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Divergence of laser-driven ion beams from nanometer thick foils
Abstract and Figures
We report on experimental studies of divergence of proton beams from
nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser
with high contrast. Proton beams with extremely small divergence (half angle)
of 2 degree are observed in addition with a remarkably well-collimated feature
over the whole energy range, showing one order of magnitude reduction of the
divergence angle in comparison to the results from micrometer thick targets. We
demonstrate that this reduction arises from a steep longitudinal electron
density gradient and an exponentially decaying transverse profile at the rear
side of the ultrathin foils. Agreements are found both in an analytical model
and in particle-in-cell simulations. Those novel features make nm foils an
attractive alternative for high flux experiments relevant for fundamental
research in nuclear and warm dense matter physics.
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Content may be subject to copyright.
... Regarding silicon (considering 28 Si only, that accounts for 92.2% of natural silicon), the first reaction channel that leads to a neutron as output is 28 Si(p, 3 He + n) 26 Al, whose reaction threshold is around 9 MeV. For oxygen ( 16 O almost 100% abundance in natural oxygen), at around 6 MeV, a few reaction channels open up leading to the production of neutrons such as: 16 O(p, 3 He + n) 13 N, 16 O(p,n + p) 15 O and 16 O(p,n + d) 14 O. Even though boron accounts only for a small fraction, it's worth mentioning that 11 B (around 90% abundance in natural boron) shows a 3 MeV cross section threshold for the 11 B(p,n) 11 C reaction. ...
... Here, reactions with 16 O and 14 N are the most probable. The (p,n + 3 He) reaction channel on 14 N opens at about 2.5 MeV and has its maximum cross section at around 10 MeV, while the (p,n) reaction channel opens at 6.3 MeV. ...
At the Center for Advanced Laser Applications (CALA), Garching, Germany, the LION (Laser-driven ION Acceleration) experiment is being commissioned, aiming at the production of laser-driven bunches of protons and light ions with multi-MeV energies and repetition frequency up to 1 Hz. A Geant4 Monte Carlo-based study of the secondary neutron and photon fields expected during LION’s different commissioning phases is presented. Goal of this study is the characterization of the secondary radiation environment present inside and outside the LION cave. Three different primary proton spectra, taken from experimental results reported in the literature and representative of three different future stages of the LION’s commissioning path are used. Together with protons, also electrons are emitted through laser-target interaction and are also responsible for the production of secondary radiation. For the electron component of the three source terms, a simplified exponential model is used. Moreover, in order to reduce the simulation complexity, a two-components simplified geometrical model of proton and electron sources is proposed. It has been found that the radiation environment inside the experimental cave is either dominated by photons or neutrons depending on the position in the room and the source term used. The higher the intensity of the source, the higher the neutron contribution to the total dose for all scored positions. Maximum neutron and photon ambient dose equivalent values normalized to 10 ⁹ simulated incident primaries were calculated at the exit of the vacuum chamber, where values of about 85 nSv (10 ⁹ primaries) ⁻¹ and 1.0 μSv (10 ⁹ primaries) ⁻¹ were found.
... The divergence angles of the single-layer target are almost constant (11.5 6 0.7 ) for different energies, which is consistent with previous work's results. 36 In the case of the double-layer target, the divergence of low-energy protons is doubled compared to that of the single-layer target. With the increase in energy, the divergence angles notably decrease. ...
Double-layer targets composed of near-critical-density carbon nanotube foams (CNFs) and solid foils have shown their advantages in laser-driven ion acceleration under high relativistic intensity. Here, we report the experimental and numerical results on the laser-accelerated proton beams from such targets under moderate relativistic intensities [Formula: see text]. 40-TW femtosecond laser pulses were used to irradiate CNF-based double-layer targets. Compared to single-layer targets, significant enhancements on the cutoff energy and numbers of ions were observed. It was found that the CNF layer also leads to a larger divergence angle and a more homogeneous spatial distribution profile of the proton beam. Particle-in-cell simulations reveal the reason for the enhanced proton acceleration. It is found that the lateral electric field and the strong magnetic field built by the directly accelerated electrons from the CNF layer contribute to the enlarged divergence angle.
