Session QI2: Intense Beams and Accelerators

High Intensity e-beam Diode Development for Flash X-ray Radiography

A variety of electron beam diodes are being used and developed for the purpose of creating high-brightness, flash x-ray radiography sources. In these diodes, high energy (multi MeV), high current (multi kA), small spot (multi mm) electron beams are generated and stopped in high atomic number anode-targets (typically Ta or W). Beam stopping in the target creates copious amounts of bremsstrahlung radiation. In addition, beam heating of the target liberates material, either in the form of low density ($\sim $10$^{12}$-10$^{14}$ cm$^{-3})$ ion emission or higher density ($>$ 10$^{15}$ cm$^{-3})$ plasma. In all cases, beam/target collective effects dominate the diode and beam characteristics, affecting the radiation properties (dose and spot-size). Recent experiments at Sandia National Laboratories have demonstrated diodes capable of producing $>$ 350 rad@m with 1.7mm FWHM x-ray source distributions. A review of our present theoretical understanding of the diode (s) operation and our experimental and simulation methods to investigate them will be presented. Emphasis will be given to e- beam sources used on state-of-the-art Inductive Voltage Adder (IVA) pulsed-power accelerators. In particular, the physics of magnetically pinched diodes (e.g. the rod-pinch [1,2]), gas-cell focusing diodes [3] and the magnetically immersed [4] diode will be discussed. Various proposed methods to optimize the x-ray intensity and the direction of future diode research will be discussed. \newline [1] G. Cooperstein, et al., Phys. Plasmas \textbf{8}, 4618 (2001).\newline [2] B.V. Oliver et al., Phys. Plasmas\textbf{ 11}, 3976 (2004)\newline [3] B.V. Oliver, et al., IEEE Trans. on Plasma Science \textbf {33}, 704 (2005).\newline [4] M.G. Mazarakis, et al., Appl. Phys. Lett. \textbf{70}, 832 (1997)   

Time Resolved Imaging of Longitudinal Modulations in Intense Beams

The longitudinal evolution of high intensity beams is not well understood despite its importance to the success of such applications as free electron lasers and light sources, heavy ion inertial fusion, and high energy colliders. ~For example any amplification of current modulations in an FEL photoinjector can lead to unwanted coherent synchrotron radiation further downstream in compression chicanes or bends. ~A significant factor usually neglected is the coupling to the transverse dynamics which can strongly affect the longitudinal evolution. ~Previous experiments at the University of Maryland have revealed much about the longitudinal physics of space-charge dominated beams by monitoring the evolution of longitudinal perturbations. ~For the first time, experimental results are presented here which reveal the effect of longitudinal perturbations on the transverse beam distribution, with the aid of several new diagnostics that capture detailed time-resolved density images.~ A longitudinal modulation of the particle density is deliberately generated at the source, and its evolution is tracked downstream using a number of diagnostics such as current monitors, high-resolution energy analyzers, as well as the transverse imaging devices.~ The latter consist of a high-resolution 16-bit gated camera coupled with very fast emitters such as prompt optical transition radiation (OTR) from an alumina screen, or fast Phosphor screens with 3-ns time resolution. ~Simulations using the particle-in-cell code WARP are applied to cross-check the experimental results.~ These experiments and especially the comparisons to simulation represent significant progress towards understanding the longitudinal physics of intense beams.   

Reduced-Order Simulation of Large Accelerator Structures

Simulating electromagnetic waves inside finite periodic or almost periodic three-dimensional structures is important to research in linear particle acceleration, high power microwave generation, and photonic bandgap structures. While eigenmodes of periodic structures can be determined from analysis of a single unit cell, based on Floquet theory, the general case of aperiodic structures, with defects or non-uniform properties, typically requires 3D electromagnetic simulation of the entire structure. When the structure is large and high accuracy is necessary this can require high-performance computing techniques to obtain even a few eigenmodes [1]. To confront this problem, we describe an efficient, field-based algorithm that can accurately determine the complete eigenmode spectrum for extended aperiodic structures, up to some chosen frequency limit. The new method combines domain decomposition with a non-traditional, dual eigenmode representation of the fields local to each cell of the structure. Two related boundary value eigenproblems are solved numerically in each cell, with (a) electrically shielded, and (b) magnetically shielded interfaces, to determine a combined set of basis fields. By using the dual solutions in our field representation we accurately represent both the electric and magnetic surface currents that mediate coupling at the interfaces between adjacent cells. The solution is uniformly convergent, so that typically only a few modes are used in each cell. We present results from 3D simulations that demonstrate the speed and low computational needs of the algorithm. \newline \newline [1] Z. Li, et al, Nucl. Instrum. Methods Phys. Res., Sect. A 558 (2006), 168-174.   

A brightness transformer using a beam driven plasma wake field accelerator

High brightness electron beams are essential for many physics applications, colliders and light sources such as X-ray FEL at SLAC Linear Coherent Light Source (LCLS), Berkeley Advanced Light Source (ALS), and Argonne Advanced Photon Source (APS). Currently operational state of the art photo injectors can produce electron beams with brightness as high as 10$^{13}$ A/(mrad)$^2$. Here we introduce a new scheme for producing an electron beam with ultra-high brightness. 2D fully parallel PIC simulations of a plasma wakefield experiment driven by an ultrarelativistic drive beam are performed. The simulations show that ultra-high gradient longitudinal fields ($>$40 GV/m) trap plasma electrons; trapped electrons form a bunch which has a brightness value ($>10^{15}$A/mrad$^2$) two orders of magnitude greater than that of the drive beam. The simulations results are supported by the experimental data taken at the Stanford Linear Accelerator Center. Using the simulations we also show how the brightness can be optimized by changing the drive beam parameters.