Roger Richardson

Beam-target interaction experiments for bremsstrahlung converter applications

For multi-pulse radiography facilities, we are investigating the possible adverse effects of (1) backstreaming ion emission from the bremsstrahlung converter target and (2) the interaction of the resultant plasma with the electron beam during subsequent pulses. These effects would primarily manifest themselves in a static focusing system as a rapidly varying X-ray spot. To study these effects, we are conducting beam-target interaction experiments on the ETA-II accelerator (a 6.0 MeV, 2.5 kA, 70 ns FWHM pulsed, electron accelerator) by measuring spot dynamics and characterizing the resultant plasma for various configurations.

Commissioning the DARHT-II scaled accelerator downstream transport

The DARHT-II accelerator will produce a 2-kA, 17-MeV beam in a 1600-ns pulse when completed mid-2007. After exiting the accelerator, the pulse is sliced into four short pulses by a kicker and quadrupole septum and then transported for several meters to a tantalum target for conversion to X-rays for radiography. We describe tests of the kicker, septum, transport, and multi-pulse converter target using a short accelerator assembled from the first available refurbished cells. This scaled accelerator was operated at ~8 MeV and ~1 kA, providing a beam with approximately the same v/gamma as the final 18-MeV, 2-kA beam, and therefore the same beam dynamics in the downstream transport. The results of beam measurements made during the commissioning of this scaled accelerator downstream transport are described.

LLNL flash X-ray radiography machine (FXR) double-pulse upgrade diagnostics

When the FXR machine was first tuned on the 1980s, a minimal amount of diagnostics was available and consisted mostly of power monitors. During the accelerator upgrade, additional beam diagnostics were added. The sensor upgrades included beam bugs (resistive wall beam motion sensors) and high-frequency B-dot. Even with this suite of measurement tools, tuning was difficult. For the current double-pulse upgrade, beam transport is a more complex problem-the beam characteristics must be measured better. Streak and framing cameras, which measure beam size and motions, are being added. Characterization of the beam along the entire accelerator is expected and other techniques are also evaluated. Each sensor has limitations and only provides a piece of the puzzle. Besides providing more beam data, the set of diagnostics used should be broad enough so results can be cross validated. Results are also compared to theoretical calculations and computer models, and successes and difficulties are reported.

DARHT II Scaled Accelerator Tests on the ETA II Accelerator

The DARHT II accelerator at LANL is preparing a series of preliminary tests at the reduced voltage of 7.8 Me V. The transport hardware between the end of the accelerator and the final target magnet was shipped to LLNL and installed on ETA II. Using the ETA II beam at 5.2 MeV we completed a set of experiments designed reduce start up time on the DARHT II experiments and run the equipment in a configuration adapted to the reduced energy. Results of the beam transport using a reduced energy beam, including the kicker and kicker pulser system will be presented.

Beam-target interaction experiments for multipulse bremsstrahlung converters applications

As part of the Dual Axis Radiography Hydrotest Facility, Phase II (DARHT II) Multipulse Bremsstrahlung Target effort, we have been performing an investigation of (1) the possible adverse effects of backstreaming ion emission from the bremsstrahlung converter target and (2) the hydrodynamic behavior of the target after the electron beam interaction. Theory predictions show that the first effect would primarily be manifested in the static focusing system as a rapidly varying X-ray spot. From experiments performed on ETA-II, we have shown that the first effect is not strongly present when the beam initially interacts with the target. Electron beam pulses delivered to the target after formation of a plasma are strongly affected, however. Secondly, we have performed measurements of the time varying target density after disassembly was initiated by the electron beam. The measurements presented show that the target density as a function of time compares favorably with our LASNEX models.

Search for backstreaming ion defocusing during a single pulse of a 2 kA relativistic electron beam

Desorption and subsequent ionization of the monolayers from the vacuum wall of an accelerator system can have a detrimental effect on the performance of the beam transport system. Ions extracted from the resultant plasma neutralize the spacecharge and dynamically perturb the net focusing forces within the beam. To study the effect, a transparent first foil, presumably with contaminants on the surface, intercepts the beam. Placing an imaging foil tens of centimeters downstream from the first foil allows observation of minor fluxuations in the envelope. Using conducting foil targets, we see no effect unless the beam radius is small enough to damage the foil. Non-conducting foils produce a strong effect.

