Rhon Keinigs

Two-dimensional dynamics of the plasma wakefield accelerator

A general analysis of the electromagnetic wakefields for an axisymmetric charge distribution moving through a cold uniform plasma is presented. Particular attention is given to the electromagnetic and relativistic effects influencing the waves. It is shown that the plasma provides shielding of the transverse wakefields on a scale length of a few electromagnetic skin depths (c/ωp). The implications for plasma wakefield accelerators are that driving beams with radii of a few skin depths will experience severe pinching, whereas larger beams will be subject to filamentation instabilities. These conclusions are supported by self‐consistent particle‐in‐cell simulations. The deleterious effects of transverse wakefields will severely limit the accelerating gradients that can be obtained even if a magnetic field is used to guide the driving beam.

A new interpretation of the alpha effect

In a magnetized plasma the alpha effect represents a turbulently generated emf directed along the mean magnetic field. This emf is central to the understanding of turbulent generation of large‐scale magnetic fields. The alpha effect is reevaluated in terms of ensemble‐averaged properties of the magnetic fluctuation spectrum. The results indicate that it is current helicity as opposed to kinetic helicity that is required to produce an alpha effect.

Ion Plasma Wave Wakefield Accelerators

The possibility of using space-charge waves on an ion beam or column as a wakefield accelerator is discussed. The primary advantages of using ion plasma waves over electron plasma waves are that the kinetic energy and fall-time requirements on the driving beam are reduced. One disadvantage in using a lower plasma frequency is that a larger current is required to achieve the same accelerating gradient. The basic aspects of wakefield accelerators are reviewed and this concept is analyzed in this context. Particle-in-cell simulations show that wakefields utilizing ion waves, although more complicated than plasma wakefields, can produce acceleration.

Field‐error induced transport in a pure electron plasma column

The long confinement times achieved in experiments on pure electron plasmas are explainable in terms of the conservation of canonical angular momentum in azimuthally symmetic systems. A low‐density, pure electron plasma is considered, and the effect of disrupting the system’s symmetry by introducing a small, magnetic field error is investigated. It is found that this external perturbation can resonantly drive low‐frequency waves that couple back to the field error and produce a change in the plasma’s angular momentum. This results in changing the radius of the column.

Scaling laws for particle growth in plasma reactors

We quantify a model which incorporates observed features of contaminant particle growth in plasma processing reactors. According to the model, large ‘‘predator’’ particles grow by adsorbing smaller, typically neutral, ‘‘prey’’ protoparticles. The latter are supplied by an assumed constant mass injection of contaminant material. Scaling laws and quantitative predictions compare favorably with published experimental results.

One- and two-dimensional simulations of imploding metal shells

We report results of one- and two-dimensional (2D) magnetohydrodynamic simulations of imploding, cylindrical metal shells. One-dimensional simulations are used to calculate implosion velocities of heavy liners driven by 30 MA currents. Accelerated by the j×B force, 45 g aluminum/tungsten composite liners achieve velocities on the order of 13 km/s. Used to impact a tungsten target, the liner produces shock pressures of approximately 14 Mbar. The first 2D simulations of these liners are also described. These simulations have focused on two problems: (1) the interaction of the liner with the electrically conducting glide planes, and (2) the effect of realistic surface perturbations on the dynamics of the implosion. The former interaction is confined primarily to the region of the contact point between the liner and glide plane, and does not seriously affect the inner liner surface. However a 0.2 μm surface perturbation has a significant effect on the implosion dynamics.

Material science experiments at the Atlas facility

Three material properties experiments that are to be performed on the Atlas pulsed power facility are described; friction at sliding metal interfaces, spallation and damage in convergent geometry, and plastic flow at high strain and high strain rate. Construction of this facility has been completed and experiments in high energy density hydrodynamics and material dynamics will begin in 2001.

A comparison of the dielectric and plasma wakefield accelerators

A comparison of two advanced accelerator concepts, the plasma wakefield accelerator (PWA) and the dielectric wakefield accelerator (DWA), is presented. Emphasis focuses on the peak accelerating gradients and transformer ratios that can be achieved in these two devices. The effect of finite plasma size on the PWA is also analyzed. For the same cavity geometry and drive beam current, it is found that the dielectric wakefield accelerator can generate a peak field that is comparable to the field that can be generated in the plasma wakefield accelerator. Provided that ideal beam pulse shaping can be achieved, the transformer ratio for the PWA is ten times larger than that for the DWA. A change in this ideal pulse shape results in closer agreement for the transformer ratios. Given these encouraging preliminary results for the DWA it is concluded that the simplicity of employing a passive structure as an accelerating medium warrants further experimental testing of the dielectric wakefield accelerator.

Simulation of the Wisconsin-Argonne Plasma Wakefield Experiment

The plasma wakefield accelerator (PWFA) is an advanced accelerator concept that uses the large electric fields that can be generated in a plasma to accelerate charged particles. We present the results of a self-consistent two-dimensional simulation of the first experiment designed to test this concept. Linear theory predicts for this experiment an accelerating gradient of approximately 95 MV/m. However, the simulations indicate that a much larger accelerating field is achieved in the plasma. This enhancement is due to strong beam pinching, which is not treated self-consistently by a linear theory. Wave steepening due to a nonlinear modulation of the background plasma is also observed. This steepening results in a phase shift that degrades the acceleration.

Shock‐Wave and Material Properties Experiments Using the Los Alamos Atlas Pulsed Power System

The Atlas facility built by Los Alamos is the world’s first and only laboratory pulsed power system designed specifically to provide capability for shock‐wave physics, materials properties, instability, and hydrodynamics experiments in converging geometry. Constructed in 2000 and commissioned in August 2001, Atlas completed its first year of physics experiments in October 2002, using ultra high precision magnetically imploded, cylindrical liners to reliably and reproducibly convert electrical energy to hydrodynamic energy in targets whose volume is many cubic centimeters. Multi‐view (transverse and axial) radiography, laser‐illuminated shadowgraphy, and VISAR measurements of liner and target surface motion, in addition to electrical diagnostics, provide a detailed description of the behavior of the experimental package. In the first year material damage and failure experiments, dynamic friction experiments, and a family of converging shock experiments were conducted in addition to a detailed series of lin...