Session NO05: Laser-Plasma Ion Accelerators

Experimental and numerical investigations of ion acceleration by ultraintense laser pulses in near-critical transparent gas jets

High-energy ion sources driven by ultra-intense laser pulses are interesting for many applications ranging from medicine to fundamental research. While laser-driven ion acceleration has been extensively investigated in solid targets, much fewer studies have addressed the case of near-critical plasmas (n_e ~ n_c) due to the technical difficulty of achieving controlled high gas densities. Such plasmas are predicted to give rise to new ion acceleration regimes combining TNSA and collisionless shock acceleration (CSA), as well as enhanced hot-electron production, beyond the standard ponderomotive scaling. 

We report on the results of two experimental campaigns conducted at the 200 TW, 30 fs VEGA II and recently at the 1 PW, 30 fs VEGA III laser systems (CLPU, Spain) together with an extensive parametric PIC study performed to explore the physics of the expected interaction. Our first campaign aimed at studying the potential for ion acceleration of a state-of-the-art gas jet coupled with shock nozzles. Preliminary time-of-flight results of our most recent campaign show ~5 MeV/a.m.u. alpha particles accelerated at a moderately high repetition rate with relatively low energy dispersion and divergence. They also validate the strong electron heating predicted by our numerical studies.   

Generating a Triton Beam by Target Normal Sheath Acceleration with a High-Energy Short Pulse Laser for Nuclear Experiments

In a novel experiment, a beam of energetic tritons was generated via the target normal sheath acceleration (TNSA) mechanism using tritiated titanium targets. Commercial 25-mm-thick Ti foil was cut into 500 x 500-mm² squares and exposed for 2 h to ~1 atm. of 99.97% pure tritium gas at 200°C. These targets were irradiated with an on-target intensity of 2 x 10¹⁸ W/cm² with the high-energy (1250-kJ), short-pulse (10-ps) OMEGA EP laser. Using a Thomson parabola velocity analyzer, the energy spectrum of the tritons was found to exponentially decrease with a mean energy of 2.3MeV and a high-energy cutoff at ~10 MeV. Approx. 0.04% of the laser energy transferred to the tritons. The total beam yield was determined to be ~10¹² tritons per pulse, comparable to other TNSA experiments with protons. In a second experiment, the triton beam was directed onto a secondary deuterated-polyethylene target, which produced 10⁸ neutrons from DT fusion nuclear reactions. Further experiments are planned to induce the T(t, 2n)α reaction of interest for ab-initio nuclear structure calculations of ⁶He.   

A micrometre long pre-plasma leads to a three-fold proton energy enhancement in Target Normal Sheath Acceleration by enabling an improved laser-to-electron coupling

Target Normal Sheath Acceleration has attracted much interest over the past twenty years as a promising, compact and affordable ion source. Important challenges still affecting this consolidated laser-driven ion acceleration scheme regard the increase of the maximum ion energy and the improvement of the beam quality.

In this work, we present novel experimental results demonstrating a three-fold energy enhancement obtained by pre-expanding in a controlled way the front surface of not-so-thin solid targets (Gizzi et al. 2021 Sci. Rep. 11, 13728). Using a pre-pulse, a 5-10 μm scale-length pre-plasma has been created on the front surface of 25 μm Titanium targets, leading to proton cutoff energies up to 12 MeV as opposite to 4 MeV achieved without pre-plasma. Particle-In-Cell simulations modelling the experimental conditions show a very complex and rich laser dynamics in the long underdense region with the creation of standing waves. This causes electrons to undergo stochastics motion thus enabling a more efficient heating mechanism. As a result, protons from the contaminant layer on the back surface of the target are accelerated to higher energies. Interestingly, the presence of a long-scale pre-plasma seems also beneficial to reduce the divergence of the accelerated ions.      

A pulse length and intensity study of proton generation from microtube foil targets

The interaction of an intense laser with a solid foil target can drive ~TV/m electric fields, accelerating ions to MeV energies. Simulations and experimental data [1,2] show that the ion energies and numbers can increase using structured targets. In this study, we experimentally observe that structured targets can dramatically enhance proton acceleration in the target normal sheath acceleration (TNSA) regime. At the Texas Petawatt Laser facility, we compared proton acceleration from a 1 µm Ag flat foil, to a fixed microtube structure 3D printed on the front side of the same foil type. A pulse length (140 – 500 fs) and intensity ( [6 – 20] ×10²⁰ W/cm²)  study found an optimum laser configuration (140 fs, 6×10²⁰ W/cm²), in which microtube targets increase the proton cutoff energy by ~2× and the energetic proton yield (>1.5 MeV) by ~3×. Experimental results are compared with 2D simulations and will be presented at the meeting.