... Proper energy selectors need to be developed and a primary task is to control the stability of the laser-target interaction and to be able to characterize the dose and the fluency delivered to the patient. Possible solutions to these issues have been already proposed both by means of target engineering 80,81 and ion optics 82,83 able to focus and energy-select the emitted ion beam. Another task to accomplish before a laser-based ion source could replace a conventional accelerator, is the enhancement of the maximum achievable energy. ...
Time-Of-Flight (TOF) methods are very effective to detect ions accelerated in laser plasma interactions, but they show significant limitations when used in experiments
with high energy and intensity lasers, where both high-energy ions and remarkable levels of ElectroMagnetic Pulses (EMPs) in the radiofrequency-microwave range are generated.
In this joined-doctoral thesis, performed at the La Sapienza University in Rome, at the institut national de la recherche scientifique (INRS) in Montréal and at ENEA Centro Ricerche Frascati, an advanced diagnostic technique for the characterization of protons accelerated by intense laser-matter interactions with high-energy and high-intensity lasers has been implemented.
The proposed method exploits and improves the advantages given by TOF technique coupled to Chemical Vapor Deposition diamond detectors and features high sensitivity, high energy resolution and high radiation hardness. Thanks to the optimization of the acquisition system and to the careful setup of the TOF line, high signal-to-noise ratios in environments heavily affected by remarkable EMP fields have been achieved.
In the first part of the work a brief overview of laser-matter interaction is given, with particular emphasis to processes leading to particle acceleration. Then, the main diagnostic techniques available for the characterization of secondary sources produced by laser-matter interaction are presented. Here the TOF technique is introduced and analyzed when coupled with different kinds of detectors, including diamonds. The choice of the latter is justified by their physical properties. Various types of diamond structures and electrode layouts have been tested and their performances characterized for application as detectors to be employed in TOF lines.
The second part of the work is dedicated to a detailed description of the proposed advanced technique and to examples of its effectiveness in reducing the EMP noise and in enhancing the dynamic range, when employed in real experimental scenarios. A novel procedure to retrieve a calibrated proton spectrum from the performed measurements is here also proposed and discussed.
Eventually, the developed technique is applied for detecting laser-plasma accelerated particles produced in different application scenarios and in the most variable laser-matter interaction conditions and the concept of a new multi-layer detector is also described.
... The source size in laser-ion acceleration from thin foils is typically in the order of a few m [15,16] and was here approximated by a point source. The divergence of the proton bunches was sampled from a Gaussian distribution with a full angle of θ = 5 • (FWHM), which is a rather modest divergence angle for such proton sources [17,18]. Since a spatially homogeneous proton fluence would be beneficial for imaging, the choice of this small divergence angle could approach a worst case scenario. ...
Laser-accelerated proton bunches with kinetic energies up to several tens of MeV and at repetition rates in the order of Hz are nowadays achievable at several research centres housing high-power laser system. The unique features of such ultra-short bunches are also arousing interest in the field of radiological and biomedical applications. For many of these applications, accurate positioning of the biological target is crucial, raising the need for on-site imaging. One convenient option is proton radiography, which can exploit the polyenergetic spectrum of laser-accelerated proton bunches. We present a Monte Carlo (MC) feasibility study to assess the applicability and potential of laser-driven proton radiography of millimetre to centimetre sized objects. Our radiography setup consists of a thin time-of-flight spectrometer operated in transmission prior to the object and a pixelated silicon detector for imaging. Proton bunches with kinetic energies up to 20 MeV and up to 100 MeV were investigated. The water equivalent thickness (WET) of the traversed material is calculated from the energy deposition inside an imaging detector, using an online generated calibration curve that is based on a MC generated look-up table and the reconstructed proton energy distribution. With a dose of 43 mGy for a 1 mm thin object imaged with protons up to 20 MeV, the reconstructed WET of defined regions-of-interest was within 1.5% of the ground truth values. The spatial resolution, which strongly depends on the gap between object and imaging detector, was 2.5 lp mm⁻¹ for a realistic distance of 5 mm. Due to this relatively high imaging dose, our proposed setup for laser-driven proton radiography is currently limited to objects with low radio-sensitivity, but possibilities for further dose reduction are presented and discussed.
