Dosimetry and spectral analysis of a radiobiological experiment using laser-driven proton beams

Laser-driven proton and ion acceleration is an area of increasing research interest given the recent development of short pulse-high intensity lasers. Several groups have reported experiments to understand whether a laser-driven beam can be applied for radiobiological purposes and in each of these, the method to obtain dose and spectral analysis was slightly different. The difficulty with these studies is that the very large instantaneous dose rate is a challenge for commonly used dosimetry techniques, so that other more sophisticated procedures need to be explored. This paper aims to explain a method for obtaining the energetic spectrum and the dose of a laser-driven proton beam irradiating a cell dish used for radiobiology studies. The procedure includes the use of a magnet to have charge and energy separation of the laser-driven beam, Gafchromic films to have information on dose and partially on energy, and a Monte Carlo code to expand the measured data in order to obtain specific details of the proton spectrum on the cells. Two specific correction factors have to be calculated: one to take into account the variation of the dose response of the films as a function of the proton energy and the other to obtain the dose to the cell layer starting from the dose measured on the films. This method, particularly suited to irradiation delivered in a single laser shot, can be applied in any other radiobiological experiment performed with laser-driven proton beams, with the only condition that the initial proton spectrum has to be at least roughly known. The method was tested in an experiment conducted at Queen's University of Belfast using the TARANIS laser, where the mean energy of the protons crossing the cells was between 0.9 and 5 MeV, the instantaneous dose rate was estimated to be close to 10(9) Gy s(-1) and doses between 0.8 and 5 Gy were delivered to the cells in a single laser shot. The combination of the applied corrections modified the initial estimate of dose by up to 40%.

Dynamics of nanometer-scale foil targets irradiated with relativistically intense laser pulses

In this paper we report on an experimental study of high harmonic radiation generated in nanometer-scale foil targets irradiated under normal incidence. The experiments constitute the first unambiguous observation of odd-numbered relativistic harmonics generated by the v x B component of the Lorentz force verifying a long predicted property of solid target harmonics. Simultaneously the observed harmonic spectra allow in-situ extraction of the target density in an experimental scenario which is of utmost interest for applications such as ion acceleration by the radiation pressure of an ultraintense laser.

High contrast plasma mirror: spatial filtering and second harmonic generation at 10(19)Wcm(-2)

Recently, the use of plasma optics to improve temporal pulse contrast has had a remarkable impact on the field of high- power laser-solid density interaction physics. Opening an avenue to previously unachievable plasma density gradients in the high intensity focus, this advance has enabled researchers to investigate new regimes of harmonic generation and ion acceleration. Until now, however, plasma optics for fundamental laser reflection have been used in the sub-relativistic intensity regime (10(15) - 10(16)Wcm(-2)) showing high reflectivity (similar to 70%) and good focusability. Therefore, the question remains as to whether plasma optics can be used for such applications in the relativistic intensity regime (> 10(18)Wcm(-2)). Previous studies of plasma mirrors (PMs) indicate that, for 40 fs laser pulses, the reflectivity fluctuates by an order of magnitude and that focusability of the beam is lost as the intensity is increased above 5 x 10(16)Wcm(-2). However, these experiments were performed using laser pulses with a contrast ratio of similar to 10(7) to generate the reflecting surface. Here, we present results for PM operation using high contrast laser pulses resulting in a new regime of operation - the high contrast plasma mirror (HCPM). In this regime, pulses with contrast ratio > 10(10) are used to form the PM surface at > 10(19)Wcm(-2), displaying excellent spatial filtering, reflected near- field beam profile of the fundamental beam and reflectivities of 60 +/- 5%. Efficient second harmonic generation is also observed with exceptional beam quality suggesting that this may be a route to achieving the highest focusable harmonic intensities. Plasma optics therefore offer the opportunity to manipulate ultra-intense laser beams both spatially and temporally. They also allow for ultrafast frequency up-shifting without detrimental effects due to group velocity dispersion (GVD) or reduced focusability which frequently occur when nonlinear crystals are used for frequency conversion.

Dynamics of charge-displacement channeling in intense laser-plasma interactions

The dynamics of transient electric fields generated by the interaction of high intensity laser pulses with underdense plasmas has been studied experimentally with the proton projection imaging technique. The formation of a charged channel, the propagation of its front edge and the late electric field evolution have been characterized with high temporal and spatial resolution. Particle-in-cell simulations and an electrostatic, ponderomotive model reproduce the experimental features and trace them back to the ponderomotive expulsion of electrons and the subsequent ion acceleration.

