Measurements on the energy loss of a relativistic electron beam in a plasma

The energy transfer from a relativistic electron beam (550 keV, 500 A, 20 ns FWHM) to a plasma (Argon, n_e = 10¹⁷ - 10¹⁸ m^-3) is investigated. The electron beam is injected into a precreated plasma column of 3 m length parallel to a magnetic field (0.16 T). Time resolved measurements of the return current in the system are used to determine phenomenologic collision frequencies. The collision frequencies found are of the order of 10⁸ s^-1, which is much larger than the classical collision frequencies. This points to high levels of turbulence. An important feature of the present experiment is the determination of the energy spectrum of the electron beam after its passage through the plasma. The time resolved energy spectrum of the beam shows rapid fluctuations on a time scale of 10 ns. Beam particles may loose up to 40% of their energy while also their scatter angle with respect to the magnetic field may increase from 30° to 60°. The broadening of the energy spectrum around the initial beam energy leads to the conclusion that electrostatic beam-plasma instabilities are present with a wave electric field E ≈ 1.8 × 10⁶ V/m.