A coupler for coupling a flexible coaxial cable, a fluid cooling system and the outer sheath of a catheter, the coupler comprising; fluid coupler body including: a fluid inlet formed in the fluid coupler body and configured to operably couple to a source of cooling fluid and receive fluid therefrom; a fluid outlet formed in the fluid coupler body and configured to operably couple to a fluid discharge; a bypass bulb forming an aperture for slidably coupling with a coaxial cable; and an outer sheath coupler forming an aperture for coupling with an outer sheath of a catheter while forming a fluid-tight seal therewith; a fluid sealing system housed in the fluid coupler body including: a distal sealing diaphragm configured to form a fluid-tight seal about an outer surface of an inflow lumen and a fluid-tight seal with an interior surface of the fluid coupler body defining an outflow plenum in fluid communication with the fluid outlet, the outflow plenum formed between a distal interior surface of the fluid coupler body, the outer surface of the inflow lumen, a distal side of the distal sealing diaphragm and the outer sheath coupler; a proximal sealing diaphragm configured to form a fluid- tight seal about an outer surface of the coaxial cable and a fluid- tight seal with an interior surface of the fluid coupler body thereby forming an inflow plenum in fluid communication with the fluid inlet, the inflow plenum formed between a proximal interior surface of the fluid coupler body, and a proximal side of the distal sealing diaphragm, and a distal side of the proximal sealing diaphragm. a fluid inlet formed in the fluid coupler body and configured to operably couple to a source of cooling fluid and receive fluid therefrom; a fluid outlet formed in the fluid coupler body and configured to operably couple to a fluid discharge; a bypass bulb forming an aperture for slidably coupling with a coaxial cable; and an outer sheath coupler forming an aperture for coupling with an outer sheath of a catheter while forming a fluid-tight seal therewith; a fluid sealing system housed in the fluid coupler body including: a distal sealing diaphragm configured to form a fluid-tight seal about an outer surface of an inflow lumen and a fluid-tight seal with an interior surface of the fluid coupler body defining an outflow plenum in fluid communication with the fluid outlet, the outflow plenum formed between a distal interior surface of the fluid coupler body, the outer surface of the inflow lumen, a distal side of the distal sealing diaphragm and the outer sheath coupler; a proximal sealing diaphragm configured to form a fluid- tight seal about an outer surface of the coaxial cable and a fluid- tight seal with an interior surface of the fluid coupler body thereby forming an inflow plenum in fluid communication with the fluid inlet, the inflow plenum formed between a proximal interior surface of the fluid coupler body, and a proximal side of the distal sealing diaphragm, and a distal side of the proximal sealing diaphragm. a distal sealing diaphragm configured to form a fluid-tight seal about an outer surface of an inflow lumen and a fluid-tight seal with an interior surface of the fluid coupler body defining an outflow plenum in fluid communication with the fluid outlet, the outflow plenum formed between a distal interior surface of the fluid coupler body, the outer surface of the inflow lumen, a distal side of the distal sealing diaphragm and the outer sheath coupler; a proximal sealing diaphragm configured to form a fluid- tight seal about an outer surface of the coaxial cable and a fluid- tight seal with an interior surface of the fluid coupler body thereby forming an inflow plenum in fluid communication with the fluid inlet, the inflow plenum formed between a proximal interior surface of the fluid coupler body, and a proximal side of the distal sealing diaphragm, and a distal side of the proximal sealing diaphragm.