Joan Marler

Monte Carlo simulation of non-conservative positron transport in pure argon

The main aim of this paper is to apply modern phenomenology and accurate Monte Carlo simulation techniques to obtain the same level of understanding of positron transport as has been achieved for electrons. To this end, a reasonably complete set of cross sections for low energy positron scattering in argon has been used to calculate transport coefficients of low energy positrons in pure argon gas subject to an electrostatic field. We have analyzed the main features of these coefficients and have compared the calculated values with those for electrons in the same gas. The particular focus is on the influence of the non-conservative nature of positronium formation. This effect is substantial, generally speaking much larger than any comparable effects in electron transport due to attachment and/or ionization. As a result several new phenomena have been observed, such as negative differential conductivity (NDC) in the bulk drift velocity, but with no indication of any NDC for the flux drift velocity. In addition, there is a drastic effect on the bulk longitudinal diffusion coefficient for positrons, which is reduced to almost zero, in contrast to the other components of the diffusion tensor, which have normal values. It is found that the best way of explaining these kinetic phenomena is by sampling real space distributions which reveal drastic modification of the usual Gaussian profile due to pronounced spatial differentiation of the positrons by energy.

Noninvasive Vibrational Mode Spectroscopy of Ion Coulomb Crystals through Resonant Collective Coupling to an Optical Cavity Field

We report on a novel noninvasive method to determine the normal mode frequencies of ion Coulomb crystals in traps based on the resonance enhanced collective coupling between the electronic states of the ions and an optical cavity field at the single photon level. Excitations of the normal modes are observed through a Doppler broadening of the resonance. An excellent agreement with the predictions of a zero-temperature uniformly charged liquid plasma model is found. The technique opens up for investigations of the heating and damping of cold plasma modes, as well as the coupling between them.

Positron transport in water vapour

Transport properties of positron swarms in water vapour under the influence of electric and magnetic fields are investigated using a Monte Carlo simulation technique and a multi-term theory for solving the Boltzmann equation. Special attention is paid to the correct treatment of the non- conservative nature of positronium (Ps) formation and its explicit and implicit influences on various positron transport properties. Many interesting and atypical phenomena induced by these influences are identified and discussed. Calculated transport properties for positrons are compared with those for electrons, and the most important differences are highlighted. The significant impact of a magnetic field on non-conservative positron transport in a crossed field configuration is also investigated. In general, the mean energy and diffusion coefficients are lowered, while for the measurable drift velocity an unexpected phenomenon arises: for certain values of the reduced electric field, the magnetic field enhances the drift. The variation of transport coefficients with the reduced electric and magnetic fields is addressed using physical arguments with the goal of understanding the synergistic effects of Ps formation and magnetic field on the drift and diffusion of positrons in neutral gases. 6 Author to whom any correspondence should be addressed.

Collective strong coupling between ion Coulomb crystals and an optical cavity field: Theory and experiment

A detailed description and theoretical analysis of experiments achieving coherent coupling between an ion Coulomb crystal and an optical cavity field are presented. The various methods used to measure the coherent coupling rate between large ion Coulomb crystals in a linear quadrupole radiofrequency ion trap and a single-field mode of a moderately high-finesse cavity are described in detail. Theoretical models based on a semiclassical approach are applied in assessment of the experimental results of P. F. Herskind et al. [Nature Phys. 5, 494 (2009)] and of complementary new measurements. Generally, a very good agreement between theory and experiments is obtained. DOI: 10.1103/PhysRevA.85.023818

Positron transport: the plasma-gas interface

Motivated by an increasing number of applications, new techniques in the analysis of electron transport have been developed over the past 30 years or so, but similar methods had yet to be applied to positrons. Recently, an in-depth look at positron transport in pure argon gas has been performed using a recently established comprehensive set of cross sections and well-established Monte Carlo simulations. The key novelty as compared to electron transport is the effect of positronium formation which changes the number of particles and has a strong energy dependence. This coupled with spatial separation by energy of the positron swarm leads to counterintuitive behavior of some of the transport coefficients. Finally new results in how the presence of an applied magnetic field affects the transport coefficients are presented.