... Compared to other published experiments with PW lasers in the TNSA regime, the proton beam divergence was significantly reduced by a factor of 3-5 [36], reaching values of advanced acceleration schemes [38][39][40]. Together with the increased proton numbers, the charge density reached values so far only accessible with single shot laser systems and/or advanced acceleration mechanisms [3]. ...
We present a study of laser-ion acceleration, where an increased laser spot size leads to sheath field geometries that accelerate ion beams of narrow and achromatic divergence at unprecedented charge densities, resulting from the high aspect ratio of the laser spot size to the acceleration distance. Matching the laser pulse length to the transverse laser spot size mitigated laser sweeping across the target front side and optimized the acceleration, with maximum energies deviating significantly from a linear intensity scaling.
... A very small normalized transverse emittance of below 0.01 mm mrad (compared to about 1 mm mrad in conventional accelerators [7]). This is resulting from the ion bunch originating from a small, micrometer source size and the (at present still rather) large divergence half angles ranging from two [8] up to some tens of degrees. ...
Laser-driven acceleration of ions has inspired novel applications, that can benefit from ion bunch properties different from conventionally (non-laser based) accelerated particle beams. Those differences range from extremely short bunch durations, broad energy spectra, large divergence angles and small source sizes to ultra-high ion bunch densities. So far, the main focus of research has been concentrating on the physics of the interaction of intense laser pulses with plasmas and the related mechanisms of ion acceleration. Now, the new Centre for Advanced Laser Applications (CALA) near Munich aims at pushing these ion bunches towards applications, including radiation therapy of tumors and the development of heavy ion bunches with solid-state-like density. These are needed for novel reaction mechanisms (‘fission-fusion’) to study the origin of heavy elements in the universe and to prepare for related studies at the upcoming EU-funded high-power laser facility ELI – Nuclear Physics in Bucharest.
... The ATLAS laser pulse is normally incident on nm-thin DLC foils with thickness of 5-20 nm. A magnetic spectrometer with a long entrance slit is employed to measure the energy and angular distributions of the protons emitted in the direction of laser propagation 32 . With an Al foil of 45 µm thickness placed in front of Image plate (IP) ion detector, protons with kinetic energies above 2 MeV are recorded. ...
Proton acceleration from nanometer thin foils with intense laser pulses is investigated experimentally. We analyzed the laser absorptivity by parallel monitoring of laser transmissivity and reflectivity with different laser intensities when moving the targets along the laser axis. A direct correlation between laser absorptivity and maximum proton energy is observed. Experimental results are interpreted in analytical estimation, exhibiting a coexistence of plasma expansion and light-sail form of radiation pressure acceleration (RPA-LS) mechanisms during the entire proton acceleration process based on the measured laser absorptivity and reflectivity.
Application experiments with laser plasma-based accelerators (LPA) for protons have to cope with the inherent fluctuations of the proton source. This creates a demand for non-destructive and online spectral characterization of the proton pulses, which are for application experiments mostly spectrally filtered and transported by a beamline. Here, we present a scintillator-based time-of-flight (ToF) beam monitoring system (BMS) for the recording of single-pulse proton energy spectra. The setup’s capabilities are showcased by characterizing the spectral stability for the transport of LPA protons for two beamline application cases. For the two beamline settings monitored, data of 122 and 144 proton pulses collected over multiple days were evaluated, respectively. A relative energy uncertainty of 5.5% (1σ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upsigma$$\end{document}) is reached for the ToF BMS, allowing for a Monte-Carlo based prediction of depth dose distributions, also used for the calibration of the device. Finally, online spectral monitoring combined with the prediction of the corresponding depth dose distribution in the irradiated samples is demonstrated to enhance applicability of plasma sources in dose-critical scenarios.
