J.C. Cochrane

Review of the Los Alamos FRX-C experiment

The FRX-C device is a large field-reversed theta pinch experiment with linear dimensions twice those of its FRX-A and FRX-B predecessors. It is used to form field-reversed configurations (FRCs), which are high-beta, highly prolate compact toroids. The FRX-C has demonstrated an R/sup 2/ scaling for particle confinement in FRCs, indicating particles are lost by diffusive processes. Particle losses were also observed to dominate the energy balance. When weak quadrupole fields were applied to stabilize the n = 2 rotational mode, FRC lifetimes >300..mu..s were observed. Detailed studies of the FRC equilibrium were performed using multichord and holographic interferometry. Measurements of electron temperature by Thomson scattering showed a flat profile and substantial losses through the electron channel. The loss rate of the internal poloidal flux of the FRC was observed to be anomalous and to scale less strongly with temperature than predicted from classical resistivity.

Flux loss during the equilibrium phase of field-reversed configurations

Field‐reversed configurations are consistently formed at low filling pressures in the FRX‐C device, with decay time of the trapped flux after formation much larger than the stable period. This contrasts with previous experimental observations.

ϑ‐pinch ionization for field‐reversed configuration formation

The behavior of a plasma produced by a ringing ϑ‐pinch discharge in the presence of a 2.3‐kG bias field is examined for the case where the net field passes through zero. Experimental studies, employing internal and external field probes, indicated ∼50% of the initially applied bias flux is excluded by the plasma with this ionization technique. A theoretical model incorporating field diffusion and elastic ion‐neutral collisions is used to describe the observed dynamics of the plasma sheath.

Liner target interaction experiments on Pegasus II

The Los Alamos High Energy Density Physics program uses capacitively driven low voltage, inductive-storage pulse power (including the 4.3 MJ Pegasus II capacitor bank facility) to implode cylindrical targets for hydrodynamics experiments. Once a precision driver liner was characterized an experimental series characterizing the aluminum target dynamics was performed. The target was developed for shock-induced quasi-particle ejecta experiments including holography. The concept for the liner shock experiment is that the driver liner is used to impact the target liner which then accelerates toward a collimator with a slit in it. A shock wave is set up in the target liner and as the shock emerges from the back side of the target liner, ejecta are generated. By taking a laser hologram the particle distribution of the ejecta are hoped to be determined. The goal for the second experimental series was to characterize the target dynamics and not to measure and generate the ejecta. Only the results from the third shot, Pegasus II-26 fired April 26th, 1994, from the series are discussed in detail. The second experimental series successfully characterized the target dynamics necessary to move forward towards our planned quasi-ejecta experiments.

PLASMA FLOW SWITCH AND FOIL IMPLOSION EXPERIMENTS ON PEGASUS II

Pegasus II is the upgraded version of Pegasus, a pulsed power machine used in the Los Alamos AGEX (Above Ground EXperiments) program. A goal of the program is to produce an intense (> 100 TW) source of soft x-rays from the thermalization of the kinetic energy of a 1 to 10 MJ plasma implosion. The radiation pulse should have a maximum duration of several 10`s of nanoseconds and will be used in the study of fusion conditions and material properties. The radiating plasma source will be generated by the thermalization of the kinetic energy of an imploding cylindrical, thin, metallic foil. This paper addresses experiments done on a capacitor bank to develop a switch (plasma flow switch) to switch the bank current into the load at peak current. This allows efficient coupling of bank energy into foil kinetic energy.

Scaling studies in field reversal experiments

The stable period of field‐reversed configurations, defined by the onset of the rotational n = 2 instability, is observed to scale with R2/ ρi over a new, wider range of experimental conditions, where R is the major radius and ρi is the ion gyro‐radius indexed to the external field. The scaling factor is approximately 6.0×10−7 sec cm−1 over a range of R2/ ρi from 18 to ∼100 cm in which 1/ ρi varied from 1 to 5 cm−1 and R varied by approximately 30%. In a complimentary study, the stable period was observed to be independent of Ti over a range of 200–1200 eV when R2/ ρi was held approximately constant. The theoretical correlation of the stable period with the particle containment time, and hence with R2/ ρi, are discussed.

Precision current measurements on Pegasus II using Faraday rotation

The authors measure the current on the Los Alamos pulsed power machine, Pegasus II, using the Faraday rotation technique in a twisted, low-birefringence optical fiber. This technique yields results which are reproducible to within about 1%. When comparing their results with data from a Rogowski loop and from B-dot loop detectors, they find discrepancies larger than the uncertainties in the measurements. They have calibrated their system in three different ways: (1) the Pegasus II experiment was driven into a shorted load in a ring-down test to measure the load inductance. The measured Faraday data were fitted to a damped sinusoidal equation and compared with current calculated from the measured voltage and capacitance; (2) a single capacitor drove about 3 kA of current into a 403 turn solenoid coil. A Pearson transformer calibrated to about 1% measured the current supplied to the coil and the Faraday data were compared with the Pearson data; and (3) on a separate machine, a calibrated Rogowski coil provided direct comparison with fiber optic Faraday measurements. The Verdet constant has been measured for bulk silica glass at a wavelength of 633 nm by several researchers. The authors extrapolated these averaged results of 4.61 radians/MA to their wavelength of 830 nm and corrected it for the 4% germania dopant in the glass from which their optical fiber was fabricated. They obtained results from all three methods consistent with a Verdet constant 6% smaller than the extrapolated value. They are continuing to investigate this discrepancy and are working with NIST to measure the Verdet constant in their glass at 830 nm.

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.

Precision solid liner experiments on Pegasus II

Pulsed power systems have been used in the past to drive solid liner implosions for a variety of applications. In combination with a variety of target configurations, solid liner drivers can be used to compress working fluids, produce shock waves and study material properties in convergent geometry. The utility of such a driver depends in part on how well-characterized the drive conditions are. This, in part, requires a pulsed power system with a well-characterized current waveform and well-understood electrical parameters. At Los Alamos, the authors have developed a capacitively driven, inductive store pulsed power machine, Pegasus, which meets these needs. They have also developed an extensive suite of diagnostics which are capable of characterizing the performance of the system and of the imploding liners. Pegasus consists of a 4.3 MJ capacitor bank, with a capacitance of 850 /spl mu/f fired with a typical initial bank voltage of 90 kV or less. The bank resistance is about 0.5 m/spl Omega/, and bank plus power flow channel has a total inductance of about 24 nH. In this paper, the authors consider the theory and modeling of the first precision solid liner driver fielded on the Pegasus pulsed power facility.

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.