Electron beam injection experiments: The beam‐plasma discharge at low pressures and magnetic field strengths

A column of enhanced density plasma, exceeding the density expected from ionization by primary beam electrons, has been observed in a large vacuum system at low magnetic fields (1 to 1.5 G) and low ambient pressures (10⁻⁶ to 10⁻⁵ torr). The peak luminosity of the discharge is about 10 times that of the beam alone, with a radius increase by a factor of 3. In the absence of the discharge, RF emission is observed at 1.1 to 1.2 f_c. A strong band of RF noise with upper frequency cutoff at about f_c is observed in the discharge mode, along with higher frequency noise at or near the plasma frequency. The onset of the plasma discharge is critically dependent on beam current. The present results agree with observations made at much higher densities and magnetic fields in fusion research studies.