P.J. Turchi

A physics exploratory experiment on plasma liner formation

Momentum flux for imploding a target plasma in magnetized target fusion (MTF) may be delivered by an array of plasma guns launching plasma jets that would merge to form an imploding plasma shell (liner). In this paper, we examine what would be a worthwhile experiment to explore the dynamics of merging plasma jets to form a plasma liner as a first step in establishing an experimental database for plasma-jets-driven magnetized target fusion (PJETS-MTF). Using past experience in fusion energy research as a model, we envisage a four-phase program to advance the art of PJETS-MTF to fusion breakeven (Q ∼ 1). The experiment (PLX) described in this paper serves as Phase 1 of this four-phase program. The logic underlying the selection of the experimental parameters is presented. The experiment consists of using 12 plasma guns arranged in a circle, launching plasma jets toward the center of a vacuum chamber. The velocity of the plasma jets chosen is 200 km/s, and each jet is to carry a mass of 0.2 mg to 0.4 mg. A candidate plasma accelerator for launching these jets consists of a coaxial plasma gun of the Marshall type.

Design, fabrication, and operation of a high-energy liner implosion experiment at 16 megamperes

We discuss the design, fabrication, and operation of a liner implosion system at peak currents of 16 MA. Liners of 1100 aluminum, with initial length, radius, and thickness of 4 cm, 5 cm, and 1 mm, respectively, implode under the action of an axial current, rising in 8 /spl mu/s. Fields on conductor surfaces exceed 0.6 MG. Design and fabrication issues that were successfully addressed include: Pulsed Power-especially current joints at high magnetic fields and the possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; Liner Physics-including the angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; Diagnostics-X-radiography through cassette insulator and outer conductor without shrapnel damage to film.

The Atlas High-Energy Density Physics Project

Atlas is a pulsed-power facility under development at Los Alamos National Laboratory to drive high-energy density experiments. Atlas will be operational in the summer of 2000 and is optimized for the study of dynamic material properties, hydrodynamics, and dense plasmas under extreme conditions. Atlas is designed to implode heavy-liner loads in a z-pinch configuration. The peak current of 30 MA is delivered in 4 µs. A typical Atlas liner is a 47-gram-aluminum cylinder with ∼ 4-cm radius and 4-cm length. Three to five MJ of kinetic energy will be delivered to the load. Using composite layers and a variety of interior target designs, a wide variety of experiments in ∼ cm3 volumes will be performed. Atlas applications, machine design, and the status of the project are reviewed.

A Ceramic Loaded Polymer Blumlein Pulser for Compact, Rep-Rated Pulsed Power Applications

The design of compact pulsed power systems involves the trade between size, pulse length and pulse shape. A stacked Blumlein line with high dielectric constant material can deliver a voltage flattop to a matched load with an energy density similar to capacitor banks. By imbedding nano-scale titanate particles in an epoxy matrix, a composite material with a relative permittivity in the range of 30 to 60 may be realized without the drastic loss in dielectric strength associated with large area ceramics. So called ceramic loaded polymer dielectric employed in a Blumlein line facilitates the fabrication of a compact pulse forming line potentially suitable for driving loads of several tens of Ohms in the GW power range for greater than 100 ns. This paper describes the initial efforts to fabricate and test a parallel plate Blumlein incorporating ceramic loaded polymer dielectric. Two single-stage parallel plate Blumlein lines were fabricated with different ceramic loading. The lines were designed to yield a 50 ns pulse into a 6.25 Omega load. The Blumlein lines were designed to be charged to 62.5 kV, and both fabricated units held the charge voltage in static tests. A small railgap switch was fabricated for use with the Blumlein lines. A mid-plane knife-edge electrode was used to trigger the switch. The results of the tests are presented along with projections for the future development of this technology.