The availability of high-power, short-pulse laser systems has created over the last two decades a novel branch of accelerator physics. Laser-driven electron and ion acceleration based on the enormous acceleration fields that can be realized over very short acceleration lengths during ultra-short pulse periods has gained enormous interest and huge efforts are conducted worldwide to realize optimized acceleration schemes and beam properties, drawing on the rapid development of the underlying laser technology. The present chapter reviews this development, starting with a primer on the basic features of relativistic laser-plasma interaction. Subsequently, laser-driven acceleration mechanisms for electrons and ions are introduced. Finally, an example for the application of laser-driven ion bunches for nuclear astrophysics is given, based on the unprecedented high density of laser-accelerated ion bunches.
We report on the experimental studies of laser driven ion acceleration from double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer thin diamond-like carbon foil. A significant enhancement of proton maximum energies from 12 to ~30 MeV is observed when relativistic laser pulse impinge on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.
Ion beams are relevant for radiobiological studies and for tumor therapy. In contrast to conventional accelerators, laser-driven ion acceleration offers a potentially more compact and cost-effective means of delivering ions for radiotherapy. Here, we show that by combining advanced acceleration using nanometer thin targets and beam transport, truly nanosecond quasi-monoenergetic proton bunches can be generated with a table-top laser system, delivering single shot doses up to 7 Gy to living cells. Although in their infancy, laser-ion accelerators allow studying fast radiobiological processes as demonstrated here by measurements of the relative biological effectiveness of nanosecond proton bunches in human tumor cells.
A complete analytical description of ion acceleration in the laser radiation-pressure regime is presented. The combined effects of hot electron and light-pressure phenomena are used to qualitatively and quantitatively describe most recent experimental results in this regime. An essential part of the developed model is exhibited in the calculation of nonlinear laser light reflection and transmission properties, as well as in the spectral characterization of the laser light after interaction. The validity of the analytical model is supported by recent experimental results and by particle-in-cell simulations.
We propose an analytical model that analyzes the divergence of fast ion beams accelerated at the rear of thin foils irradiated with ultra-short intense laser pulses. We demonstrate the critical role played by the non-stationary character of the side components of the electric field, which is responsible for ion acceleration from the back of the foil. The model predictions are in very good agreement with 2D PIC simulations and with the experiments performed in the ultra-high-contrast regime as well.
The acceleration of ions by ultra-intense lasers has attracted great attention due to the unique properties and the unmatched intensities of the ion beams. In the early days the prospects for applications were already studied, and first experiments have identified some of the areas where laser accelerated ions can contribute to the ongoing inertial confinement fusion (ICF) research. In addition to the idea of laser driven proton fast ignition (PFI) and its use as a novel diagnostic tool for radiography the strong dependence on the electron transport in the target was found to be helpful in investigating the energy transport by electrons in fast ignitor scenarios. More recently an additional idea has been presented to use laser accelerated ion beams as the next generation ion sources, and taking advantage of the luminosity of the beams, to develop a test bed for heavy ion beam driven inertial confinement fusion physics. We review our recent experiments and simulations relevant to ICF research presenting a possible scenario for PFI as well as the prospects for next generation ion sources.
Using a pulse power solenoid, we demonstrate efficient capture of laser accelerated proton beams and the ability to control their large divergence angles and broad energy range. Simulations using measured data for the input parameters give inference into the phase-space and transport efficiencies of the captured proton beams. We conclude with results from a feasibility study of a pulse power compact achromatic gantry concept. Using a scaled target normal sheath acceleration spectrum, we present simulation results of the available spectrum after transport through the gantry.
We present experimental results from an all-optical diagnostic method to directly measure the evolution of the hot-electron distribution driving the acceleration of ions from thin foils using high-intensity lasers. Central parameters of laser ion acceleration such as the hot-electron density, the temperature distribution and the conversion efficiency from laser pulse energy into hot electrons become comprehensively accessible with this technique.
