Contents I. Introduction
Flash X-ray radiography has been used in USA and around the world for many years as a diagnostic tool for imaging highly kinetic experiments where temporal imaging resolution and spatial resolutions < 1 mm are required for imaging high- materials. Hydrodynamic testing facilities, such as the Lawrence Livermore National Laboratory (LLNL) flash X-ray (FXR) linear induction accelerator (LIA) [1]–[4] and the Los Alamos National Laboratory (LANL) Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT/DARHT-II) [5], [6], have been developed and operated for many years. FXR is a single beamline that can support two independent high-current and high-energy electron pulses by partitioning its Marx banks and driving interleaving accelerator cells to run two pulses at half of the nominal single pulse beam energy. DARHT-II is a long pulse machine that can chop the beam by quickly kicking it between a beam dump and a foil target. All LIAs until now have been driven by Marx banks, which are able to generate large voltages by charging a set of capacitors in parallel and then quickly switching them into series to add the voltage. Marx generators tend to be prone to noise and it can be difficult to control the beam parameters. There are limitations on the performance of Marx generator pulsers that manifest as energy variation in the pulse flat-tops and pulse configuration limitations (i.e., pulse spacing and pulsewidth). The Scorpius LIA concept has been conceived to improve the performance characteristics of the accelerator power systems by replacing the Marx generators with solid-state pulsed power (SSPP) [7]. This allows the pulses to be modulated to improve interpulse spacing, pulse rise times, energy variation, and current variation. These improvements in the pulse quality translate to superior multipulse radiographic performance by generating a sharper and flatter high voltage pulse. However, one major engineering tradeoff associated with using SSPP is the injector grows in size compared to Marx generator-driven systems. This leads to the requirement of a longer drift for electron beam transport before reaching the accelerator. The beam is at lower energy for a longer period of time and can be prone to seeding, or development of instability in the injector, and will be more susceptible to engineering misalignment as a larger number of transport magnets are used to deliver the beam to the accelerator. There is existing simulation work on DARHT-II in the injector [8] and the accelerator [9] that have focused on analyzing component misalignment and instability growth. There also exists work for Scorpius to simulate the electron beam from cathode to target [10], [11] using simulation tools such as Trak, XTR, and LSP-slice [12], [13]. This article focuses on analyzing the engineering alignment requirements for the Scorpius injector transport magnets, cathode stalk, and anode beam pipe, and how those requirements impact beam quality. A 3-D particle-in-cell (PIC) model of the Scorpius injector has been developed in Warp, which includes electron self-generated electric and magnetic fields that can resolve beam centroid motion to be less than 1 mm. This work is complementary to previous simulation studies, which utilize azimuthal symmetry in the simulations to decrease simulation time. It will be demonstrated that the pulses generated by the SSPP system drive a high-quality beam out of the injector in the presence of transverse error fields that are introduced by physical solenoid misalignment. It will be shown that the beam that can be deflected off-axis significantly by the error fields can be corrected and steered back onto axis by computing an optimal steering solution and adjusting the dipole steering coils in the injector.
