Session YO06: Laser-plasma ion accelerators

Experimental investigation of the magnetic vortex acceleration regime

Magnetic Vortex Acceleration (MVA), an advanced ion acceleration mechanism, requires an ultra-high intensity and contrast laser pulse combined with a near-critical density target that is matched to the laser parameters. Theoretically, these conditions result in high energy, well-collimated ion beams, directional electron beams, and gamma-ray generation. A series of 3D particle-in-cell simulations were performed to aid with experimental preparation and to study the electromagnetic field evolution and the properties of accelerated beams under realistic experimental conditions. We report on simulation and experimental results, investigating this regime using the newly commissioned high intensity beamline at the BELLA Center, iP2. The acceleration mechanism is studied with a diverse set of diagnostics monitoring the properties of the generated ions, electrons, x-rays and gamma-rays as well as the transmitted and reflected laser light.   

Stable collimation of MeV proton beams by self-driven magnetic pinching

We report the generation of a multi-MeV proton beam from a novel continuously-flowing ambient-temperature liquid water jet target [Treffert et al., Physics of Plasmas 29, 123105 (2022)]. Compared to those generated from a more typical polyimide tape target, proton beams from this water target were less divergent (≤ 20 mrad), higher dosage (55 Gy), stable (peak dose variation of 11% rms), high-energy (4-6 MeV), and could operate reliably at 5 Hz with the potential to scale up to kHz rates. The presence of a low-density vapor surrounding the target aided in the generation of these desirable proton beams. Here, we report on 2D OSIRIS simulations used to study the collimation mechanism. Through proton collisional ionization, the beam was able to maintain an amount of neutrality via the newly ionized electrons that helped to mitigate electrostatic fields that would otherwise cause the beam to expand. It does not, however, fully negate the beam current, which generates an azimuthal magnetic field that acts to pinch the proton bunch much like the ion Weibel instability would. This allows for the self-focusing of a single filament. And while these simulations are inherently simplified, they offer an exciting opportunity to explore experimental conditions to allow for the control of proton beam propagation.   

Acceleration and focusing of multispecies ion beams using over-dense targets with curved front surfaces

High-flux, low-emittance ion beams are of interest to a wide range of applications. In particular, Fast Ignition (FI) of Inertial Confinement Fusion (ICF) requires a sub-ps ignitor beam with high energy flux to ignite a compressed DT fuel. We propose a scheme to generate high-flux ion beams via hole-boring radiation pressure acceleration using over-dense plasma targets with curved front surfaces. These low-emittance (on the order of 0.01 mm mrad) beams can focus to a few-micron spot size at a predefined focal length. The focal length and ion mean energy can be independently controlled: the former by changing the front-surface curvature and the latter by tuning the laser- plasma parameters. We interpret the results using simple models and validate them using first-principles PIC simulations. We demonstrate the applicability of the scheme for different laser transverse profiles and multi-ion species targets, with laser powers in the 100TW to 100 PW range, and the focal length in the 20-60 micron range. We identify factors limiting the ion beam spot size at focus and discuss methods to extend beam focal length to mm range – in accordance with ion beam stand-off distance required for igniting a FI target.   

Light Sail Acceleration of Ions

The applicative potential of high energy ion beams involving proton probing, production of warm dense matter, fast ignition of fusion targets, nuclear physics and biomedical applications, and the thrust on investigating the novel regimes of ion acceleration is the motive to  focus on non-linear interaction of an ultra-short, ultra-intense (intensity of the order of 10²²-10²⁴ W cm^-2) laser pulse with an ultra-thin solid target in radiation pressure dominant  regime(RPD). The ~ tera bar pressure of such super-intense laser pulses causes the direct acceleration of ions making it the most efficient mechanism. The relativistic non-linear laser-plasma foil interaction depends on many parameters; laser intensity pulse profile, pulse group velocity, target density etc.  The analytical and numerical results exhibit the ion momentum/energy, their numbers, and dependence upon the pulse profile.  For optimum energy transfer from the pulse to the ion during the interaction, the foil should be opaque requiring proper matching of the target to the laser pulse.  Also, a Lorentzian laser pulse incident on a thin hydrogen foil can generate ~ 10¹⁰ protons, almost monoenergetic in the energy range ~ 230 MeV,  resulting in a proton beam  that makes  it a suitable candidate for hadron therapy applications. The momentum/energy that a laser pulse transfers to the ions depends on the  pulse profile as well as the group velocity of the incident  laser pulse. In the RPD regime, excluding the group velocity effec the energy imparted to the plasma ions by the Lorentzian pulse came out to be ~ 2.67 times greater than those transferred by the Gaussian pulse. On the other hand taking into account the group velocity, this value becomes ~ 2.46 times. Interaction of an intense laser pulse with flat target leads to an expansion of the plasma foil in the transverse direction resulting in decrease in the accelerated ions in longitudinal direction and hence energy per ion is increased. The surface density of the expanded plasma foil also affects the transparency of the thin foil; and with laser group velocity effect, the transverse expansion of the plasma foil is reduced due to early termination of the ion acceleration process. The group velocity effect is dominant over the transverse expansion effect.   

