High Power RF Processing Studies of 3 GHz Niobium Superconducting Accelerator Cavities

Joel H. Graber, Ph.D.

Cornell University 1993

Abstract

Superconducting radio-frequency (SRF) cavities are a promising technology for future high energy particle accelerators. SRF technology is presently limited by field emission (FE), a quantum mechanical tunneling effect wherein electrons are emitted from a metal surface into vacuum in the presence of a strong electric field. A systematic study is presented of the effects of pulsed high power RF processing (HPP) as a method of understanding and reducing field emission in SRF cavities. The HPP experimental apparatus was built to provide up to 200 kW peak RF power to 3 GHz cavities, for pulse lengths of hundreds of microseconds. Single-cell, two-cell, and nine-cell cavities were tested extensively. HPP proved to be a highly successful method of reducing FE loading in SRF cavities. Attainable continuous wave (CW) fields are increased by as much as 80% from their pre-HPP limits. Analysis of HPP results and data increases our understanding of the nature of RF processing. Clear correlations are obtained linking FE reduction with the maximum electric field attained during processing. Analysis of the pulsed behavior of the cavities indicates that thermal breakdown, initiated by high surface magnetic fields, is a dominant limitation on the attainable fields. A thermal model is developed which accurately predicts the limitations. A special two-cell cavity with a reduced magnetic to electric field ratio is successfully tested. During HPP, pulsed fields reach E _peak= 113 MV/m ( H _peak= 1600 Oe), and subsequent low power measurement reaches E _peak= 100 MV/m ( H _peak= 1420 Oe), the highest CW field ever measured in a superconducting accelerator cavity. Additional studies improve our understanding of the microscopic effect of HPP. Thermometry measurements of the outer wall of single-cell cavities reveal that processing gains are made by reduction in emission from localized sites. Scanning electron microscope examination of RF surfaces reveals craters and other phenomena which indicate that processing occurs through a violent melting/vaporization phenomenon. A "model" for RF processing is presented based upon the experimental evidence, both from this study and from other related experiments.

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* Complete Dissertation (PDF format, 2.7 MB)

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