M. Cavenago

Plasma-beam traps and radiofrequency quadrupole beam coolers

Two linear trap devices for particle beam manipulation (including emittance reduction, cooling, control of instabilities, dust dynamics, and non-neutral plasmas) are here presented, namely, a radiofrequency quadrupole (RFQ) beam cooler and a compact Penning trap with a dust injector. Both beam dynamics studies by means of dedicated codes including the interaction of the ions with a buffer gas (up to 3 Pa pressure), and the electromagnetic design of the RFQ beam cooler are reported. The compact multipurpose Penning trap is aimed to the study of multispecies charged particle samples, primarily electron beams interacting with a background gas and/or a micrometric dust contaminant. Using a 0.9 T solenoid and an electrode stack where both static and RF electric fields can be applied, both beam transport and confinement operations will be available. The design of the apparatus is presented.

Cylindrical Penning trap for the study of electron plasmas

The ELTRAP device installed at the Department of Physics of the University of Milan is a Malmberg–Penning trap, with a magnetic field up to 0.2 T, equipped with charge coupled device optical diagnostics. It is intended to be a small scale facility for electron plasma and beam dynamics experiments, and in particular for the study of collective effects, equilibrium states, and the formation of coherent structures in these systems. The device features a relatively long solenoid, corrected by 4 shims and 16 dipole coils, in order to obtain a large uniform magnetic field region. The modular electrode design allows several variations of the experimental configuration. The first experiments which assess the operation of the facility are described. Plasma confinement times up to several minutes have been obtained and an electron temperature of 4–8 eV has been measured.

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.

Excitation of the l=2 diocotron mode with a resistive load

The resistive wall instability of the l=2 diocotron mode in a pure electron plasma has been investigated with a systematic variation of the parameters of the external impedance connected to a pair of sectored electrodes. The measured growth rate is well described by a linear perturbation theory of the two-dimensional drift-Poisson system.

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.

Completion of the medium-β section of the ALPI SC-booster at LNL

Abstract The status of the project for the construction of the ALPI superconducting linear accelerator is described, with information on the building, the radiation shield, the beam transport system, the cryogenic system, the machine cryostats, the power and cooling plants, the pulsing system, the beam diagnostic system, the control system, the installation of the resonators in the machine, the niobium QWR development, and the 14.4 GHz electron cyclotron resonance ion source. Details on the plating experience with large scale production of resonators at LNL are also given.

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.

Excitation of the l=2 azimuthal mode in a pure electron plasma

The effect of an externally imposed time-varying quadrupole electrostatic perturbation on a pure electron plasma trapped in a Malmberg-Penning trap is investigated experimentally. A resonance occurs when the frequency of the drive matches the frequency of the azimuthal l=2, kz=0 mode. Very good agreement is found between the experimentally determined resonance frequency and the real part of the frequency calculated from a two-dimensional linear model for a top-hat density profile, while nonlinear deviations for the mode amplitude are found especially close to the resonance.

Characterization of a pulsed electron beam with a planar charge collector

Pulsed electron beams produced by a photocathode source in the 1–10 keV energy range have been experimentally characterized by means of an electrostatic diagnostic system. A Malmberg–Penning trap in an open configuration, equipped with a planar charge collector has been used for the experiments. The relevant physical properties of the beams and their dependence on the injection conditions have been inferred through the numerical analysis of the electric signal measured across the overall load impedance of the charge collector. The indirect measurement technique presented here gives a general method to overcome the resolution limits of capacitive charge collectors.

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.