Dynamics of Self-Generated, Large Amplitude Magnetic Fields Following High-Intensity Laser Matter Interaction

The dynamics of magnetic fields with an amplitude of several tens of megagauss, generated at both sides of a solid target irradiated with a high-intensity (~1019W/cm2) picosecond laser pulse, has been spatially and temporally resolved using a proton imaging technique. The amplitude of the magnetic fields is sufficiently large to have a constraining effect on the radial expansion of the plasma sheath at the target surfaces. These results, supported by numerical simulations and simple analytical modeling, may have implications for ion acceleration driven by the plasma sheath at the rear side of the target as well as for the laboratory study of self-collimated high-energy plasma jets. © 2012 American Physical Society.

Bright Laser-Driven Neutron Source Based on the Relativistic Transparency of Solids

Neutrons are unique particles to probe samples in many ?elds of research ranging from biology to material sciences to engineering and security applications. Access to bright, pulsed sources is currently
limited to large accelerator facilities and there has been a growing need for compact sources over the recent years. Short pulse laser driven neutron sources could be a compact and relatively cheap way to
produce neutrons with energies in excess of 10 MeV. For more than a decade experiments have tried to obtain neutron numbers suf?cient for applications. Our recent experiments demonstrated an ion acceleration mechanism based on the concept of relativistic transparency. Using this new mechanism, we produced an intense beam of high energy (up to 170 MeV) deuterons directed into a Be converter to
produce a forward peaked neutron ?ux with a record yield, on the order of 1010 n=sr. We present results comparing the two acceleration mechanisms and the ?rst short pulse laser generated neutron radiograph.

Parametric study of non-relativistic electrostatic shocks and the structure of their transition layer

Nonrelativistic electrostatic unmagnetized shocks are frequently observed in laboratory plasmas and they are likely to exist in astrophysical plasmas. Their maximum speed, expressed in units of the ion acoustic speed far upstream of the shock, depends only on the electron-to-ion temperature ratio if binary collisions are absent. The formation and evolution of such shocks is examined here for a wide range of shock speeds with particle-in-cell simulations. The initial temperatures of the electrons and the 400 times heavier ions are equal. Shocks form on electron time scales at Mach numbers between 1.7 and 2.2. Shocks with Mach numbers up to 2.5 form after tens of inverse ion plasma frequencies. The density of the shock-reflected ion beam increases and the number of ions crossing the shock thus decreases with an increasing Mach number, causing a slower expansion of the downstream region in its rest frame. The interval occupied by this ion beam is on a positive potential relative to the far upstream. This potential pre-heats the electrons ahead of the shock even in the absence of beam instabilities and decouples the electron temperature in the foreshock ahead of the shock from the one in the far upstream plasma. The effective Mach number of the shock is reduced by this electron heating. This effect can potentially stabilize nonrelativistic electrostatic shocks moving as fast as supernova remnant shocks. 

Radiobiology at ultra-high dose rates employing laser-driven ions

The potential that laser based particle accelerators offer to solve sizing and cost issues arising with conventional proton therapy has generated great interest in the understanding and development of laser ion acceleration, and in investigating the radiobiological effects induced by laser accelerated ions. Laser-driven ions are produced in bursts of ultra-short duration resulting in ultra-high dose rates, and an investigation at Queen's University Belfast was carried out to investigate this virtually unexplored regime of cell rdaiobiology. This employed the TARANIS terawatt laser producing protons in the MeV range for proton irradiation, with dose rates exceeding 10 Gys on a single exposure. A clonogenic assay was implemented to analyse the biological effect of proton irradiation on V79 cells, which, when compared to data obtained with the same cell line irradiated with conventionally accelerated protons, was found to show no significant difference. A Relative Biological effectiveness of 1.4±0.2 at 10 % Survival Fraction was estimated from a comparison with a 225 kVp X-ray source. © 2013 SPIE.

Azimuthal asymmetry in collective electron dynamics in relativistically transparent laser-foil interactions

Asymmetry in the collective dynamics of ponderomotively-driven electrons in the interaction of an ultraintense laser pulse with a relativistically transparent target is demonstrated experimentally. The 2D profile of the beam of accelerated electrons is shown to change from an ellipse aligned along the laser polarization direction in the case of limited transparency, to a double-lobe structure aligned perpendicular to it when a significant fraction of the laser pulse co-propagates with the electrons. The temporally-resolved dynamics of the interaction are investigated via particle-in-cell simulations. The results provide new insight into the collective response of charged particles to intense laser fields over an extended interaction volume, which is important for a wide range of applications, and in particular for the development of promising new ultraintense laser-driven ion acceleration mechanisms involving ultrathin target foils.

