Radiation therapy is currently the most utilized technique for the treatment of tumors by means of ionizing radiation, such as electrons, protons and x/gamma rays, depending on the type, size and depth of the cancer mass. Radiation therapy has in general fulfilled the main requirement of targeting thus damaging the malignant cells and sparing the healthy tissues as best as possible. In this scenario, electron linear accelerators have been operated as viable tools for the delivery of both high-energetic electrons and x-ray beams, which are obtained via the bremsstrahlung process of the electrons hitting on a high-Z material. Recently, it has been experimentally demonstrated that ultrahigh dose-rate bursts of electrons and x-ray beams increase the differential response between healthy and tumor tissues. This beneficial response is referred to as the FLASH effect. For this purpose, we have developed the first dedicated compact $S$-band linear accelerator for FLASH radiotherapy. This linac is optimized for a nominal energy of 7 MeV and a pulsed electron beam current of 100 mA and above. The accelerator is mounted on a remote-controlled system for preclinical research studies in the FLASH regime. We will show the rf and beam dynamics design of the $S$-band linac as well as the commissioning and high-power rf tests. Furthermore, the results of the dosimetric measurements will be illustrated.

The production of high spectral brilliance radiation from electron beam sources depends critically on the electron beam qualities. One must obtain very high electron beam brightness, implying simultaneous high peak current and low emittance. These attributes are enabled through the use of very high field acceleration in a radio-frequency (rf) photoinjector source. Despite the high fields currently utilized, there is a limit on the achievable peak current in high brightness operation, in the range of tens of Ampere. This limitation can be overcome by the use of a hybrid standing wave/traveling wave structure; the standing wave portion provides acceleration at a high field from the photocathode, while the traveling wave part yields strong velocity bunching. This approach is explored here in a C-band scenario, at field strengths ($>100\mathrm{MV}/\mathrm{m}$) at the current state-of-the-art. It is found that one may arrive at an electron beam with many hundreds of Amperes with well-sub-micron normalized emittance. This extremely compact injector system also possesses attractive simplification of the rf distribution system by eliminating the need for an rf circulator. We explore the use of this device in a compact 400 MeV-class source, driving both inverse Compton scattering and free-electron laser radiation sources with unique, attractive properties.