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Experimental setup. A linearly polarized laser pulse is focused by an f/2.8 off-axis-parabola (OAP) mirror on targets at normal incidence. Electrons emitted from the back of the target are detected with electron spectrometers located at three different angles.
Source publication
The irradiation of few nm thick targets by a finite-contrast high-intensity
short-pulse laser results in a strong pre-expansion of these targets at the
arrival time of the main pulse. The targets decompress to near and lower than
critical densities plasmas extending over few micrometers, i.e. multiple
wavelengths. The interaction of the main pulse...
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Proton-driven plasma wakefield accelerators have numerically demonstrated substantially higher accelerating gradients compared to conventional accelerators and the viability of accelerating electrons to energy frontier in a single plasma stage. However, due to the intrinsic strong and radially varying transverse fields, the beam quality is still fa...
Citations
... However, the beam charge is limited to tens of pC due to beam loading effects [22,23]. When irradiating laser pulses onto targets with a higher density, such as a near-critical-density target or a solid target, electron beams with several nC even hundreds of nC charge can be produced [24][25][26]. The acceleration process is dominated by the DLA [26][27][28][29][30][31][32], such as ponderomotive acceleration [27,31], resonance acceleration [5,33] and vacuum acceleration [32,34]. ...
... When irradiating laser pulses onto targets with a higher density, such as a near-critical-density target or a solid target, electron beams with several nC even hundreds of nC charge can be produced [24][25][26]. The acceleration process is dominated by the DLA [26][27][28][29][30][31][32], such as ponderomotive acceleration [27,31], resonance acceleration [5,33] and vacuum acceleration [32,34]. In the DLA mechanisms, massive electrons gain energy directly from the laser electromagnetic fields with acceleration gradient up to 10s TV/m [32], which is different from the energy gain from plasma waves in the ILA regime. ...
High-quality ultrashort electron beams have diverse applications in a variety of areas, such as 4D electron diffraction and microscopy, relativistic electron mirrors and ultrashort radiation sources. Direct laser acceleration (DLA) mechanism can produce electron beams with a large amount of charge (several to hundreds of nC), but the generated electron beams usually have large divergence and wide energy spread. Here, we propose a novel DLA scheme to generate high-quality ultrashort electron beams by irradiating a radially polarized laser pulse on a nanofiber. Since electrons are continuously squeezed transversely by the inward radial electric field force, the divergence angle gradually decreases as electrons transport stably with the laser pulse. The well-collimated electron bunches are effectively accelerated by the circularly-symmetric longitudinal electric field and the relative energy spread also gradually decreases. It is demonstrated by three-dimensional (3D) simulations that collimated monoenergetic electron bunches with 0.75° center divergence angle and 14% energy spread can be generated. An analytical model of electron acceleration is presented which interprets well by the 3D simulation results.
... GeV、电 量6 pC的准单能电子束 [18] . 强激光与固体靶作用通常 可以产生电量高达几十pC甚至nC、温度MeV的超热 电子脉冲 [19,20] . [21] . ...
Traditional electron accelerators usually use the radio-frequency field as the driving field. In contrast, the terahertz radiation, with a shorter wavelength and a higher accelerating gradient, has the capability to work as the driver of compact electron accelerators in the future. In addition, the terahertz pulse can also provide an ultrafast modulated field to compress and measure the electron pulse width. Recently, the interactions between terahertz filed and electrons have attracted increasing attention. Intense laser-solid interactions can produce both a high-energy terahertz pulse and a high-charge electron beam simultaneously. This advantage enables a unique platform for the terahertz control of electrons and the terahertz pump-electron probe experiments. Here, by taking a feasible experimental layout as an example, we first investigate the interactions of a terahertz pulse with a co-propagating electron beam. Systematic discussions on the influences of several parameters on the terahertz field-induced electron deflection are presented via numerical simulations. We find that a strong terahertz electric field or the long pulse duration will enhance the deflection. Furthermore, the deflection depends strongly on the terahertz waveform. A preliminary experiment is performed to qualitatively demonstrate the relevant analysis.
