Twenty Five Years of Vibrational Kinetics and Negative Ion Production in H 2 Plasmas: Modelling Aspects (original) (raw)
Related papers
Nuclear Fusion, 2006
Old and new problems in the physics of multicusp magnetic sources for the production of negative H -/Dions are presented and discussed. We emphasize particularly, in this kind of plasmas, both the vibrational and electron non equilibrium energy distributions, the role of Rydberg states in enhancing the negative ion production, the production of vibrationally excited states by the Eley-Rideal mechanism, and the enhancement of negative ion concentrations in pulsed discharges. In appendix I recent cross sections calculations for elementary processes and the theoretical determination of hydrogen recombination probability on graphite surface are illustrated. In appendix II two types of sources are modeled: the first one is a classical negative ion source in which the plasma is generated by thermoemitted electrons; in the second one, electrons already present in the mixture are accelerated by an RF field to sufficiently high energy to ionize the gas molecules.
Vibrational excitation and negative ion production in radio frequency parallel plate H2 plasmas
The European Physical Journal D, 2005
A theoretical study of the vibrational kinetics and attachment in low pressure hydrogen plasmas produced by Radio Frequency (RF) discharges is performed. In particular we study the influence of gas/surface kinetic processes such as the vibrational deactivation and the atomic recombination of molecules. The production of vibrationally excited molecules by the surface recombination of atoms is also considered. The study is realized by means of a self-consistent one dimensional kinetic model, and a parallel plate RF discharge test case is implemented. Results show that surface processes are able to affect the vibrational distribution function (vdf) and the negative ion (H − ) density. The effect of vibrational exothermicity of H atom recombination is also discussed as a way to reduce the gap between theory and experimental results. Moreover, it is shown that the H − ion heating by the electric field strongly affects the detachment rate: this effect is specially important for negative ions produced through Rydberg states in this kind of discharges.
From dynamics to modeling of plasma complex systems: negative ion (H-) sources
Chemical Physics, 1992
We present an approach for calculating the HZ vibrational distribution, electron energy distribution function and negative ion concentration (H-) based on the self-consistent solution ofthe vibrational master equation and of the Boltzmann equation. Most of the input data of the present model have been obtained by our group in these last years by using classical, semiclassical and quantum approaches. In particular we discuss the most important process involving vibrationally excited H2 molecules colliding with themselves as well as with atomic hydrogen, electrons and metallic surfaces. Part of these data have been obtained by usmg dynamic calculations. The main result of our approach, which mixes different techniques, is the ability of reproducing the main experimental features of negative ion (H-) sources.
Roberto Zorat and David Vender ------------------------- A study of the gas-phase kinetics and the plasma chemistry for an rf inductively coupled H_2 plasma discharge, in the deuterium negative ion source experiment (DENISE), has been carried out by means of a global model of the discharge implemented by the numerical code, Global Model Solver (GMS), which gives results for the spatially averaged densities of the species included in the model and the electron temperature in the steady state. The modelling of the discharge includes H− , with the production mechanism assumed to be dissociative attachment of H 2 (v). The effect of the magnetic filter in the source is taken into account for obtaining an estimate of the H− density also in the extraction chamber. The differences in the cross-section data assumed for some processes, between this paper and previous work, are pointed out, along with the consequences on H− density. The ratio between the H − density and the electron density obtained with GMS for the considered negative ion production mechanism is always far lower than 0.1, even with the most optimistic assumptions for H− production that can be put forward.
Modeling of a negative ion source. I. Gas kinetics and dynamics in the expansion region
Physics of Plasmas, 2007
The vibrational population distribution of the electronic ground state of H 2 in the expansion region of a negative ion source is investigated using a kinetic Monte Carlo model. Operative conditions are referred to the inductively coupled plasma radio frequency negative ion source developed at IPP-Garching. The different excitation and relaxation processes are discussed, both bulk and surface contributions. In particular, due to the relatively high plasma density, the relevant role of direct low energy electron-impact excitation, surface Auger neutralization, and vibration-translation deactivation are recovered. Results of the present model will be used as input data for the neutral source model in the extraction region.
