The general concept of radiation therapy used in conventional cancer treatment is to increase the therapeutic index by creating a physical dose differential between tumors and normal tissues through precision dose targeting, image guidance, and radiation beams that deliver a radiation dose with high conformality, e.g., protons and ions. However, the treatment and cure are still limited by normal tissue radiation toxicity, with the corresponding side effects. A fundamentally different paradigm for increasing the therapeutic index of radiation therapy has emerged recently, supported by preclinical research, and based on the FLASH radiation effect. FLASH radiation therapy (FLASH-RT) is an ultra-high-dose-rate delivery of a therapeutic radiation dose within a fraction of a second. Experimental studies have shown that normal tissues seem to be universally spared at these high dose rates, whereas tumors are not. While dose delivery conditions to achieve a FLASH effect are not yet fully characterized, it is currently estimated that doses delivered in less than 200 ms produce normal-tissue-sparing effects, yet effectively kill tumor cells. Despite a great opportunity, there are many technical challenges for the accelerator community to create the required dose rates with novel compact accelerators to ensure the safe delivery of FLASH radiation beams.

A highly efficient, high-voltage power switching technology, the Optical Transconductance Varistor (OTV) is being developed based on the photoconductive property of 6H-SiC. The behavior of the dislocations in 6H-SiC under the application of voltage and laser in such devices is of particular interest. In this study, both ex-situ and in-situ synchrotron X-ray topography were applied to characterize dislocations and investigate their behaviors when the sample was electrically and photonically stressed. Threading dislocations (TDs) and basal plane dislocations (BPDs) were revealed in transmission topographs and grazing topographs. When the samples were connected to external voltage ranging from 1kV to 4kV, there were no observable signs of dislocation movement. This indicates that the energy released from the transitioning of Vanadium states is lower than the activation energy for dislocation gliding.

Conventional cancer therapies include surgery, radiation therapy, chemotherapy, and, more recently, immunotherapy. These modalities are often combined to improve the therapeutic index. The general concept of radiation therapy is to increase the therapeutic index by creating a physical dose differential between tumors and normal tissues through precision dose targeting, image guidance, and high radiation beams that deliver radiation dose with high conformality, e.g., protons and ions. However, treatment and cure are still limited by normal tissue radiation toxicity, with many patients experiencing acute and long-term side effects. Recently, however, a fundamentally different paradigm for increasing the therapeutic index of radiation therapy has emerged, supported by preclinical research, and based on the FLASH radiation effect. FLASH radiation therapy (FLASH-RT) is an ultra-high dose-rate delivery of a therapeutic radiation dose within a fraction of a second. Experimental studies have shown that normal tissues seem to be universally spared at these high dose rates, whereas tumors are not. The dose delivery conditions are not yet fully characterized. Still, it is currently estimated that large doses of 10 Gy or more delivered in 200 ms or less produce normal tissue sparing effects yet effectively kill tumor cells. There is a great opportunity, but also many technical challenges, for the accelerator community to create the required dose rates with novel and compact accelerators to ensure the safe delivery of FLASH radiation beams.

Wide bandgap semiconductor, 6H-SiC, is being applied in photoconductive semiconductor switches (PCSS) due to its semi-insulating properties. The behavior of the dislocations in 6H-SiC under the application of voltage and laser in such devices is of particular interest. In this study, synchrotron X-ray transmission topography, grazing incidence topography and rocking curve topography are applied to characterize the types and distribution of defects in (1 1 -2 0) oriented wafers. Threading edge dislocations (TED), threading screw dislocations (TSD), threading mixed dislocation (TMD) and basal plane dislocations (BPD) in 6H-SiC axial samples are revealed in the X-ray topographs. TEDs are observed to be more likely off c-axis compared to TSDs/TMDs. Contrast features of BPDs differ depending on their Burgers vectors and orientation with respect to wafer surface on rocking curve topographs and grazing incidence topographs. Applying ray tracing simulations, the configurations of BPDs can be determined, and the threading dislocations (TED, TSD and TMD) can be distinguished in grazing incidence topographs. The understanding of the nature and the distribution of these dislocations will help predict their propagation and movement under the application of voltage and laser, providing guidance for device fabrication.

Unabated, worldwide trends in CO 2 production project growth to > 43-BMT per year over the next two decades. Efficient power electronics are crucial to fully realizing the CO 2 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 × 10 7 -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.

This project was a collaborative effort between Lawrence Livermore National Security, LLC as manager and operator of Lawrence Livermore National Laboratory (LLNL) and Raytheon Company, Missile Systems Rancho Innovations, to develop High Power Microwave (HPM) source technology. Under this effort, we sought to combine joint expertise to achieve scientific goals, develop new specialized knowledge, and utilize facilities and resources more effectively, in order to research, develop and deploy advanced technologies to improve US National Security. The HPM source technology used in this effort was developed outside of this CRADA under several efforts sponsored by the US government and internal Laboratory funding.

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.

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 potential movement of the defects will have an impact on the performance of the device. Therefore, 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 will help predict their propagation and movement under the application of voltage and laser, and eventually help improve the performance of the device.

The Optical Transconductance Varistor (OTV) represents a new class of photonically controlled, high-voltage power electronic device. It takes advantage of the bulk photonic properties of wide bandgap (WBG) materials, eliminating the traditional semiconductor control junction. Without drift region limitations, carrier excitation occurs on the order of picoseconds and in the bulk of the crystal; decay of the carriers is dependent on doping. Conductivity is therefore proportional to optical intensity so the device exhibits a transconductance-like property, in contrast to conventional photoconductive semiconductor switches (PCSS). The device is bidirectional and inherent optical isolation provides scalability in voltage and current capability. Recent testing demonstrated switching for bioelectric applications of kilovolt levels at 1 MHz repetition rate with a 10 ns rise time. A second device with a 50% duty cycle demonstrated operation at 20 kV and 2.5 A at over 125 kHz switching frequency. The OTV has use in pulsed power applications such as electroporation and accelerators and also in higher duty cycle cases such as power conversion for the electrical grid. Device background, present status and future development are set forth.

Free carrier absorption spectra are measured along the different polarization directions with respect to the c-axis of 4H-SiC using ultrafast differential transmission spectroscopy. Probing of excited carrier spectra in an undoped material reveals intrinsic resonances within the conduction band. Widths of the detected resonance peaks are shown to be wider than their theoretical estimates and more comparable to the ones observed in a low doped material. Relative strength of the peaks, on the other hand, is shown to be nearly excitation independent in contrast to the doping induced absorption weakening for the same transitions in n-type samples. Free carrier cross sections are extracted from the excitation dependency of the detected spectra and linked to the individual electron and hole contributions in the near infrared range.

The Optical Transconductance Varistor (OTV) represents a new class of power electronic device that is photonically controlled. It eliminates the traditional semiconductor control junction by exploiting the bulk photonic properties of wide bandgap (WBG) materials. Without drift region limitations, carrier excitation occurs on the order of picoseconds and in the bulk of the crystal; decay of the carriers is dependent on doping. Conductivity is therefore proportional to optical intensity so the device exhibits a transconductance-like property. The device is bi-directional and inherent optical isolation provides scalability in voltage and current capability. Recent testing demonstrated operation at 20 kV and 2.5 A at over 125 kHz switching frequency; our target is a 20 kV, 10 A device. The OTV has broad application in medical equipment, radar, and HVDC and other equipment for the Smart Grid, reducing energy losses and the size and cost of equipment. Device background, present status and future development are set forth.

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.

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.