D. Sanders

High gradient induction accelerator

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 stimulated by the desire for compact flash X-ray radiography sources. Research describing an extreme variant of this technology aimed at proton therapy for cancer will be described. Progress in applying this technology to several applications will be reviewed.

Development of a Compact Radiography Accelerator Using Dielectric Wall Accelerator Technology

We are developing an inexpensive compact accelerator system primarily intended for pulsed radiography. Design characteristics are an 8 MeV endpoint energy, 2 kA beam current, a cell gradient of approximately 3 MV/m (for an overall accelerator length is 2-3 m), and <

Multilayer High-Gradient Insulators

1/Volt capital costs. Such designs have been made possible with the development of high specific energy dielectrics (>10J/cm3), specialized transmission line designs and multi-gap laser triggered low jitter (<1 ns) gas switches. In this geometry, the pulse forming lines, switches, and insulator/beam pipe are fully integrated within each cell to form a compact, stand-alone, stackable unit. We detail our research and modeling to date, recent high voltage test results, and the integration concept of the cells into a radiographic system.

Vacuum Insulator Development for the Dielectric Wall Accelerator

Multilayer high-gradient insulators are vacuum insulating structures composed of thin, alternating layers of dielectric and metal. They are currently being developed for application to high-current accelerators and related pulsed power systems. This paper describes some of the high-gradient insulator research currently being conducted at Lawrence Livermore National Laboratory

Electrical Strength of Multilayer Vacuum Insulators

At Lawrence Livermore National Laboratory, we are developing a new type of accelerator, known as a dielectric wall accelerator, in which compact pulse-forming lines directly apply an accelerating field to the beam through an insulating vacuum boundary. The electrical strength of this insulator may define the maximum gradient achievable in these machines. To increase the system gradient, we use “high-gradient insulators” composed of alternating layers of dielectric and metal for the vacuum insulator. In this paper, we present our recent results from experiment and simulation, including successful testing of a high-gradient insulator in a functioning dielectric wall accelerator cell. Our results indicate that proper high-voltage conditioning of the insulators can delay the onset of flashover, that the observed conditioning consists of both a permanent and a temporary part, and that the insulators’ voltage-holding capability increases with increasing dielectric layer thickness.

Beam-target interaction experiments for bremsstrahlung converter applications

The electrical strength of vacuum insulators is a key constraint in the design of particle accelerators and pulsed power systems. Vacuum insulating structures assembled from alternating layers of metal and dielectric can result in improved performance compared to conventional insulators, but previous attempts to optimize their design have yielded seemingly inconsistent results. Here, we present two models for the electrical strength of these structures, one assuming failure by vacuum arcing between adjacent metal layers and the other assuming failure by vacuum surface flashover. These models predict scaling laws which are in agreement with the experimental data currently available.

TH‐C‐AUD‐09: A Proposal for a Novel Compact Intensity Modulated Proton Therapy System Using a Dielectric Wall Accelerator

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.

Development of a Low Loss, High Dielectric Strength Microwave Substrate

Purpose: A novel compact CT‐guided intensity modulated protonradiotherapy (IMPT) system is introduced. The system is being designed to deliver motion‐managed IMPT to large target volumes. The system will be ideal for large and complex target volumes in young patients. Method and Materials: The basis of the design is the dielectric wall accelerator (DWA) system being developed at Lawrence Livermore National Laboratory (LLNL). The DWA will use fast switched high voltage transmission lines to generate pulsed electric fields on the inside of a high gradient insulating (HGI) acceleration tube. High electric field gradients are achieved alternating insulators and conductors and short pulse times. The system will produce individual pulses that can be varied in intensity, energy and spot width, all of which will be optimized in the IMPT planning system. It is anticipated that no magnets will be required and the neutron contamination will be very low. The system will be capable of being sited in a conventional linac vault. Results: The design specifications have been met in some component tests. Gradients of 100 MV/m have been achieved in small HGI samples. Optical switches based on fast laser switched SiC has been achieved. Feasibility tests of an optimization system for selecting the position, energy, intensity and spot size for a collection of spots comprising the treatment are underway. A prototype is being designed and concept designs of the envelope and environmental needs of the unit has commenced. Conclusion: The DWA accelerator represents breakthrough technology for intensity modulated proton therapy. The system is being designed from the ground up to be capable of CT‐guided intensity modulated proton therapy and to be housed in a conventional linac vault. Conflict of Interest:Some of the authors have financial interest in TomoTherapy Inc., which has licensed the DWA technology from LLNL.

Beam-target interaction experiments for multipulse bremsstrahlung converters applications

This work describes a comparison of two candidate materials for pulse forming line fabrication with respect to bulk dielectric breakdown, frequency response of relative permittivity, and dielectric loss. One material is a commercially available microwave substrate material that can be procured in sheet form without a high voltage specification, while the other is a newly developed material that also comes in sheet form that can also be cast between the electrodes

Breakdown Performance Statistics of a Nanoparticle Composite System

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.