A new electron gun is currently being designed for the S-band linac injector for ELSA. The objective of this development is to realize a new single-bunch injection mode in addition to the standard long pulse (multi-bunch) mode along with an improvement of the current beam parameters (e.g. emission current & transverse emittance) achieved by the existing gun. A dual mode design is being developed that utilizes a caesium dispenser cathode both as a thermionic and a photocathode using thermally assisted photoemission. In addition to the novel electron gun, a dedicated test stand is currently being designed to allow detailed characterization of both operating modes. The refined design of the gun and the current status of the test stand including beam parameter simulations are presented.

Ultra-high energy electrons (UHEE) are used to investigate their effect on tumor cells and healthy tissue in short pulses of microseconds at the electron accelerator facility ELSA. This may enable highly efficient treatment of deep-seated tumors due to the FLASH effect. In a preliminary setting electrons with an energy of 1.2 GeV are used to irradiate cell samples which are located inside a water volume, representing the human body. Irradiation occurs with dose rates of up to 10 MGy/s due to the short pulse lengths of 250 ns. The relative biological effectiveness (RBE) can be determined by assessing the cell survival of tissues under FLASH and conventional conditions. For a precise dose determination, dose measurements via radiochromic films are utilized and compared to simulations with Geant4, that reproduce the electromagnetic shower process.