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In radiation therapy, x-ray dose must be precisely sculpted to the tumor, while simultaneously avoiding surrounding organs at risk. This requires modulation of x-ray intensity in space and/or time. Typically, this is achieved using a multi leaf collimator (MLC)-a complex mechatronic device comprising over one hundred individually powered tungsten 'leaves' that move in or out of the radiation field as required. Here, an all-electronic x-ray collimation concept with no moving parts is presented, termed "SPHINX": Scanning Pencil-beam High-speed Intensity-modulated X-ray source. SPHINX utilizes a spatially distributed bremsstrahlung target and collimator array in conjunction with magnetic scanning of a high energy electron beam to generate a plurality of small x-ray "beamlets."A simulation framework was developed in Topas Monte Carlo incorporating a phase space electron source, transport through user defined magnetic fields, bremsstrahlung x-ray production, transport through a SPHINX collimator, and dose in water. This framework was completely parametric, meaning a simulation could be built and run for any supplied geometric parameters. This functionality was coupled with Bayesian optimization to find the best parameter set based on an objective function which included terms to maximize dose rate for a user defined beamlet width while constraining inter-channel cross talk and electron contamination. Designs for beamlet widths of 5, 7, and 10 mmThe optimized 5-, 7-, and 10-mm models had beamlet widths of 5.1 , 7.2 , and 10.1 mmBayesian optimization was coupled with Monte Carlo modeling to generate SPHINX geometries for various beamlet widths. A complete Monte Carlo simulation for one of these designs was developed, including electron beam transport of all beamlets through scanning magnets, x-ray production and collimation, and dose in water. These results demonstrate that SPHINX is a promising candidate for sculpting radiation dose with no moving parts, and has the potential to vastly improve both the speed and robustness of radiotherapy delivery. A multi-beam SPHINX system may be a candidate for delivering magavoltage FLASH RT in humans.

We are developing a high-gradient, THz-driven electron gun. THz-frequency accelerator structures can enable the development of compact, bright, and efficient particle sources. Our two cell standing-wave gun is powered by a 110 GHz gyrotron and will produce 350 keV electrons with 500 k $W$ of input power. The field emission cathode is a copper tip located in the first cell. High power measurements will include diagnostics to measure the energy spread, charge, and size of the resulting bunch.

SLAC is pursuing high efficiency, low cost RF source designs that are suitable for mass production, in order to enable the next generation of linear accelerators. These efforts have driven the development of compact linac systems with integrated modulators, klystrons, and accelerator structures, based on a modular, low voltage klystron topology. There is substantial demand for compact linacs for X-ray radiography with security applications, and for new radiation therapy machines that reduce treatment times by orders of magnitude and may yield beneficial biological effects. In this presentation, details and challenges for these new compact accelerator systems and the corresponding RF sources are discussed.