title: Development of a Cryogenic DC Photoelectron Gun for GaAs Cathodes with Increased Charge Lifetime
Creators: name: Eggert, Tobias
Publisher: name: UNSPECIFIED
Year: 2023

description: Electron beams can be generated by utilizing the internal photoelectric effect. In this matter, a special photocathode material is necessary for generating polarized electrons. Gallium arsenide (GaAs) proved to be the best choice. However, the production of polarized electrons requires a photo-emission close to the band gap, which in turn requires a negative-electron-affinity (NEA) coating consisting of caesium (Cs) and either oxygen or nitrogen trifluoride (NF3). This layer limits the possible operation time of the electron source as the layer deteriorates over time due to interactions with residual gas particles, with contamination through desorption and degradiation through ionized particles hitting the surface being the two main mechanisms. The latter is called ion back-bombardment (IBB) and is the main lifetime-limiting factor during high-current electron emission. Therefore, a reduction of IBB in order to increase operational lifetime and performance of electron sources is of great interest for many accelerator-based applications with polarized electrons. In this dissertation the design, assembly and first experimental results of a new kind of electron source is presented, which aims at reducing IBB by placing the cathode inside an almost closed cryogenic sub-volume. The inner walls of this volume are coated with activated charcoal making them act as a cryopump, reducing the local pressure significantly. The closed design limits the flux of particles from the outer, higher vacuum into the volume. Simulations of the hydrogen partial pressure, which contributes the most to IBB, shows a reduction of up to two magnitudes compared to conventional sources. As IBB intensity directly scales with the amount of residual gas molecules, a significant increase in lifetime can is anticipated. In addition, the cooling of the cathode itself compensates the heat load deposited by the laser beam, hence reducing heat-induced desorption of the NEA layer that would otherwise severely limit cathode lifetime for high-current applications. A pressure below 1 ⋅ 10^-11 mbar outside the sub-volume could be achieved after a four-week bake-out at 120 °C inside an aluminum vacuum chamber equipped with a 45 l/s ion-getter pump and four 430 l/s non-evaporable getter panels. Electrostatic field simulations for electrode voltages ranging from -5 kV to -30 kV have been conducted, showing a maximum field gradient of 11.1(1) MV/m at -30 kV. Experimentally, a stable voltage of -12 kV without and -14 kV with cooling of the electrode has be achieved. Simulations of the electron beam at potentials from -12 kV to -30 kV showed no beam loss for currents from 10 µA to 1 mA. The required cooling power was provided by a 1.1 W closed-cycle cryostat. A temperature of 21.3(1) K at the far end of the sub-volume has be reached allowing an effective pumping of hydrogen. A dedicated test stand was designed and setup at the Institut für Kernphysik of the Technische Universität Darmstadt to conduct measurements evaluating a potential lifetime enhancing effect. It does not feature a chamber to conduct the process of applying the NEA layer to the photocathode surface, also called photocathode activation, hence requiring the use of the existing test stand for Photo-Cathode Activation, Testing, and Cleaning using atomic Hydrogen (Photo-CATCH) for the cleaning and activation procedure. A measurement series demonstrated that a transport chamber, equipped with a NEG pump, is capable of retaining enough of the NEA layer, to allow a transfer of activated cathode from Photo-Catch to the new electron source. After transferring and storing for 20 h a quantum efficiency (QE) of up to 67 % using a re laser with 780 nm and 99 % with a blue 450 nm laser could be maintained. For a foreseen integration of the source into Photo-CATCH, simulations show that with minor modifications of the beamline an operation of both the cryogenic and the conventional source, already in place, is possible. A measurement demonstrating a life time enhancing effect could not be conducted, as the beam was lost at an unknown location in the system. Multiple approaches were conducted to identify the origin of the beam loss, offering room for further exploration.