  SRF cavities for particle acceleration are conventionally operated immersed
in a bath of liquid helium at 4.2 K and below. Although this cooling
configuration is practically and economically viable for large scientific
accelerator installations, it may not be so for smaller accelerators intended
for industrial applications such as the treatment of wastewater, sludge, flue
gases, etc. In this paper, we describe a procedure to operate SRF cavities
without liquid helium that can be used to construct electron-beam sources for
industrial applications of electron irradiation (1-10 MeV electron energy). In
this procedure, an elliptical single-cell 650 MHz niobium-tin coated niobium
cavity is coupled to a closed-cycle 4 K cryocooler using high purity aluminum
thermal links. The cryocooler conductively extracts heat (RF dissipation) from
the cavity without requiring liquid helium around the cavity. We present
construction details of this cryocooler conduction-cooling technique and
systematic experiments that have demonstrated ~10 MV/m cw gradient on the
cavity. By straightforward scaling up the cavity length and number of
cryocoolers, the technique will provide the complete range of 1-10 MeV electron
energy for industrial applications.