Journal of Physics D: Applied Physics, 2004
We present the results of a study of a capacitively coupled hydrogen discharge by means of a one-dimensional numerical fluid model and experiments. The model includes a detailed description of the gas-phase chemistry taking into account the production of H − ions by dissociative attachment of H 2 vibrational levels. The population of these levels is described by a Boltzmann vibrational distribution function characterized by a vibrational temperature T V . The effect of the dissociative-attachment reaction on the discharge dynamics was investigated by varying the vibrational temperature, which was used as a model input parameter. Increasing the vibrational temperature from 1000 to 6000 K affects both the chemistry and the dynamics of the electrical discharge. Because of dissociative attachment, the H − ion density increases by seven orders of magnitude and the H − ion density to electron density ratio varies from 10 −7 to 6, while the positive ion density increases slightly. As a consequence, the atomic hydrogen density increases by a factor of three, and the sheath voltage drops from 95 to 75 V. Therefore, clear evidence of a strong coupling between chemistry and electrical dynamics through the production of H − ions is demonstrated. Moreover, satisfactory agreement between computed and measured values of atomic hydrogen and H − ion densities gives further support to the requirement of a detailed description of the hydrogen vibrational kinetics for capacitively coupled radio frequency discharge models in the Torr regime.
Numerical simulation of the RF plasma discharge in the Linac4 H− ion source
AIP Conference Proceedings, 2017
This paper presents a Particle-In-Cell Monte Carlo Collision simulation of the Radio-Frequency (RF) plasma heating in the Linac4 H ion source at CERN. The model self-consistently takes into account the electromagnetic field generated by the RF coil, the external static magnetic fields and the resulting plasma response, including a kinetic description of the charged species (e , H + , H + 2 , H + 3 , H), as well as the atomic and molecular (vibrationally resolved) populations. The simulation is performed for the nominal operational condition of 40 kW RF power and 3 Pa H 2 pressure. Results show that the plasma spatial distribution is non-uniform in the plasma chamber, with a density peak of n e = 5 • 10 19 m 3 in the RF coil region. In the filter field region the electron density drops by two orders of magnitude, with a substantial reduction of the electron energy as well. This results in a ratio e/H ⇡ 1 in the extraction region. The vibrational population is characterized by a two temperature distribution, with the high vibrational states showing a factor 2 higher termperature. A very good agreement is found between the simulation results and optical emission spectroscopy measurement performed on a dedicated test stand at CERN.
In the frame work of a development project for ITER neutral beam injection system a radio frequency (RF) driven negative hydrogen (H-/D-) ion source, (BATMAN ion source) is developed which is designed to produce several 10s of ampere of H-/Dbeam current. This PhD work has been carried out to understand and optimize BATMAN ion source. The study has been done with the help of computer simulations, modeling and experiments. The complete three dimensional Monte-Carlo computer simulation codes have been developed under the scope of this PhD work. A comprehensive description about the volume production and the surface production of Hions is presented in the thesis along with the study results obtained from the simulations, modeling and the experiments. One of the simulations is based on the volume production of Hions, where it calculates the density profile of the vibrationally excited H 2 molecules, the density profile of Hions and the transport probability of those Hions along the source axis towards the grid. The other simulation studies the transport of those Hions which are produced on the surface of the plasma grid. It is expected that if there is a plasma flow in the source, the transport of plasma components (molecules and ions) would be influenced. Experimentally it is observed that there is a convective plasma flow exists in the ion source. A transverse magnetic filter field which is present near the grid inside the ion source reduces the flow velocity. Negative ions and electrons have the same sign of charge; therefore the electrons are co-extracted with the negative ions through the grid system, which is not desirable. It is observed that a magnetic field near the grid, magnetized the electrons and therefore reduce the co-extracted electron current. It is also observed experimentally that if the plasma grid is biased positively with respect to the source body, the electron density near the plasma grid is reduced and therefore the co-extracted electron current is also reduced. A double layer is formed near the positively biased plasma grid in the plasma, which would have an influence on the negative ion extraction mechanisms. In the process, two phase-sensitive diagnostic methods have been developed based on a new