Status of Vertical Electroplishing at Cornell University

Abstract

Cornell's SRF group has continued R&D on vertical electropolishing (VEP) of Niobium SRF Cavity with electrolyte of HF(40w%):H2SO4(96w%) in Volume proportions 1: 9. Compared with conventional horizontal EP system for SRF cavities, VEP could eliminate rotary acid seals, sliding electrical contact, cavity vertical/horizontal position control fixtures. And also, the acid plumbing/containment could be simplified. So, the cost reduction of capital equipment is highly expectable. Many of single- and multi-cell cavities have been processed with VEP and performed RF test at Cornell. VEP’ed multi-cell cavity performance was limited by too much Qo degradation above 25MV/m before. But in the analysis of removal vs. field gradient against previous VEP’ed cavity tests, we have found a tendency that small removal as final etches could provide high gradient without Qo degradation. The ILC 9-cell cavity “ACCEL9” with TESLA shape processed by VEP at Cornell achieved Eacc of about 40MV/m by minimizing final removal. This is the first 9-cell cavity that achieved ILC base-line specification (Qo=1.0e10 at 31.5MV/m, Qo=8.0e9 at 35MV/m) by VEP in the world. We have also fabricated demountable half-cell coupon cavity. This cavity consists of Nb half-cell and dummy cavity assemblies. This coupon cavity provides us more information about VEP process details, such as the difference of I-V curve between cell orientations. Four demountable Nb coupons with diameter of 11mm are attached on half-cell and processed. These coupons are very strong tool for further surface analysis to optimize process parameters. We will present our current status of VEP R&D, cavity test results, and fundamental study results with Nb coupons.