High-intensity laser plasma-based ion accelerators provide unsurpassed field gradients in the megavolt-per-micrometer range. They represent promising candidates for next-generation applications such as ion beam cancer therapy in compact facilities. The weak scaling of maximum ion energies with the square-root of the laser intensity, established for large sub-picosecond class laser systems, motivates the search for more efficient acceleration processes. Here we demonstrate that for ultrashort (pulse duration ~30 fs) highly relativistic (intensity ~10(21) W cm(-2)) laser pulses, the intra-pulse phase of the proton acceleration process becomes relevant, yielding maximum energies of around 20 MeV. Prominent non-target-normal emission of energetic protons, reflecting an engineered asymmetry in the field distribution of promptly accelerated electrons, is used to identify this pre-thermal phase of the acceleration. The relevant timescale reveals the underlying physics leading to the near-linear intensity scaling observed for 100 TW class table-top laser systems.
Ion beam therapy for cancer has proven to be a successful clinical approach, affording as good a cure as surgery and a higher quality of life. However, the ion beam therapy installation is large and expensive, limiting its availability for public benefit. One of the hurdles is to make the accelerator more compact on the basis of conventional technology. Laser acceleration of ions represents a rapidly developing young field. The prevailing acceleration mechanism (known as target normal sheath acceleration, TNSA), however, shows severe limitations in some key elements. We now witness that a new regime of coherent acceleration of ions by laser (CAIL) has been studied to overcome many of these problems and accelerate protons and carbon ions to high energies with higher efficiencies. Emerging scaling laws indicate possible realization of an ion therapy facility with compact, cost-efficient lasers. Furthermore, dense particle bunches may allow the use of much higher collective fields, reducing the size of beam transport and dump systems. Though ultimate realization of a laser-driven medical facility may take many years, the field is developing fast with many conceptual innovations and technical progress.
Ion acceleration phenomena at relativistic intense laser interaction with thin foil targets are studied to find an efficient laser-target interaction concept at the conditions, where neither the ponderomotive pressure of the laser light nor the hot electron pressure is negligible. Particle in cell simulations and the analytical model are allowing to predict optimum laser-target parameters and suggesting a significant increase of proton energy if a hybrid proton acceleration scheme is used. In the proposed scenario, the laser polarisation is changed during the acceleration process: First with circularly polarised laser light the target is accelerated as a whole by the ponderamotive pressure, and then with linearly polarised laser light the electrons are heated which additionally increases the accelerating field. The calculations are in good agreement with experimental findings.
A laser-accelerated dense electron sheet with an energy
E=[(g)\tilde] mc2E=\tilde{\gamma} mc^2 can be used as a relativistic mirror to
coherently reflect a second laser with photon energy ħω,
thus generating by the Doppler boost [A.Einstein, Annalen der Physik 17, 891 (1905); D. Habs etal., Appl. Phys. B 93, 349 (2008)] brilliant high-energy photon beams with
(h/2p)w¢=4[(g)\tilde]2(h/2p)w\hbar\omega^{\prime}=4\tilde{\gamma}^2\hbar\omega and
short duration for many new nuclear physics experiments.
While the shortest-lived atomic levels are
in the atto-second range, nuclear levels can have lifetimes
down to zeptoseconds. We discuss how the modulation of
electron energies in phase-locked laser fields
used for as-measurements [E. Goulielmakis etal., Science 317, 769 (2007)] can be carried over to the new direct measurement of fs–zs nuclear lifetimes by modulating the
energies of accompanying conversion electrons or emitted protons.
In the field of nuclear spectroscopy we discuss the new perspective
as a function of increasing photon energy.
In nuclear systems a much higher sensitivity is predicted to the time
variation of fundamental constants compared to atomic systems [V. Flambaum, arXiv:nucl-th/0801.1994v1 (2008)]. For energies up to 50keV Mössbauer-like
recoilless absorption allows to produce nuclear bosonic ensembles
with many delocalized coherent polaritons [G.V. Smirnov etal., Phys. Rev. A 71, 023804 (2005)] for the first
time. Using the (γ,n) reaction to produce
cold, polarized neutrons with a focusing ellipsoidal device [P. Böni, Nucl. Instrum. Meth. A 586, 1 (2008); Ch. Schanzer etal., Nucl. Instrum. Meth. 529, 63 (2004)], brilliant cold polarized micro-neutron
beams become available. The compact and relatively cheap
laser-generated γ beams may serve for
extended studies at university-based facilities.