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A major limitation towards achieving the theoretical critical field in niobium superconducting rf cavities is represented by a strong degradation of the quality factor Q 0 starting at peak surface magnetic field of the order of 90 mT (so called “Q-drop”). This anomalous heating of the cavity surface is not accompanied by field emission and seems to be independent of the chemical etching method (Buffered Chemical Polishing or Electropolishing) used to “clean” the cavity. An empirical method to reduce these losses is a low temperature (100-140 ° C) baking of the cavity in ultrahigh vacuum.
An R&D program is currently underway at Jefferson Lab to understand the basic phenomena behind the Q-drop and the benefit of the low temperature baking.
Measurements of surface impedance as function of temperature on cavities before and after baking at several temperatures yield in formations on material parameters such as the energy gap, the penetration depth, the residual resistance, the mean free path and those can be compared with predictions from the BCS theory of superconductivity.
In order to clarify the nature of the Q-drop, a single cell cavity has been tested in the usual TM 010 and in the TE 011 mode. The TE mode is characterized by having zero electric field on the cavity surface, therefore allowing to exclude any electric field effect on the Q-drop. These tests have been repeated for different baking parameters and surface treatments.
The distribution of the losses on the cavity surface will be explored using an array of 576 resistor used to monitor the local temperature of the cavity.
Typical plot of the quality factor Q 0 as function of the peak surface magnetic field for a CEBAF single cell cavity measured at 1.37 K showing a strong degradation of Q 0 at high fields in absence of field emission (red symbols) recovered by baking the cavity at 120 ° C for 48 h (blue symbols).
Quality factor Q 0 as function of the peak surface magnetic field for a CEBAF single cell cavity measured at 1.7 K for the TM 010 mode (blue triangles) and for the TE 011 mode (red squares) showing Q-drop in both modes.