Thomas P. Hughes

Implementation of an non-iterative implicit electromagnetic field solver for dense plasma simulation

The implementation of an implicit electromagnetic field solver in the particle-in-cell code Lsp is presented. This solver is adapted for use in dense plasma simulation through the direct implicit scheme. The new implicit field solver involves two half-timestep field advances with convenient time centering for the ∇×E and ∇×B terms. Although making use of the alternating direct implicit technique, the new solution technique is faster because no iterations are required.

Small‐angle multiple scattering of charged particle beams

The scattering of self‐pinched particle beams propagating in gas is investigated using Moliere’s treatment of multiple scattering. Nordsieck lengths significantly longer than those predicted on the basis of less accurate scattering formalisms are obtained numerically. An improved analytic expression for the Nordsieck length is derived and agrees well with the numerical results. Experimental data on beam expansion from the ASTRON experiment show good agreement with numerical simulations and analytic results. The evolution of a thin annular beam due to scattering is studied, and the beam is found to acquire a Bennett profile in a distance on the order of one Nordsieck length.

Initial electron-beam results from the DARHT-II linear induction accelerator

The DARHT-II linear-induction accelerator has been successfully operated at 1.2-1.3 kA and 12.5-12.7 MeV to demonstrate the production and acceleration of an electron beam. Beam pulse lengths for these experiments were varied from 0.5 /spl mu/s to 1.2 /spl mu/s full-width half-maximum. A low-frequency inductance-capacitance (LC) oscillation of diode voltage and current resulted in an oscillation of the beam position through interaction with an accidental (static) magnetic dipole in the diode region. There was no growth in the amplitude of this oscillation after propagating more than 44 m through the accelerator, and there was no loss of beam current that could be measured. The results of these initial experiments are presented in this paper.

Status of the DARHT phase 2 long-pulse accelerator

The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will employ two perpendicular electron linear induction accelerators to produce intense, bremsstrahlung X-ray pulses for flash radiography. We intend to produce measurements containing three-dimensional information with sub-millimeter spatial resolution of the interior features of very dense, explosively-driven objects. The facility will be completed in two phases with the first phase having become operational in July 1999 utilizing a single-pulse, 20-MeV, 2 -kA, 60-ns accelerator, a high-resolution electrooptical X-ray imaging system, and other hydrodynamics testing systems. The second phase will be operational in 2004 and features the addition of a 20-MeV, 2-kA, 2-microsecond accelerator. Four short electron micropulses of variable pulse-width and spacing will be chopped out of the original, long accelerator pulse for producing time-resolved X-ray images. The second phase also features an extended, high-resolution electro-optical X-ray system with a framing speed of 1.6-MHz. Production of the first beam from the Phase 2 injector will occur this year. In this paper we will present the overall design of the Phase 2 long-pulse injector and accelerator as well as some component test results. We will also discuss the downstream transport section that contains the fast kicker used to separate the long-pulse beam into short bursts suitable for radiography as well as the X-ray conversion target assembly. Selected experimental results from this area of the project will also be included. Finally, we will discuss our plans for initial operations.

Electromagnetic wave propagation through an overdense magnetized collisional plasma layer

The results of investigations into the feasibility of using a magnetic window to propagate electromagnetic waves through a finite-sized overdense plasma slab are described. We theoretically calculate the transmission coefficients for right- and left-handed circularly polarized plane waves through a uniform magnetized plasma slab. Using reasonable estimates for the plasma properties expected to be found in the ionized shock layer surrounding a hypersonic aircraft traveling in the earth’s upper atmosphere (radio blackout conditions), and assuming a 1 GHz carrier frequency for the radio communications channel, we find that the required magnetic field for propagation of right-handed circularly polarized, or whistler, waves is on the order of a few hundred gauss. Transmission coefficients are calculated as a function of sheath thickness and are shown to be quite sensitive to the electron collision frequency. One-dimensional particle-in-cell simulations are shown to be in good agreement with the theory. These s...

Long‐wavelength negative mass instabilities in high current betatrons

Growth rates of negative mass instabilities in conventional and modified betatrons are calculated by analytic methods and by performing three‐dimensional particle simulations. In contrast to earlier work, toroidal corrections to the field equations are included in the analytic model. As a result, good agreement with numerical simulations is obtained. The simulations show that the nonlinear development of the instabilities can seriously disrupt the beam.

Electromagnetic instability in a quadrupole-focusing accelerator

The addition of helical quadrupole focusing to a modified betatron configuration is shown to give rise to an electromagnetic instability under certain conditions. The instability arises from a three‐wave coupling between the helical field, a transverse mode on the beam, and a transverse‐electric waveguide mode. An analytic dispersion relation is derived. Several features of the instability are confirmed using three‐dimensional computer simulations.

Long-pulse beam stability experiments on the DARHT-II linear induction accelerator

When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2-kA, 17-MeV electron beam in a 1600-ns flat-top pulse. In initial tests, DARHT-II accelerated beams with current pulse lengths from 500 to 1200 ns full-width at half-maximum (FWHM) with more than 1.2-kA, 12.5-MeV peak current and energy. Experiments have now been done with a /spl sim/1600-ns pulse length. These pulse lengths are all significantly longer than any other multimegaelectronvolt LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated insignificant beam-breakup instability (BBU), the pulse length was too short to determine whether ion-hose instability would be present toward the end of a long, 1600-ns pulse. The 1600-ns pulse experiments reported here resolved these issues for the long-pulse DARHT-II LIA.

High‐brightness electron‐beam generation and transport

Experimental results for a high‐brightness electron‐beam source are compared to results of numerical and analytic calculations. The 4‐MV, 5‐kA beam is generated from a velvet cathode and focused by a solenoidal magnet. Data for the diode impedance and the beam focusing length and spot size agree well with numerical simulations. The minimum spot size is consistent with a normalized Lapostolle emittance of 0.12 cm rad, yielding a brightness of 3.5×108 A/m2  rad2 . Transverse beam oscillations are observed in streak photographs and are thought to be caused by electromagnetic dipole modes in the diode cavity. The oscillation amplitude is significantly reduced by making the current rise more slowly.

Operation of a five-stage 40000-cm 2 -area insulator stack at 158 kV/cm

We have demonstrated operation of a 3.35-m-diameter insulator stack at 158 kV/cm with no total-stack flashovers on five consecutive Z-accelerator shots. The stack consisted of five +45/spl deg/-profile 5.715-cm-thick crosslinked-polystyrene (Rexolite-1422) insulator rings, and four anodized-aluminum grading rings shaped to reduce the field at cathode triple junctions. The width of the voltage pulse at 89% of peak was 32 ns. We compare this result to a new empirical flashover relation developed from previous small-insulator experiments conducted with flat unanodized electrodes. The relation predicts a 50% flashover probability for a Rexolite insulator during an applied voltage pulse when E/sub max/e/sup -0.27/d/(t/sub eff/C)/sup 1/10/=224, where E/sub max/ is the peak mean electric field (kV/cm), d is the insulator thickness (cm), t/sub eff/ is the effective pulse width (/spl mu/s), and C is the insulator circumference (cm). We find the Z stack can be operated at a stress at least 19% higher than predicted. This result, together with previous experiments conducted by Vogtlin, suggest anodized electrodes with geometries that reduce the field at both anode and cathode triple junctions would improve the flashover strength of multi-stage insulator stacks.