Modeling, Fabrication, and Optimization of Niobium Cavities: Phase III Third Quarterly Report

Accelerator driven transmutation of waste is one complementary approach to deal with spentnuclear fuel as compared to permanent storage. High-energy protons generated by a particleaccelerator collide with a heavy metal target producing neutrons. Long-lived radioactive isotopesinteracting with the neutrons transmute into shorter-lived isotopes. To generate the high-energyprotons efficiently, linear accelerators use multi-cell superconducting radio frequency (RF) cavitiesmade of niobium. Superconducting niobium cavities have several advantages, including smallpower dissipation. The high electromagnetic fields present in these cavities may result in undesiredfield emission from surface imperfections with the probability of generating an avalanche ofsecondary 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 inturn leads to an increase in cavity wall temperature and the eventual breakdown of the wall’ssuperconductivity. In addition, this can result in structural damage to the cavity surface and thedegradation of cavity vacuum. As a result, the Q0 (quality factor) of the cavity is significantlyreduced. A good cavity design should be able to eliminate, or at least minimize multipacting. Thefactors that affect multipacting include shape, surface finish and conditioning, and the secondaryelectron yield of the material. It is desired to measure the distributed secondary electron yield from a Los Alamos NationalLaboratory surface prepared niobium test piece in the superconducting state under ultra highvacuum (UHV). A micro-channel plate/delay-line-anode detector (MCP/DLD) capable of singleparticle position and timing will be used to determine, with the aid of particle tracking codes, thesecondary electron yield. The experimental setup primarily evolves around the detector to measurethe secondary electron beam and the physics to be studied. Simulation studies using an electromagnetic particle tracking code will be presented to establishthe system parameters and geometry, and examine constraints and resolutions of the experimentalsetup. With the aid of a biasing grid, secondary electrons with 1 eV increments in initial energiesbetween 1 and 20 eV for a wide range of launch angles can be captured and distinguished on a 4.5cm diameter MCP/DLD detector. An experimental setup is presented.