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Automated control and optimization of laser-driven ion acceleration
Published in High Power Laser Science and Engineering, 2023
The interaction of relativistically intense lasers with opaque targets represents a highly non-linear, multi-dimensional parameter space. This limits the utility of sequential 1D scanning of experimental parameters for the optimization of secondary radiation, although to-date this has been the accepted methodology due to low data acquisition rates. High repetition-rate (HRR) lasers augmented by machine learning present a valuable opportunity for efficient source optimization. Here, an automated, HRR-compatible system produced high-fidelity parameter scans, revealing the influence of laser intensity on target pre-heating and proton generation. A closed-loop Bayesian optimization of maximum proton energy, through control of the laser wavefront and target position, produced proton beams with equivalent maximum energy to manually optimized laser pulses but using only 60% of the laser energy. This demonstration of automated optimization of laser-driven proton beams is a crucial step towards deeper physical insight and the construction of future radiation sources.
Recommended citation: B. Loughran, M. J. V. Streeter, H. Ahmed, S. Astbury, M. Balcazar, M. Borghesi, N. Bourgeois, C. B. Curry, S. J. D. Dann, S.DiIorio, N. P. Dover, T. Dzelzainis, O. C. Ettlinger, M. Gauthier, L. Giuffrida, G. D. Glenn, S. H. Glenzer, J. S. Green, R. J. Gray, G. S. Hicks, C. Hyland, V. Istokskaia, M. King, D. Margarone, O. McCusker, P. McKenna, Z. Najmudin, C. Parisuaña, P. Parsons, C. Spindloe, D. R. Symes, A. G. R. Thomas, F. Treffert, N. Xu, and C. A. J. Palmer. "Automated control and optimization of laser-driven ion acceleration." High Power Laser Science and Engineering 11, E35 (2023).
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On the Role of Ionization Physics in Intense Laser-Plasma Interactions
Published at The University of Michigan, Ann Arbor, 2024
Ionization is critical in the formation and evolution of plasma dynamics; collisional ionization, in particular, is an often overlooked source of electrons when dealing with laser-plasma interactions. It, however, plays a crucial role in understanding the complex plasma kinetics, ranging from cold and sparse astrophysical settings to hot and dense fusion systems. This dissertation presents a new, deterministic algorithm that adds collisional ionization physics to particle-in-cell (PIC) codes. This algorithm offers improved accuracy, achieving up to two orders of magnitude decrease in the error of the ionization rate calculations versus the alternatives, scales linearly in execution time with the number of macro-particles per cell, and was rigorously tested for physical correctness. The first simulation study we present using this new algorithm examines a method of measuring the collisional relaxation time of a plasma with picosecond resolution through the evolution of short-pulse laser-generated plasma. We describe the evolution of a two-temperature plasma, created due to above-threshold ionization, that expands from ∼1 μm to a radius of ≈50 μm and is sustained due to the balancing currents of these “hot”, 500 eV, and “cold”, 100 eV, electrons. Our second study offers a model and supporting simulations for the stable generation of low divergence (≤20 mrad) proton beams from a novel liquid sheet target. Through the generation of a cold electron plasma (≲100 eV) via proton-impact ionization of a background water vapor, these proton beams drive a single filament Weibel instability, which causes the rapid growth of an azimuthal magnetic field that focuses these protons over long distances (cm scale). These studies provide a novel look at laser-plasma interactions that explore the dynamics of collisional ionization and its interplay with the plasma kinetics and are in good agreement with experimental data. Finally, our algorithm offers an alternative means of simulating collisional ionization inside a PIC framework that could easily be expanded, along with its described benefits, to include any other ionization or recombination scheme a user may desire.
Recommended citation: S. DiIorio, On the Role of Ionization Physics in Intense Laser-Plasma Interactions, Ph.D. dissertation, The University of Michigan, Ann Arbor, 2024.
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Stable laser-acceleration of high-flux proton beams with plasma collimation
Published in Nature Communications, 2025
Laser-plasma acceleration of protons offers a compact, ultra-fast alternative to conventional acceleration techniques, and is being widely pursued for potential applications in medicine, industry and fundamental science. Creating a stable, collimated beam of protons at high repetition rates presents a key challenge. Here, we demonstrate the generation of multi-MeV proton beams from a fast-replenishing ambient-temperature liquid sheet. The beam has an unprecedentedly low divergence of 1° (≤ 20 mrad), resulting from magnetic self-guiding of the proton beam during propagation through a low density vapour. The proton beams, generated at a repetition rate of 5 Hz using only 190 mJ of laser energy, exhibit a hundred-fold increase in flux compared to beams from a solid target. Coupled with the high shot-to-shot stability of this source, this represents a crucial step towards applications.
Recommended citation: M. J. V. Streeter, G. D. Glenn, S. DiIorio, F. Treffert, B. Loughran, H. Ahmed, S. Astbury, M. Balcazar, M. Borghesi, N. Bourgeois, C. B. Curry, S. J. D. Dann, N. P. Dover, T. Dzelzainis, O. C. Ettlinger, M. Gauthier, L. Giuffrida, S. H. Glenzer, J. S. Green, R. J. Gray, G. S. Hicks, C. Hyland, V. Istokskaia, M. King, D. Margarone, O. McCusker, P. McKenna, Z. Najmudin, C. Parisuaña, P. Parsons, C. Spindloe, D. R. Symes, A. G. R. Thomas, N. Xu, and C. A. J. Palmer. "Stable laser-acceleration of high-flux proton beams with plasma collimation." Nature Communications 16, 1004 (2025).
