R.J. Faehl

Intense space-charge beam physics relevant to relativistic klystron amplifiers

In this paper, we examine intense space-charge beam physics that is relevant to beam bunching and extraction in a mildly relativistic klystron amplifier, and give numerical examples for a 5 kA, 500 keV electron beam in a 1.3 GHz structure. Much of the peculiar beam physics in these types of devices results from the partitioning of beam energy into kinetic and potential parts. Both tenuous-nonrelativistic and intense-relativistic beams produce effects different in nature from those produced by intense, mildly relativistic beams because the potential energy requirements are either negligible or fixed. In particular, we demonstrate anomalous beam bunching aided by the nonlinear potential requirements and we discuss maximum power extraction as a function of beam bunching. We show that although the space-charge effects can produce quite high harmonic current content, the maximum power extraction from the beam into RF typically occurs at relatively modest bunching. >

Beam-cavity interaction physics for mildly relativistic, intense-beam klystron amplifiers

We examine beam-cavity interaction physics relevant to mildly relativistic, intense-beam klystron amplifiers. This is an interesting but difficult regime of operation, because of the combination of high beam current and low voltage. The advantage of this regime is that it is relatively easy to access high beam powers (and potentially high microwave output powers) at relatively low beam energy. We calculate the effect of the extremely high beam loading in the input and idler cavities. The output cavitys shunt impedance must match the low beam impedance in order to prevent high output gap voltages that will reflect electrons back upstream. This leads to very low cavity Q factors ( >

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.

Pegasus II experiments and plans for the Atlas pulsed power facility

Atlas will be a high-energy (36 MJ stored), high-power (/spl sim/10 TW) pulsed power driver for high energy-density experiments, with an emphasis on hydrodynamics. Scheduled for completion in late 1999, Atlas is designed to produce currents in the 40-50 MA range with a quarter-cycle time of 4-5 /spl mu/s. It will drive implosions of heavy liners (typically 50 g) with implosion velocities exceeding 20 mm//spl mu/s. Under these conditions, very high pressures and magnetic fields are produced. Shock pressures in the 50 Mbar range and pressures exceeding 10 Mbar in an adiabatic compression will be possible. By performing flux compression of a seed field, axial magnetic fields in the 2000 T range may be achieved. A variety of concepts have been identified for the first experimental campaigns on Atlas. Experimental configurations, associated physics issues, and diagnostic strategies are all under investigation as the design of the Atlas facility proceeds. Near-term proof-of-principle experiments employing the smaller Pegasus II capacitor bank have been identified, and several of these experiments have now been performed. This paper discusses a number of recent Pegasus II experiments and identifies several areas of research presently planned on Atlas.

The Challenge of Wall-Plasma Interaction with Pulsed MG Fields Parallel to the Wall

Experiments suitable for a variety of pulsed power facilities are being developed to study plasma formation and stability on the surface of typical liner materials in the megagauss (MG) regime. Understanding the plasma properties near the surface is likely to be critical for the design of Magnetized Target Fusion experiments, where the plasma density in the region near the wall can play an important role in setting the transport from hot fuel to the cold boundary. From the perspective of diagnostic access and simplicity, the surface of a stationary conductor with large enough current to generate MG surface field offers advantages compared with studying the surface of a moving liner. This paper reports on recent experiments at UNR that have generated magnetic fields in the range of about 0.2 to 3 MG, which confirm the viability of future experiments planned at Atlas and/or Shiva Star. Diagnostics reported here involve electrical measurements, streak camera photography, and surface luminosity. Additional diagnostic measurements and numerical modeling will be reported in the future.

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...

