R.R. Bartsch

Intense ion beam optimization and characterization with infrared imaging

We have developed two-dimensional calorimetry with infrared imaging of beam targets to optimize and measure the energy-density distribution of intense ion beams. The technique, which measures a complete energy-density distribution on each machine firing, has been used to rapidly develop and characterize two very different beams—a 400 keV beam used to study materials processing and an 80 keV beam used for magnetic fusion diagnostics. Results of measurements, using this technique, varying the diode applied magnetic field strength and geometry, anode material type and configuration, and anode-cathode gap spacing are presented and correlated with other observations. An assessment of calorimeter errors due to target ablation is made by comparison with Faraday cup measurements and computer modeling of beam-target interactions.

Microsecond pulse width, intense, light-ion beam accelerator

A relatively long‐pulse width (0.1–1 μs) intense ion beam accelerator has been built for materials processing applications. An applied Br, magnetically insulated extraction ion diode with dielectric flashover ion source is installed directly onto the output of a 1.2 MV, 300‐kJ Marx generator. The diode is designed with the aid of multidimensional particle‐in‐cell simulations. Initial operation of the accelerator at 0.4 MV indicates satisfactory performance without the need for additional pulse shaping. The effect of a plasma opening switch on diode behavior is considered.

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.

Experimental confirmation of the reditron concept

A description is given of experiments demonstrating a method for producing high-power microwave emission. The unstable oscillations of a virtual cathode, which forms when a magnetized relativistic electron beam is injected into a circular waveguide, generates the microwave radiation. In contrast to other virtual-cathode microwave-generation techniques, electrons in the waveguide are prevented from reflexing back into the diode region by use of a slotted range-thick anode. Electrons injected into the waveguide are guided through the slot by an applied magnetic field, while reflected electrons, under the proper conditions, are intercepted by the anode. Several advantages of this approach are described, and experimental confirmation of this mode of high-power microwave generation is demonstrated. Data showing frequency scaling with beam parameters and magnetic field are also presented. Using this technique, 1.4 GW was produced at 3.9 GHz with several hundred megawatts radiated in harmonic radiation. >

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.

The Atlas project-a new pulsed power facility for high energy density physics experiments

Atlas is a facility being designed at Los Alamos National Laboratory (LANL) to perform high-energy-density experiments in support of weapon physics and basic research programs. It is designed to be an international user facility, providing experimental opportunities to researchers from national laboratories and academic institutions. For hydrodynamic experiments, it will be capable of achieving a pressure exceeding 30 Mbar in a several cubic centimeter volume. With the development of a suitable opening switch, it will be capable of producing more than 3 MJ of soft X-rays. The capacitor bank design consists of a 36 MJ array of 240 kV Marx modules. The system is designed to deliver a peak current of 45-50 MA with a 4-5-/spl mu/s rise time. The Marx modules are designed to be reconfigured to a 480-kV configuration for opening switch development. The capacitor bank is resistively damped to limit fault currents and capacitor voltage reversal. An experimental program for testing and certifying prototype components is currently under way. The capacitor bank design contains 300 closing switches. These switches are a modified version of a railgap switch originally designed for the DNA-ACE machines. Because of the large number of switches in the system, individual switch prefire rates must be less than 10/sup -4/ to protect the expensive target assemblies. Experiments are under way to determine if the switch-prefire probability can be reduced with rapid capacitor charging.

Rotational instability in a linear theta pinch

The m=1 ’’wobble’’ instability of the plasma column in a 5‐m linear theta pinch has been studied using an axial array of orthogonally viewing position detectors to resolve the wavelength and frequency of the column motion. The experimental results are compared with recent theoretical predictions that include finite Larmor orbit effects. The frequency and wavelength characteristics at saturation agree with the predicted dispersion relation for a plasma rotating faster than the diamagnetic drift speed. Measurements of the magnetic fields at the ends of the pinch establish the existence of currents flowing in such a way that they short out the radial electric fields in the plasma column. The magnitude of rotation, the observed delay in the onset of m=1 motion, and the magnitude of end‐shorting currents can all be understood in terms of the torsional Alfven waves that communicate to the central plasma column the information that the ends have been shorted. The same waves are responsible for the torque which r...

Collisionless flow and end loss from a high‐energy theta‐pinch plasma

End‐loss experiments on the high‐energy (Te+Ti=3.3 keV, ne=1.5×1016 cm−3) 5 m Scylla IV‐P theta pinch are reported. The evolution of the theta‐pinch plasma parameters in the presence of axial losses and the behavior of the exhausting plasma near the ends of the device have been investigated. The measured decay of the theta‐pinch plasma electron temperature agrees with code predictions based on classical axial thermal conduction losses. However, the axial ion heat flux is found to be unmeasurably small in the collisionless ion plasma. Energy‐line‐density measurements at the coil midplane also agree with code predictions and provide evidence of inward traveling rarefaction‐like waves. At the theta‐pinch ends, the exhausting plasma is comprised of a collimated plasma core which remains radially confined for tens of centimeters, strongly convects magnetic fields, and contains the bulk of the ejected plasma. This collimated core is surrounded by a plasma annulus that expands rapidly to the walls after leaving ...

Progress toward a microsecond duration, repetitively pulsed, intense-ion beam for active spectroscopic measurements on ITER

We describe the design of an intense, pulsed, repetitive, neutral beam based on magnetically insulated diode technology for injection into ITER for spectroscopic measurements of thermalizing alpha particle and thermal helium density profiles, ion temperature, plasma rotation, and low Z impurity concentrations throughout the confinement region. The beam is being developed to enhance low signal-to-noise ratios expected with conventional steady-state ion beams because of severe beam attenuation and intense bremsstrahlung emission. A 5 GW (e.g., 100 keV, 50 kA) 1 μs duration beam would increase the charge exchange recombination signal by 103 compared to a conventional 5 MW beam.

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.