Electron beam/converter target interactions in radiographic accelerators

Linear induction accelerators used in X-ray radiography have single-pulse parameters of the order 20 MeV of electron beam energy, 2 kA of beam current, pulse lengths of 50-100 ns, and spot sizes of 1-2 mm. The thermal energy deposited in a bremsstrahlung converter target made of tantalum from such a pulse is /spl sim/80 kJ/cc, more than enough to bring the target material to a partially ionized state. The tail end of a single beam pulse, or any subsequent pulse in a multi-pulse train, undergoes a number of interactions with the target that can affect beam transport and radiographic performance. Positive ions extracted from the target plasma by the electron beam space charge can affect the beam focus and centroid stability. As the target expands on the inter-pulse time scale, the integrated line density of material decreases, eventually affecting the X-ray output of the system. If the target plume becomes sufficiently large, beam transport through it is affected by macroscopic charge and current neutralization effects and microscopic beam/plasma instability mechanisms. We will present a survey of some of these interactions, as well as some results of an extensive experimental and theoretical campaign to understand the practical amelioration of these effects, carried out at the ETA-II accelerator facility at the Lawrence Livermore National Laboratory.

High Intensity Beam and X-Ray Converter Target Interactions and Mitigation

Ions extracted from a solid surface or plasma by impact of an high intensity and high current electron beam can partially neutralize the beam space charge and change the focusing system. We have investigated ion emission computationally and experimentally. By matching PIC simulation results with available experimental data, our finding suggests that if a mix of ion species is available at the emitting surface, protons dominate the backstreaming ion effects, and that, unless there is surface flashover, ion emission is source limited. We have also investigated mitigation, such as e‐beam cleaning, laser cleaning and ion trapping with a foil barrier. The temporal behavior of beam spot size with a foil barrier and a focusing scheme to improve foil barrier performance are discussed.

MO‐D‐BRD‐02: Dielectric Wall Accelerators for Proton Therapy

A new type of compact induction accelerator is under development at the Lawrence Livermore National Laboratory that promises to increase the average accelerating gradient by at least an order of magnitude over that of existing induction machines. The machine is based on the use of high gradient vacuum insulators, advanced dielectric materials and switches and is being developed for a variety of applications. Research describing an extreme variant of this technology aimed at producing a compact, variable output linear accelerator for proton therapy for cancer will be described along with the technical challenges and issues. The goal of the development is to produce a proton accelerator that will fit in a standard linac vault and deliver intensity modulated proton therapy. Tomotherapy, Inc. has licensed the new accelerator technology from the Lawrence Livermore National Laboratory and the Compact Particle Acceleration Corporation (CPAC) is supporting development of the system. Research sponsored by Tomotherapy, Inc. and CPAC. Conflict of Interest: Some of the co‐authors have a financial interest in Tomotherapy, Inc. and/or CPAC.

Parallel Measurement and Modeling of Transport in the Darht II Beamline on ETA II

To successfully tune the DARHT II transport beamline requires the close coupling of a model of the beam transport and the measurement of the beam observables as the beam conditions and magnet settings are varied. For the ETA II experiment using the DARHT II beamline components this was achieved using the SUICIDE (Simple User Interface Connecting to an Integrated Data Environment) data analysis environment and the FITS (Fully Integrated Transport Simulation) model. The SUICIDE environment has direct access to the experimental beam transport data at acquisition and the FITS predictions of the transport for immediate comparison. The FITS model is coupled into the control system where it can read magnet current settings for real time modeling. We find this integrated coupling is essential for model verification and the successful development of a tuning aid for the efficient convergence on a useable tune. We show the real time comparisons of simulation and experiment and explore the successes and limitations of this close coupled approach.