Beam profiles of proton and carbon ions in the relativistic transparency regime

Ion acceleration from relativistic laser solid interactions has been of particular interest over the last decade. While beam profiles have been studied for target normal sheath acceleration (TNSA), such profiles have yet to be described for other mechanisms. Here, experimental data is presented, investigating ion beam profiles from acceleration governed by relativistic transparent laser plasma interaction. The beam shape of carbon C6+ ions and protons has been measured simultaneously with a wide angle spectrometer. It was found that ion beams deviate from the typical Gaussian-like shape found with TNSA and that the profile is governed by electron dynamics in the volumetric laser-plasma interaction with a relativistically transparent plasma; due to the ponderomotive force electrons are depleted from the center of the laser axis and form lobes affecting the ion beam structure. The results are in good agreement with high resolution three-dimensional-VPIC simulations and can be used as a new tool to experimentally distinguish between different acceleration mechanisms.

Transport of laser accelerated proton beams and isochoric heating of matter

The acceleration of intense proton and ion beams by ultra-intense lasers has matured to a point where applications in basic research and technology are being developed. Crucial for harvesting the unmatched beam parameters driven by the relativistic electron sheath is the precise control of the beam. We report on recent experiments using the PHELIX laser at GSI, the VULCAN laser at RAL and the TRIDENT laser at LANL to control and use laser accelerated proton beams for applications in high energy density research. We demonstrate efficient collimation of the proton beam using high field pulsed solenoid magnets, a prerequisite to capture and transport the beam for applications. Furthermore we report on two campaigns to use intense, short proton bunches to isochorically heat solid targets up to the warm dense matter state. The temporal profile of the proton beam allows for rapid heating of the target, much faster than the hydrodynamic response time thereby creating a strongly coupled plasma at solid density. The target parameters are then probed by X-ray Thomson scattering (XRTS) to reveal the density and temperature of the heated volume. This combination of two powerful techniques developed during the past few years allows for the generation and investigation of macroscopic samples of matter in states present in giant planets or the interior of the earth.

Observation of the inhomogeneous spatial distribution of MeV ions accelerated by the hydrodynamic ambipolar expansion of clusters

An inhomogeneous spatial distribution of laser accelerated carbon/oxygen ions produced via the hydrodynamic ambipolar expansion of CO₂ clusters has been measured by using CR-39 detectors. An inhomogeneous etch pits spatial distribution has appeared on the etched CR-39 detector installed on the laser propagation direction, while homogeneous ones are appeared on those installed at 45°and 90°from the laser propagation direction. From the range of ions in CR-39 obtained by using the multi-step etching technique, the averaged energies of carbon/oxygen ions for all directions are determined as 0.78 ± 0.09 MeV/n. The number of ions in the laser propagation direction is about 1.5 times larger than those in other directions. The inhomogeneous etch pits spatial distribution in the laser propagation direction could originate from an ion beam collimation and modulation by the effect of electromagnetic structures created in the laser plasma.

Picosecond metrology of laser-driven proton bursts

Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.

Buffered high charge spectrally-peaked proton beams in the relativistic-transparency regime

Spectrally-peaked proton beams of high charge (E_p ≈ 8 MeV, ΔE ≈ 4 MeV, N ≈ 50 nC ) have been observed from the interaction of an intense laser (>10¹⁹Wcm^-2) with ultrathinCHfoils, as measured by spectrally-resolved full beam profiles. These beams are reproducibly generated for foil thicknesses 5-100 nm, and exhibit narrowing divergence with decreasing target thickness down to ≈8° for 5 nm. Simulations demonstrate that the narrow energy spread feature is a result of buffered acceleration of protons. The radiation pressure at the front of the target results in asymmetric sheath fields which permeate throughout the target, causing preferential forward acceleration. Due to their higher chargeto-mass ratio, the protons outrun a carbon plasma driven in the relativistic transparency regime.

Radiation-Pressure Acceleration of Ion Beams from Nanofoil Targets: The Leaky Light-Sail Regime

A new ion radiation-pressure acceleration regime, the "leaky light sail," is proposed which uses sub-skin-depth nanometer foils irradiated by circularly polarized laser pulses. In the regime, the foil is partially transparent, continuously leaking electrons out along with the transmitted laser field. This feature can be exploited by a multispecies nanofoil configuration to stabilize the acceleration of the light ion component, supplementing the latter with an excess of electrons leaked from those associated with the heavy ions to avoid Coulomb explosion. It is shown by 2D particle-in-cell simulations that a monoenergetic proton beam with energy 18 MeV is produced by circularly polarized lasers at intensities of just 10(19) W/cm(2). 100 MeV proton beams are obtained by increasing the intensities to 2 x 10(20) W/cm(2).