., yttrium, zirconium, niobium, and molybdenum, at 24.01, 22.02, 20.33, and 18.89 nm, respectively. The output shows a gain of 21 cm 1 , 26 cm 1 , and 17 cm 1 for Mo, Nb, and Zr, respectively, with the usual roll-off starting at a target length of about 3 mm. For Y no experimental measurements were reported. A factor common to all these measurements is that a rela-tively high gain, but still much lower in most cases an order of magnitude than that predicted, is obtained only for very short target lengths (3 mm), while no fit to the Linford formula is ever possible when using a single value for the gain coefficient. This behavior, i.e., a roll-off in the intensity output, cannot be due to saturation because of the small value of the gain-length product generally obtained. Recently, Dunn et al. 8 published results for a Pd TCE-x-ray laser, showing a comparison between TWE and instan-taneous excitation. TWE is obtained using a five-step reflec-tion echelon adjusted to produce an excitation with velocity c along the line focus. During the experiment the echelon could be replaced with a flat mirror, thus producing instan-taneous excitation along the line focus. The authors observed that soft-x-ray laser output, in the case of TWE, smoothly
The properties and oscillation characteristics of very long cw xenon lasers are discussed and pertinent long laser experiments described. High-gain 3.508-μm xenon laser amplifiers were used in these experiments. Laser cavity lengths of up to 30 km were studied. Spectrum analyses revealed complex oscillation spectra exhibiting the characteristics of both homogeneous and inhomogeneous laser mode structures. Long laser Doppler experiments, Q-switching studies, and modulation experiments were performed. Applications to atmospheric pollution detection are discussed.
Using a series of 150-ps pulses from the Nova laser to illuminate neodymium slab targets, we observe a 38-ps-duration lasing pulse at 7.906 nm in nickellike neodymium on the J = 0 → 1, 4d →4p transition. To overcome the low gain in this system, we used a traveling-wave geometry in conjunction with two slab targets that were coupled lengthwise to increase the effective length and whose surfaces were curved to compensate partially for refraction effects.
We report what is to our knowledge the first demonstration of a transient x-ray laser pumped by a 350-fs pulse in a traveling-wave irradiation geometry. For a 500-fs pump pulse the traveling-wave irradiation was found to have a strong effect on enhancing the Ni-like silver 4 d – 4 p lasing emission at 13.9 nm. The signal enhancement was significantly less when the pulse duration was lengthened to 1.7 ps. The experimental observations are well reproduced by a simple model when the duration of gain is taken of the order of 15–20 ps. For the 500-fs pulse a gain coefficient of 14.5 cm⁻¹ was measured for plasma lengths up to 7 mm. Refraction of the amplified photons is believed to be the main cause of the limitation of the effective amplification length.
We report on measurements of the output power from the J = 2–1 (λ = 15.5 nm) neonlike yttrium soft-x-ray laser. The results show peak powers of 32 MW in an output pulse width of 200 ps (FWHM) and an integrated energy of 7 mJ. The output is consistent with the expected saturation intensity of this laser and makes this system one of the brightest XUV sources available.
Laser energy requirements for the creation of active plasma media with
Ne- and Ni-like ions with a `long' subnanosecond prepulse (100 - 1000
ps) and a `short' picosecond main pulse (0.1 - 10 ps) have been
determined for foil and low density targets (foams, gases, vapors,
etc.). RADEX hydrodynamics and atomic kinetics calculations show that
X-ray laser action in the 200 - 400 A can be obtained with 1 - 2 J in
100 - 1000 ps prepulse followed of an approximately 1 ps pulse of
similar energy. Transient gains of approximately 100 cm-1
level have been calculated. For the wavelengths 38 - 46 A (`red' edge of
`water window' spectral band) a short pulse of only approximately 5 - 10
J is required.
