David Mosher

Plasma Erosion Opening Switch Research at NRL

This paper is a review of plasma erosion opening switch (PEOS) research performed at the Naval Research Laboratory (NRL). Several experimental and theoretical results are described to illustrate the present level of understanding and the best switching results obtained to date. Significant power multiplication has been achieved on the Gamble II generator, producing 3.5 TW with less than 10-ns rise time. Switching after nearly 1-¿s conduction time has been demonstrated on Pawn, producing a 0.2-TW 100-ns pulse. Scaling the switch to higher current, power, and conduction time should be possible based on theoretical analysis and the favorable results of scaling experiments performed thus far.

A two-level model for K-shell radiation scaling of the imploding Z-pinch plasma radiation source

A simple, first-principle treatment is presented for estimating, within about a factor-of-two, how K-shell X-radiation from the plasma radiation source scales with plasma, pulsed-power, and imploding-load parameters. The transparency of this two-level model clarifies the relations between X-ray yield and underlying physical processes, provides simple analytic expressions for optimal load parameters and associated yields, and establishes links between radiating-load performance and system constraints imposed by other processes. The two-level model provides results similar to another one-zone model based on phenomenological extrapolations of selected one-dimensional radiation-hydrodynamic calculations. The model compares well with K-shell radiation results from Hawk neon gas puff and Saturn aluminum large-wire-number array experiments, provided that assumed compression ratios are less than those inferred from X-ray pinhole images. The reduced compression ratios required by the model can be traced to its one-zone nature.

X radiation from high‐energy‐density exploded‐wire discharges

Exploded‐wire discharges of tungsten and titanium driven by a high‐power pulse generator have been used to produce intense x‐ray continuum and line radiation. A calibrated LiF crystal spectrograph recorded the radiation spectrum in the 3‐ to 25‐keV range. More than 20 J of x radiation are emitted in this photon energy band by tungsten plasmas in less than 50 nsec. The source of emission is less than 1 mm in diameter and about 3.5 cm long.

Interactions of relativistic electron beams with high atomic-number plasmas

A relativistically correct Fokker–Planck analysis is used to develop fluid equations which model the interaction of relativistic electron beams with high atomic‐number plasmas. The derived collision terms can be used to describe scattering and energy loss in materials ranging from the solid to high‐temperature plasma forms. The full set of equations discussed can be used to study electron‐beam‐initiated pellet fusion in a completely self‐consistent fashion. Specifically, the model is applicable to study of beam pinching in plasma‐filled diodes, the interaction of focused beams with target plasmas, and the transport of high ν/γ beams in high atomic‐number plasmas. Although the general equations are amenable to solutions only by computational techniques, analytic solutions describing the time‐dependent, collisional interaction of a beam in an infinite plasma and the one‐dimensional equilibrium of a beam in a plasma with applied electric field have been obtained.

Evaluation of self-magnetically pinched diodes up to 10 MV as high-resolution flash X-ray sources

The merits of several high-resolution, pulsed-power-driven, flash X-ray sources are examined with numerical simulation for voltages up to 10 MV. The charged particle dynamics in these self-magnetically pinched diodes (SMPDs), as well as electron scattering and energy loss in the high-atomic-number target, are treated with the partic by coupling the output from LSP with the two-dimensional component of the integrated tiger series of Monte Carlo electron/photon transport codes, CYLTRAN. The LSP/CYLTRAN model agrees well with angular dose-rate measurements from positive-polarity rod-pinch-diode experiments, where peak voltages ranged from 5.2-6.3 MV. This analysis indicates that, in this voltage range, the dose increases with angle and is a maximum in the direction headed back into the generator. This suggests that high-voltage rod-pinch experiments should be performed in negative polarity to maximize the extracted dose. The benchmarked LSP/CYLTRAN model is then used to examine three attractive negative-polarity diode geometry concepts as possible high-resolution radiography sources for voltages up to 10 MV. For a 2-mm-diameter reentrant rod-pinch diode (RPD), a forward-directed dose of 740 rad(LiF) at 1 m in a 50-ns full-width at half-maximum radiation pulse is predicted. For a 2-mm-diameter nonreentrant RPD, a forward-directed dose of 1270 rad(LiF) is predicted. For both RPDs, the on-axis X-ray spot size is comparable to the rod diameter. A self-similar hydrodynamic model for rod expansion indicates that spot-size growth from hydrodynamic effects should be minimal. For the planar SMPD, a forward-directed dose of 1370 rad(LiF) and a similar X-ray spot size are predicted. These results show that the nonreentrant RPD and the planar SMPD are very attractive candidates for negative-polarity high-resolution X-ray sources for voltages of up to 10 MV.

