Time Resolved Beam Distribution Functions in a Beam-Plasma Experiment

Cyclotron instability and its harmonics appear under certain conditions in injection machines, whether the injection is parallel to the axial magnetic field B0 (Demirkhanov and Alexeff at the Culham Conference) or perpendicular to B0 (DCX, Phoenix experiments). The paper describes measurements on these instabilities, around the transition from the first to the second regime. This experiment is characterized by a rapid alternation of different instabilities and plasma-density and temperature changes with each instability. Investigations of these variations have only recently been forthcoming (Kharchenko, Seidl, Hopman at the Conference on Ionized Gases, Vienna, 1967). Main results of this paper are the distribution functions of the beam electrons, correlated with the different types of instabilities. Drastic changes in the distribution function are found. In the second regime the distribution function of axial energies jumps between a narrow plateau between 0.6 and 1.1 eVb, and a wide plateau extending from 0 to 1.4 eVb. These plateaus develop in about 100 ns. The wide plateau is found in correlation with an instability whose frequency is twice the electron cyclotron frequency. The narrow plateau is established by the electron cyclotron instability. Only the plateau belonging to the electron plasma instability is symmetric around eVb; it has a width of 0.1 eVb. In all three cases the plateaus are found to be flat, independent of their width, and of the radius on which they are measured. In correlation with the cyclotron instability are found beam electrons with transverse energies of the order of 0.3 eVb, indicating the possibility to build up a high-energy electron plasma, directly from beam electrons, in cases where a magnetic bottle is applied. In connection with the experimental data, numerically calculated dispersion diagrams are presented for bounded beam-plasma systems including temperature effects. (author)