  Bulk niobium is the material mostly used in RF superconducting cavities for
accelerator. Predicting and reducing the surface dissipation in RF is
mandatory, since it has a tremendous cost impact on most of the large
accelerator projects. The theoretical approach of superconducting
radiofrequency (SRF) behavior has been far less explored than DC behavior and
is still based on the description of relatively simple systems, quite remote
from the realistic material in use. Nevertheless, because the actual
crystalline substructure is not taken into account, it is still difficult to
predict surface dissipation accurately. Moreover, Niobium with its large lambda
(~40 nm), exhibit an original behavior compared to the usual superconductors
used in applied superconductivity, and generalities (e.g. pinning at grain
boundaries) needs to be reconsidered. In this paper we hope to demonstrate that
sources of dissipation usually attributed to external causes (mainly flux
trapping during cooldown and hydrides precipitates) are related to the same
type of crystalline defects which are not grain boundaries; that they affect
the local superconducting properties and can also be at the source of early
vortex penetration at the surface. We want also to stretch out how those
defects can explain some of the discrepancies observed from lab to lab on
recent results obtained i.e. in doping experiments. Understanding the origin
and the role of these defects could provide indications for updating
specification as well as fabrication follow up. Rather than new results the
author wishes to present to the superconducting accelerators community the
synthesis of multiple experimental results scattered in the literature that
provide a new lighting on recently published work, in particular in the domain
of SRF cavity doping as well as issues such as sensitivity to trapped flux
during cooldown.
