Preparation Studies for Secondary Electron Emission Experiments on Superconducting Niobium

Accelerator driven transmutation of waste is one complementary approach to deal with spent nuclear fuel as compared to permanent storage. High-energy protons generated by a particle accelerator collide with a heavy metal target producing neutrons. Long-lived radioactive isotopes interacting with the neutrons transmute into shorter-lived isotopes. To generate the high-energy protons efficiently, linear accelerators use multi-cell superconducting radio frequency (RF) cavities made of niobium. Superconducting niobium cavities have several advantages, including small power dissipation. The high electromagnetic fields present in these cavities may result in undesired field emission from surface imperfections with the probability of generating an avalanche of secondary electrons from a localized resonant process of impacting known as multipacting. Undesirably, this localized electron current absorbs the RF power supplied to the cavity. This in turn leads to an increase in cavity wall temperature and the eventual breakdown of the wall’s superconductivity. In addition, this can result in structural damage to the cavity surface and the degradation of cavity vacuum. As a result, the Q0 (quality factor) of the cavity is significantly reduced. A good cavity design should be able to eliminate, or at least minimize multipacting. The factors that affect multipacting include shape, surface finish and conditioning, and the secondary electron yield of the material. It is desired to measure the distributed secondary electron yield from a Los Alamos National Laboratory surface prepared niobium test piece in the superconducting state under ultra high vacuum (UHV). A micro-channel plate/delay-line-anode detector (MCP/DLD) capable of single particle position and timing will be used to determine, with the aid of particle tracking codes, the secondary electron yield. The experimental setup primarily evolves around the detector to measure the secondary electron beam and the physics to be studied. Simulation studies using an electromagnetic particle tracking code will be presented to establish the system parameters and geometry, and examine constraints and resolutions of the experimental setup. With the aid of a biasing grid, secondary electrons with 1 eV increments in initial energies between 1 and 20 eV for a wide range of launch angles can be captured and distinguished on a 4.5 cm diameter MCP/DLD detector. An experimental setup is presented