Relativistic distorted wave collision strengths are given for the 10 transitions among the 3sââ, 3pââ, 3pââ, 3dââ, and 3dââ levels and for the 80 transitions from these levels to the excited levels nlj with n = 4 and 5 in the 71 Na-like ions with nuclear charge number Z in the range 22 ⤠Z ⤠92. The calculations were made for the six final, or scattered, electron energies Eâ² = 0.0025, 0.015, 0.04, 0.10, 0.21, and 0.40, where Eâ² is in units of Z²{sub eff} rydbergs with Z{sub eff} = Z - 8.34. In addition, the transition energies and electric dipole oscillator strengths are given. To their knowledge the present work is the first comprehensive publication of the results of fully relativistic calculations of the collision strengths for excitation of Na-like ions.
Ne-like Se X-ray laser experiments have been performed to examine the effects in line focus width narrowing on amplification in a collisional excitation scheme. Variation from 40 micrometers up to 180 micrometers has been investigated. Significant changes in temperature and ionization balance have been observed and explained from theoretical considerations.
The basic theory of amplified spontaneous emission (ASE) from long lived laser media was established several years ago enabling a clear understanding of the effects of saturation on output irradiance and line profile to be achieved. The advent of ultra-short pulse laser pumping for X-ray lasers using a travelling wave has necessitated the extension to a time dependent description. Using the simple model developed earlier, we have investigated the key features of time dependent ASE laser action, with particular emphasis on the matched travelling case. We have found that, initially, substantial pulse shortening occurs, but that after saturation the pulse slowly lengthens. Concomitantly the output energy is reduced below that predicted by the steady state theory and the gain pulse duration. Line width narrowing is consistent with the static case. Comparison with more detailed simulations shows that the beam is also broadened transverse to its propagation direction after saturation. Good agreement is found between the simulation and experimental values.
The example of multiply charged Ne-like ions and a detailed model of radiative collisions are used to consider three approximations employed in determination of the level populations and of the gain G in the short-wavelength part of the far ultraviolet range. If the plasma heating process is fast, which corresponds to the transient approximation, the gain can reach G ~ 102 cm − 1. A population inversion appears then not because of strong radiative depletion of the lower active level, but because of transient radiative collisional processes. There is therefore no need to limit the transverse dimensions of a plasma in establishing optical transparency conditions and there are several other consequences of practical importance.
We report experimental results of x-ray amplification of spontaneous emission in a Ni-like transient collisional excitation scheme. The Ni-like plasma formation, ionization, and collisional excitation requires irradiation of a slab target by two laser pulses: a formation beam with 5 J energy of 800 ps duration and a pump beam of 5 J energy in 1.1 ps. A gain of 35 cm-1 and a gL product of 12.5 are measured on the 4d-->4p J = 0-->1 transition for Ni-like Pd at 147 Å with an 8 mm line focus. The high efficiency of this scheme at ``table-top'' laser energies is a direct consequence of the nonstationary population inversion produced by the high intensity picosecond pulse.
A novel two-step excitation scheme for an efficient table-top x-ray laser has been realized for the first time. A nanosecond pulse creates a plasma of neonlike ions of titanium, followed by a subpicosecond pulse which excites a nonstationary population inversion. With only a few joules of pump energy, a compact x-ray laser at 32.6 nm with a very high gain coefficient of g\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}19{\mathrm{cm}}^{-{}1} and a gain-length product of was achieved.
The Ni-like Gd X-ray laser is investigated in detail primarily through a 1.5-dimensional combined fluid and atomic physics code EHYBRID. Directly measurable parameters are calculated using a combined optical raytracing and saturation calculation code RAYTRACE. As experimental data for Gd are not widely available, we compare the predictions of our model with experimental results for Sm. There has been considerable success reproducing the results of experiment with this model. Much has been made of the similarities between the operating conditions for Ne- and Ni-like lasers. This approach has been quite successful from an experimental viewpoint. In this paper we consider the degree of similarity. Areas of difference are highlighted particularly the relative stability of the lasant ion stages.
Short pulses of 100 ps FWHM duration at 1.06 μm wavelength are used as the pump source for driving the J = 0–1, 19.6nm, Ne-like germanium X-ray laser. Different combinations of short pulses are investigated and quantitatively compared. Configurations investigated include a single pulse, double pulses at 400 ps and 800 ps separation, single pulses with prepulses and double pulses with prepulses. Data are presented in the form of integrated energy measurements, and supported by modelling. The most efficient short pulse configurations are shown to be orders of magnitude more effective than pumping with nanosecond duration pulses.