Full Axial Coverage Radiography of Deformable Contact Liner Implosion Performed with 8 cm Diameter Electrode Apertures

diameter ratio, radial convergence, uniformity, and implosion velocity suitable forcompressing anFRC[3]. We obtained full axial coverage radiography ofa Ourrecent progress hasbeentoreplace themorestandard deformable contact imploding liner. Thisradiographic data sliding liner-electrode contacts withdeformable linerindicates thefeasibility ofusing avarying thickness inalong electrode contacts, whichenables theuseoflarge cylindrical solid liner, driven asa 12megampZ-pinch, to electrode apertures, suitable forFRCinjection. SeeFig. 1 achieve factor - 16cylindrical convergence, while using 8cm foraillustration ofthis concept. diameter aperture electrodes. TheAlliner was30cmlong, with9.78cminner diameter forits full length, 10.0cmouter Research ontheuseofimploding liners to diameter forthecentral 18cm ofitslength, andouter compress plasmas hasbeenreported byanumberof diameter increased linearly to10.2cmat1cmfromeitherresearchers. Thisincludes suggesting thegeneral concept electrode, andto11cmatelectrode contacts. Theelectrode ofusing liners tocompress plasma, andresearch on apertures allow injection ofField Reversed Configurations in shorter orlowervelocity liner implosions [4-17], and proposed future experiments onmagnetized target fusion. implosion ofaCu-Wliner withexplosives tocompress Indexterms: capacitor bank, Field Reversed Configuration, flux to200T[18]. FRC,Magnetized Target Fusion, MTF,imploding liner, radiography, megamp Uniform-thickness liner Variable-thickness

A new category of high-voltage pulser based on dynamic plasma techniques

High-speed plasmas generated by low-impedance, pulsed electrical sources are used to displace and compress magnetic flux in a variety of arrangements involving multi-turn coils. The output impedance of power from these coils is much higher than the original source impedance, and the output pulse time is much shorter than the input pulse. Dynamic plasma techniques thus provide a new category of high-voltage pulser, capable of simultaneously achieving voltage multiplication and pulsetime compression. Arrangements based on implosion, inverse-pinch and coaxial plasma-gun flows are discussed.

High voltage pulses for high impedance loads using explosive formed fuses

Explosive formed fuses (EFFs) use conducting elements that are deformed by explosive pressure (typically, against dielectric dies). This causes the fuse geometry to change, so that the conducting element cross section decreases. This enables a higher ratio of current conduction to current interrupt time than for normal fuses, and it enables more control of when current interruption occurs. In combination with a suitable output closing switch, EFFs can be used to obtain several hundred kilovolt voltage pulses from inductive stores to drive several ohm loads. With proper choices of inductive store, EFF geometry and material, and output closing switch features, such a voltage pulse can be approximately flat topped for microsecond duration and have a small fraction of microsecond risetime. We present theoretical analysis and circuit simulations which illustrate this, using scaled empirical EFF parameters for inductive stores in the 1 weber flux, several hundred nanohenry range. The circuit simulations were done using MicroCap-IV, with user defined elements. These simulations were done with static inductive stores and with explosive magnetic flux compression generators driving inductive stores.

Modeling of impedance collapse in high-voltage diodes

Electron-beam diodes driven by fast-rising, high-voltage pulses often operate with cold cathodes for which the presence of a plasma adjacent to the cathode surface is essential to obtain adequate electron emission. A consequence of such surface plasma, however, is closure of the interelectrode gap by plasma motion. The diode impedance decreases with time, adversely affecting the efficiency of coupling to the power source. Plasma closure of the diode gap also limits the length of the electron beam pulse, and the ability to operate the diode repetitively at high frequency. Resistive heating of the plasma competes with work performed in expanding the plasma and heat transfer to the cold-cathode boundary. The resulting closure speed is calculated, using an MHD code, and found to agree well with results of experiments using organic-cloth cathodes at 35 kV. Computed plasma speeds are typically 8-12 km/s, and are relatively insensitive to the applied voltage. Gap closure due to the plasma motion calculated numerically corresponds to estimates based on impedance collapse in the experiments.