Low divergence and high energetic proton acceleration with vortex lasers

Exotic laser beams, carrying angular momentum, can give rise to novel physics in high-intensity laser-plasma interactions [1,2]. The angular momentum leads to a new degree of freedom, yielding from angular momentum transfer and complex non-linear optical processes. Various applications ranging from laser-driven particle accelerators to high-energy-density physics and medical applications can benefit from using exotic laser beams [3,4]. 

We have demonstrated ultra-low divergence (≲ 0.04) proton beams with enhanced proton energies (multi-10-MeV) through the interaction of a laser with orbital angular momentum (OAM) with a double-layer target, relying on three-dimensional particle-in-cell simulations in OSIRIS. We have investigated the reduced relativistic self-focusing [1] of a laser with OAM in near-critical plasmas and determined its crucial role in generating high-quality proton beams.

Lastly, we studied the angular momentum transfer of exotic lasers in underdense plasmas for intense magnetic field generation through the inverse Faraday effect [2]. 

[1] L. Sa and J. Vieira, Phys. Rev. A 100, 013836 (2019).

[2] S. Ali et al., Phys. Rev. Lett. 105, 035001 (2010).

[3] J. Vieira et al., Phys. Rev. Lett. 117, 265001 (2016). 

[4] C. Willim et al., Phys. Rev. Research 5, 023083 (2023).   

Modeling the effect of spectral phase shaping of ultra-short laser pulses for laser-driven ion acceleration on ensemble scales

In this work we performed 1D and 2D particle-in-cell  modeling of ion acceleration by spectrally shaped laser pulses in a multi-fidelity framework that covered several orders of magnitude of parameter space. Spectral shaping by means of a dazzler allows us to modify the temporal shape of a short-pulse laser on femtosecond to picosecond time scales. An ensemble of several thousand 1D PIC  simulations was used as support for several hundred 2D PIC simulations. The 1D dataset is used to inform a neural-network surrogate model which is then elevated to 2D fidelities via ad-hoc transfer learning, whereby the trends embedded in the 1D data are used to inform the 2D surrogate model at higher accuracies. More complex network architectures allow us to synthesize non-congruent datasets in ways otherwise not possible and may allow for the integration of more realistic data in future work. Pulse shaping suggests that we can not only achieve higher ion energies than otherwise accessible with just a Gaussian pulse but also that typical quantities of interest may be partially tuned independently of one another to a limited degree.

    

Relativistic two-wave resonant acceleration of electrons at large-amplitude standing whistler waves during laser-plasma interaction

The interaction between a thin foil target and a circularly polarized laser light injected along an external magnetic field is investigated numerically by particle-in-cell simulations. A standing wave appears at the front surface of the target overlapping the injected and partially reflected waves. Hot electrons are efficiently generated at the standing wave due to the relativistic two-wave resonant acceleration if the magnetic field amplitude of the standing wave is larger than the ambient field. A phase transition occurs in the gyration motion of electrons, allowing all electrons with non-relativistic velocities to acquire relativistic energy through the cyclotron resonance. The optimal conditions for the highest energy and the most significant fraction of hot electrons are derived precisely through a simple analysis of test-particle trajectories in the standing wave. Since the number of hot electrons increases drastically by many orders of magnitude compared to the conventional unmagnetized cases, this acceleration could be a great advantage in laser-driven ion acceleration and its applications.   

