Preliminary studies of electric and magnetic field effects in superconducting niobium cavities

Superconducting cavities made from high purity niobium with RRR > 200 often show pronounced features in the Q vs. E_acc dependence such as a peak at low gradients, a B²-slope at intermediate fields and a steep degradation of Q-values ("Q-drop") at gradients above E_acc ∼ 20 MV/m without field emission loading. Whereas the B²-slope is in line with \´global\´ heating there are still different models to explain the observed "Q-drop". The model of Knobloch, et al., (1999) is based on magnetic field enhancements at grain boundaries in the equator weld region of the cavity and local heating. These grain boundaries become normal conducting, when their critical magnetic field is reached and contribute gradually to the losses in the cavity as long as they are thermally stable. The model proposed by Halbitter, et al., (2001) is based on effects taking place in the metal-oxide interface on the niobium surface. The major contribution to the RF absorption is coming from interface tunnel exchange between electronic states of superconducting Nb with their energy gap and localized states of the dielectric Nb₂O₅. An experimental program was started at JLab to settle the mechanisms behind B²-slope and the Q-drop. A modified CEBAF single cell cavity is excited in either TM₀₁₀ or TE₀₁₁ modes and the Q vs. E_acc dependences are measured as a function of various surface treatments such as BCP, electropolishing, high temperature heat treatment and "in-situ" baking. In addition, a special two-cell cavity was designed, which allows the excitation of the 0- and πmodes of the TM₀₁₀ passband, which "scan" different areas of the cavity surface with high electric and magnetic fields, respectively. This contribution reports about the design and first measurements with both types of cavities.