Study of Coulomb collisions and magneto-ionic propagation effects on incoherent scatter radar measurements at Jicamarca

In this dissertation, Coulomb collisions and magneto-ionic propagation effects on the incoherent scatter radar measurements have been studied and analyzed in detail. The present study aims at modeling radar observations of the equatorial ionosphere carried out at the Jicamarca Radio Observatory (Lima, Peru) using antenna beams pointed perpendicular to the Earth's magnetic field B. A Monte Carlo procedure based on the simulation of charged particle trajectories in a magnetized plasma (with suppressed collective interactions) was developed to account for the effects of Coulomb collisions on the shape of the incoherent scatter spectrum. Statistics of simulated electron and ion trajectories, single-particle ACF's, and associated Gordeyev integrals are utilized in the general framework of incoherent scatter spectrum models (e.g., Kudeki and Milla, 2006) to produce theoretical spectra for different plasma configurations. Our simulation method effectively extends the procedure of Sulzer and Gonz??lez (1999) into three dimensions and is valid for all magnetic aspect angles including the direction perpendicular to B. The 3D trajectories, randomized by Coulomb collisions, are described by a generalized Langevin equation with velocity-dependent friction and diffusion coefficients taken from the standard Fokker-Planck collision model of Rosenbluth et al. (1957). A statistical analysis of the simulated trajectories shows that the ion motion is well modeled as a Brownian-motion process with Gaussian displacement distributions (and constant friction and diffusion coefficients), in which case, an analytical expression for the single-ion ACF can be obtained (e.g., Woodman, 1967). However, the simulated electron motions do not fit a Brownian model because the electron displacement distributions in the direction parallel to B are sharper than a Gaussian. To account for these effects on our incoherent scatter spectrum model, a numerical library of electron statistics in an oxygen plasma (single-electron ACF's) had to be developed. The library spans a set of densities, temperatures, and magnetic fields as needed for Jicamarca F-region applications. The antenna beams used in perpendicular-to-B radar observations at Jicamarca have angular widths of the order of a degree. Within this range of small magnetic aspect angles, different modes of magneto-ionic wave propagation are excited. These characteristic modes vary from linearly polarized in the direction perpendicular to B (Cotton-Mouton regime) to circularly polarized at aspect angles greater than 0.5 deg (Faraday rotation regime). In order to model the magneto-ionic propagation effects on incoherent scatter radar measurements, a computer algorithm based on the Appleton-Hartree equation for electromagnetic wave propagation in a magnetized plasma was developed. Simulation studies show that magneto-ionic propagation effectively modifies the shapes of the radar beams and does have an impact on the incoherent scatter radar measurements because the polarization of the incident and backscattered fields vary as they propagate through the ionosphere. A soft-target radar equation, which incorporates our collisional incoherent scatter spectrum and magneto-ionic propagation models, is formulated to model the radar measurements collected at Jicamarca. Voltages detected by the radar antenna are represented as the beam-weighted sum of ionospheric backscattered signals corresponding to the range of magnetic aspect angle directions illuminated by the antenna beam. This integration is carried out numerically using a finite-element-like integration method that takes advantage of the slow variation of physical parameters in the direction transverse to the geomagnetic field. The resultant radar model is utilized in the inversion of ionospheric parameters in a three-beam radar experiment conducted at Jicamarca. The experiment interleaves radar observations with perpendicular-to-B and off-perpendicular antenna beams. The data model matches very closely the different features of the measured data; for instance, it predicts the enhancement of the measured power in the direction perpendicular-to-B at ionospheric altitudes where the electron temperature is greater than the ion temperature. F-region electron density and temperature ratio (T_e/T_i) estimates were obtained using a least-squares inversion algorithm. The inversion results show a good agreement with ionosonde data, validating our model for incoherent scatter radar measurements

Particle-in-cell simulations of electron dynamics in low pressure discharges with magnetic fields

