Session UO6: AB: Laser-Plasma Ion Acceleration

Ion acceleration from pre-expanded thin foils irradiated by petawatt laser

In this contribution, we demonstrate that the ions in the pre-expanded foils with near-critical density plasma before its interaction with the main ultrashort petawatt laser pulse may be accelerated to higher energies than that from intact ultrathin foils. In order to investigate the mechanisms responsible for the acceleration of the most energetic ions, we used tracking of particles in multidimensional particle-in-cell simulations. These simulations show that high-energy ions originate from a small region of the depth below one micron and the width about the laser focal spot size in the case of targets with a steep density gradient on its rear side. On the other hand, the depth of this region exceeds a few microns for pre-expanded target with long density gradients on both sides. When the laser pulse propagates through near-critical density pre-expanded targets, a high-density electron bunch is formed and travels with the laser pulse behind the target. Behind this electron bunch, a relatively long longitudinal electric field accelerates ions. Moreover, additional ion acceleration can be observed later due to expanding transverse magnetic field generated by propagating electrons.   

A new regime of short-pulse laser-particle acceleration: an overview of results from the NIF-ARC protons campaign

A high energy, high flux short-pulse driven proton source has been demonstrated and characterized on the NIF's Advanced Radiographic Capability (ARC) laser. The ARC laser resides in a unique parameter space: 4 separate beamlets, very high-energy (kJ), relatively long (multi-ps), large focal spot, quasi-relativistic (\textasciitilde 10\textasciicircum 18 W/cm\textasciicircum 2) intensities. The proton campaign at the NIF-ARC has focused on exploring proton acceleration via TNSA and investigating the underlying time-dependent physics of particle acceleration via an integrated experimental and simulation effort. A significant (\textasciitilde 5x) enhancement of maximum proton energy over that predicted by conventional scalings is observed at these laser intensities, definitively establishing a new superponderomotive acceleration regime. Furthermore, a high conversion efficiency of \textasciitilde 2.5-5{\%} laser energy into protons yields a record flux (\textasciitilde 50 J) of laser-accelerated protons. This opens exciting new applications such as proton isochoric heating of solids to several 100's of eV temperatures, and 3D tomography of evolving plasma conditions with \textless 10 ps temporal resolution. We will show results from proton heating and radiography and provide a prospective for future HED experiments with the NIF-ARC protons.   

Colliding laser pulses for enhancing proton acceleration in thin foils

Despite having been the most commonly used accelerating method for protons in laser-matter interaction for the past decades, target normal sheath acceleration (TNSA) still produces proton beams whose energy is not high-enough for many interesting applications, in particular due to the poor scaling of their maximum energy with the laser energy. In this contribution, we describe a modified TNSA scheme, in which two laser pulses are simultaneously incident on a solid target with opposite angles. Based on 2 and 3-dimensional simulations with the EPOCH Particle-In-Cell code, we show that this setup leads to a doubling of the proton energy and a strong enhancement of the proton number compared with using a single pulse with the same total energy. This is explained by a much more efficient hot-electron generation, due to a combined effect of the increased electric field in the standing wave in front of the target and the modification of the relative phases of the electric and magnetic fields. This physical process remains valid for a large range of incident angles, which should lead to straightforward implementation in experiments. Finally, we show that this concept remains valid when applied to ultra-thin targets in the relativistic self-induced transparency regime.   

Ion Acceleration in Laser Generated Mega Tesla Magnetic Vortex.

Magnetic Vortex Acceleration (MVA) from near critical density targets is one of the promising schemes of laser-driven ion acceleration. 3D particle-in-cell simulations are used to explore a more extensive laser-target parameter space than previously reported on in the literature as well as to study the laser pulse coupling to the target, the structure of the fields, and the properties of the accelerated ion beam in the MVA scheme. The efficiency of acceleration depends on the coupling of the laser energy to the self-generated channel in the target. The accelerated proton beams demonstrate high level of collimation with achromatic angular divergence, and carry a significant amount of charge. For PW-class lasers, this acceleration regime provides favorable scaling of maximum ion energy with laser power for optimized interaction parameters. The mega Tesla-level magnetic fields generated by the laser-driven co-axial plasma structure in the target are a prerequisite for accelerating protons to the energy of several hundred MeV.   