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talks
Laser wakefield acceleration and plasma collisions: What they are, how the two relate, and methods of simulation
Published:
Laser wakefield acceleration (lwfa) is a means of accelerating electrons to energies up to several GeV. It is becoming an attractive experimental tool because it can produce electron bunches comparable to large facilities like SLAC or CERN in a few centimeters versus several kilometers. Additionally, with the advancement of laser technology, we are now reaching a point where we can create denser plasmas, which, combined with lwfa electron bunches, leads to collisions and collisional ionization effects that become important. In this talk, I will give a brief overview of laser wakefield acceleration: how it is accomplished, how it compares to the alternative rf means of accelerating particles, and its future prospects. Then, I will discuss the physics and computational methods behind collisions and collisional ionization within plasmas. In particular I will focus on the models I am implementing to realize this physics within the particle-in-cell code OSIRIS, i.e. a modified binary encounter Bethe model to calculate collisional cross-sections and the approach proposed by Pérez (2012) to implement the collisional interactions. Finally, I will discuss the propagation of high-energy, high-density electron bunches in air and how collisions (and other effects) may play an important role in helping or hindering the total distance they can travel.
Ultrafast Probing of Non-Equilibrium Plasmas Using Laser-Wakefield-Accelerated Electron Bunches
Published:
We explore the predominant physics behind the generation and evolution of an optical-field ionized plasma using electron bunches produced from a laser-plasma accelerator. The delay between the pump pulse and when the electron probe passes through the generated plasma has a direct effect on the measured energy of said electrons. Thus, we can use these electron bunches as a diagnostic to understand the dynamics of these non-equilibrium plasmas with picosecond temporal resolution. We employ particle-in-cell codes, including collisions and ionization, to model the plasma’s electric fields that cause such an effect. We then simulate electrons passing through these electric fields at various points in its evolution and compare with experimental results.
Modeling Collisional Ionization Using a Modified Binary-Encounter-Bethe Model in the Particle-in-Cell Code OSIRIS
Published:
Collisions, and subsequently collisional ionization, have become necessary to a proper understanding of plasma dynamics in a variety of situations. For example, collisional ionization must be used to properly model electron bunch propagation over long distances (up to several meters or more) outside vacuum. To correctly simulate these problems, it is important to develop and implement computational models that accurately depict the complex atomic physics of these interactions. However, difficulties can arise when the atomic structure and electron configuration of an atom greatly alters the binding energy and cross sections to be used in these formulations. We have implemented a collisional ionization routine in the particle-in-cell code OSIRIS that draws on examples and advancements from other particle-in-cell codes. We use a modified binary-encounter-Bethe model to calculate atomic cross-sections along with the Monte Carlo collisional scheme in order to model inter- and intra-species collisional ionization in both relativistic and non-relativistic regimes. We present details of the implementation and results from running OSIRIS using this new collisional ionization module.
Work Towards a Collisional Ionization Model for Particle-in-Cell Codes
Published:
The necessity for modeling collisional processes in plasmas is becoming ever more important as experimental efforts using higher density plasmas and solid targets come to fruition. We present progress towards an efficient module for simulating collisional ionization events within a particle-in-cell (PIC) framework. Our model has been tested rigorously for physical accuracy and does not suffer from statistical noise, thus decreasing the number of particles needed for a given simulation. This is done by calculating the rate of ionization deterministically and then adjusting the species densities within the simulation accordingly, which acts as a “smoothing” process reducing noise generated. Our model also includes proper momentum transfer due to the collisional process. This module has been integrated into the PIC code OSIRIS and has been benchmarked against other PIC codes, such as EPOCH and Smilei. We also use our model to simulate a variety of physical situations including electron beam propagation through air, electron stopping power through collisional ionization, and fast electron propagation through solids.
A Deterministic Collisional Ionization Module for Particle-in-Cell Codes
Published:
We present updates to our collisional ionization module for particle-in-cell (PIC) codes. Our method treats and calculates ionization events deterministically as each particle’s rate is calculated explicitly and deposited onto a grid. This grid of ionization rates is then used to advance ion densities, which allows us to track how much new charge is generated each timestep, so we can create newly ionized electrons accordingly. Additionally, the ionization rate grid, with little modification, keeps track of how much energy is lost per grid cell due to ionization physics. We interpolate this information back onto the particles; this allows for a continuous decrease in the energy of macro-particles as they participate in ionization events and allows for the easy calculation of the new momentum of ionized electrons. Collectively, this particle-to-grid and grid-to-particle information transfer act as a “smoothing” process, reducing noise considerably compared to other current algorithms. This module has been tested for its accuracy and integrated into the PIC code OSIRIS. In addition to this, we present several simulations highlighting the new physics that is captured when considering collisional ionization in different scenarios.
Stable Collimation of MeV Proton Beams by Self-Driven Magnetic Pinching
Published:
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.