Implications of recent improvements in conductivity models on liner, wire, and fuse design

Summary form only given. The pulse power community is conducting numerous Z-pinch experiments to explore critical issues in both hydrodynamics and magnetohydrodynamics. Adequately predicting the behavior of metals (i.e. copper, aluminum, tungsten, etc.) as intense current/fields are introduced is crucial for designing such experiments. Our simulations have used a variety of resistivity models including SESAME tables as well as analytic models. Most models to date were derived from approximate theory and adjusted to match available data. Results of our simulations have shown that the behavior of aluminum, for example, in the region from a 0.1 to 0.5 eV and densities from about 0.2 to 3.0 g/cc is not reproduced well and impacts our ability to accurately predict high performance liners, exploding wires, and fuses using our MHD simulation codes. Recent examination of improve conductivity models from Desjarlais at SNL suggests that the nature of conductivity in these low/intermediate temperatures and near normal densities may be a significant problem. This was demonstrated by calculations of high current (>30 MA) driven liner experiments which showed inaccuracies in the employed conductivity in this region can lead to 10-20 % discrepancies in predicting the liner velocity at impact. This inferred problem with conductivity models is endemic to all three of the discussed cases, since any solid/liquid surface heated ohmically into a plasma state must traverse this region in phase space. This paper will attempt to compare simulations of liners, wires and fuses using a variety of conductivity models against a limited set of actual experiments.

Improved modeling of electrically exploded fuse opening switches in flux compression generator experiments

In previously reported computational studies of fuse opening switches and other exploding metallic foils, apparent inaccuracies in the constitutive relations for the fuse material limited the accuracy of the modeling. In this paper, we use constitutive property models to re-examine experiments that used flux compression generators to deliver 25-35 MA to a copper fuse. Based upon this preliminary study, it is difficult to conclude that the newly developed models offer substantial improvement. For some considerations, models developed more than 15 years ago remain the most accurate.

Linear experiment on verification of Rayleigh-Taylor instability magnetic stabilization effect (joint LANL/VNIIEF experiment Pegasus-2)

A liner implosion experiment was conducted on facility Pegasus-2, in which two perturbation type growth was compared. On one half (through height) of the cylindrical liner sinusoidal azimuthally symmetric perturbations were produced. On the other liner half the perturbations were of the same wavelength and the same amplitude, but the angle between the wave vector and the cylinder axis was 45/spl deg/ (screw perturbations). The experimental radiographs show that there is essentially no screw perturbation growth, while the azimuthally symmetric perturbations grow many-fold. This result agrees with the theoretical predictions.

Examination of liner stability during magnetic implosion using experiments and simulations

Los Alamos has been conducting a number experiments to examine dynamic properties of materials using high-energy pulse power generator systems. These experiments are conducted in a Z-pinch configuration typically with an outer aluminum liner to carry the current, develop the acting force, and act as the driving element. The peak magnetic fields produced by these systems have ranged from 0.5 to 1.7 mega gauss. The onset of what has been called Magneto-Raleigh-Taylor (MRT) instabilities in the outer aluminum liner, when excessive current is applied, has been considered a limitation on the design of these liners. However, in several cases where the material of the liner was calculated to be completely melted the outside liner surface remained stable. Analysis of the data from this and several other experiments and comparison to 1D MHD simulations has already permitted us to examine how the drive conditions on this aluminum layer appear to effect the likelihood of onset of these instabilities. Additionally, careful variations of drive conditions, initial liner surface conditions, and EOS properties (including conductivity) suggest two phenomenons that appear to cause onset of instability. First, while the nature of the instability may still be fundamentally driven by the acceleration of a fluid interface, the effect may be drastically accentuated by the onset of liquid to vapor phase change if the material is allowed to approach too closely to the saturated liquid line. Furthermore, several observed cases which remained stable even after melting suggest that there may be drive conditions which maintain the aluminum at densities and temperatures above the saturated liquid line and significantly delay the onset of MRT instabilities. Second, the gradient of distribution of forces within the melted liner may also impact the growth of instabilities. We will also present the results of 2D simulations of these conditions and examine in greater detail the apparent mechanisms by which these instabilities grow.