Simulation and experimental validation of a new type of laser target that produces collimated and accelerated proton bunches.

The Target Normal Sheath Acceleration is the most robust and well-known production process of laser driven proton beams, but the application of such a scheme to isochoric heating, isotope production, proton radiography or proton therapy, suffers from two major limits: a broad energy spectrum and a large beam divergence. In order to optimize the properties of the proton beam, a new scheme of post-acceleration proposes to add an helical coil connected at the rear side of the target. The discharge current induced by the electron charge ejection propagates through this helix and generates an electromagnetic pulse which collimates, post-accelerates and energy selects the protons emitted from the rear side of the target. We present the results of the PACMAN1 campaign carried out at the LULI 2000 facility, in March 2019, where the pico2000 laser beam (70J, 1ps) irradiated gold foils attached to helixes of different diameters, lengths or pitches. The experimental data are compared to the results of numerical simulations carried out with the PIC code SOPHIE developed at the CEA.   

Ultra-High Intensity Laser Research at BELLA

This presentation will review the status of ion acceleration at the BELLA petawatt (PW) facility with a large laser spot (f\65) and give an outlook on science enabled by a short-focal length (f\2.5) laser beamline, currently under construction. Proton beams from the long-focal length beam line exhibit a strongly reduced divergence and increased ion numbers and are hence, ideally suited for subsequent capture and transport with an active plasma lens (APL). As part of our development of an experimental platform for investigating radiobiological effects of laser-accelerated ions, we were able to irradiate normal and radioresistant prostate cancer tumor cell samples with over 1500 PW shots using the APL. The new the short-focal length beamline will be equipped with a re-collimating double-plasma mirror to study laser-plasma interactions at ultra-high temporal contrast. The BELLA center is now part of LaserNetUS, providing international user access.   

Focusing of laser-accelerated proton beams with active plasma lens.

We report on the first experimental demonstration of radially symmetric focusing of laser-accelerated proton beams with an active plasma lens which provides tunable field gradients of the order of kT/m. MeV level proton beams generated from micrometer solid density targets interacting with a relativistically intense laser were used to examine the focusability and robustness of this new approach. By varying field gradients of the active plasma lens, proton beams with mm focus spot size at selected energies were achieved at a distance of 1.5 meters behind the source. Results were supported by simple numerical calculations. Work towards retrieval of the proton beam source properties (source size, emittance) by combining these methods with dipole magnetic dispersion will be discussed as well.   

Performance and limits of micro-structured targets at high laser intensities for superior sources of light and heavy ions.

The generation of high-intensity ion beams driven by short pulse lasers has emerged as an important area of plasma research due to their unique short duration ($\sim$ ps) and small source size ($\sim\mu$m). In response to the demand for higher laser-to-ion conversion efficiencies, there has been growing interest in using structured targets. They have demonstrated enhanced number and energy of accelerated electrons and protons compared to flat foils. Yet, the previous experimental work on structured targets used moderate laser intensities ($<10^{20}$ W/cm$^2$). We will present results of light and heavy ion acceleration from experimental campaigns at the PHELIX laser (150~J, 0.5~ps, $10^{21}$ W/cm$^2$), Germany, and at the ALEPH laser (10~J, 45~fs, $10^{21}$ W/cm$^2$), Colorado State University, using micro-pillars and micro-tubes structured foils. We will also present particle-in-cell simulations for these experiments and insights about our future experimental studies at laser intensities of $\sim 10^{22}$ W/cm$^2$.   

The effect of pulse length on laser-proton acceleration from microstructured targets

An intense laser pulse incident on a foil target creates plasma structures with TV/m fields, accelerating ions to MeV energies. Laser coupling to the target can be enhanced with structures engineered on the front side of the target. These targets were studied with two 10$^{\mathrm{21}}$ W/cm$^{\mathrm{2}}$ lasers, PHELIX (500fs) at GSI and ALEPH (45fs) at CSU. Relative to flat foils, microtube targets double the proton cutoff energy and quadruple the yield. Micropillar targets, however, produce only a 30{\%} enhancement in proton cutoff energy. ALEPH, with its shorter pulse length, predominantly accelerated protons, while PHELIX also generated energetic heavy ions. PHELIX performed optimally with microtube diameters on the order of the spot size, and further study will determine the optimum structure for ALEPH parameters. 2D PIC simulations investigate the mechanisms responsible for the enhanced proton acceleration from structured targets. Acknowledgments: UC Office of the President (LFR-17-449059; DOE NNSA (DE-NA0003842); DOE Office of Science, FES (DE-SC0019076).   