In modern low pressure plasma discharges, the electron mean free path often exceeds the device dimensions. Under such conditions the electron velocity distribution function may significantly deviate from Maxwellian, which strongly affects the discharge properties. The description of such plasmas has to be kinetic and often requires the use of numerical methods. This thesis presents the study of kinetic effects in inductively coupled plasmas and Hall thrusters carried out by means of particle-in-cell simulations. The important result and the essential part of the research is the development of particle-in-cell codes. An advective electromagnetic 1d3v particle-in-cell code is developed for modelling the inductively coupled plasmas. An electrostatic direct implicit 1d3v particle-in-cell code EDIPIC is developed for plane geometry simulations of Hall thruster plasmas. The EDIPIC code includes several physical effects important for Hall thrusters: collisions with neutral atoms, turbulence, and secondary electron emission. In addition, the narrow sheath regions crucial for plasma-wall interaction are resolved in simulations. The code is parallelized to achieve fast run times. Inductively coupled plasmas sustained by the external RF electromagnetic field are widely used in material processing reactors and electrodeless lighting sources. In a low pressure inductive discharge, the collisionless electron motion strongly affects the absorption of the external electromagnetic waves and, via the ponderomotive force, the density profile. The linear theory of the anomalous skin effect based on the linear electron trajectories predicts a strong decrease of the ponderomotive force for warm plasmas. Particle-in-cell simulations show that the nonlinear modification of electron trajectories by the RF magnetic field partially compensates the effects of electron thermal motion. As a result, the ponderomotive force in warm collisionless plasmas is stronger than predicted by linear kinetic theory. Hall thrusters, where plasma is maintained by the DC electric field crossed with the stationary magnetic field, are efficient low-thrust devices for spacecraft propulsion. The energy exchange between the plasma and the wall in Hall thrusters is enhanced by the secondary electron emission, which strongly affects electron temperature and, subsequently, thruster operation. Particle-in-cell simulations show that the effect of secondary electron emission on electron cooling in Hall thrusters is quite different from predictions of previous fluid studies. Collisionless electron motion results in a strongly anisotropic, nonmonotonic electron velocity distribution function, which is depleted in the loss cone, subsequently reducing the electron wall losses compared to Maxwellian plasmas. Secondary electrons form two beams propagating between the walls of a thruster channel in opposite radial directions. The secondary electron beams acquire additional energy in the crossed external electric and magnetic fields. The energy increment depends on both the field magnitudes and the electron flight time between the walls. A new model of secondary electron emission in a bounded plasma slab, allowing for emission due to the counter-propagating secondary electron beams, is developed. It is shown that in bounded plasmas the average energy of plasma bulk electrons is far less important for the space charge saturation of the sheath than it is in purely Maxwellian plasmas. A new regime with relaxation oscillations of the sheath has been identified in simulations. Recent experimental studies of Hall thrusters indirectly support the simulation results with respect to the electron temperature saturation and the channel width effect on the thruster discharge

A new numerical description of the interaction of an ion beam with a magnetized plasma in an ECR-based charge breeding device

The ion beam-plasma interaction is a relevant topic in several fields of plasma physics, from fusion devices to modern ion sources. This paper discusses the numerical modelling of the whole beam-plus-plasma-target system in case of 1+ ions entering an ECR-based charge breeder (ECR-CB). The model is able to reproduce the ion capture and the creation of the first charge states in the selected physics case, i.e. the interaction of a 85Rb1+ ions with the plasma of the 14.5 GHz PHOENIX ECR-CB installed at the Laboratoire de Physique Subatomique et de Cosmologie (LPSC) of Grenoble. The results show that a very narrow window of physical parameters for both the beam (energy and energy spread especially) and plasma (ion temperature, density, density structural distribution, self-generated ambipolar fields) exists which is able to reproduce very well the experimental results, providing an exhaustive picture of the involved phenomena. Possible non-linear interactions and the role played by the eventual onset of instabilities are also discussed

Cold molecular ion-neutral collisions in a dynamic ion-atom hybrid trap

This thesis explores the fundamental physical and chemical processes between molecular ions and neutral atoms in the gas phase at very low energies. At temperatures in the mK regime, these are dominated by long-range interactions and only a few partial waves. This study contributes to the understanding of the details of intermolecular interactions and provides valuable data for benchmarking theoretical models and quantum-chemical calculations. Although there is a wide range of previous atomic collision system studies, the investigations of cold collisions with molecular ions are limited. The experimental setup used in this work enabled the measurement of rate coefficients for molecular ion-neutral collisions. The development of a dynamic ion-neutral hybrid trap with improved control over collision energy led to new insights into charge-transfer rate coefficients for molecular ion-neutral atom collisions involving N$^+_2$+Rb, O$^+_2$+Rb and N$_2$H$^+$+Rb. Two different experimental settings enable state-and collision-energy dependent measurements of rate coefficients for all systems revealing an intriguing interplay between long-and short-range effects for the homonuclear diatomics by comparison to theoretical calculations and surprising experimental results for the polyatomic study. A fit of a constant function to the measured CT rate coefficients in the state-dependent experiment of N$_2$H$^+$+Rb yielded the similar result as for N$^+_2$+Rb($5s$)($^2S_{1/2}$). In the collision-energy dependent experiment the CT rate coefficient for N$_2$H$^+$+Rb($5s$)($^2S_{1/2}$) and N$_2$H$^+$+Rb($5p$)($^2P_{3/2}$) are both similar to the results for N$^+_2$+Rb($5s$)($^2S_{1/2}$), and also the rate coefficients increase as the collision-energy increases

