Assembling NEG, Ion Pump and Bankable BNNT
Cryopump System to Reach XHV
Student: Anahi Segovia Miranda
School: Universidad Autonoma de Zacatecas
Mentored By: Marcy Stutzman
New accelerator initiatives require spin polarized electron photoinjectors. The gallium arsenide (GaAs) photoguns must provide high average current (≫1mA) and a long operating lifetime. Its performance depends on our ability to improve vacuum inside the chamber. The Center of Injectors and Sources (CIS) at Thomas Jefferson National Accelerator Facility has nearly reached extreme high vacuum (XHV) of P = 1 × 10-12 Torr by combining Non-Evaporable Getter (NEG) pumps and ion pumps (IP) in the Continuous Electron Beam Accelerator Facility (CEBAF) polarized source. Obtaining XHV pressure requires careful material selection and preparation and appreciation for pump characteristics, capability and limitations. Measuring pressure at XHV is also a challenge so gauges must be used that have low x-ray limits, and the x-ray contribution to the pressure must be measured. In this project we investigated cryopump technology to maintain an XHV on the electron gun to achieve P < 10-12 Torr, using a bakeable cryopump with mechanically attached BNNT (Boron Nitride Nanotubes), and the NEG-ion pump system. We also measured the x-ray limit of the extractor and bent belt-beam (3BG) gauge that we used to measure the pressure. Additionally, we used the Monte Carlo simulation software MolFlow+ to model the pressure distribution in the chamber , using the expected outgassing rates, measured temperatures, and expected pump speeds for the NEG, ion and cryopumps. We found that the system reaches a pressure of 1.6 × 10-11 Torr with the cryopump off, and a pressure of 2.7 × 10-12 Torr with the cryopump on. This cryopumped chamber did not reached the pressures previously measured in JLab NEG/ion pump chambers with the cryopump at room temperature, but when the cryopump was turned on, the chamber achieved pressure near 1 × 10-12 quickly. We measured the same pressure as before: This first attempt at using BNNT cryopumping in addition to the NEG/ion pump system previoulsy used at Jefferson Lab showed that we can achieve similar pressures with the prior technology, and that the limitations to ion pump speed at low pressures are not the limiting factor for chamber base pressure.