Experimental realization of nearly steady-state toroidal electron plasmas

Electron plasmas with densities of 5×106 cm−3 are trapped in the Lawrence Non-neutral Torus II (LNT II) for times exceeding 1 s. LNT II is a high aspect ratio (R0/a≳10) partially toroidal trap (270° arc, B0=670 G). The m=1 diocotron mode is launched and detected using isolated segments of a fully sectored conducting boundary and its frequency is used to determine the total trapped charge as a function of time. The observed confinement time (≈3 s) approaches the theoretical limit (≈6 s) set by the magnetic pumping transport mechanism of Crooks and O’Neil [Phys. Plasmas 3, 2533 (1996)]. We also present equilibrium modeling and numerical simulations of the toroidal m=1 mode constrained by experimental data. Future work includes the identification of the dominant transport mechanisms via confinement scaling experiments and measurement of the m=2 mode frequency and development of a strategy for making a transition to fully toroidal confinement.

On new developments in the physics of positron swarms

Recently a new wave of swarm studies of positrons was initiated based on more complete scattering cross section sets. Initially some interesting and new physics was discovered, most importantly negative differential conductivity (NDC) that occurs only for the bulk drift velocity while it does not exist for the flux property. However the ultimate goal was to develop tools to model positron transport in realistic applications and the work that is progressing along these lines is reviewed here. It includes studies of positron transport in molecular gases, thermalization in generic swarm situations and in realistic gas filled traps and transport of positrons in crossed electric and magnetic fields. Finally we have extended the same technique of simulation (Monte Carlo) to studies of thermalization of positronium molecule. In addition, recently published first steps towards including effects of dense media on positron transport are summarized here.

Using numerical simulations to extract parameters of toroidal electron plasmas from experimental data

Measurements of the image charge induced on electrodes provide the primary means of diagnosing plasmas in the Lawrence Non-neutral Torus II (LNT II) [Phys. Rev. Lett. 100, 155001 (2008)]. Therefore, it is necessary to develop techniques that determine characteristics of the electron plasma from features of the induced image charge signal. This paper presents a numerical study which finds that the frequency of the image charge signal due to the toroidal version of the m=1 diocotron mode is proportional to the total trapped charge and inversely proportional to magnetic field strength, as in the cylindrical case. In the toroidal case, additional information about the m=1 motion of the plasma can be obtained by analysis of the image charge signal amplitude and shape. Finally, results from the numerical simulations are compared to experimental data from the LNT II and plasma characteristics are reported.

Low energy positron interactions - trapping, transport and scattering

Recent experiments, theory and modelling of positron interactions with atoms and molecules are discussed. The first half of the paper is devoted to binary collisions between positrons and crossed beams of atoms or molecules (in this case neon) and the second half deals with ensembles of non-interacting positrons, otherwise known as swarms which are transported through the background gas. We review the recent results on measurements of the cross sections based on obtained from collisional positron traps and subsequent calculations of transport properties of positron swarms that may be used to model thermalization experiments, collisional traps and possible applications of positrons in materials science and biomedicine. It was found that kinetic phenomena occur in positron transport that are mainly the result of the positronium (Ps) formation which has a larger cross section than elastic scattering in most gases and at the same time is a non-conservative process. Most importantly negative differential conductivity (NDC) occurs only for the bulk drift velocity while it does not exist for the flux property, a phenomenon that has not been observed for electrons.

Cavity QED experiments with ion Coulomb crystals

Cavity QED experimental results demonstrating collective strong coupling between ensembles of atomic ions cooled into Coulomb crystals and optical cavity fields have been achieved. Collective Zeeman coherence times of milliseconds have furthermore been obtained.