Measurement of the high-field Q drop in the TM_{010} and TE_{011} modes in a niobium cavity

• Main Authors: Gianluigi Ciovati, Peter Kneisel
• Format: Article
• Language: English
• Published: American Physical Society 2006-04-01
• Series: Physical Review Special Topics. Accelerators and Beams
• Online Access: http://doi.org/10.1103/PhysRevSTAB.9.042001

In the last few years superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio>200) niobium achieved accelerating gradients close to the theoretical limits. An obstacle towards achieving reproducibly higher fields is represented by “anomalous” losses causing a sharp degradation of the cavity quality factor when the peak surface magnetic field (B_{p}) is above about 90 mT, in the absence of field emission. This effect, called “Q drop” has been measured in many laboratories with single- and multicell cavities mainly in the gigahertz range. In addition, a low-temperature (100–140 °C) “in situ” baking of the cavity was found to be beneficial in reducing the Q drop. In order to gain some understanding of the nature of these losses, a single-cell cavity has been tested in the TM_{010} and TE_{011} modes at 2 K. The feature of the TE_{011} mode is to have zero electric field on the cavity surface, so that electric field effects can be excluded as a source for the Q drop. This article will present some of the experimental results for different cavity treatments and will compare them with existing models.