An X-ray source with a cathode (2) that includes a first wire (4) having optionally thermal loops (12, 14) between an emission loop (10) and first and second ends (6, 8). A spiral second wire (30) is wound around the wire (4) and a low work function coating (32) is provided on both wires. The first and second wires may be of refractory material, such as tungsten, and the low work function coating may include barium oxide.