Bulletin of the American Physical Society
61st Annual Gaseous Electronics Conference
Volume 53, Number 10
Monday–Friday, October 13–17, 2008; Dallas, Texas
Session ET2: Negative Ion Plasmas |
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Chair: Nicholas Braithwaite, The Open University, UK Room: Salon A-D |
Tuesday, October 14, 2008 4:00PM - 4:30PM |
ET2.00001: Discharges In Electronegative Gases Invited Speaker: This talk will come in three parts. First, the early work in electronegative plasmas, principally by Emeleus and co-workers in Iodine, and by Massey and co-workers in Oxygen. They were at opposite ends of the ``spectrum'' of electronegativity - the ratio of negative ion density to electron density. Secondly, we cover in more detail work in Oxygen, where in retrospect we know that too many parameters were included to reveal the underlying structure of electronegative plasmas. That is associated with Edgley and von Engel, and later with Ferriera and co-workers. From there until the present day we describe work coming from different directions, showing that by questioning prior assumptions, we have arrived at our present understanding. The basic elements are, that in general there is a negative ion core, surrounded by a conventional plasma, and that at low pressures the situation is significantly different from higher pressures. The talk will seek to avoid mathematical complexity and concentrate on the physics, explaining the reason for previous differences, and show the way forward for a more Complete understanding of the very complex problem of strongly electronegative plasmas and their structure when diluted by rare gases. All of this involves a multiplicity of ion species of both signs, and a variety of reaction rates. [Preview Abstract] |
Tuesday, October 14, 2008 4:30PM - 5:00PM |
ET2.00002: Electronegative Plasma Discharge Equilibria Invited Speaker: The equilibrium of electronegative discharges is studied in the plane-parallel approximation over a wide range of pressure and electron densities, encompassing a number of regimes that have previously been modeled analytically. The transitions between the various regimes (models) have been determined in the input parameter space. It is shown that for a given feedstock gas, these transitions can be found in terms of the two input parameters plp and ne0lp, where p is the pressure, ne0 the electron density, and lp the system half-length. Here ne0 is used as a convenient input related to the power, and the conversion from electron power to ne0 is given. The input parameter space is partitioned by whether ion flux to the wall or positive-negative ion recombination is the dominant positive ion loss mechanism. For each of the principal regimes, scaling laws are developed for the most important plasma parameters in terms of the input parameters. Extensions to 2D discharges and to plasmas with axial magnetic fields are briefly considered. [Preview Abstract] |
Tuesday, October 14, 2008 5:00PM - 5:15PM |
ET2.00003: Global Models for Electronegative Discharges Derek D. Monahan, Miles M. Turner The first step towards understanding a complex plasma is usually to develop a zero-dimensional or global model. This is difficult when the plasma is electronegative, because the literature contains many proposed models with different and sometimes contradictory detailed assumptions, and different domains of applicability. The appropriateness of such models in a given context is often hard to assess. In this paper, we present a set of detailed kinetic simulations spanning a wide of range of parameters, especially with respect to electronegativity, collisionality, and type of negative ion destruction mechanism. We use these simulations as a benchmark to investigate the validity of a variety of proposed models for electronegative discharges. We reach two important conclusions: (i) that an an accurate electron kinetics model is more important than any consideration relating to plasma transport in the presence of negative ions and (ii) that there exists a simple and robust transport model that is in reasonable agreement with all of our benchmark simulations, when the electrons are treated properly. We therefore commend this approach as likely to offer reasonable accuracy for modelling any electronegative discharge where a global model is likely to be useful. [Preview Abstract] |
Tuesday, October 14, 2008 5:15PM - 5:30PM |
ET2.00004: Negative Ion Extraction from a Pulsed NF3 Plasma Svetlana Radovanov, Ludovic Godet, Rajesh Dorai, Vikram Singh Negative ions that are ordinarily trapped inside a plasma can be extracted during the afterglow period immediately following the end of a pulse. During this time the ratio of negative ions to electrons dramatically increases because of the electrons higher mobility, enabling them to reach the walls more promptly than the ions. In the later phase of the afterglow an ion ion plasma is formed and negative ions become the dominant negative charge carriers. In this paper we report time resolved measurements of negative ion energy distributions during the on and off period of a pulsed radio frequency inductively coupled discharge at pressures from 20 to 150 mTorr of NF3, peak powers of 1-3 kW at pulse repetition frequencies between 1 and 10 kHz and duty cycles of 20 to 70\%. We show a large flux of negative ions immediately following plasma turnoff. We find that RF pulse repetition frequency, duty cycle and power, can be adjusted to produce efficient negative ion formation. These parameters also control the transition from electron-ion to ion ion plasma. We also show a comparison to a Hybrid Plasma Equipment Model of the steady state plasma properties during the on period and comment on the transient effects observed. [Preview Abstract] |
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