... Injection mechanisms are well developed for the case of low-density plasma and LWFA [7,21], whereas in the DLA case they are still under investigation. Although at relatively moderate laser intensity (a 0 < 5) the particle acceleration in the plasma channel was reported theoretically and experimentally [22][23][24], the injection into the channel is poorly discussed. Jiang [25] considered DLA dynamics at ultra-relativistic intensity (a 0 = 20) in overdense plasma (that is feasible due to relativistic transparency). ...
The efficient injection of electrons into a propagating relativistic laser pulse with normalized vector potential a 0 ∼ 2 is demonstrated numerically and experimentally in a thin plasma layer with density 0.15–0.3 of the critical value. The injection is due to the wavebreaking of parametric plasma waves. The trapped particles gain multi-MeV (up to 20 MeV) energies by the direct laser acceleration in the plasma channel formed by the laser pulse in the lower density plasma tail. Numerical calculations were supported by experiments with micron-scale films pre-evaporated by an additional nanosecond laser pulse and a TW femtosecond laser facility. The experimentally observed bunch of electrons with energy above 1.6 MeV had a divergence of ∼0.05 rad and charge of ∼50 pC measured with photoneutron Be(g,n) reaction.
... Ultraintense and ultrashort laser-driven accelerators have become one of the most attractive topics in the accelerator community in recent years, due to their high accelerating gradients [1]. Since the first experiments on interactions between high-intensity laser pulses and solid targets, new research into different acceleration regimes has produced some meaningful experimental results [2][3][4][5][6][7]. Laser-driven accelerators with radiation pressure acceleration (RPA) have generated proton energies up to 93 MeV [8][9][10]. ...
A new laser-driven proton therapy facility is being designed by Peking University. The protons will be produced by laser-plasma interaction, using a 2-PW laser to reach proton energies up to 100 MeV. We hope that the construction of this facility will promote the real-world applications of laser accelerators. Based on the experimental results and design experience of existing devices in Peking University, we propose a beam transmission system which is suitable for the beam produced by laser acceleration, and demonstrate its feasibility through theoretical simulation. It is designed with two transport lines to provide both horizontal and vertical irradiation modes. We have used a locally-achromatic design method with new canted-cosine-theta (CCT) magnets. These two measures allow us to mitigate the negative effects of large energy spread produced by laser-acceleration, and to reduce the overall weight of the vertical beamline. The beamline contains a complete energy selection system, which can reduce the energy spread of the laser-accelerated beam enough to meet the application requirements. The users can select the proton beam energy within the range 40-100 MeV, which is then transmitted through the rest of the beamline. A beam spot with diameter of less than 15 mm and energy spread of less than 5% can be provided at the horizontal and vertical irradiation targets.
... Being the plasma collisionless and the dynamics of hot electron generation ultra-fast (∼ 10 − 100 fs), then the hot electron population does not have enough time to thermalize. Hence, in that case, the assumption of thermal equilibrium is unlikely to be accurate, as is also suggested by other works [22,23]. ...
... In equation (22) the (normalized) maximum ion energy is written as the sum of two contributions: ϕ * − 1/ζ, which does not depend on α, and a combination of other terms that do depend on α. Among the latter, the term proportional to (1 + 2α) would vanish if all hot electrons were trapped (note that, at equilibrium, if all electrons were trapped, i.e. if ζϕ * 1, then ϕ 0 → ϕ * − 1/ζ). ...
Plasma sheaths characterized by electrons with relativistic energies and far from thermodynamic equilibrium are governed by a rich and largely unexplored physics. A reliable kinetic description of relativistic non-equilibrium plasma sheaths—besides its interest from a fundamental point of view—is crucial to many application, from controlled nuclear fusion to laser-driven particle acceleration. Sheath models proposed in the literature adopt either relativistic equilibrium distribution functions or non-relativistic non-equilibrium distribution functions, making it impossible to properly capture the physics involved when both relativistic and non-equilibrium effects are important. Here we tackle this issue by solving the electrostatic Vlasov–Poisson equations with a new class of fully-relativistic distribution functions that can describe non-equilibrium features via a real scalar parameter. After having discussed the general properties of the distribution functions and the resulting plasma sheath model, we establish an approach to investigate the effect of non-equilibrium solely. Then, we apply our approach to describe laser–plasma ion acceleration in the target normal sheath acceleration scheme. Results show how different degrees of non-equilibrium lead to the formation of sheaths with significantly different features, thereby having a relevant impact on the ion acceleration process. We believe that this approach can offer a deeper understanding of relativistic plasma sheaths, opening new perspectives in view of their applications.
