R. C. Shah

All-optical Compton gamma-ray source

Scientists demonstrate a Compton-based electromagnetic source based on a laser-plasma accelerator and a plasma mirror. The source generates a broadband spectrum of X-rays and is 10,000 times brighter than Compton X-ray sources based on conventional accelerators.

High-temporal contrast using low-gain optical parametric amplification

We demonstrate the use of low-gain optical parametric amplification (OPA) as a means of improving temporal contrast to a detection-limited level 10(-10). 250 microJ, 500 fs pulses of 1053 nm are frequency doubled and subsequently restored to the original wavelength by OPA with >10% efficiency.

Efficient carbon ion beam generation from laser-driven volume acceleration

Experimental data on laser-driven carbon C6+ ion acceleration with a peak intensity of 5???1020?W?cm?2 are presented and compared for opaque target normal sheath acceleration (TNSA) and relativistically transparent laser?plasma interactions. Particle numbers, peak ion energy and conversion efficiency have been investigated for target thicknesses from 50?nm to 25??m using unprecedented full spectral beam profile line-out measurements made using a novel high-resolution ion wide-angle spectrometer. For thicknesses of about 200?nm, particle numbers and peak energy increase to 5???1011 carbon C6+ particles between 33 and 700?MeV (60?MeV?u?1), which is a factor of five higher in particle number than that observed for targets with micron thickness. For 200?nm thick targets, we find that the peak conversion efficiency is 6% and that up to 55% of the target under the laser focal spot is accelerated to energies above 33?MeV. This contrasts with the results for targets with micron thickness, where surface acceleration with TNSA is dominant. The experimental findings are consistent with two-dimensional particle-in-cell simulations.

Laser-driven ion acceleration from relativistically transparent nanotargets

Here we present experimental results on laser-driven ion accel- eration from relativistically transparent, overdense plasmas in the break-out afterburner (BOA) regime. Experiments were preformed at the Trident ultra-high contrast laser facility at Los Alamos National Laboratory, and at the Texas Petawatt laser facility, located in the University of Texas at Austin. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the

Laser-driven 1 GeV carbon ions from preheated diamond targets in the break-out afterburner regime

Experimental data are presented for laser-driven carbon C6+ ion-acceleration, verifying 2D-PIC studies for multi-species targets in the Break-Out Afterburner regime. With Tridents ultra-high contrast at relativistic intensities of 5 × 1020 W/cm2 and nm-scale diamond targets, acceleration of carbon ions has been optimized by using target laser-preheating for removal of surface proton contaminants. Using a high-resolution wide angle spectrometer, carbon C6+ ion energies exceeding 1 GeV or 83 MeV/amu have been measured, which is a 40% increase in maximum ion energy over uncleaned targets. These results are consistent with kinetic plasma modeling and analytic theory.

Experimental demonstration of particle energy, conversion efficiency and spectral shape required for ion-based fast ignition

Research on fusion fast ignition (FI) initiated by laser-driven ion beams has made substantial progress in the last years. Compared with electrons, FI based on a beam of quasi-monoenergetic ions has the advantage of a more localized energy deposition, and stiffer particle transport, bringing the required total beam energy close to the theoretical minimum. Due to short pulse laser drive, the ion beam can easily deliver the 200 TW power required to ignite the compressed D–T fuel. In integrated calculations we recently simulated ion-based FI targets with high fusion gain targets and a proof of principle experiment [1]. These simulations identify three key requirements for the success of ion-driven fast ignition (IFI): (1) the generation of a sufficiently high-energetic ion beam (≈400–500 MeV for C), with (2) less than 20% energy spread at (3) more than 10% conversion efficiency of laser to beam energy. Here we present for the first time new experimental results, demonstrating all three parameters in separate experiments. Using diamond nanotargets and ultrahigh contrast laser pulses we were able to demonstrate >500 MeV carbon ions, as well as carbon pulses with ΔE/E < 20%. The first measurements put the total conversion efficiency of laser light into high energy carbon ions on the order of 10%.

Break-out afterburner ion acceleration in the longer laser pulse length regime

Kinetic simulations of break-out-afterburner (BOA) ion acceleration from nm-scale targets are examined in a longer pulse length regime than studied previously. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the time-evolving sound speed, from which these times may be estimated. The maximum ion energy attainable is controlled by the finite acceleration volume and time over which the BOA acts.

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.

A novel high resolution ion wide angle spectrometer

A novel ion wide angle spectrometer (iWASP) has been developed, which is capable of measuring angularly resolved energy distributions of protons and a second ion species, such as carbon C(6 +), simultaneously. The energy resolution for protons and carbon ions is better than 10% at ∼50 MeV/nucleon and thus suitable for the study of novel laser-ion acceleration schemes aiming for ultrahigh particle energies. A wedged magnet design enables an acceptance angle of 30°(∼524 mrad) and high angular accuracy in the μrad range. First, results obtained at the LANL Trident laser facility are presented demonstrating high energy and angular resolution of this novel iWASP.

Mono-energetic ion beam acceleration in solitary waves during relativistic transparency using high-contrast circularly polarized short-pulse laser and nanoscale targets

In recent experiments at the Trident laser facility, quasi-monoenergetic ion beams have been obtained from the interaction of an ultraintense, circularly polarized laser with a diamond-like carbon target of nm-scale thickness under conditions of ultrahigh laser pulse contrast. Kinetic simulations of this experiment under realistic laser and plasma conditions show that relativistic transparency occurs before significant radiation pressure acceleration and that the main ion acceleration occurs after the onset of relativistic transparency. Associated with this transition are a period of intense ion acceleration and the generation of a new class of ion solitons that naturally give rise to quasi-monoenergetic ion beams. An analytic theory has been derived for the properties of these solitons that reproduces the behavior observed in kinetic simulations and the experiments.