Radio-frequency generation of an electron plasma in a Malmberg-Penning trap and its interaction with a stationary or pulsed electron beam.

Experiments and numerical investigations on trapped electron plasmas and traveling electron bunches are discussed. A Thomson backscattering diagnostics set up was installed in the ELTRAP (Electron TRAP) device, a Penning-Malmberg trap operating at the Department of Physics of the University of Milano since 2001. Here, an infrared (IR) laser pulse collides with nanosecond electron bunches with an energy of 1-20 keV traveling through a longitudinal magnetic field in a dynamical regime where space-charge effects play a significant role. The backscattered radiation is optically filtered and detected by means of a photomultiplier tube. The minimum sensitivity of the backscattering diagnostics has been estimated for the present set-up configuration. Constraints on the number of photons and thus on the information one can obtain with the Thomson backscattering technique are determined by the relatively low density of the electron beam as well as by noise issues. Solutions to increase the signal level and to reduce the noise are briefly discussed. The generation of an electron plasma by stochastic heating was realized in ELTRAP under ultra-high vacuum conditions by means of the application of low power RF (1-20 MHz) drives on one of the azimuthally sectored electrodes of the trap. The relevant experimental results are reviewed. The electron heating mechanism has been studied by means of a two-dimensional (2D) particle-in-cell (PIC) code, starting with a very low electron density, and applying RF drives of various amplitudes in the range 1-15 MHz on different electrodes. The axial kinetic energy of the electrons is in general increasing for all considered cases. Of course, higher temperature increments are obtained by increasing the amplitude of the RF excitation. The simulation results indicate in particular that the heating is initially higher close to the cylindrical wall of the device. These results on the electron heating point in the same direction of the experimental findings, where the plasma formation due to the ionization of the residual gas is found to be localized close to the trap wall. The simulations indicate also major heating effects when the RF drive is applied close to one end of the trap. Similar results are obtained for an electron plasma at higher densities, simulating a situation in which the RF is applied to an already formed plasma. With the aim to extend these RF studies to the microwave range, a bench test analysis has been performed of the transmission efficiency of a microwave injection system up to a few GHz. The test was based on the use of a prototype circular waveguide with the same diameter and length of the ELTRAP electrode stack and of a coupled rectangular waveguide with dimensions suitable for a future installation in the device. Electromagnetic PIC simulations have also been performed of the electron heating effect, again both at very low and relatively high electron densities, applying a microwave drive with a frequency of approximately 3 GHz close to the center and close to one end of the trap. Both the bench test of the injection system and the numerical simulations indicate that the new microwave heating system will allow the extension of the previous RF studies to the GHz range. In particular, the electron cyclotron resonance heating of the electrons will be aimed to increasing the electron temperature, and possibly its density as a consequence of a higher ionization rate of the residual gas. The installation of the new RF system will open up the possibility to study, e.g., the interaction between the confined plasma and traveling electron bunches.