Frank Bieniosek

Technological Improvements in the DARHT II Accelerator Cells

The DARHT facility will employ two perpendicular electron Linear Induction Accelerators to produce intense, bremsstrahlung x-ray pulses for flash radiography. The second axis, DARHT II [1], features an 18 MeV, 2-kA, 2-microsecond pulse accelerator. DARHT II accelerator cells have undergone a series of testing and modeling efforts to fully understand their unacceptable initial failure rate. These R&D efforts are part of the DARHT 2ndAxis Refurbishment and Commissioning project, and they have led to a better understanding of Linear Induction Accelerator technology and physics for the unique DARHT II design. Specific improvements have been identified and tested. Improvements in the cell oil region, the cell vacuum region, and the Pulse Forming Networks (PFNs) have been implemented in the prototype units that have doubled the cell’s performance. The prototype acceptance tests have been completed, and the lifetime reliability requirements were exceeded. The shortcomings of the previous design are summarized. The improvements to the original design, their resultant improvement in performance, and various test results are included.

Electron-beam diagnostic for space-charge measurement of an ion beam

An electron beam diagnostic system for measuring the charge distribution of an ion beam without changing its properties is presently under development for Heavy Ion Fusion (HIF) beam physics studies. Conventional diagnostics require temporary insertion of sensors into the beam, but these capture it, or significantly alter its properties. In this new diagnostic a low energy, low current electron beam is scanned transversely across the ion beam; the measured electron beam deflection is used to calculate the line-integrated charge density of the ion beam, assuming at present a circular charge distribution that is functionally dependent only on radius. The initial application of this diagnostic is being made to the Neutralized Transport Experiment (NTX), which is exploring the physics of space charge dominated beam focusing through neutralizing plasma onto a small spot. The diagnostic system is able to scan an ion beam of up to 3 cm radius. Design and performance of this diagnostic system is presented.

Beam imaging diagnostics for heavy ion beam fusion experiments

We are developing techniques for imaging beams in heavy-ion beam fusion experiments in the HIF-VNL in 2 to 4 transverse dimensions. The beams in current experiments range in energy from 50 keV to 2 MeV, with beam current densities from <10 to 200 mA/cm/sup 2/, and pulse lengths of 4 to 20 /spl mu/s. The beam energy will range up to 10 MeV in near-future beam experiments. The imaging techniques, based on kapton films and optical scintillators, complement and, in some cases, may replace mechanical slit scanners. The kapton film images represent a time-integrated image on the film exposed to the beam. The optical scintillator utilizes glass and ceramic scintillator material imaged by a fast, image-intensified CCD-based camera. We will discuss the techniques, results, and plans for implementation of the diagnostics on the beam experiments.

Experiments in warm dense matter using an ion beam driver

Warm dense matter (WDM) conditions are to be achieved by combined longitudinal and transverse neutralized drift compression of an intense ion beam pulse to provide a hot spot on a target with a beam spot size of about 1 mm, and pulse length about 1-2 ns. The range of the beams in solid matter targets is about 1 micron, which can be lengthened by using reduced density porous targets. Initial experiments in ion-beam-driven WDM will be at low beam velocity, below the Bragg peak, increasing toward the Bragg peak in subsequent higher-energy accelerators. Initial experiments include a transient darkening experiment and a experiment in porous targets at GSI. Further experiments will explore target temperature and other properties such as electrical conductivity to investigate phase transitions and the critical point.

Optical Faraday Cup for Heavy Ion Beams

We have been using alumina scintillators for imaging beams in heavy-ion beam fusion experiments in 2 to 4 transverse dimensions [1]. The scintillator has a limited lifetime under bombardment by the heavy ion beams. As a possible replacement for the scintillator, we are studying the technique of imaging the beam on a gas cloud. A gas cloud for imaging the beam may be created on a solid hole plate placed in the path of the beam, or by a localized gas jet. It is possible to image the beam using certain fast-quenching optical lines that closely follow beam current density and are independent of gas density. We describe this technique and show preliminary experimental data. This approach has promise to be a new fast beam current diagnostic on a nanosecond time scale.