Simulations are presented to compare the output of the 62 Å inner shell transition in germanium with the 66 Å traditional J=0→1 gadolinium transition, using 280 ps pulses with a maximum of 100 J energy and a 30 J, 3 ps pulse, of peak intensity 1.5×1015 W cm−2, focussed onto a 1 cm long, 100 μm wide slab target. Saturated output appears possible for gadolinium assuming the implementation of a suitable travelling wave pumping scheme. Saturation was achieved in this configuration using two long pulses of peak intensity 8×1012 W cm−2 and the CPA pulse, leading to an output power of ∼14 MW. Under these pulse restrictions, the germanium inner shell does not achieve saturation. Simulations indicate that saturation may be achieved if the CPA pulse intensity is fractionally increased (to ∼2×1015 W cm−2). The output power of germanium is ∼1 MW, due to a smaller near field area. Raytracing for both schemes predicts a far field deflected angle of ∼3 mrad. Simulations indicate the potential of these schemes for successful X-ray lasing at wavelengths of ∼65 Å.
Simulations are presented for a collisionally pumped germanium slab target X-ray laser, irradiated with a 1 ns, 5 × 1012 W cm-2 background pulse and a 2 ps, 1015 W cm-2 main pulse, which starts 900 ps after the beginning of the background pulse. Plasma regions with electron temperatures and densities of ~1 keV and 1021 cm-3 result in high small signal gain coefficients on the J = 0-1 transitions at 196, 193 and 132 Å. Raytracing calculations indicate the possibility of obtaining saturated output at 196 Å for ~5 ps from a 1 cm length target if a travelling wave pumping scheme can be implemented. The calculations also suggest that significant output at 132 Å should be observed with a far field deflected angle of ~ 8 mrad.
This paper models recent experiments in which a solid titanium target was illuminated by several joules of combined energy from a nanosecond laser pulse to create a preplasma followed by a picosecond laser pulse to drive the gain. Gains greater than 200 cm-1 are predicted for the Ne-like Ti 3p 1S0→3s 1P1 transition at 326 Å, which is driven by the monopole collisional excitation. High gain is also predicted for the 3d 1P1→3p 1P1 transition at 301 Å, which is driven by a combination of collisional excitation and self-photopumping. We also discuss the possibilities for driving a Ne-like Ge laser using this approach.
A monodimensional amplified spontaneous emission model of traveling-wave pumped soft-x-ray lasers in transient collisional excitation (TCE) is presented. The model explicitly includes the influence of the local gain on the group velocity of the x-ray pulses propagating in the active medium and the saturation of the amplified x-ray signal. Complete analytical solutions of the model in a number of physically interesting cases are derived. The important parameters governing the behavior of these lasers are identified, their influence on the output is investigated, and the optimum configuration required in experiments for efficient traveling-wave pumping is discussed for conditions typical of soft-x-ray lasers in the TCE regime.
Collisionally pumped soft x-ray lasers now operate over a wavelength range extending from 35 - 300 angstroms. These sources have high peak brightness and are now being utilized for x-ray imaging and plasma interferometry. In this paper we will describe our efforts to probe long scalelength plasmas using Moire deflectometry and soft x-ray imaging. The progress in the development of short pulse x-ray lasers using a double pulse irradiation technique which incorporates a travelling wave pump will also be presented.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Laser energy requirements for the creation of active plasma media with Ne- and Ni-like ions with a `long' subnanosecond prepulse (100 - 1000 ps) and a `short' picosecond main pulse (0.1 - 10 ps) have been determined for foil and low density targets (foams, gases, vapors, etc.). RADEX hydrodynamics and atomic kinetics calculations show that X-ray laser action in the 200 - 400 A can be obtained with 1 - 2 J in 100 - 1000 ps prepulse followed of an approximately 1 ps pulse of similar energy. Transient gains of approximately 100 cm-1 level have been calculated. For the wavelengths 38 - 46 A (`red' edge of `water window' spectral band) a short pulse of only approximately 5 - 10 J is required.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
The effects of double pulse irradiation upon the performance of the Ne-like collisional excitation X-ray laser are investigated through the use of a 3d raytracing code used as a post-processor to the coupled hydrodynamics-atomic physics code EHYBD3. The driving laser pulses are separated by 300 ps peak to peak, delivering a total energy of 165 → 285 J to a 3 cm long, 100 μm wide germanium slab target. Calculated output characteristics for both the and lines are presented. The effects of curvature along the target length are also considered.
