Session GO04: Beams: Laser-Driven Ion Acceleration

Live

Laser-driven ion acceleration is constrained by the difficulty and expense involved not only in experiments but also in modeling. High-resolution simulations in 1D can take hundreds of hours to resolve and this cost only increases for more complex problems. Using multi-layer neural networks, one can effectively map out large swaths of parameter space in ways not computationally accessible to a traditional parameter scan. An ensemble of 1D simulations focusing on Target-Normal Sheath Acceleration was built for a large spread of initial conditions, varying initial parameters such as intensity, pulse-length, target thickness, etc. The crux of the technique is the continuous, data-informed function generated by the neural network that can rapidly produce output parameters for various times and inputs. The function can accurately reproduce sampled datapoints as well as interpolate to untrained points within the parameter space. As a diagnostic, it can pinpoint discrepancies between its results and the simulation ensemble. It can be used to identify unique physics suggested by the network or simulation failure due to numerical error or insufficient resolution.   

Live

High-intensity, short pulse lasers serve as phenomenal tools to produce high-brightness, high-energy laser-driven particle sources. Recent simulation results have shown that two pulses separated by a narrow spatial gap and relative temporal delay can produce an increase in the maximum proton energy from target normal sheath acceleration (TNSA) [1]. Earlier this year, off-axis spiral phase mirrors were used to generate high intensity optical vortices, which carry orbital angular momentum (OAM) and have annular spatial profiles and helical wave-fronts [2]. Taken together, these two findings suggest that optical vortices could be used to achieve higher maximum proton energies from TNSA. Motivated by these results, this work examines these annular optical pulses and simulates the characterization and retrieval of these pulses using a novel technique, Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH). Based on these simulations, STRIPED FISH correctly retrieves optical vortices, and can be leveraged in HED experiments to provide deeper insight into laser plasma interactions.   

Live

Rapid advancements in high-repetition rate laser systems necessitate improvement in target capabilities. Using liquid micro-jets and micron-scale liquid droplets, we produce thin film sheet targets and free standing structured targets for high-repetition rate laser-plasma experiments. We present an overview of the target system used with the Extreme Light Group at Wright Patterson Air Force Base and several achievable structured target geometries. We also present particle-in-cell simulations on the reduced mass structured targets to demonstrate their applicability in laser driven sources of ions and electrons.   

Live

Energetic (10s MeV) neutron-beam sources have important applications including the non-destructive probing of dense materials and nuclear waste transmutation, but these applications require high average flux currently achievable only using large accelerator facilities or reactors. The use of small-scale laser facilities to generate these neutron beams is still an ongoing research effort. At the same time, free-standing thin films (\textless 1 um) of liquid crystals (LC) have been used on PW-class lasers as plasma mirrors and ion-acceleration targets and are ideal for high repetition rate, in-situ target formation. We have developed a new technique to synthesize deuterated liquid crystals with \textless 2{\%} remaining hydrogen content. Here we report on an experiment conducted at the Scarlet laser facility where thin films of LC were used to generate a beam of \textgreater 4 MeV deuterons directed into a Be converter producing fast neutrons through the nuclear stripping reaction. We measured a n$^{\mathrm{0}}$ yield of \textgreater 2 x 10$^{\mathrm{7}}${\#}/sr.   

Live

Ultra-intense laser technology has the potential to serve as a compact source of energetic proton/ion beams ($>$ MeV) for applications in science, medicine, defense, and industry. High scientific data-rates could accelerate our understanding and enable use of "big data" statistical techniques and machine learning. Currently, there is a need for high quality, single-shot proton spectra from the most-compact, highest-repetition rate laser-plasma accelerator systems (1 Hz, 10 Hz, kHz). We explore whether low-level hardware development by our own scientists may meet the needs of our field. We have prototyped a network of inexpensive, compact magnetic proton spectrometers with digital proton detection by scintillator-coupled line CCDs driven by STM32 Cortex-M7 microcontrollers. Our system is designed to scale to large numbers of simultaneous proton spectrometer diagnostics and to meet the needs of high data-rate experiments in laser-driven proton acceleration, e.g. (1) experiments capturing the correlation between single-shot proton spectra at multiple locations within the experimental chamber, (2) identifying and filtering results in real-time based on "rare" spectra due to shot-to-shot fluctuations, and (3) real-time integration with active feedback systems.   

