A coupler for coupling a flexible coaxial cable (32), a fluid cooling system (40) and the outer sheath (35) of a catheter (30), the coupler comprising:a fluid coupler body (845a) including:a fluid inlet (842a) configured to operably couple to a source of cooling fluid and receive fluid therefrom；a fluid outlet (843a) configured to operably couple to a fluid discharge； andan outer sheath coupler (845b) forming an aperture for coupling with an outer sheath (35) of a catheter while forming a fluid-tight seal therewith； anda fluid sealing system (819) housed in the fluid coupler body including:a distal sealing diaphragm (819a) configured to form a fluid-tight seal about an outer surface of an inflow lumen (842) and a fluid-tight seal with an interior surface of the fluid coupler body (845a) defining an outflow plenum (843b) in fluid communication with the fluid outlet (843a)； anda proximal sealing diaphragm (819b) configured to form a fluid-tight seal about an outer surface of the coaxial cable (32) and a fluid-tight seal with an interior surface of the fluid coupler body(845a) thereby forming an inflow plenum (842b) in fluid communication with the fluid inlet (842a).
