PURPOSE: To maintain predetermined electron discharging state for long time and to obtain stable electron discharging state, by employing a composition particularly stable for thermal stress on the surface layer of heating wire.

CONSTITUTION: Spiral heating wire 1 is supported at both ends with end hat member 2. Surface layer is formed through carbonization on the surface of wire 1 while having layer composition. In other word W2C and W layers are alternatively laminated. Said surface layer must be constructed such that W2C-layer contributing to electron discharge is thicker than W-layer.