All-optical structuring of laser-driven proton beam profiles

Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact multi-MeV proton accelerators with unique bunch characteristics. Protons are accelerated in TV/m fields that are established within the micrometer-scale vicinity of the high-power laser focus. Substantially extending this picture, our recent results show a critical influence of the millimeter scale vacuum environment on the accelerated proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation.   

An Analytical Model Connecting Spectral Multi-Species Modulations with Microscopic Electron-Properties in PW-Class Laser-Ion Acceleration

Spectral signatures of laser-accelerated ion beams are frequently used to characterize underlying acceleration mechanisms. Yet regularly, more than just one ion species are accelerated in experiments, e.g. from hydro-carbon contamination layers, multiple charge states or mixed materials. Such presence of multiple ion species (q/m) in the accelerating field leads to characteristic modulations in observed proton spectra due to electro-static repulsion during co-propagation. Resulting typical spectral modulations from these effects are presented with an analytical model for PW-class laser-ion acceleration. We improve previous predictions with explicit multi-species interaction for arbitrary mixtures, enabling us to connect important ensemble properties of laser-accelerated electrons with spectral signatures of accelerated ions. We support our new model with extensive particle-in-cell simulations and propose an experimental implementation with a novel cryogenic target, allowing systematic verification of our predictions in an environment without the strong influence of hardly controllable processes such as ionization dynamics.   

Transport of intense laser-driven proton beams in warm, dense plastic foams

Chirped pulse amplified lasers have reached the kilojoule-petawatt class, and the secondary sources of radiation they produce are themselves now capable of driving unexplored physics. Laser-driven proton beams with MeV to 10s of MeV particle energy and 10s of J beam energy can now be the pump in innovative experiments such as isochoric heating of foams or solids to warm (\textgreater 1 eV) or even hot (\textgreater 100 eV), dense matter states. However, the transport behavior of such an intense beam through the plasma target is complex. To study the transport, we carried out an experiment with the OMEGA EP laser and simulations with the LSP particle-in-cell code. We present experimental measurements of laser-accelerated proton spectra from curved, engineered targets before and through two areal densities of carbonized resorcinol formaldehyde (CRF) foam and simulations of the beam transport within the foam. 2D images of K$\alpha $ emission from a solid Cu layer at the back of the foam shed insight into the beam transport, indicating a bright, directional beam.   

Clusterized surface transformation under intense heating generated by laser-accelerated proton irradiation

The acceleration of protons using ultra-intense (I\textgreater 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}})$ short pulse (duration\textless 1 ps) lasers, is a growing field of interest, in particular since their short bunch duration and their very intense and localized heating properties are perfectly suited for studies in warm dense matter or material science. In this letter we use laser-accelerated protons to analyze the effect of an intense and short (ns-scale) energy deposition process occurring on solid metal surfaces and studying its evolution on a ns and nm scale. We show that the thermal shock generates on the surface a uniformly distributed clustered heating, with dimensions of the clusters depending on the irradiated dose. When cooling down, the clusters produce large nanostructured surfaces. Controlling the dose allows obtained nanostructured surfaces with a low dispersion in particle dimension, high density of particles and polycrystallinity morphology.   

Enhancement of laser driven proton acceleration by double-layer target

Experimental results of proton acceleration driven by picosecond laser on SGII-UP laser facility are presented. The laser beam which delivering 150J in 1 picosecond pulse was focused onto the foil target. The laser intensity on target was about 3×10$^{\mathrm{18}}$ W/cm2. A 0.3$\mu $m plastic film was placed 300 $\mu $m in front of the 5 $\mu $m Copper target, which was used as a filter to cut off the prepulse and protect the target rear surface, and also to produce a large-scale near-critical-density preplasma before the picosecond main pulse arriving. With plastic filter in front of the target, the maximum energy of proton increased from 10 MeV to 18 MeV, which indicates that the proton acceleration is enhanced by the plastic film in front of the Copper target.