References:

[1] L. L. Ji, et al. Scientific Reports, Scientific reports 6.1 (2016): 1-7.

[2] M. Bailly-Grandvaux, et al. Physical Review E 102.2 (2020): 021201.   

Three-dimensional effects in the Radiation Pressure Acceleration.

Radiation Pressure Acceleration (RPA) is considered one of the most efficient regimes of laser driven ion acceleration. Previous analytical and computer simulation results single out this regime as a way of generating GeV-class ion beams. Here a simple analytical model is presented, which describes multi-dimensional effects, especially the transverse expansion of the target, and advances our understanding  of the properties of the RPA-generated ion source. It is shown how laser pulse temporal and spatial profiles can be used to alter the divergence of the ion beam and optimize the energy transfer from the laser to the ions.   

Effects of Laser Polarization on Target Focusing and Acceleration in a Laser-Ion Lens and Accelerator

We present the process of ion acceleration with ultra-thin foils irradiated by elliptically polarized, highly intense laser pulses. Recently, efficient generation of monoenergetic ion beams was introduced using the concept of laser-ion lensing and acceleration (LILA)[1]. LILA is a novel scheme where the target has a radially-dependent thickness, resulting in simultaneous acceleration and focusing of a proton beam. We extend the LILA concept to include elliptically polarized (EP) laser pulses. It is widely believed that an elliptically polarized (EP) laser is unacceptable for radiation pressure acceleration (RPA) because strong electron heating compromises ion acceleration [2].  By multidimensional particle-in-cell simulations, we demonstrate that the LILA scheme can be driven by EP laser pulses as long as the average target thickness is optimized. We also demonstrate that with a non-uniform thickness target, even linearly polarized laser pulses can efficiently generate low-emittance focused ion beams,  with the overall laser-to-ions energy conversion comparable to those predicted for circularly polarized laser pulses.

[1] Tianhong Wang, Vladimir Khudik and Gennady Shvets, Phys. Rev. Lett. 126, 024801(2021).

[2] S G Rykovanov et al, New J. of Phys. 10 (2008) 113005   

3D Simulations of Magnetic Vortex Acceleration with BELLA IP2 parameters

Magnetic Vortex Acceleration (MVA) is an advanced ion acceleration mechanism requiring ultra-relativistic intensities combined with near-critical density targets that are well matched to the laser conditions. 3D particle-in-cell simulations were conducted in this regime using the WarpX code. Simulations were done with realistic parameters for a newly constructed short-focal length beamline at the BELLA PW facility (IP2) to plan upcoming experiments. We studied the robustness and performance of MVA under varied density ramps to investigate acceleration conditions with and without laser contrast cleaning. The direction and collimation of ion beams in the case of an off-normal angle of incidence was studied, identifying potentially observable signatures in experiments. Simulations were performed with hydrogen and mixed targets to investigate multi-species effects.   

Efficient Ion Acceleration by Continuous Fields in Target Transparency Regime

Ion beams driven by short-pulse lasers have been an increasingly active area of research as the beams have potential applications where maximizing energy and yield of ions would be beneficial. Here, we present a new feasible scheme of short-pulse laser-driven ion acceleration with multi-ps pulses and ultra-thin targets in which the synergetic effects of laser-induced target transparency and continuous field acceleration efficiently enhance the flux and peak cutoff energy of accelerated ions. Recent experiment conducted on the OMEGA EP lasers demonstrated the new approach showing the maximum proton energy of higher than 70MeV from the moderate-intensity, ~10^19 W/cm^2 with sub-micron thick targets, and significantly higher yield of protons than expected from a typical TNSA mechanism. The computational study indicates the enhanced temperature of electrons from laser interaction with expanding plasma drives a strong electric field for further ion acceleration. Detailed experimental results including various measurements and systematic simulations will be presented.   

Enhancement of laser-driven ion acceleration by laser pulse shaping via plasma shutter

A plasma shutter [1,2] is usually a thin solid foil that is placed in front of the main target in the laser-target interaction. The laser pulse with its prepulses then needs to burn through it which can improve the laser pulse contrast and its intensity profile. This includes the generation of a steep rising front [3] and intensity increase [4]. The new pulse shape can improve the consequent ion acceleration from the additional target [5] and reduce the development of short-wavelength instabilities [6].

In this work, we study the effects of the plasma shutter on ion acceleration from an additional target with the help of 2D and 3D PIC simulations using code EPOCH. We also consider a setup using two plasma shutters, where the first of them is expanded by prepulses simulated by hydrodynamic simulations. We demonstrate a substantial increase in maximal energy of accelerated heavy ions when a thin plasma shutter is added into the interaction of a PW-class laser pulse with the main target.   

