Stephen E. Sampayan

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.

Mixed electron emission from lead zirconate–titanate ceramics

Simultaneous ferroelectric and plasma emission from Pb(Zr,Ti)O3 was observed with only a negative driving pulse applied to the sample, and without an extraction potential on the electron collector. Plasma emission was a strong, inconsistent, and self-destructive process. In addition, a positive ion current was detected. Comparatively, ferroelectric emission was a relatively stable self-emission process, exhibiting no apparent delay time, and no positive ion current. The relationship between the switching and emission current of ferroelectric samples measured simultaneously cannot only be used to determine the existence of ferroelectric emission, but can also give direction to choosing suitable ferroelectric materials for emitter applications.

The Dielectric Wall Accelerator

Dielectric wall accelerators, a class of induction accelerators, employ a novel insulating beam tube to impress a longitudinal electric field on a bunch of charged particles. The surface flashover characteristics of this tube may permit the attainment of accelerating gradients on the order of 100 MV/m for accelerating pulses on the order of a nanosecond in duration. A virtual traveling wave of excitation along the tube is produced at any desired speed by controlling the timing of pulse-generating modules that supply a tangential electric field to the tube wall. Because of the ability to control the speed of this virtual wave, the accelerator is capable of handling any charge-to-mass-ratio particle; hence it can be used for electrons, protons and any ion. The accelerator architectures, key technologies and development challenges will be described.

Neutron production from feedback controlled thermal cycling of a pyroelectric crystal

The LLNL Crystal Driven Neutron Source is operational and has produced record ion currents of approximately 10 nA and neutron output of 1.9(+/-0.3)x10(5) per thermal cycle using a crystal heating rate of 0.2 degrees C/s from 10 to 110 degrees C. A 3 cm diameter by 1 cm thick LiTaO(3) crystal with a socket secured field emitter tip is thermally cycled with feedback control for ionization and acceleration of deuterons onto a deuterated target to produce D-D fusion neutrons. The entire crystal and temperature system is mounted on a bellows which allows movement of the crystal along the beam axis and is completely contained on a single small vacuum flange. The modular crystal assembly permitted experimental flexibility. Operationally, flashover breakdowns along the side of the crystal and poor emitter tip characteristics can limit the neutron source. The experimental neutron results extend earlier published work by increasing the ion current and pulse length significantly to achieve a factor-of-two higher neutron output per thermal cycle. These findings are reviewed along with details of the instrument.

Intense pulsed neutron emission from a compact pyroelectric driven accelerator

Intense pulsed D–D neutron emission with rates of >1010 n/s during the pulse, pulse widths of approximately hundreds of nanoseconds and neutron yields of greater than 10 000 per pulse, are demonstrated in a compact pyroelectric accelerator. The accelerator consists of a small pyroelectric LiTaO3 crystal that provides the accelerating voltage and an independent compact spark plasma ion source. The crystal voltage versus temperature is characterized and compares well with theory. Results show neutron output per pulse that scales with voltage as V∼1.7. These neutron yields match a simple model of the system at low voltages but are lower than predicted at higher voltages due to charge losses not accounted for in the model. Interpretation of the data against modeling provides understanding of the accelerator and in general pyroelectric LiTaO3 crystals operated as charge limited negative high voltage targets. The findings overall serve as the proof of principle and basis for pyroelectric neutron generators that...

Response to “Comment on ‘Mixed electron emission from lead zirconate–titanate ceramics’ ” [J. Appl. Phys. 85, 8495 (1999)]

Understanding the mechanism of electron emission from ferroelectrics is in deed crucial for the application of ferroelectric cathodes. We appreciated the comments of Dorfman et al. on our recent article [J. Appl. Phys. 85, 6055 (1998)]. We believe the original proposed model is consistent and reasonable, i.e., the coexistence of ferroelectric and plasma emission from the same sample.

Enhanced ionization freeman ion source

Abstract We developed modifications to a standard Freeman ion source which increased the plasma electron temperature and density by minimum factors of 1.5 and 1.1, respectively. The improvement in these fundamental plasma parameters resulted in a greater than 40% increase in B+ currents from boron trifluoride gas and substantial usable beam current increases in B2+, P2+, and As2+. A large increase in triple charge currents was also present in the extracted beam. Ion source lifetime tests were performed at 1 mA scanned B+ wafer current with a 125 mm aperture on an Eaton NV-6200 Ion Implanter. Filament lifetime was observed to be in excess of 50 h. Further, erosion was more uniformly distributed along the filament length and tests indicate that the extracted ion beam uniformity was increased along the extraction slit. Therefore, we conclude that our modifications to the standard Freeman ion source also increased discharge uniformity.

High Voltage Wide Bandgap Photoconductive Switching

High gain photoconductive switching using Si and GaAs was studied previously for pulsed high voltage switching. A laser is used to generate charge carriers within the material to render the bulk conductive. We have begun the study of photoconductive switching using wide bandgap materials. These materials appear to operate in a non-high gain mode and the on resistance can be directly controlled with the laser intensity over many decades. It is presently believed that the conduction mechanism may be due to (a) excitation of deep states or (b) multi-photon pumping of carriers from the valance band. We present the study of the physics processes and development of a device operating at >20-kV.

Crystal Driven Neutron Source: A New Paradigm for Miniature Neutron Sources

Neutron interrogation techniques have specific advantages for detection of hidden, shielded, or buried threats over other detection modalities in that neutrons readily penetrate most materials providing backscattered gammas indicative of the elemental composition of the potential threat. Such techniques have broad application to military and homeland security needs. Present neutron sources and interrogation systems are expensive and relatively bulky, thereby making widespread use of this technique impractical. Development of a compact, high intensity crystal driven neutron source is described. The crystal driven neutron source approach has been previously demonstrated using pyroelectric crystals that generate extremely high voltages when thermal cycled [1–4]. Placement of a sharpened needle on the positively polarized surface of the pyroelectric crystal results in sufficient field intensification to field ionize background deuterium molecules in a test chamber, and subsequently accelerate the ions to ener...

Probing of Carrier Recombination in n- and p-Type 6H-SiC Using Ultrafast Supercontinuum Pulses

Excess carrier dynamics in 6H-SiC substrates with n- and p-type moderate doping were detected using femtosecond pump-probe measurements with supercontinuum probing. Band-to-band recombination and carrier trapping were determined as the main recombination processes in both materials. Spectral fingerprints corresponding to each of these recombination components were obtained using the global and target analysis. It was shown that, in spite of background doping, the band-to-band recombination in 6H-SiC is dominated by the excess electron absorption component and the carrier trapping is dominated by the excess hole absorption.