G. Maero

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.

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.

Stabilizing effect of a nonresonant radio frequency drive on the m=1 diocotron instability

It has been experimentally shown that the rotation radius of a non-neutral plasma column around the longitudinal axis of a Malmberg–Penning trap experiences a growth in amplitude (m=1 diocotron instability), leading to the loss of the plasma on the surface of the confining electrodes. A new stabilization mechanism has been investigated with the help of systematic experiments in the ELTRAP (ELectron TRAP) device where a high-frequency, low-amplitude drive has been applied on an azimuthally sectored electrode. An effective confining force is created, which reduces the offset of the column from the center. This interpretation and its theoretical analysis show a qualitative agreement with the experimental findings, where a net confinement effect is present for a wide range of drive amplitudes and frequencies.

Electrostatic diagnostics of nanosecond pulsed electron beams in a Malmberg–Penning trap

A fast electrostatic diagnostic and analysis scheme on nanosecond pulsed beams in the keV energy range has been developed in the Malmberg-Penning trap ELTRAP. Low-noise electronics has been used for the detection of small induced current signals on the trap electrodes. A discrete wavelet-based procedure has been implemented for data postprocessing. The development of an effective electrostatic diagnostics together with proper data analysis techniques is of general interest in view of deducing the beam properties through comparison of the postprocessed data with the theoretically computed signal shape, which contains beam radius, length, and average density as fit parameters.

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.

Axial heating and temperature of RF-excited non-neutral plasmas in Penning-Malmberg traps

Electro-magnetostatic traps have been used for decades to provide long-term storage of charged particle samples or non-neutral plasmas. The dynamics and equilibrium states of these ideally simple systems can be strongly diverted from the usual working conditions (i.e. single-species, quiescent samples) in the presence of oppositely charged particles or external electric field perturbations. Both these conditions occur when the plasma is generated by means of a radio-frequency (RF) excitation continuously applied on a trap electrode. The application of RF drives of some volts over periods larger than typical collisional time scales leads to residual-gas ionization and to the accumulation of an electron plasma, a process that has previously been exploited as an alternative to thermionic or photoemission electron sources. The analysis of the axial energy distribution shows a deviation of the continuously excited final state from maxwellianity dependent on the radial position and the subsequent relaxation to equilibrium after the interruption of the drive. Systematic measurements also indicate the high sensitivity to the residual gas pressure of both the total confined charge and of the attainable densities and plasma profiles. The results are compared to the information obtained from a very simple one-dimensional electron heating model and show the validity of its most basic features together with its shortcomings.

Effect of initial conditions on electron–plasma turbulence: a multiresolution analysis

The transverse dynamics of a pure electron plasma confined in a Penning–Malmberg trap is investigated, taking advantage of two-dimensional particle-in-cell numerical simulations. The evolution of the electron plasma turbulence is studied by means of a wavelet-based multiresolution analysis. In particular, a modified recursive denoising algorithm is developed to separate coherent and incoherent (not necessarily homogeneous) components of the flow. A set of simulations have been carried out changing systematically the radii of an initial annular density distribution of the electrons. The results of the multiresolution analysis indicate that the initial density configuration may have a considerable effect on the evolution of turbulence. Even very small initial density fluctuations can lead to quite different final states, especially in the presence of multiple active diocotron modes characterized by similar growth rates.

Experimental and numerical investigation of non-neutral complex plasmas

A plasma of particles with the same sign of charge, can be easily confined under ultra-high vacuum conditions in Penning-Malmberg traps, where the time evolution of the system is monitored for very long times by means of electrostatic and optical diagnostic systems. Complex (dusty) plasmas are ionized gases that contain a distribution of micrometer-sized particles with a surface charge of the order of a few thousand electron charges. The interplay between a wide range of scales in time and space gives rise to new characteristic physical phenomena. Laboratory complex plasmas generally satisfy a global (quasi-)neutrality condition. A different concept is represented by a non-neutral complex plasma. To investigate the dynamics of this system, we are currently developing the DuEl (Dust-Electron) device, where negatively charged dust particles will be present together with a population of electrons. The experimental set-up will include a dust injection system and a Penning-Malmberg trap for the confinement of ...

Thomson backscattering diagnostic set-up for the study of nanosecond electron bunches in high space-charge regime

A Thomson backscattering diagnostic apparatus was set up in the ELTRAP (ELectron TRAP) device for the measurement of density as well as longitudinal energy and energy spread of nanosecond electron bunches traveling through a longitudinal magnetic field in a dynamical regime where space-charge effects play a significant role. We discuss the main features of the apparatus as well as the challenging technical issues of the backscattering experiment. In particular, we detail the technical solutions and the characterization of both bunch and laser injection in the vacuum chamber, photon detection, space and time coincidence of electron and laser pulses. Then we summarize the results with an estimate of the minimum sensitivity of the set-up.