technique (modulation technique). One diagnostic is for measuring the Hion density and the other one is for measuring the electron temperature. Zusammenfassung Im Rahmen eines größeren Projekts wird am Institut für Plasmaphysik, Garching, eine HF-getriebene Plasmaquelle (BATMAN-Quelle) zur Erzeugung negativer Wasserstoffionen (Hbzw. D-) entwickelt, die im Endausbau mit H-Stromstärken von mehreren 10 Ampère für die Neutralteilchen-Injektionsheizung am Fusionsexperiment ITER eingesetzt werden soll. Ziel der vorliegenden Arbeit war es, ein tieferes Verständnis der sehr komplexen physikalischen Vorgänge in einer solchen H-Ionenquelle zu erzielen, um damit die Leistungsfähigkeit dieser Quelle entscheidend zu verbessern. Die Studie basiert auf Computersimulationen, Modell-Rechnungen und dem jeweiligen Vergleich mit den experimentellen Resultaten. Dabei wurden im Verlauf der Arbeit vollständige dreidimensionale Monte-Carlo Computercodes entwickelt, die eine detaillierte Beschreibung von Erzeugung und Transport der H-Ionen liefern. Ein erster Code beschreibt die Erzeugung von H-Ionen im Volumen (Volumenprozeß) und berechnet dazu das Dichteprofil der Vibrations-angeregten H 2-Moleküle, das daraus resultierende Dichteprofil der H-Ionen und deren Transportwahrscheinlichkeit auf die Extraktionsfläche (Plasmagitter). Eine zweite Simulation untersucht den Transport von H-Ionen, die unmittelbar an der Oberfläche eines (Cäsium-aktivierten) Gitters erzeugt werden (Oberflächenprozeß). Mit zu berücksichtigen war dabei der experimentell beobachtete konvektive Teilchenfluss, der offensichtlich neben positiven und negativen Ionen auch Neutrale (Atome, Moleküle) in Richtung Gitter mitführt. Da am Gitter selbst anstelle der gewünschten negativen Ionen in weit höherem Maße die leichten Elektronen extrahiert würden, sind hier geeignete Gegenmaßnahmen zu treffen. Diese bestehen aus einem schwachen Magnetfeld (zur Magnetisierung der Elektronen) und einer elektrischen Vorspannung des Plasmagitters. Dabei zeigt sich, dass die Elektronendichte bei positiven Werten der Gittervorspannung (gegen das Quellengehäuse) in Gitternähe stark abnimmt und der co-extrahierte Elektronenstrom damit deutlich unter den negativen Ionenstrom gedrückt werden kann. Zugleich bildet sich nahe dem Gitter eine Doppelschicht aus, die sich günstig auf die Extraktion der negativen Ionen auswirkt. Zur Detailanalyse wurden im Rahmen dieser Arbeit wurden zwei spezielle Diagnostikmethoden entwickelt, die auf ein neues, hoch-empfindliches Modulationsverfahren zurückgreifen. Hiermit lassen sich die Dichte der H-Ionen und die lokale Elektronentemperatur erfassen. i Contents 1. Introduction 23-35 2.1. Volume production of Hion.….……………………………………….23 2.1.1. Creation of vibrationally excited H 2 * (v") states……………………...23 2.1.2. Destruction of vibrationally excited states…………….……………..25 2.1.3. Creation of H-ions…………………………………………………...27 2.2. Surface production of Hion.….………………………………………..28 2.2.1. Atomic process………………………………………………………28 2.2.2. Ionic process…………………………………………………………30 2.3. Destruction of the H-ions………………………………………………..31 3. Description of the ion source 36-47 3.1. Introduction……………………………………………………………....36 3.2. Description of the source……………………………………………...…36 3.3. Description of diagnostic and other subsystems…………………………41 3.3.1. Diagnostic system…………………………………………………....41 3.3.2. Other subsystems…………………………………………………….42 3.4. Typical characteristics of the source……………………………………..43 4. Model and simulations for Hion production and transport 48-78 4.1. Introduction…………………………………………………………...….48 4.2. Geometrical model of the source….……………………………………...50 4.3. Physical model…………………………………………………………...50 4.4. Input and output structure of the computer code…….…………………...55 4.5. Neutral transport code for volume process………….……………………56 4.6. Hion production code for volume process……….……...………………57 4.7. Volume produced Hion transport code…………………………………57 4.8. Particle balance model………….………………………………………...60 4.9. Results and discussion on simulation and model………………………...62 4.10. Transport of surface produced H-ions…………………………………...68 4.11. Results and discussion on surface produced Hion transport……………71 4.12. Conclusion………………………………………………………………..77 i 5. Plasma flow measurements 79-97 5.1. Introduction………………………………………………………...…….79 5.2. Plasma flow measurements………………………………………………80 5.3. Force balance…………………………………………………….……….84 5.4. Effect of plasma flow on the plasma sheath ....…………………….…….91 5.5. Influence of plasma flow on negative ion production ……………….…..93 5.6. Conclusion…………….………………………………………………….96 6. Grid bias experiments 98-103 6.1. Introduction……….……………………………………………………...98 6.2. Experimental setup…….…………………………………………………98 6.3. Results ……………...…….…………………………………………….100 6.4. Conclusion………………………………………………………………102 7. Ion source diagnostics by modulation technique 104-118 7.1. Negative ion density measurement…………………………………..…104 7.1.1. Introduction…………………………………………………………104 7.1.2. Principle of the Hion diagnostic system…………………………...105 7.1.3. Experimental setup……………………………………………….…106 7.1.4. Results of the Hion diagnostic system…………………………….107 7.1.5. Discussion the Hion diagnostic system……………………………109 7.
