Magnesium diboride (MgB 2 ) has a transition temperature (T c ) of 40 K, i.e., about 4 times as high as that of niobium (Nb). We have been evaluating MgB 2 as a candidate material for radio-frequency (RF) cavities for future particle accelerators. Studies in the last 3 years have shown that it could have about one order of magnitude less RF surface resistance (R s ) than Nb at 4 K. A power dependence test using a 6 GHz TE 011 mode cavity has shown little power dependence up to 12 mT (120 Oe), limited by available power, compared to other high-T c materials such as YBCO. A recent study showed, however, that the power dependence of R s is dependent on the coating method. A film made with on-axis pulsed laser deposition (PLD) has showed rapid increase in R s compared to the film deposited by reactive evaporation method. This paper shows these results as well as future plans.

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Magnesium diboride (MgB 2 ) is a superconducting material that has a transition temperature (T c ) of ~40 K, which is ~30 K higher than niobium (Nb) that has been used for most superconducting RF cavities in the past decades. Last year, it was demonstrated that the RF surface resistance of MgB 2 can be lower than Nb at 4 K.One of the problems with other high-T c materials such as YBCO was its rapid increase in RF surface resistance with higher surface magnetic fields. Recently, we have shown that MgB 2 shows little increase in the surface resistance up to ~120 Oe, equivalent of an accelerating field of ~3 MV/m. The highest field tested was limited by available power. This result is encouraging and has made us consider fabrication of a cavity coated with MgB 2 and test it. Also, there is a potential that this material has a higher critical magnetic field that enables the cavity to run at a higher gradient than Nb cavities in addition to the possibility of operation at higher temperatures.

As the surface magnetic field in niobium cavities approaches the theoretical critical field, RF losses grow sensitive to increasingly subtle features of the material and the surface. A striking example is the familiar onset of the high field Q-slope, where RF losses increase exponentially with field. A surprising feature of the high field Q-slope is its positive response to a mild baking at 100-120 • C. But the Q-slope returns after the first 20 nm of the niobium metal surface is converted to loss-less pentoxide via anodization. The latter result suggests that the cause of the fast growing losses resides in the first 20 nm of the RF surface. Although there are several propositions, the exact mechanism for the high field Q-slope is not yet fully understood and demands further research. We are conducting surface analytic studies with optical profilometry, EBSD, XPS, Auger and SIMS to shed light on the mechanism of the high field Q-slope. We are comparing the behavior of fine-grain niobium with single crystal niobium, buffered chemical polishing (BCP) treatments with electropolishing (EP) treatments and properties before and after 110 • C bake. Our approach is based on identifying lossy regions, dissecting of these regions and range of analysis.

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