High efficiency ion acceleration in foil plasma expansion driven by kJ petawatt lasers

Kilojoule petawatt lasers with relativistic intensities such as LFEX and NIF-ARC have demonstrated efficient ion accelerations from thin foil plasmas. In the laser-plasma interaction, the foil plasma expands significantly to make a large-scale coronal plasma, in which fast electrons are accelerated much beyond the scaling for sub-ps laser-plasma interactions. Owing to the large laser spot, fast electrons are confined in the laser spot area [1]. In this new stage, the plasma expansion structure changes to a non-isothermal, fast expansion mode [2]. We here study the temporal evolution of energies of fast electrons and ions in the expanding plasma under a continuous laser energy input in 1-10 picosecond scale, to model the efficient proton acceleration seen in the kJ laser experiments. Particle-in-cell simulations show that fast electron energy density in the expanding foil plasma increases temporally, and a strong sheath electric field is maintained in the expansion. On a picosecond time scale, about one half of the absorbed laser energy is converted to the kinetic energy of over-MeV fast ions during the laser irradiation, resulting a high laser-to-ion energy coupling.   

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

Today, there are multi-hundred-TW table-top short-pulse laser systems that can generate on-target intensities of around 10¹⁹ to 10²¹ W/cm² and can routinely produce proton beams in the multi-MeV range[1]. The short bunch duration and the very intense and localized heating properties of these beams are perfectly suited for studies in warm dense matter or material science. Here, 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 thermal shocks generate a uniformly distributed clustered heating on a surface[2], with the dimensions of the clusters depending on the irradiation dose and on the duration of the thermal shock. When quickly cooling down, the clusters can produce large nanostructured surfaces[3], [4]. Controlling the dose allows us to obtain nanostructured surfaces with a low dispersion in particle dimension, a high density of particles and a polycrystalline morphology.   

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High-intensity (>10¹⁸ W/cm²) lasers can be used to generate energetic proton sources via the Target Normal Sheath Acceleration (TNSA) mechanism. The PROton Beam Imager and Energy Spectrometer (PROBIES) [D. Mariscal et al., PPCF 2021] uses a stepped filter design to diagnose these sources by providing simultaneous energy and spatial information in a compact form-factor. PROBIES can be fielded with radiochromic film (RCF) stacks to achieve higher energy resolution and larger bandwidths or with a scintillator to provide data for each shot at a high laser repetition rate (>1 Hz). Here we present developments in the methodology for the design and interpretation of PROBIES data by including detailed calculations of proton transport and scattering. This work discusses the deconvolution of the proton beam from PROBIES data used in an RCF stack to characterize high energy, high divergence proton beams generated at OMEGA EP.   

Optimization of light-emitting defects in silicon with plasmas, ion beams, and lasers

Light-emitting defects (or color centers) in semiconductors are promising qubit candidates for applications in quantum sensing and quantum communication.  Color centers often form when dopants are introduced into the host crystal matrix combined with energetic radiation and thermal annealing.  Quantum information science (QIS) applications benefit from high

quality color centers that can be formed reliably and this poses new challenges and opportunities for material processing including with ion beams and plasmas. We report on the synthesis of high-quality color centers using terawatt to petawatt laser-plasma-driven ion beams to implant selected elements including boron, titanium, and carbon into silicon. Color centers, including qubit candidates such as W, G, and C-centers in silicon, form directly under these conditions of laser-ion doping. 

Color center synthesis with the laser-ion plasma-driven approach complements and can be combined with processing methods including conventional ion implantation, thermal annealing, exposure to H₂ plasmas, and fs lasers for local formation and passivation of high-quality color centers. We outline directions toward scalable integration of color centers in silicon for applications in QIS.   

Laser-ion-doping and qubit synthesis in semiconductors

Intense ion pulses from laser acceleration can damage, dope, heat and anneal semiconductors in one step. Excitation during nanosecond ion pulses is followed by rapid quenching on a microsecond time scale where desired phases and defect structures can stabilize. We report on direct formation of quantum defects in silicon and diamond samples processed with ion pulses from laser acceleration at the BELLA Center [1] and at the PHELIX facility (GSI) [2]. We characterized the ex situ quantum defect optical spectra, structural and compositional changes in the samples. In situ time-resolved ion current measurements at the BELLA Center 100 TW laser show a dual pulse structure with MeV ions from target normal sheath acceleration, followed by low energy ions from plasma expansion of the laser foil. Boron concentrations up to several atomic percent are observed in the samples from PW shots with boron foils, which enable the formation of a superconducting phase. We will discuss next steps in laser-ion doping optimization and color center qubit integration based on our recent results [1-5].