||The effects of low temperature heat
treatment (baking) and electropolishing (EP) on the high-gradient
performance of superconducting niobium cavities are of high interest
for future electron-positron colliders and X ray free electron lasers.
Niobium samples are investigated at 4.2 K using ac susceptibility
measurements. The surface critical field and the shielding currents are
investigated with respect to the duration (12 hours to 28 days)
and the temperature (100 C, 123 C and 144 C) of the UHV heat treatment.
Secondly, the difference between chemically etched and electropolished
samples has been studied. The surface superconductivity in a thin
surface layer is utilized to study the influence of various surface
treatments on the superconducting properties. Electropolishing raises
the surface critical field by about 8%. A further increase by up to 20%
is caused by the baking procedure.