Mark A. Rhodes

A compact linac for intensity modulated proton therapy based on a dielectric wall accelerator

A novel compact CT-guided intensity modulated proton radiotherapy (IMPT) system is described. The system is being designed to deliver fast IMPT so that larger target volumes and motion management can be accomplished. The system will be ideal for large and complex target volumes in young patients. The basis of the design is the dielectric wall accelerator (DWA) system being developed at the Lawrence Livermore National Laboratory (LLNL). The DWA uses 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 by the use of alternating insulators and conductors and short pulse times. The system will produce individual pulses that can be varied in intensity, energy and spot width. The IMPT planning system will optimize delivery characteristics. The system will be capable of being sited in a conventional linac vault and provide intensity modulated rotational therapy. 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 are beginning. The status of the developmental new technologies that make the compact system possible will be reviewed. These include, high gradient vacuum insulators, solid dielectric materials, SiC photoconductive switches and compact proton sources.

PERFORMANCE OF LARGE-APERTURE OPTICAL SWITCHES FOR HIGH-ENERGY INERTIAL-CONFINEMENT FUSION LASERS

We describe the design and performance of large-aperture (>30 cm × 30 cm) optical switches that have demonstrated, for the first time to our knowledge, active switching of a high-energy (>5 kJ) optical pulse in an inertial-confinement fusion laser. These optical switches, which consist of a plasma-electrode Pockels cell (PEPC) and a passive polarizer, permit the design of efficient, multipass laser amplifiers. In a PEPC, plasma discharges on the faces of a thin (1-cm) electro-optic crystal (KDP or KD*P) act as highly conductive and transparent electrodes. These plasma electrodes facilitate rapid (<100 ns) and uniform charging of the crystal to the half-wave voltage and discharging back to 0 V. We discuss the operating principles, design, optical performance, and technical issues of a 32 cm × 32 cm prototype PEPC with both KDP and KD*P crystals, and a 37 cm × 37 cm PEPC with a KDP crystal for the Beamlet laser. This PEPC recently switched a 6-kJ, 3-ns pulse in a four-pass cavity.

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.

Scaling studies and time-resolved microwave measurements on a relativistic backward-wave oscillator

A relativistic backward-wave oscillator (BWO) operating at a frequency near 8 GHz has been built. The parameters of the 60-ns electron beam driving this microwave source are varied over the ranges 0.8-1.5 MV and 2-10 kA. Several different annular cathodes for launching the electron beam are tried, varying the outer radius and shape. The axial magnetic field guiding the beam through the BWO is varied between 0.6 and 3 T. The power transfer downstream to an output waveguide is investigated as a function of the shape of the transition from the BWO to the waveguide. The scaling of the output power and frequency with these variations is discussed. Time-resolved measurements of 2-ns-long segments of the microwave output are shown. In observations of the microwave signal, it is found that the frequency shifts as the output power envelope passes through a sharp dip. It is proposed that this shift corresponds to a change in the longitudinal operating mode of the BWO. >

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 <

Modeling of plasma behavior in a plasma electrode Pockels cell

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.

Ferrite-Free Stacked Blumlein Pulse Generator for Compact Induction Linacs

We present three interrelated models of plasma behavior in a plasma electrode Pockels cell (PEPC). In a PEPC, plasma discharges are formed on both sides of a thin, large-aperture electro-optic crystal (typically KDP). The plasmas act as optically transparent, highly conductive electrodes, allowing uniform application of a longitudinal field to induce birefringence in the crystal. First, we model the plasma in the thin direction, perpendicular to the crystal, via a one-dimensional fluid model. This yields the electron temperature and the density and velocity profiles in this direction as functions of the neutral pressure, the plasma channel width, and the discharge current density. Next, me model the temporal response of the crystal to the charging process, combining a circuit model with a model of the sheath which forms near the crystal boundary. This model gives the time-dependent voltage drop across the sheath as a function of electron density at the sheath entrance. Finally, we develop a two dimensional MHD model of the planar plasma, in order to calculate the response of the plasma to magnetic fields. We show how the plasma uniformity is affected by the design of the current return, by the longitudinal field from the cathode magnetron, and by fields from other sources. This model also gives the plasma sensitivity to the boundary potential at which the top and bottom of the discharge are held. We validate these models by showing how they explain observations in three large Pockels cells built at Lawrence Livermore National Laboratory.

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

Stacked Blumlein pulse generators comprised of parallel-plate transmission lines are potentially a useful pulsed-power architecture for high-gradient, compact, electron-beam induction accelerators. However, like induction accelerators driven by other pulsed-power architectures, it is generally a system requirement that the multi-stage accelerator structure be enclosed in a grounded metal enclosure so that the full beam voltage is not developed on the exterior of the machine. In the past, this has been accomplished by using magnetic cores to prevent the external metal case from shorting the accelerating field. However, magnetic cores are heavy, bulky, expensive, lossy, nonlinear, and therefore generally undesirable. Various core-free pulse architectures have been reported in the past. One class uses pairs of lines with widely different dielectric constants while another class uses combinations of open-circuit lines combined with short-circuit lines. These designs are encased in metal and support stackable output pulses without the need for magnetic isolation cores. These configurations are also known as bi-polar or zero-integral configurations because they produce a positive and negative voltage pulse with a net time integral of zero. Some of these designs are inefficient leaving substantial stored energy in the lines while others have never been realized as practical accelerating structures. We present here a particular, realizable, magnetic-core-free induction linac geometry that is based on a parallel-plate, stacked Blumlein-like structure, with a symmetric bi-polar, zero-integral output pulse, and an outer metal enclosure. Our design is, in theory, 100% efficient into a matched load. We have evaluated the electromagnetic operation of this geometry by computer modeling. We present the results of this modeling.

Plasma electrode pockels cell for the National Ignition Facility

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.

Oil-Switched Planar Blumlein Pulse Generators for Dielectric Wall Accelerators

The NIF, now under construction at LLNL, will be the largest laser fusion facility ever built. The NIF laser architecture is based on a multi-pass power amplifier to reduce cost and maximize performance. A key component in this laser design is an optical switch that closes to trap the optical pulse in the cavity for four gain passes and then opens to divert the optical pulse out of the amplifier cavity. The switch is comprised of a Pockels cell and a polarizer and is unique because it handles a beam that is 40 cm X 40 cm square and allows close horizontal and vertical beam spacing. Conventional Pockels cells do not scale to such large apertures or the square shape required for close packing. Our switch is based on a Plasma-Electrode Pockels Cell (PEPC). In a PEPC, low-pressure helium discharges are formed on both sides of a thin slab of electro-optic material. Typically, we use KH2PO4 crystals (KDP). The discharges form highly conductive, transparent sheets that allow uniform application of a high-voltage pulse across the crystal. A 37 cm X 37 cm PEPC has been in routine operation for two years on the 6 kJ Beamlet laser at LLNL. For the NIF, a module four apertures high by one wide is required. However, this 4 X 1 mechanical module will be comprised electrically of a pair of 2 X 1 sub-modules. Last year, we demonstrated full operation of a prototype 2 X 1 PEPC. In this PEPC, the plasma spans two KDP crystals. A major advance in the 2 X 1 PEPC over the Beamlet PEPC is the use of anodized aluminum construction that still provides sufficient insulation to allow formation of the planar plasmas. In this paper, we discuss full 4 X 1 NIF prototypes.