Monoenergetic High-Energy Ion Source via Femtosecond Laser Interacting With a Microtape

Laser-based ion sources are characterized by unsurpassed acceleration gradient and exceptional beam emittance. They are promising candidates for next-generation accelerator towards diverse applications. However, ion beams achieved currently have limitations in energy spread and peak energy. In this talk, we report a novel method to achieve monoenergetic proton beams with energy spread at 1% level and peak energy of hundred MeV by using a readily available femtosecond laser interacting with a microtape. As the laser sweeps along the tape, it drags out a huge charge (∼100 nC) of collimated electrons and accelerates them to superponderomotive energies. When this dense electron current reaches the rear edge of the tape, it induces a strong electrostatic field. Due to the excessive space charge of electrons, the longitudinal field becomes bunching while the transverse is focusing for protons. Together, this leads to a highly monoenergetic energy spectrum and much higher proton energy as compared to results from typical target normal sheath acceleration and radiation pressure acceleration at the same parameters.   

High-Repetition-Rate Two-Stage Ion Acceleration

We propose a parameter-rich experiment uniquely possible at a high-repetition rate (> 1 Hz) laser-plasma science facility. A single laser is unequally split and tightly focused on opposing sides of two closely spaced thin-film targets. The first target emits an ion beam whose energy spectrum, flux, and spatial divergence is modified as it passes through the second laser-target interaction. We use 2D PIC simulations to demonstrate that resulting quasi-monoenergetic ion features have central energies robust to total laser energy and laser pointing fluctuations, and are finely controllable by target separation and relative laser timing. We propose three elements for practical experimental success: (1) high-repetition-rate real-time monitoring of particle beam mode to fine-tune relative laser alignment, (2) transforming shot-to-shot fluctuations, especially focal spot overlap, into an automatic parameter scan monitored in an equivalent plane, and (3) focusing the second laser on the opposing side of the second target, enabling tighter target separation for larger solid angles and stronger experimental signatures. The result will be an experiment with a rich, quickly-scanned parameter space and the potential for machine learning applications.   

High repetition-rate neutron beam generation using micron-scale converging heavy water jet targets

Laser-driven neutron sources offer an attractive, alternative approach for generation of short, intense bursts of neutrons with narrow energy ranges. Such beams are desirable especially for damage cascade and pump-probe studies of fusion-relevant materials. In order to achieve necessary neutron beam fluxes, the utilization of petawatt lasers as drivers promises to surpass current recorded particle yields. This is due to the proportionality of laser-to-neutron conversion efficiency to the ion beam peak brightness. Together with recently available high-repetition rate laser systems, laser-driven neutron sources could outperform existing conventional sources.

Here, we present first results of reliable generation of deuteron beams using a converging heavy water sheet target at 0.5 Hz with the ALEPH laser at Colorado State University (400 nm, 45 fs, 10 J). Proof of principle measurements show efficient neutron generation through deuteron breakup and nuclear reactions.   

Not Participating

The peak on-target laser intensity is a key parameter for applications such as laser driven particle acceleration and x-ray sources. We have tested Compound Parabolic Concentrator [1] (CPC) targets which act as a non-imaging focusing optic. We demonstrate experimental measurements [2] collected at Texas Petawatt laser facility that show a hot-electron temperature enhancement of approximately a factor of 9 from the CPC target when compared to planar target. Using PIC simulations, we describe this hot-electron enhancement from a purely geometric intensity enhancement and existing temperature-intensity scaling laws. Using the CPC geometry, enhancements in proton acceleration and maximum proton acceleration are predicted due to the increase in electron temperature and reduction in electron source size.

[1] MacPhee et al., “Enhanced laser-plasma interactions using non-imaging optical concentrator targets,” Optica, vol. 7 issue 1 (2020).

[2] Rusby et al., “Enhancements in laser-generated hot-electron production via focusing cone targets at short pulse and high contrast,” PRE 103, 053207 (2021)   

Color Center Qubit Synthesis with Ion Pulses from Laser-Plasma Acceleration

We report on the formation of color centers, such as the G-center, a spin-photon qubit candidate in silicon, with ion pulses from laser-plasma acceleration at the BELLA PW laser (~10¹⁹ W/cm², up to 1 Hz).  Proton and carbon ion pulses with intensities in the 10¹² ions/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 followed by ex-situ analysis and find that carbon ions with energies below 2 MeV are implanted with fluences of 10¹⁴ - 10¹⁵ atoms/cm²/shot.  Intense ion pulses excite and heat targets, leading to the direct formation of color centers without consecutive thermal annealing [1].  We compute the electronic structure of color centers within density functional theory.  We discuss the potential impacts of this form of local qubit synthesis far from equilibrium.