# Transformative Technology for FLASH Radiation Therapy
## 7 High-Current Electron Linear Accelerator for X-ray FLASH-RT Developed by RadiaBeam and UCLA
### Very-High-Energy Electron (VHEE) Flash-RT
Advances in both the distributed coupling design and cryogenic operation of the normal conducting structure are necessary to maximize the power efficiency of the linac, allowing SLAC's one-meter X-band structure to reach a gradient of 100 MeV/m while using a peak power of only 20 MW. A preliminary design of the cryogenic X-band linac is shown in Figure 17.  
The SLAC system will rely on RF pulse compression to achieve a 20 MW peak power using the initial 6 MW pulse produced by the commercial klystron. RF pulse compression is a mature technology that has been used extensively for large-scale accelerator applications to reconcile the need for short high-peak-power pulses with a cost-efficient long-pulse, low-power sources [160; 161; 162]. Recent advances in RF compressor technology have opened the door to compact structures that could dramatically reduce the system footprint while maintaining the capability to produce a four-fold pulse compression and isolate the source from the reflected RF signal from the cavities [163; 164]. The SLAC VHEE program investigates multiple cavity designs, such as the spherical cavity in Ref. [163], focusing on structures designed for high intrinsic quality factors, Q0 up to 400,000, and high coupling factors, \(\beta\) up to 10. Active research in this area will continue to benefit efforts to design a new generation of compact, cost-efficient medical accelerators and the broader accelerator community that relies on pulse compressors to supply the peak powers needed for high-gradient operation. Proposed programs such as the Cool Copper Collider (C\({}^{3}\)) proposal [165] to realize a e\({}^{+}\)e\({}^{-}\) collider for the study of the Higgs boson offer an exciting opportunity for synergistic research efforts on accelerator technologies, from the distributed coupling linac to the pulse compression system, which could enhance power efficiency for a single compact cancer therapy system up to a large-scale facility such as C\({}^{3}\). In each case, the same underlying techniques are used to push the limits on achievable accelerating gradients with a cost-efficient system. Research from a C\({}^{3}\) linac R&D effort [166] would provide insight into  
Figure 16: (**a**) Schematic of cryogenic test setup at XTA using liquid nitrogen. (**b**) Photo of linac and cryostat assembly prior to installation at XTA. (**c**) Measurements of accelerating gradient as a function of RF power. Figure 17: Schematic of the cryogenic X-band linac for VHEE under development at SLAC. features needed for a high-gradient VHEE system and vice versa, including stringent performance reliability criteria optimized for a substantial beam current under cryogenic operating conditions. The fabrication of SLAC's X-band distributed coupling linac for VHEE will rely on a split-block approach which allows significant flexibility for the CNC machining of the linac cavities and power coupling manifold into the copper slabs. This flexibility is critical for implementing a 135\({}^{\circ}\) phase advance linac design which further enhances the power efficiency, increasing the shunt impedance by nearly 10% compared to the \(\pi\)-mode. On-going collaboration with industry partners will facilitate the transition of SLAC's prototype VHEE system into modular industrialized equipment. Mass production will be an important feature not only for commercialization generally, but also for achieving FLASH capability with the VHEE system. In order to eliminate gantry motion and reach an ultra-high dose rate, the proposed FLASH VHEE system utilizes an array of 16 linacs, in the same architecture as the PHASER system for photon therapy [141]. VHEE beam energies as high as 250 MeV could be needed for treatment scenarios that use advanced techniques such as spatially fractionated radiotherapy in combination with FLASH dose rates [167]. The demand for equipment that can deliver these beam energies on the meter scale has motivated a search for technology that can provide gradients exceeding 100 MeV/m at the FLASH dose rate.