In the last few years superconducting radio-frequency cavities made of high purity (RRR>200) niobium achieved accelerating gradients close to the theoretical limits. An obstacle towards reproducibly achieve higher fields is represented by some anomalous losses causing a sharp degradation of the cavity quality factor when the peak surface magnetic field is above about 90 mT, in the absence of field emission. This effect, called Q-drop has been measured in many laboratories on single- and multi-cell cavities mainly in the gigahertz range. In order to gain some understanding of the nature of these losses, a CEBAF single cell cavity has been tested in the TM010 and TE011 modes at 2 K. The feature of the TE011 mode is to have zero electric field on the cavity surface, allowing to exclude any electric field effect on the Q-drop. This paper will present some of the experimental results for different cavity treatments and will compare them with existing models for the Q-drop.
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