A. Illiberi

Experimental investigation of the ion resonance instability in a trapped electron plasma

An investigation of the ions induced diocotron instability in an electron plasma confined in a Malmberg–Penning trap is presented. The detection of the instability is based on the spectral analysis of the induced charge signals on the walls of the confining electrodes, which allows tracking of the plasma displacement from the axis. The dependence of the instability on the electron energy is analysed by three different methods: (i) injecting electrons with different energies, (ii) heating the electrons with a single radio frequency burst, (iii) varying the ramp-up time of the confining voltage. An experimental technique to limit the ion resonance instability, based on the application of suitable potentials on a set of electrodes, is presented.

Experimental investigation of coherent structures in a low-energy electron beam

A sharp transition to a space-charge dominated regime is induced in a low-energy electron beam produced in a Malmberg–Penning trap by increasing the emission current of the source. The transition is characterized by the appearance of a region, around the axis of the beam, not accessible to beam electrons, and by the fast development of coherent structures in the remaining electron plasma, due to the sharp increase of local vorticity. The results are interpreted in the framework of a cold fluid drift–Poisson model, and using a three-dimensional particle-in-cell simulation code.

Experimental and numerical analysis of the electron injection in a Malmberg-Penning trap

The injection phase in a Malmberg-Penning trap is investigated both experimentally in the ELTRAP [M. Amoretti et al., Rev. Sci. Instrum. 74, 3991 (2003)] device, and numerically. The resulting plasma density distribution is studied by varying the source parameters, the external magnetic field strength, and the axial position of the external potential barrier. Space charge phenomena dominate the dynamics of the system; formation of hollow plasma columns and three-dimensional structures are observed. The processes are interpreted using a three-dimensional particle-in-cell code which solves the drift-Poisson system.

Active control of the ion resonance instability by ion removing fields

The off-axis bulk rotation (l=1 diocotron mode) of an electron plasma column confined in a Malmberg-Penning trap is strongly destabilized by a small population of positive ions formed by energetic electron-neutral collisions. The instability, known as ion resonance instability, drives the plasma against the wall, destroying the confinement. A new experimental technique based on the static or time dependent application of low voltages to the inner conductors of the trap is shown to be effective in controlling the instability. The efficiency of the control technique is experimentally investigated by a systematic variation of the amplitudes, time duration, and periodicity of the additional potentials.

INVESTIGATION OF FREE DECAYING TURBULENCE IN A TRAPPED PURE ELECTRON PLASMA

An electron plasma confined in a Malmberg-Penning trap can be a good experimental setup for the study of the two-dimensional fluid dynamics, since a magnetized plasma in this geometry behaves like an eulerian fluid in a wide range of experimental conditions. Plasma turbulence is triggered by the diocotron instability. Here, the results of a Fourier spectral analysis of the energy and enstrophy distributions are reported and interpreted using theoretical models of two dimensional turbulence.

Analysis of the electron injection in a Malmberg-Penning trap

The phase of injection of the electrons in a Malmberg‐Penning trap is investigated both experimentally (in the ELTRAP device) and numerically. The resulting plasma density distribution is studied by varying the source parameters, the external magnetic field strength, the time of injection of the electrons and the axial position of the external potential barrier. The observed processes are interpreted using a three‐dimensional particle‐in‐cell code which solves the drift‐Poisson system where kinetic effects in the motion parallel to the magnetic field are taken into account.

Active control of the ion-resonance instability in a trapped pure electron plasma

The fundamental diocotron mode in a pure electron plasma confined in a Malmberg‐Penning trap is destabilized when a small population of positive charges is produced (transient ion resonance instability). The electron column undergoes a bulk rotation around the axis of the trap (m = 1 diocotron mode) with an increasing offset, until it reaches the confining walls, loosing particles. The instability has been analyzed using both optical and electrostatic diagnostics, and has been controlled by the application of a proper external electric field. This technique is effective in removing the instability and increasing the plasma meanlife.

Coherent structures in the ELTRAP experiment

The formation and evolution of coherent structures in an electron plasma is investigated in the Malmberg‐Penning trap ELTRAP in different regimes of operation, using a CCD diagnostic system. In the trapped plasma configuration the experimental results are in agreement with the two dimensional Euler fluid behavior of the (bounce averaged) electron columns, as found in the literature. In the beam regime, where a low energy electron plasma continuously flows from the emitting thermionic cathode to a collector, a rapid development of coherent structures is found when a part of the beam is reflected back to the source, due to space charge effects. The convective evolution of the structures along the beam is studied by varying the strength of the confined magnetic field. The observed nonlinear processes are interpreted using a tridimensional particle‐in‐cell code which solves the fluid, drift‐Poisson system.

Coherent Structures in low Energy Electron Beams in ELTRAP

The experimental investigation of the formation of coherent structures in an electron beam of very low energy produced in a Malmberg‐Penning trap is presented. A rapid development of the structures is observed when, by increasing the emission current of the cathode, a sharp transition to a space charge dominated regime occurs, where a region unaccessible to beam electrons is found around the axis, and is characterized by a sharp density gradient at the edge. The transport of the structures along the beam is studied by varying the strength of the axial magnetic field. A 3D PIC simulation code is used to interpret the results. A modification of the configuration is under way, in which a pulsed electron source is used, and plasma effects in the beam transport, of relevance for X‐ray SASE‐FELs, can be investigated.