In order to ionize molecular deuterium a photon energy greater than 15.6 eV is required. This is created by radiation from bombardment of a thick wall by energetic electrons, as in x‐ tubes. A high voltage pulse extracts the electrons by field emission from the sharp pointed electrode and accelerates them to the cylindrical anode. The degree of ionization is measured with an ionization cell and has been measured at pressures from 5×10−6up to 5×−4Torr in D at a distance of 70 cm from the source, averaged over the volume of the chamber. This made it possible to start discharges in the T.F.R. (Tonomav de Fontenaya‐Tokomak from as low as 10−5, giving an appreciable increase in the range of working pressures of the T.F.R. experiment.