... Nanometer scale thick solid foil targets were also suggested recently as hot electron source targets. 11 Trapping electrons in the plasma wave requires minimum initial electron energy. 12 For moderate laser intensity of about 10 17 W/cm 2 the minimum electron energy required for trapping in plasma wave is about 1 MeV. ...
We are proposing hot electrons source, which are suitable for external injection into a wakefield accelerator. Hot electrons with energies up to 3 MeV were generated by the interaction of femtosecond laser at an intensity of I=3.5×1018 W/cm2 with the preplasma produced in 25 μm holes, drilled in 1 μm Au foils targets. The preplasma created by the 1 ns prepulse preceding the intense main laser pulse generates an elongated plasma under the critical density and scale length of tens of microns. This plasma channel enables generation of high energy and collimated electron beam. The proposed approach can allow minimizing current, laser based electron accelerators, to produce a new X –ray source, to generate relatively long, high density plasma source, which important for study of nonlinear effects related to Laser Fusion and other applications.
... One of the possibilities to increase the electron beam charge above a nC level keeping the electron energy at a level of tens up to hundreds of MeV, is to use the advantage of relativistic laser interaction with plasmas of subcritical and near critical density (NCD) [21][22][23][24]. The critical electron density is defined as n m e 4 L cr 2 2 w p = ( ) / where m and e are the mass of electron at rest and its charge and L w is the laser frequency. ...
Experiments were performed to study electron acceleration by intense sub-picosecond laser pulses propagating in sub-mm long plasmas of near critical electron density (NCD). Low density foam layers of 300–500 μm thickness were used as targets. In foams, the NCD-plasma was produced by a mechanism of super-sonic ionization when a well-defined separate ns-pulse was sent onto the foam-target forerunning the relativistic main pulse. The application of sub-mm thick low density foam layers provided a substantial increase of the electron acceleration path in a NCD-plasma compared to the case of freely expanding plasmas created in the interaction of the ns-laser pulse with solid foils. The performed experiments on the electron heating by a 100 J, 750 fs short laser pulse of 2–5 × 10¹⁹ W cm⁻² intensity demonstrated that the effective temperature of supra-thermal electrons increased from 1.5–2 MeV in the case of the relativistic laser interaction with a metallic foil at high laser contrast up to 13 MeV for the laser shots onto the pre-ionized foam. The observed tendency towards a strong increase of the mean electron energy and the number of ultra-relativistic laser-accelerated electrons is reinforced by the results of gamma-yield measurements that showed a 1000-fold increase of the measured doses. The experiment was supported by 3D-PIC and FLUKA simulations, which considered the laser parameters and the geometry of the experimental set-up. Both, measurements and simulations showed a high directionality of the acceleration process, since the strongest increase in the electron energy, charge and corresponding gamma-yield was observed close to the direction of the laser pulse propagation. The charge of super-ponderomotive electrons with energy above 30 MeV reached a very high value of 78 nC.
... There is a wide disparity of experimental results, and various mechanisms have been proposed, including resonant absorption, 34,35 J Â B heating, 36 ponderomotive acceleration, 37 stochastic heating, 38,39 acceleration by surface quasistatic fields, 40 or direct laser acceleration. 41,42 However, it is still unclear what mechanisms actually arise in experiments and the precise experimental conditions under which they appear are not known. This may be due to the lack of control and measurement of the density gradients, which makes the interpretations difficult. ...
