  A detailed thermal analysis of a Niobium (Nb) based superconducting radio
frequency (SRF) cavity in a liquid helium bath is presented, taking into
account the temperature and magnetic field dependence of the surface resistance
and thermal conductivity in the superconducting state of the starting Nb
material (for SRF cavity fabrication) with different impurity levels. The drop
in SRF cavity quality factor (Q_0) in the high acceleration gradient regime
(before ultimate breakdown of the SRF cavity) is studied in details. It is
argued that the high field Q_0-drop in SRF cavity is considerably influenced by
the intrinsic material parameters such as electrical conductivity, and thermal
diffusivity. The detail analysis also shows that the current specification on
the purity of niobium material for SRF cavity fabrication is somewhat over
specified. Niobium material with a relatively low purity can very well serve
the purpose for the accelerators dedicated for spallation neutron source (SNS)
or accelerator driven sub-critical system (ADSS) applications, where the
required accelerating gradient is typically up to 20 MV/m,. This information
will have important implication towards the cost reduction of superconducting
technology based particle accelerators for various applications.
