R. Pozzoli

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.

MHD simulations of plasma dynamics in pinch discharges in capillary plasmas

Magnetohydrodynamic simulation results related to the capillary discharge dynamics are presented. The main physical process that should be taken into account is the ablation of the capillary wall material evaporated by the heat flux from the capillary plasma. The possible applications of the capillary discharges related to the physics of the X-ray lasers and the use of the capillary plasma to provide a guiding for ultrashort high-intensity laser pulses over a distance greater than the defocusing length are discussed.

Asymmetric vortex merger: Experiments and simulations

The two-dimensional (2-D) merging of an intense, pointlike vortex with a diffuse, extended vortex is investigated with experiments using strongly magnetized electron columns in a Malmberg–Penning trap, and with numerical simulations using a 2-D particle-in-cell code. The study is restricted to highly nonlinear conditions, where the perturbative approach does not apply. A very good agreement between experiment and simulation is obtained. The pointlike vortex wraps the extended vortex about itself, moving toward the center of the system during the process. The interaction generates filaments of zero vorticity within the extended vortex that subsequently evolve into vorticity holes. During the evolution, energy is fed to the extended vortex from the background curl-free flow via the stirring action of the pointlike vortex, whose energy remains approximately constant.

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.

Nonlinear electron‐cyclotron power absorption

The process of nonlinear power absorption of a localized electron‐cyclotron wave is investigated on the basis of the analysis of the electron motion in the wave field, by means of a relativistic Hamiltonian formalism under adiabatic conditions, for a wave frequency close to a cyclotron harmonic, and in perpendicular propagation. The computation of the absorbed power density relies on the investigation of the electron energy transitions due to a single crossing of the radiation beam. The absorbed power profiles are explicitly given for the ordinary mode at the first cyclotron harmonic and the extraordinary mode at the second cyclotron harmonic, and are shown to be in good agreement with those obtained by numerical simulation codes. The behavior of the absorbed power versus the frequency and the amplitude of the wave is interpreted by means of the main features of the nonlinear interaction process in momentum space.

Diocotron modulation in an electron plasma through continuous radio-frequency excitation

The application of a radio-frequency (RF) excitation to any electrode of a Penning-Malmberg trap may result in significant electron heating and ionization of the residual gas with the formation of a plasma column when the RF frequency is of the order or larger than the typical axial bounce frequencies of few-eV electrons. The use of a quadrupolar excitation can induce additional phenomena, like formation of dense, narrow-cross section columns which exhibit an mθ=1 diocotron mode, i.e., a rotation of their center around the trap axis. A series of experiments is presented and discussed showing that the continuous application of such excitation causes a dramatic perturbation of the plasma equilibrium also involving continuous production and loss of particles in the trapping region. In particular, the growth of the first diocotron mode is suppressed even in the presence of ion resonance and resistive instability and the mode exhibits steady-state or underdamped amplitude and frequency modulations, typically i...

Broadband radio frequency plasma generation in a Penning–Malmberg trap

Plasma generation is observed in a cylindrical Penning-Malmberg trap in the ultra-high vacuum pressure regime for a large bandwidth of low-power radio frequency (RF) excitations. The process of plasma formation is investigated by measuring the density profiles and a simplified model is developed to characterize the electron heating mechanism. The total charge confined at equilibrium is systematically studied for RF drives in the 0.1-20 MHz range and with different geometrical configurations. With electron densities of some 10 6 cm -3 at least, the scheme represents an alternative source of non-neutral plasmas for Penning-Malmberg traps.

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.