We measure the emission of energetic electrons from the interaction between relativistic-intensity ultrashort laser pulses and a solid density plasma with a tunable density gradient scale length. We detect an electron beam that only appears with few-cycle pulses (<10 fs) and large plasma scale lengths (L > λ0). Numerical simulations, in agreement with the experiments, reveal that these electrons are accelerated by a laser wakefield. Plasma waves are indeed resonantly excited by the few-cycle laser pulses in the near-critical density region of the plasma. Electrons are then injected by ionization into the plasma waves and accelerated to relativistic energies. In this laser wakefield acceleration regime, the plasma waves are rotated by the plasma density gradient, which results in the electrons not being emitted in the same direction as the driving laser pulse.
... 12 For electron transport, various theoretical models have been developed, such as the classical electron transport model of Spitzer-H€ arm, 13,14 the flux limit model, 15 the nonlocal electron transport model 16 and the non-Maxwellian model. [17][18][19] Traditionally, in ICF study and simulations, it uses the AA atomic package and the flux limit model with a flux limiter f L of around 0.08, 20,21 and the simulations matched rather well with the hohlraum data. 22 However, the hohlraum energetic campaign in 2009 at the National Ignition Facility (NIF) 23 showed discrepancies with expectations from the traditional simulation model, including the hohlraum temperature, the level and spectrum of the Stimulated Raman light, and the tendency towards pancake-shaped implosions. ...
We coupled the one-dimensional multi-group radiation hydrodynamic code RDMG with the MBDCA atomic physics package, which uses the Matrix-Block Method to solve the coupled rate equations of the Detailed Configuration Accounting (DCA) non-LTE model, and applied the coupled code RDMG-MBDCA with different flux limiters fe to simulate a laser-irradiated CH-tamped Au disk experiment at the SGII laser facility. From our simulations, we found that a higher fe leads to faster laser ablation, earlier x-ray breakout time with a higher maximum x-ray flux, and an x-ray spectrum with a higher intensity. However, for the same fe, the simulation from RDMG with the DCA model shows a slower electron thermal conduction between the laser absorption region and the electron thermal conduction than that with the average-atom model. From our investigation, we can say that it is the lower ionization from DCA in the electron thermal conduction region which causes the slower electron thermal conduction between the two regions. The electron thermal conduction from DCA can be increased remarkably when the atomic processes of dielectronic capture and auto-ionization are turned off in simulation. This indicates that the atomic transition rate coefficients are important in determining the heat conduction and the plasma status for laser generated plasmas.
... According to existing works, there are two major schemes for producing large-current relativistic electrons: DLA (Gahn et al., 1999;Pukhov et al., 1999;Naseri et al., 2012;Toncian et al., 2016) and SMLWFA (Santala et al., 2001;Masuda et al., 2007;Shen et al., 2012). They will compete with each other or even co-exist in the same interaction Pukhov & Meyer-ter-Vehn, 2002;Adachi et al., 2006;Kneip et al., 2008;Shaw et al., 2014), and their underlying physics can be understood as below. ...
... Therefore, near-critical density (NCD) plasma is put forward to obtain high-yield energetic electrons (Gu et al., 2011;Iwawaki et al., 2014;Wang et al., 2015;Toncian et al., 2016). This density region not only enables the efficient acceleration mechanisms to happen, but also takes advantage of both the superiorities of gas and dense plasmas, resulting in intense energy absorption along with stronger and faster processes (Gahn et al., 1999;Liu et al., 2015). ...
The origin and characteristics of near-microcoulomb multi-MeV electrons accelerated by short pulse lasers interacting with near-critical density plasma in self-formed channels are studied using three-dimensional particle-in-cell simulations. According to the analysis on interaction phenomena and electron dynamics, the dominant mechanism turns out to be direct laser acceleration, which ensures the outstanding energy coupling. Additionally, self-channeling is found to be a decisive factor for the acceleration performance, as electrons obtain ultra-high energy through betatron resonance inside the channels. In our findings, by using a relativistic short laser pulse and near-critical plasma, a large amount of energetic electrons can be generated, presenting a promising and accessible route to ultraintense, high-spatial-resolution radiation pulses.
