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Particle-in-cell (PIC) simulations of a high current density (I > 1 kA), and highly relativistic electron beam (E ~ 2-20 MeV) in the Scorpius Linear Induction Accelerator (LIA) are presented. The simulation set consists of a 3D electrostatic/magnetostatic simulation coupled to a 2D XY slice solver that propagates the beam through the proposed accelerator lattice for Scorpius, a next-generation flash X-ray radiography source. These simulations focus on the growth of azimuthal modes in the beam (e.g. Diocotron instability) that arise when physical ring distributions manifest in the beam either due to electron optics or solenoidal focusing and transport. The saturation mechanism appears to lead to the generation of halo particles and conversion down to lower mode numbers as the width of the ring distribution increases. The mode growth and saturation can contribute to the generation of hot spots on the target as well possible azimuthal asymmetries in the radiograph. Simulation results are compared to linear theory and tuning parameters are investigated to mitigate the growth of azimuthal modes in the Scorpius electron beam., Proceedings of the 5th North American Particle Accelerator Conference, NAPAC2022, Albuquerque, NM, USA

The performance of an accelerator is limited by the quality of the beam produced at the injector. For a Pierce-type diode structure, the cathode-shroud interface and the anode pipe entrance are sources for undesired, irreversible phase space nonlinearities that lead to emittance growth. In this contribution, we present ways to mitigate these nonlinearities by adjusting the cathode-shroud interface to meet the beam edge boundary conditions and by adjusting the solenoidal focusing magnet in the diode region such that the nonlinear focusing magnetic fringe fields compensate the nonlinear defocusing electrical fields of the anode pipe entrance., Proceedings of the North American Particle Accelerator Conference, NAPAC2019, Lansing, MI, USA

In this paper, we describe an envelope code, ENSOLVE. It solves the rms beam envelope equation by including space change depression of the potential, spherical aberration of the so-lenoidal lens, emittance growth and focusing effects of backstreaming ions in the final focus region. In this paper, we focus on the physics included for beam transport simulations in the downstream section of flash x-ray radiography linear induction accelerators, such as FXR LIA. We have used ENSOLVE to design final focus tunes for FXR LIA downstream section, Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

The current interest for the next generation linear induction radiography accelerator (LIA) is to generate multiple electron beam pulses with high peak currents. The beam energy and current may vary from pulse to pulse. Conse-quently, the transport and control of multi-pulsing intense electron beams through a focus-ing lattice over a long distance on such machine becomes challenging. Simulation studies of multi-pulse LIAs using AMBER [1] and BREAKUP Code [2] are described. These include optimized focusing magnetic tune for beams with energy and current variations, and steering correction for corkscrew motion. The impact of energy variation and accelerating voltage error on radiograph performance are discussed., Proceedings of the North American Particle Accelerator Conf., NAPAC2016, Chicago, IL, USA

The Flash X-ray Radiography (FXR) [1] line-ar induction accelerator at Lawrence Livermore National Laboratory produces x-ray bursts for radiographs. The machine is able to produce x-ray spot sizes less than 2mm. Using the spot sizes measured from the magnet scanning, the beam parameters are unfolded by modelling the FXR LINAC with the simulation code AMBER [2] and the envelope code XENV [3]. In this study, the most recent spot size measurement results and techniques used to extract the beam parameters are described. Using the unfolded beam parameters as the initial condition, the backstreaming ions' neutralization factor f = 0.3 is found by comparing the calculated spot sizes with measured spot sizes at the target., Proceedings of the North American Particle Accelerator Conf., NAPAC2016, Chicago, IL, USA

Knowing the initial beam parameters entering an accelerator or a downstream beamline allows us to select transport tunes optimized for a desired accelerator performance. In this study, we report unfolding LLNL's FXR [1] beam parameters by using the tomography technique [2, 3] to construct the beam phase space along the accelerator's downstream beamline. The unfolded phase spaces from tomography and simulations are consistent., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

Energy variation study for beam transport from the entrance of a conceptual induction radiography accelerator to the x-ray target has been reported previously [1]. In this report, we have extended the study upstream to the injector. To achieve minimum emittance growth and to obtain a desired final beam size, we have developed three optimal tunes. Among them, one optimal tune, capable of supressing beam break-up instability and producing acceptable corkscrew motions, is used to study the energy variation effects on radiography performance. The study shows that ±3% energy variation is acceptable., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

Vacuum insulators composed of alternating layers of metal and dielectric, known as high-gradient insulators (HGIs), have been shown to withstand higher electric fields than conventional insulators. Generally, vacuum insulator failure is due to surface flashover, initiated by electrons emitted from the triple junction. These electrons strike the insulator surface and produce secondary electrons, which also strike the insulator surface to create more secondary electrons and lead to avalanche. Magnetic field from the external sources, the high-current electron beam, the conduction current in the transmission line or the displacement current in the insulator can deflect primary and secondary electrons’ trajectories either toward to or away from the insulator surface, and hence affect the performance of the high-voltage vacuum insulator. The displacement current effects are particularly interesting for short pulse applications. This paper presents the displacement current effects with various short applied voltage pulses on performance of high-gradient insulators. Optimal HGI configurations will also be discussed., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan