Talks and presentations

Stable Collimation of MeV Proton Beams by Self-Driven Magnetic Pinching

November 03, 2023

Talk, 65^th Annual meeting of the APS Division of Plasma Physics, Denver, Colorado

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.

A Deterministic Collisional Ionization Module for Particle-in-Cell Codes

November 12, 2021

Talk, 63^rd Annual meeting of the APS Division of Plasma Physics, Pittsburgh, Pennsylvania

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.

Work Towards a Collisional Ionization Model for Particle-in-Cell Codes

November 09, 2020

Talk, 62^nd Annual meeting of the APS Division of Plasma Physics, Remote

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.

Modeling Collisional Ionization Using a Modified Binary-Encounter-Bethe Model in the Particle-in-Cell Code OSIRIS

October 23, 2019

Poster, 61^st Annual meeting of the APS Division of Plasma Physics, Fort Lauderdale, Florida

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.

Ultrafast Probing of Non-Equilibrium Plasmas Using Laser-Wakefield-Accelerated Electron Bunches

November 07, 2018

Poster, 60^th Annual meeting of the APS Division of Plasma Physics, Portland, Oregon

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.

Laser wakefield acceleration and plasma collisions: What they are, how the two relate, and methods of simulation

August 21, 2018

Talk, Air Force Research Lab, Albuquerque, New Mexico

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.

Stephen DiIorio