In accordance with one embodiment, the hollow cathode is comprised of a
first tantalum tube, tantalum foil, and a second tantalum tube. The foil
is in the form of a spiral winding around the outside of the first tube
and is held in place by the second tube, which surrounds the foil. One
end of the second tube is approximately flush with one end of the first
tube. The other end of the second tube extends to a cathode support
through which the working gas flows. To start the cathode, a flow of
ionizable inert gas, usually argon, is initiated through the hollow
cathode and out the open end of the first tube. An electrical discharge
is then started between an external electrode and the first tube. When
the first tube is heated to operating temperature, electrons are emitted
from the open end of the first tube.