A three-dimensional raytracing code is used to examine the effects of refraction compensation obtained by bending the target along the lasing axis. The simulations relate to an experiment conducted at Osaka University, where germanium slab targets, curved along the target length, were irradiated. At large curvatures the normally weak J = 0 → 1, 196 Å line dominated the signal. General agreement with experiment is obtained.
The complex problem of a collisionally pumped Ne-like germanium laser is examined through several detailed models. The central model is EHYBRID; a 1 1/2 D fluid code which self consistently treats the plasma expansion with the atomic physics of the Ne-like ion for 124 excited levels through a collisional radiative treatment. The output of EHYBRID is used as data for ray-tracing and saturation codes which generate experimental observables. A detailed description of the models is given. The atomic physics is investigated through a three-level approximation, a steady state collisional radiative treatment and a time-dependent solution within the fluid model. The accurate calculation of the non-steady state ionization balance is identified as a key issue. Time resolved and time integrated output profiles are generated for various experimental configurations, and the effects of saturation and gain narrowing are examined. The agreement with experiment is excellent in virtually every respect.
We have studied the effect of prepulses in enhancing the efficiency of generating ASE beams in soft X-ray laser plasma amplifiers based on pumping Ne-like ions. Slab targets were irradiated with a weak prepulse followed by a main plasma heating pulse of nanosecond duration. Time-integrated; time and spectrally resolved and time and angularly resolved lasing emissions on the 3p-3s (J=0-1) XUV lasing lines of Ne-like Ni, Cu and Zn at wavelengths 232 Aring, 221 Aring and 212 Aring respectively have been monitored. Measurements were made for pre-pulse/main-pulse intensity ratios from 10 -5-10 -1 and for pump delay times of 2 ns and 4.5 ns. Zinc is shown to exhibit a peak in output intensity at ~2times10 -3 pre-pulse fraction for a 4.5 ns pump delay, with a main pulse pump intensity of ~1.3times10 13 Wcm -2 on a 20 mm target. The Zn lasing emission had a duration of ~240 ps and this was insensitive to prepulse fraction. The J=0-1 XUV laser output for nickel and copper increased monotonically with prepulse fraction, with copper targets showing least sensitivity to either prepulse level or prepulse to main pulse delay. Under the conditions of the study, the pre-pulse level was observed to have no significant influence on the output intensity of the 3p-3s (J=2-1) lines of any of the elements investigated
A saturated nickel-like samarium X-ray laser beam at 7 nanometers has been demonstrated with an output energy of 0.3 millijoule in 50-picosecond pulses, demonstrating that saturated operation of a laser at wavelengths shorter than 10 nanometers can be achieved. The narrow divergence, short wavelength, short pulse duration, high efficiency, and high brightness of this samarium laser make it an ideal candidate for many X-ray laser applications
A comparison is presented of the temporally resolved resonance-line emission from the Ne-like Ge XUV laser (pumped with nanosecond pulses) with the predictions for the same emission from the hydro-atomic code EHYBRID. The specific lines chosen were the two 3s - 2p Ne-like lines at 10.01 and 9.762 Å, and the 3s - 2p F-like group of lines in the 9.4 - 9.6 Å region. Modification of the code to include 112 excited levels of the F-like ion facilitated a direct comparison between experiment and model of (i) the temporal variation of the emissions and (ii) the variation of the peak intensity ratios of the F-like to Ne-like emissions with irradiance on target.
Agreement between experiment and model was generally good, with the trends of the F/Ne ratio variation with irradiance being clearly reproduced. The results show that the model is realistically simulating the hydrodynamics and atomic kinetics of the plasma. This provides further support that the model can simultaneously calculate the and the (sub-keV) transition intensities through population dynamics which are acceptably accurate.
