The apparatus is a capillary loop evaporator in which the vapor space is the internal volume of a cup shaped evaporator wick with sidewalls in full contact with the outer casing of the evaporator. Liquid is furnished to the wick through thicker wick wall sections, slabs protruding from the liquid-vapor barrier wick, eccentric wick cross sections, or tunnel arteries. The tunnel arteries can also be formed within heat flow reducing ridges protruding into the vapor space. The tunnel arteries can be fed liquid by bayonet tubes or cable arteries, and can be isolated from the heat source with regions of finer wick to impede vapor flow into the liquid. Tunnel arteries also enable separation of the evaporator and the reservoir for thermal isolation and structural flexibility. A wick within the reservoir aids collection of liquid in low gravity applications.