1-MeV electrostatic ion energy analyzer

We describe a high-resolution 90-degree cylindrical electrostatic energy analyzer for 1-MeV (singly ionized) heavy ions for experiments in the Heavy Ion Fusion Science Virtual National Laboratory (HIFS-VNL). By adding a stripping cell, the energy reach of the analyzer can be extended to 2 MeV. This analyzer has high dispersion in a first-order focus with bipolar deflection- plate voltages in the range of plusmn50 kV. We present calculations of vacuum-field beam trajectories, space- charge effects, field errors, and a multipole corrector. The corrector consists of 12 rods arranged in a circle around the beam. Such a corrector has excellent properties as an electrostatic quadrupole, sextupole, or linear combination. The improved energy diagnostic will allow measurements of beam energy spread, such as caused by charge exchange or temperature anisotropy, and better understanding of experimental results in planned longitudinal beam studies. Examples for such experiments include investigations of a beam patching pulser to correct errors in the head and tail of the transported beam bunch, and energy errors derived from ripples in the injector voltage waveform.

Experiments Studying Desorbed Gas and Electron Clouds in Ion Accelerators

Electron clouds and gas pressure rise limit the performance of many major accelerator rings. We are studying these issues experimentally with ∼ 1 MeV heavy-ion beams, coordinated with significant efforts in self-consistent simulation and theory. The experiments use multiple diagnostics, within and between quadrupole magnets, to measure the sources and accumulation of electrons and gas. In support of these studies, we have measured gas desorption and electron emission coefficients for potassium ions impinging on stainless steel targets at angles near grazing incidence. Our goal is to measure the electron particle balance for each source – ionization of gas, emission from beam tubes, and emission from an end wall – determine the electron effects on the ion beam and apply the increased understanding to mitigation. We describe progress towards that goal.

Initial Results on Neutralized Drift Compression Experiments (NDCX-IA) for High Intensity Ion Beam

Ion beam neutralization and compression experiments are designed to determine the feasibility of using compressed high intensity ion beams for high energy density physics (HEDP) experiments and for inertial fusion power. To quantitatively ascertain the various mechanisms and methods for beam compression, the Neutralized Drift Compression Experiment (NDCX) facility is being constructed at Lawrence Berkeley National Laboratory (LBNL). In the first neutralized drift compression experiment, a 280 KeV, 25 mA, K+ion beam is longitudinally 50-fold compressed using an induction core to produce a velocity tilt. This compression ratio is measured using various diagnostics.

Developing high brightness beams for heavy ion driven inertial fusion

Heavy ion fusion (HIF) drivers require large currents and bright beams. In this paper we review the two different approaches for building HIF injectors and the corresponding ion source requirements. The traditional approach uses large aperture, low current density ion sources, resulting in a very large injector system. A more recent conceptual approach merges high current density mini-beamlets into a large current beam in order to significantly reduce the size of the injector. Experiments are being prepared to demonstrate the feasibility of this new approach.

Gas poisoning of 612-M and 311-XM cathodes

A 2 kA cathode was successfully developed for the DARHT-II injector [1]. Since the DARHT injector cannot be baked and there may be virtual leaks, the local pressure near the cathode was not ideal even though the system pressure was in the 10-8 Torr range. In a series of experiments using quarter-inch size button cathodes, we showed that gas poisoning was a significant factor in this pressure range. Furthermore we found that the 311-XM (doped with scandium and has an M coating) cathode was less affected by gas poisoning than the 612-M, corresponding to a lower effective work function. Water vapor was found to be the worst contaminant among the various gases that we have tested. With a 6.5rdquo diameter 311-XM cathode, the DARHT-II injector produced > 2 kA corresponding to a current density of 10 A/cm2.