The complex problem of refraction in collisionally pumped x-ray lasers is examined through a three-dimensional ray-tracing code, used as a post-processor to a 1 1/2-dimensional hydro/atomic code, to generate experimentally observable beam characteristics such as divergence and output intensity. A detailed description of the model is presented. Results from the model are compared with a series of experiments carried out at Rutherford Appleton Laboratories (RAL), where germanium slab targets of lengths 0.9–2.2 cm were irradiated. Good agreement between theory and experiment is found.
The Oxford MCP/MCDF and MCBP/BENA packages have been rewritten in FORTRAN 77 and combined in the new code, GRASP. This is more versatile than its predecessors, contains more stable and accurate numerical procedures and a simplified but more flexible interface. Array dimensions and installation-dependent parameters may be set by the user. All known errors in previous versions have been eliminated.A comprehensive user's manual is now provided as supplementary documentation.
Amplified spontaneous emission lasers normally operate in one of two modes. Either the travelling wave mode whereby the short duration pump and laser fields propagate simultaneously along the lasant towards the output; there is thus a strong forward beam, and negligible backward – uni-directional output. Or the quasi-steady mode in which the gain is uniform in space and time and identical beams propagate forward and backward – bi-directional output. If the pump is applied over times comparable to the beam transit time, the mode of operation will change from one of these modes to the other. We examine this transition concentrating particularly on the case of strongly saturated emission. It is found that if the laser is started in the uni-directional mode, the transition to bi-directional is an exponential decay, and may be slow if the gain-length product is large.
It is found that ultrashort pulses in a high-gain 3.51-μm xenon laser propagate through the amplifying medium at a velocity less than the vacuum speed of light by as much as a factor of 2.5. The pulse velocity is a function of the gain and agrees with the group velocity.
We report the first demonstration of saturation in a Ni-like x-ray laser, specifically Ni-like Ag x-ray laser at 14 nm. Using high-resolution spatial imaging and angularly resolved streaking techniques, the output source size as well as the time history, divergence, energy, and spatial profile of the output beam have been fully characterized. The output intensity of the Ag laser was measured to be about 70GWcm-2. The narrow divergence, short pulse duration, high efficiency, and high brightness of the Ag laser make it an ideal candidate for many x-ray laser applications.
A ∼3 ps travelling wave chirped pulse amplified pulse at 6×1014 W cm−2 superimposed on ∼300 ps background pulses is shown to be an efficient method to pump transient collisional excitation X-ray lasers in both Ni-like and Ne-like ions. Measurements of X-ray laser output as a function of plasma length are fitted with results of an amplified spontaneous emission model of the laser output taking account of travelling wave pumping effects. A small signal gain coefficient ∼42 cm−1 and a effective gain length product of ∼18 are measured for the Ni-like Sn laser at 120 Å. Simulations from a hydrodynamic and atomic physics code (EHYBRID) coupled to a ray trace code show that a spatially averaged small signal gain ∼65 cm−1 can be obtained in Ne-like Ge provided the optimum pumping pulse arrangement is used.
The development of X-ray lasers is marked by the progression toward shorter wavelengths using less drive energy. A recent X-ray laser
experiment at the Rutherford Appleton Laboratory (RAL) demonstrated saturated X-ray laser operation in the Ne-like Ti and Ge
X-ray laser schemes at 32.6 and 19.6 nm respectively, with a drive energy of only a few joules. In this paper we describe the laser development to generate the laser drive pulses and travelling wave line-focus target irradiation geometry required for this experiment.
Saturated operation of a laser at a wavelength shorter than 6 nm is demonstrated. The output of the Ni-like Dy laser at 5.86 and 6.37 nm, pumped by the VULCAN 1.05-mum Nd-glass laser in a 75-ps double-pulse configuration at 2×1013 W cm-2 peak irradiance, is measured experimentally and studied theoretically using a rate equation and one-dimensional amplified spontaneous-emission model. The experimental results and modeling show that, upon saturation, the output intensities from the lower gain lasing transition at 6.37 nm decrease with increasing gain medium length. This is a different signature of gain saturation for Ni-like lasers and is desirable for applications, as it shows that one lasing transition will dominate with Ni-like lasers in saturation.