Collisions of Slow Highly Charged Ions with Surfaces

Progress in the study of collisions of multiply charged ions with surfaces is reviewed with the help of a few recent examples. They range from fundamental quasi-one electron processes to highly complex ablation and material modification processes. Open questions and possible future directions will be discussed.Comment: 13 pages, 16 figures, review pape

Sheath parameters for non-Debye plasmas: simulations and arc damage

This paper describes the surface environment of the dense plasma arcs that damage rf accelerators, tokamaks and other high gradient structures. We simulate the dense, non-ideal plasma sheath near a metallic surface using Molecular Dynamics (MD) to evaluate sheaths in the non-Debye region for high density, low temperature plasmas. We use direct two-component MD simulations where the interactions between all electrons and ions are computed explicitly. We find that the non-Debye sheath can be extrapolated from the Debye sheath parameters with small corrections. We find that these parameters are roughly consistent with previous PIC code estimates, pointing to densities in the range $10^{24} - 10^{25}\mathrm{m}^{-3}$. The high surface fields implied by these results could produce field emission that would short the sheath and cause an instability in the time evolution of the arc, and this mechanism could limit the maximum density and surface field in the arc. These results also provide a way of understanding how the "burn voltage" of an arc is generated, and the relation between self sputtering and the burn voltage, while not well understood, seems to be closely correlated. Using these results, and equating surface tension and plasma pressure, it is possible to infer a range of plasma densities and sheath potentials from SEM images of arc damage. We find that the high density plasma these results imply and the level of plasma pressure they would produce is consistent with arc damage on a scale 100 nm or less, in examples where the liquid metal would cool before this structure would be lost. We find that the sub-micron component of arc damage, the burn voltage, and fluctuations in the visible light production of arcs may be the most direct indicators of the parameters of the dense plasma arc, and the most useful diagnostics of the mechanisms limiting gradients in accelerators.Comment: 8 pages, 16 figure

Prediction of corona and multipactor RF breakdown thresholds using the CEST (Corona Simulation Electron Tool) software

This is an electronic version of the paper presented at the International workshop in Multipactor, Corona and Passive Intermodulation (MULCOPIM), held in Valencia (Spain) on 2008.This work was supported by ESA-ESTEC under program A=4025 ITT ESA entitled Surface treatment and coating for the reduction of multipactor and passive intermodulation (PIM) effects in RF components

Physics and Technology of the SPES Charge Breeder

This thesis has been developed in the framework of the SPES Project, an ISOL-Facility in the pipeline at the Legnaro National Laboratories of the INFN: the aim of the project is the production, ionization and post-acceleration of radioactive elements for Nuclear Physics experiments. In particular, the work has dealt with the theoretical - numerical study of a device, called Charge Breeder (CB), essential for the post-acceleration of the radioactive species produced. Among the available methods, a technique based on ECR sources has been chosen for the SPES Project. After having examined in depth the physical phenomena at the base of the Charge Breeding, a code able to reproduce the process has been developed in the Matlab environment: the ECR sources plasma has been gradually introduced with more and more complex physical models, obtaining a good agreement with the theoretical predictions on ions dynamics and their confinement. Finally, the complete model has been able to reproduce, with satisfactory correspondence, the experimental results obtained at the LPSC Laboratory in Grenoble, by injecting a Rb1+ ions beam inside the charge breeder there installed, and called PHOENIX. The experience matured during this PhD has also been enriched by an intense and fruitful experimental activity, carried out in the framework of the European Project EMILIE, at the LPSC in Grenoble, that has contributed to better understand the mechanisms at the base of the Charge Breeding process. This experimental work has been aim of a publication on a peer-reviewed journal and another article is currently under consideration by the referees. Eventually, these three years of PhD have seen a collaboration with LPSC, culminated in June 2014, with the signature of a Research Collaboration Agreement for the supply, to LNL, of an up-to-date version of the PHOENIX CB complete of ancillary systems (SPES-CB). The activity in this framework has implied both my management of this collaboration on behalf of the Laboratories, and the choice of different technological solutions that will be adopted on the SPES-CB