Ion kinetic‐energy distributions and Balmer‐alpha (Hα) excitation in Ar‐H2radio‐frequency discharges
Journal of Applied Physics, 1995
Excited neutrals and fast ions produced in a 13.56 MHz radio-frequency discharge in a 90% argon-10% hydrogen gas mixture were investigated, respectively, by spatially and temporally resolved optical emission spectroscopy, and by mass-resolved measurements of ion kinetic energy distributions at the grounded electrode. The electrical characteristics of the discharge were also measured and comparisons are made with results obtained for discharges in pure H2 under comparable conditions. Measurements of Balmer-alpha (Ha) emission show Doppler-broadened emission that is due to the excitation of fast atomic hydrogen neutrals formed from ion neutralization processes in the discharge. Temporally and spatially resolved emission profiles of the Ha radiation from the Ar-H2 mixture are presented for the "slow" component produced predominately by electron-impact dissociative excitation of H2' and for the "fast" component corresponding to energies much greater than can be derived from dissociative excitation. For the Ar-H2 mixture, the fast component is significantly enhanced relative to the slow component. The measured kinetic-energy distributions and fluxes of predominant ions in the Ar-H2 mixture, such as H;, H;, H+, and ArH+, suggest mechanisms for the formation of fast hydrogen atoms. The interpretation of results indicate that H+ and/or H; , neutralized and backscattered by collision with the powered electrode, are the likely sources of fast hydrogen atoms that produce Doppler-shifted Ha emission in the discharge. There is also evidence at the highest pressures and voltages of "runaway" H+ ions formed near the powered electrode, and detected with kinetics energies exceeding 100 eV at the grounded electrode. I. INTRODUCTION Radio-frequency (rt) discharges produced in argonhydrogen mixtures are useful for surface cleaning applications,I while discharges involving mixtures of argon, hydrogen, and methane have been used for etching of GaAs wafers.2 An understanding of these processes requires a knowledge of the role of collisions of ions and energetic neutrals with surfaces and other particles in the plasma. Particularly, in collision dominated discharges, ion and neutral transport in the sheath region are important in determining the discharge-surface interactions. In addition to industrial applications, ion transport in Ar-H2 gas mixtures has been considered as a prototype system for experimental inv~stigations and rigorous quantumtheoretical studies.3-1OA recent review article on stateselected and state-to-state cross-section measurements for several ion-molecule reaction systems shows that an increasing interest has been paid to the Ar-H2 systemY Few investigations in Ar-H2 mixtures have been performed in low pressure dC12-14 or rfl5 discharges. A recent optical emission studyl5 of an Ar-H2 mixture in 13.56 MHz rf glow discharges shows an increase in Doppler-broadened Balmer-alpha (Ha, A= 656.3 nm) emission when argon is a
Modeling of a negative ion source. II. Plasma-gas coupling in the extraction region
Physics of Plasmas, 2008
The production, destruction, and transport of H − in the extraction region of a negative ion source are investigated with a 1D͑z͒-3V particle-in-cell electrostatic code. The motion of charged particles ͑e, H + , H 2 + , and H − ͒ in their self-consistent electric field is coupled with the neutral particles ͓H͑n =1͒ and H 2 ͑X 1 ͚ g + , v =0, ... ,14͔͒ dynamics and vibrational kinetics of H 2. Neutral influxes into the domain are determined by the simulation of the expansion region. Surface and volumetric processes involving plasma and neutrals have been included by using different Monte Carlo collision methods. Calculations show the influence of the plasma grid bias and of the magnetic filter on the plasma parameter profiles. In particular, a transition from classical to complete reverse sheath is observed using a positively biased plasma grid. The influence of the magnetic filter is small. The importance of the hot-atom mechanism on the surface negative ion production is shown.