Live

A super-intense laser pulse, incident on a microtube target, can accelerate protons to tens of MeV. Microtube targets have an advantage over flat foils because additional hot electrons are accelerated from the tube surface, strengthening the accelerating sheath field. The ALEPH laser at Colorado State University (40 fs, 3\texttimes 10$^{\mathrm{21\thinspace }}$W/cm$^{\mathrm{2}} \quad \lambda =$400 nm) was used to accelerate ions from 3D-printed microtube targets. At best performance, the microtube targets increase the proton cutoff energy relative to flat foils by \textasciitilde 65{\%}, and increase the proton yield by \textasciitilde 50{\%}. A wide parameter scan of microtube targets, varying tube dimensions, determined an optimum microtube size for accelerating protons on ALEPH. 2D particle-in-cell simulations show that electrons from the tube surface are accelerated to higher energy than the ponderomotive scaling, and are collimated to the center of the tube target. For the simulated optimum tube case, this process doubles the maximum proton energy relative to flat foils. \textit{This work is supported by the DOE National Nuclear Security Administration under Award Number DE-NA0003842; and by the DOE Office of Science, Fusion Energy Sciences under Contract No. DE-SC0019076.}   

Live

Using an active plasma lens we demonstrate a single-shot method to measure the energy-dependent source size of laser driven proton beams. The active plasma lens acts as an imaging system, yielding magnified proton beam spots at a distance of 1.5 meters behind the target. A simple Monte Carlo code is used to simulate the transport and retrieve the source size from the measurement. In the energy range of 3 to 7 MeV we show the measured proton beam source size is 210 to 60 micrometers, with an incident laser spot size of 52 micrometers and a peak laser intensity of 2*1 \textasciicircum 19 W/cm \textasciicircum 2.   

Live

The generation of proton beams with 10s to 100s MeV with controllable spectral bandwidth in a compact system is important for applications that range from radiography of dense plasmas to tumor therapy. We use large-scale 2D and 3D particle-in-cell (PIC) simulations to explore the development of a hybrid accelerator that would combine the advantages of laser-driven (high-charge, low-emmitance, 10s MeV) proton beams with high-gradient RF acceleration (controllable spectral bandwidth) in a meter-scale compact system. We have used an adaptive mesh technique to model the full system self-consistently, from the laser-solid interaction, to transport, to the meter-scale acceleration in the RF structure. We have found that space-charge effects are minimized during transport due to the screening of accelerated electrons, but can be very important in the RF-acceleration stage. By tuning the distance of laser-plasma foil to the RF entrance and the injection phase we show the possibility to control space charge effects in order to obtain high-quality, high-charge protons beams in a compact hybrid accelerator.   

Live

First experiments were conducted on the Laboratory for Laser Energetics' Multi-Terawatt (MTW) laser to produce a deuteron beam by target normal sheath acceleration (TNSA) using deuterated Ti targets. Commercial 20-$\mu $m-thick Ti foil was cut into 500 \texttimes 500 $\mu $m$^{\mathrm{2}}$ squares and exposed to atomic deuterium at different temperatures ranging from 60\textdegree C to 350\textdegree C and pressures from 0.1 mTorr to 300 mTorr. An alternative loading method of condensing a titanium-deuterate layer onto the Ti foils was also examined. The MTW laser emitting at 1053 nm was operated in high-energy (24-J), short-pulse (8-ps) mode to produce an on-target intensity of 3 \texttimes 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$. Using a Thomson-parabola ion spectrometer, the energy spectra and total yields of all relevant ion species were determined as a function of deuterium loading. These experiments represent pilot studies for an experimental platform under development at this laboratory to generate a tritium beam on its more-powerful OMEGA EP laser. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

We established an experimental platform for the investigation of the radiobiological effects of laser-accelerated ions at the Bella PW laser. Stable few-MeV proton beams accelerated at peak laser intensities 2x10$^{\mathrm{19}}$ W cm$^{\mathrm{-2}}$ in the comparably large Bella PW laser focus exhibit reduced divergence at increased ion numbers and are hence ideally suited for subsequent capture and transport with an active plasma lens (APL). Combined with our high shot rate capability (0.2 Hz),~thousands of shots at ultra-high dose rates (10$^{\mathrm{7}}$ G/s), with a uniform dose distribution over a 1 cm diameter lateral area, could thus be delivered to biological cell samples, located in air, at 1.7 m distance from the laser-target interaction. The proton beamline was complemented by online (integrating current transformer and scintillator) and offline (radiochromic films) beam diagnostics for dosimetry. This assembly was used to investigate the differential sparing of healthy tissues versus the tumor response. This talk gives details on the proton beamline, dosimetry as well as cell irradiation results.   