Experimental evaluation of a megavolt rod-pinch diode as a radiography source

The rod-pinch diode is a cylindrical pinched-beam diode that provides an intense pulsed small-diameter bremsstrahlung source for radiography. For this work, the diode consists of a 1- to 6.4-mm-diameter anode rod that extends through the hole of an annular cathode. After exiting the cathode, wider anodes taper down to a 1 mm diameter. All of the anode rods then have a 1-mm-diameter tungsten tip that is usually tapered to a point. Rod-pinch diodes with anode rods of different materials, lengths, and diameters were powered by the Gamble II generator at peak voltages of 1.0 to 1.8 MV and peak currents of 30 to 60 kA. The radiation was characterized with temporally and spatially resolved X-ray diagnostics. Pinhole-camera images and time-resolved pin-diode measurements indicate that the radiation is emitted primarily from the vicinity of the rod tip. The dose measured with thermoluminescent detectors through a plexiglass transmission window ranges from 0.6 to 2.8 R at 1 m from the rod tip and the dose/charge scales faster than linearly with the diode voltage. The full-width at half-maximum (FWHM) of the radiation pulse is 30 to 50 ns. The size of the radiation source-is determined by measuring its edge spread function. The source diameter, defined here as the FWHM of the derivative of the edge spread function, decreases from 2 mm for a 6.4-mm-diameter rod to 1 mm or less for a 1-mm-diameter rod. Analysis suggests that the central portion of the radiation distribution at the source can be approximated by a uniformly radiating circular disc.

Propagation of intense ion beams in straight and tapered z-discharge plasma channels

A preformed z‐discharge plasma channel can be used to transport focused ion beams appropriate for a pellet fusion device. During transport, the beam can be compressed axially by time‐of‐fight bunching when appropriate ion accelerating voltage waveforms are employed. Single‐particle orbits in such channels are expressible in terms of simple harmonic functions for small ion injection angles. In this work, orbit analysis is used to investigate how nonuniformities or tapering of the channel and electric fields present in the channel affect radial beam confinement and power multiplication by bunching.

Microstability of a focused ion beam propagating through a z-pinch plasma

A beam‐plasma system consisting of a focused light ion beam propagating through a z‐pinch plasma is analyzed for microinstabilities. Two instabilities are discussed, one driven by the relative streaming between beam ions and electrons and the other driven by streaming between plasma ions and electrons. Conditions for stability of both modes are derived and are used to demonstrate that ion beams appropriate for use in a pellet fusion device can be propagated to the pellet through a z‐pinch plasma without disruptive microturbulence.

Results of radius scaling experiments and analysis of neon K-shell radiation data from an inductively driven Z-pinch

The K-shell radiated energy (yield) from neon Z-pinch implosions with annular, gas-puff nozzle radii of 1, 1.75, and 2.5 cm was measured for implosion times from 50 to 300 ns while systematically keeping the implosion kinetic energy nearly constant. The implosions were driven by the Hawk inductive-storage generator at the 0.65-MA level. Initial neutral-neon density distributions from the nozzles were determined with laser interferometry. Measured yields are compared with predictions from zero-dimensional (0-D) scaling models of ideal. One-dimensional (1-D) pinch behavior to both benchmark the scaling models, and to determine their utility for predicting K-shell yields for argon implosions of 200 to >300 ns driven by corresponding currents of 4 to 9 MA, such as envisioned for the DECADE QUAD. For all three nozzles, the 0-D models correctly predict the Z-pinch mass for maximum yield. For the 1and 1.75-cm radius nozzles, the scaling models accurately match the measured yields if the ratio of initial to final radius (compression ratio) is assumed to be 8:1. For the 2.5-cm radius nozzle, the measured yields are only one-third of the predictions. Analysis of K-shell spectral measurements suggest that as much as 70% (50%) of the imploded mass is radiating in the K-shell for the 1-cm (1.75-cm) radius nozzle. That fraction is only 10% for the 2.5-cm radius nozzle. The 0-D scaling models are useful for predicting 1-D-like K-shell radiation yields (better than a factor-of-two accuracy) when a nominal (/spl ap/10:1) compression ratio is assumed. However, the compression ratio assumed in the models is only an effective quantity, so that further interpretations based on the 0-D analysis require additional justification. The lower-than-predicted yield for the 2.5-cm radius nozzle is associated with larger radius and not with longer implosion time, and is probably a result of two-dimensional effects.

Development of a sodium-pump/neo-lasant photopumped soft X-ray laser

An intense source of sodium pump-line radiation has been created and used to photopump a neon plasma for application to a pulsed-power driven sodium/neo X-ray laser. Properties of the sodium-pump plasma and the neon-lasant plasma required to optimize fluorescence and lasing are determined. The implosion of a sodium-bearing plasma with a megampere pulsed-power driver (Gamble II) is used to produce a linear Z-pinch with up to 25 GW of sodium-pump-line radiation. A separate neon plasma, driven by part of the return current from the imploding sodium plasma, is created parallel to the sodium line source at a distance of 5 cm. Evidence for population inversion is indicated by fluorescence enhancement of the 11-AA resonance line from the n=4 level of neon when pumped by sodium. >