Saturation of a low pump energy x-ray laser utilizing a transient inversion mechanism on the 3p-3s transition at 32.63 nm in Ne-like Ti has been demonstrated. A close to saturation amplification was simultaneously achieved for the 3d-3p, J=1-->1 transition at 30.15 nm. Small signal effective transient gain coefficients of g~46 and ~35 cm-1 and gain-length products of 16.7 and 16.9 for these lines were obtained. Experiments demonstrate that it is possible to achieve saturated laser action in a transient regime with Ne-like Ti for a pump energy as low as ~5 J.
We report an application of the prepulse technique which uses a low-intensity prepulse before the main optical drive pulse to prepare the plasma prior to lasing in low-[ital Z], Ne-like ions. Ne-like x-ray lasers are now available over a previously inaccessible range of wavelengths. As an illustration of this technique we report an observation of lasing in Cr[sup 14+] and Fe[sup 16+] on the Ne-like [ital J]=0[r arrow]1, 3[ital p][r arrow]3[ital s] transitions at 285 and 255 A as well as gain measurements for Ti[sup 12+].
The interaction between radiation and matter in an x-ray laser occurring in a cylindrical laser-produced plasma is investigated by means of the Maxwell-Bloch equations with level degeneracies explicitly taken into account. Radiation is initiated by spontaneous emission and amplified while propagating along the cylinder axis. This theory generalizes the current simple theory of the small-signal gain coefficient and is suited to correctly describe the gradual transition to the saturation regime. A detailed numerical study of the germanium x-ray laser is performed to illustrate the theory for the case of the Ne-like ion collisional laser. The effect of electron-ion collisions leading to transitions between degenerate states on linewidths, as well as on saturation behavior is outlined.
The authors have been developing the physics and technology of a class of short wavelength laser where the upper laser level is embedded within the continuum of a valence electron. The advantage of this type of laser, compared with more typical ionic lasers, is that the upper laser level occurs at much lower energy and therefore may be pumped by cooler X-rays or electrons. This allows the operation of such laser at pumping energies and powers much lower than would otherwise be possible. A 12.8-ev laser in neutral Cs is described. This laser achieves an equivalent small signal gain of exp(83) at a pumping energy of 2.5 J in 20 ps. The authors have calculated an upper level autoionizing time of 62 ps and estimated an upper level population of approximately 1014 atoms/cm3. With 2.5 J of 1064-nm pumping energy in a 20-ps pulse, the Cs laser saturates in approximately 4 cm and output grows linearly with length for the remainder of the 17 cm.
The \rightarrow\rightarrow transition at 182 \AA{} in neonlike selenium is observed to completely dominate the lasing output, as predicted but never observed before this work. Instead of a single long pulse we use a series of short 100 ps pulses from the Nova laser to illuminate slab targets of selenium. The 182 \AA{} line is more than an order of magnitude brighter than the usual 206 and 209 \AA{} laser lines. Similar behavior is observed for neonlike ions of germanium, zinc, and nickel.
We report the demonstration of efficient soft x-ray lasing in nickel-like lanthanide elements (Nd, Sm, Gd, Tb, and Dy) covering the spectral range between 6 and 8 nm. A curved slab target was pumped by 1.053- mum-wavelength multiple laser pulses: two or three 100-ps-duration pulses separated by 400 ps. A gain coefficient of 3.1 cm-1 and a gain-length product of 7.8 have been achieved at 7.97 nm in the Nd ions with 250 J pumping energy on a 2.5-cm-length target. The result represents close to an order of magnitude reduction in the energy required to pump these soft x-ray lasers.
We have demonstrated small signal gain saturation on several transient-gain Ni-like ion x-ray lasers by using a high-power, chirped-pulse amplification, tabletop laser. These results have been achieved at wavelengths from 139-203 A using a total of 5-7 J energy in a traveling-wave excitation scheme. Strong amplification is also observed for Ni-like Sn at 119 A. Gain of 62 cm(-1) and gL product of 18 are determined on the 4d-->4p transition for Ni-like Pd at 147 A with an output energy of 12 &mgr;J. A systematic evaluation of the laser driver parameters yields optimum beam divergence and small deflection angles of 2-5 mrads, in good agreement with simulations.
- H. L. Zhang