Recent studies indicate better efficacy and healthy tissue sparing with high dose-rate FLASH radiotherapy (FLASH-RT) cancer treatment. This technique delivers a prompt high radiation dose rather than fractional doses over time. While some suggest thresholds of > 40 Gy s−1 with a maximal effect at > 100 Gy s−1, accumulated evidence shows that instantaneous dose-rate and irradiation time are critical. Mechanisms are still debated, but toxicity is minimized while inducing apoptosis in malignant tissue. Delivery technologies to date show that a capability gap exists with clinic scale, broad area, deep penetrating, high dose rate systems. Based on these trends, if FLASH-RT is adopted, it may become a dominant approach except in the least technologically advanced countries. The linear induction accelerator (LIA) developed for high instantaneous and high average dose-rate, species independent charged particle acceleration, has yet to be considered for this application. We review the status of LIA technology, explore the physics of bremsstrahlung-converter-target interactions and our work on stabilizing the electron beam. While the gradient of the LIA is low, we present our preliminary work to improve the gradient by an order of magnitude, presenting a point design for a multibeam FLASH-RT system using a single accelerator for application to conformal FLASH-RT.

Unabated, worldwide trends in CO2 production project growth to > 43-BMT per year over the next two decades. Efficient power electronics are crucial to fully realizing the CO2 mitigating benefits of a worldwide smart grid (~ 18% reduction for the United States alone). Even state-of-the-art SiC high voltage junction devices are inefficient because of slow transition times (~ 0.5-μs) and limited switching rates at high voltage (~ 20-kHz at ≥ 15-kV) resulting from the intrinsically limited charge carrier drift speed (< 2 × 107-cm-s−1). Slow transition times and limited switch rates waste energy through transition loss and hysteresis loss in external magnetic components. Bulk conduction devices, where carriers are generated and controlled nearly simultaneously throughout the device volume, minimize this loss. Such devices are possible using below bandgap excitation of semi-insulating (SI) SiC single crystals. We explored carrier dynamics with a 75-fs single wavelength pump/supercontinuum probe and a modified transient spectroscopy technique and also demonstrated a new class of efficient, high-speed, high-gain, bi-directional, optically-controlled transistor-like power device. At a performance level six times that of existing devices, for the first time we demonstrated prototype operation at multi-10s of kW and 20-kV, 125-kHz in a bulk conduction transistor-like device using direct photon-carrier excitation with below bandgap light.

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A transconductance-like behavior similar to that of junction semiconductor devices is observed in photonically excited wide bandgap (WBG) semi-insulating material without a junction. This property offers the possibility of power electronic devices capable of virtually unlimited voltage and current carrying capability due to intrinsic electrical isolation of the controlling voltage from the switched high voltage. A proof of concept experiment demonstrated the transconductance-like property in burst mode switching to >16 kV, 50% duty cycle, and 75 kHz. Our eventual goal is to combine the light source, optics and the WBG material to form a compact module that is functionally equivalent to junction power electronic devices. In this paper, we present the background, our generalized approach for implementing photoconductive switching for potential applications to high repetition rate (>50 kHz), high voltage (>15 kV) power switching, our associated material measurements, and our path forward to multi-10s of ampere devices.

Recent studies indicate better efficacy and healthy tissue sparing with high dose-rate FLASH radiotherapy (FLASH-RT) cancer treatment. This technique delivers a prompt high radiation dose rather than fractional doses over a longer period of time. The threshold is >40 Gy-s-1 with a maximal effect at >100 Gy-s-1 that must be maintained in the treatment volume. Mechanisms are still widely debated, but toxicity is minimized while inducing apoptosis in malignant tissue. Delivery technologies to date show that a capability gap exists with clinic scale, broad area, deep penetrating, high dose rate capability. Based on present trends, if FLASH-RT is adopted, it may become a dominant approach except in the least technologically advanced countries. The linear induction accelerator (LIA) developed for high current, high repetition rate, species independent charged particle acceleration, has yet to be considered for this application. We briefly review the status of LIA technology, explore the physics of bremsstrahlung-converter-target interactions and our work on stabilizing the electron beam. While the gradient of the LIA is low, we present our preliminary work to improve the gradient by an order of magnitude, presenting a point design for a multibeam FLASH-RT system using a single accelerator for application to conformal FLASH-RT.

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Wide bandgap semiconductor, 6H-SiC, is being applied in photoconductive semiconductor switches (PCSS) due to its semi-insulating properties. This material is normally insulating but when illuminated, charge carriers are pumped to the conduction band and the material becomes conductive in proportion to the light intensity. Under the application of voltage and laser, the density as well as the movement of the defects will have an impact on the performance of the device. Characterization and understanding the mechanism of this potentially destructive defect process is of great scientific interest as well to the development of the device. X-ray topography is a powerful, non-destructive technique for the characterization of extended defects in large, single crystals. In this study, threading edge dislocations (TED), threading screw dislocations (TSD) as well as basal plane dislocations (BPD) in 6H-SiC axial samples are revealed using synchrotron rocking curve topography, where the atomic structure information (Burgers vector) of these dislocations are investigated using ray tracing simulation. The understanding of the nature of these dislocations is of great importance as to predict their propagation and movement under the application of voltage and laser, and eventually helps to improve the performance of the device. Figure 1