Live

We report on ion acceleration at the BELLA PW laser (\textasciitilde 10$^{\mathrm{19}}$ W/cm$^{\mathrm{2}}$, up to 1 Hz). Proton and carbon ion pulses with intensities in the 10$^{\mathrm{12}}$ ion/shot range are characterized with a Thomson parabola for energies above 2 MeV. We quantify the flux of lower energy ions through implantation into silicon wafers and ex situ analysis and find that carbon ions with energies below 2 MeV are implanted with fluences of \textasciitilde 10$^{\mathrm{14}}$ atoms/cm$^{\mathrm{2}}$/shot. Intense ion pulses excite and heat targets, leading to evaporation of aluminum and annealing of defects in silicon. We discuss opportunities for materials processing and qubit synthesis with intense ion pulses from laser-plasma acceleration. The work is supported by the U.S. Department of Energy Office of Science, under Contract No. DE-AC02-05CH11231.   

Live

Micron-scale liquid droplets synchronized with an intense laser system provide a free-standing, reduced-mass target for high-repetition rate laser plasma interactions. We show that colliding two droplets can produce new geometries including reduced-mass sheet and tube-like targets that may be beneficial for ion acceleration and other applications. We present computational fluid dynamics (CFD) simulation results modelling the collision of two droplets and compare it to previous experimental measurements [Pan et al. 2009, Phys. Rev. E] with the same Weber number (a dimensionless number describing the relationship between inertia and surface tension in the collision). We then present particle-in-cell (PIC) simulation modeling laser plasma interactions with target geometries based on the CFD simulation result and discuss the possibilities of this type of target for high-repetition-rate interactions.   

On Demand

Laser-driven ion beams produced via Target Normal Sheath Acceleration (TNSA) are suboptimal for direct injection into an RF linear particle accelerator in terms of ion energy, spatial control, and 6-D brightness. 2-D/3-D particle-in-cell (PIC) simulations have identified more favorable regimes using higher peak laser intensities and advanced target designs. Sub-micron planar cryogenic low-Z jets are used to explore ion acceleration in the relativistic transparency regime where high energy, low divergence ion beams are predicted. We demonstrate improved target characterization, precise directional control of the ion beam, and high brightness in experiments using the Texas Petawatt Laser. This work establishes a clear path towards hybrid particle accelerators operating at high repetition rate applications.   

On Demand

Laser-driven ion beams generated during relativistic laser-interactions with solid targets exhibit superior brightness compared to conventional accelerators, but currently cannot simultaneously deliver the ion energy required for applications. In a hybrid accelerator, a laser-accelerated ion beam is injected into an RF linear particle accelerator to reach ion energies of 230+ MeV on a meter scale. This new concept has the potential to revolutionize proton radiographic imaging and hadron therapy. For optimal coupling, the ion beam should be manipulated to match the acceptance angle and energy bin width of the linac. Here, we demonstrate all-optical spatial and temporal manipulation of the forward propagating ion beam resulting from its overlap with the extreme electric and magnetic fields generated during the laser-plasma interaction.   

Live

Coupling of laser energy directly into ions has been a challenging task due to huge mass difference between electron and ion species. However, it has been recently reported by us that with the application of an external magnetic field, it is possible to couple laser energy directly into ions [1]. Upon further investigation of the coupling mechanism, we identify the accessibility condition for the mechanism to be operative. Furthermore, we demonstrate how excitation of lower hybrid helps in the proposed mechanism. This work can find application where high energy needs to be dumped into plasma through ions instead of energetic electrons, thereby avoiding current generated instabilities. Also, our study explains various astrophysical observations where ion heating or soliton formation due to lower hybrid mode has been reported. References: [1]A. Vashistha, D. Mandal, A. Kumar, C. Shukla, and A. Das, ``A new mechanism of direct coupling of laser energy to ions,'' New Journal of Physics, vol. 22, p. 063023, jun 2020.