Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session AM2: Workshop on Plasma Cross Field Diffusion |
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Chair: Rod Boswell, Australian National University, and Igor Kaganovich, Princeton Plasma Physics Laboratory Room: Classroom 203 |
Monday, October 22, 2012 8:30AM - 8:45AM |
AM2.00001: Introduction |
Monday, October 22, 2012 8:45AM - 9:15AM |
AM2.00002: Expansion of a plasma across a transverse magnetic field in a negative hydrogen ion source for fusion Ursel Fantz, Loic Schiesko, Dirk W\"underlich Negative ion sources are a key component of the neutral beam injection systems for the international fusion experiment ITER. To achieve the required ion current of 40 A at a tolerable amount of co-extracted electrons (electron to ion ratio below one) the source is separated into a plasma generation region and an expansion chamber equipped with a magnetic filter field (up to 10 mT). The field is needed for: (1) cooling the electrons down and thus minimize the H$^{-}$ destruction by collisions, (2) to reduce the co-extracted electron current, and (3) to enhance the extraction probability for the surface produced negative ions. The area of the ITER source will be approximately 1m width and 2 m height, the IPP prototype source is a 1/8-size source. The recently installed flexible magnetic filter frame allows for systematic filter field studies (strength, position, polarity). Two Langmuir probes have been used to measure the plasma parameters simultaneously in axial direction. The profiles in the upper and lower part of the expansion chamber show beside the expected electron temperature and density decrease a drop in the plasma potential and a drift depending on the polarity, which vanishes when removing the filter field. The data interpretation is supported by modeling activities. [Preview Abstract] |
Monday, October 22, 2012 9:15AM - 9:30AM |
AM2.00003: Modeling plasma transport across the magnetic filter of the ITER negative ion source G.J.M. Hagelaar, F. Gaboriau, G. Fubiani, B. Chaudhury, J.P. Boeuf This presentation gives an overview of numerical modeling work at LAPLACE Toulouse on the negative filter operation of the ITER negative ion source developed at IPP Garching [U. Fantz et al, Rev. Sci. Instrum. 79, 02A511 (2008)]. The magnetic filter separates the source driver region (where plasma is produced by an inductive RF discharge) from the low electron temperature region in front of the extraction grids (where negative ions are produced and extracted from the plasma). The plasma transport across the filter is a key issue for the source performance. Both fluid and particle-in-cell models have been developed in order to improve fundamental understanding of this transport. The models demonstrate the dominant role of magnetic drift, increasing the cross field transport and inducing asymmetry in the plasma. Plasma instabilities are also observed but presumably do not contribute significantly to the cross field transport. A dedicated experimental set-up is under construction in order to obtain additional data to test and validate the models. [Preview Abstract] |
Monday, October 22, 2012 9:30AM - 10:00AM |
AM2.00004: An Overview of Negative Ion Beams and Sources Robert Welton This report will provide a broad introduction to the field of negative ion beams and sources first by summarizing their use in scientific research and industrial applications ranging from thermonuclear fusion, high energy physics, nuclear physics, neutron production, organic mass spectroscopy, accelerator mass spectroscopy, radioactive ion beam generation and others. Specific examples of important light and heavy ion negative sources will be discussed as well as the physics of negative ion formation. Finally, a summary of the recent 3rd International Symposium on Negative Ions, Beams and Sources in Jyvaskyla, Finland will be presented identifying some of the most important issues currently facing the field. [Preview Abstract] |
Monday, October 22, 2012 10:00AM - 10:15AM |
AM2.00005: A High Brightness Negative Ion Source for SIMS Noel Smith, Rod Boswell, Paul Tesch, Noel Martin Secondary ion mass spectrometry (SIMS) often utilizes a negative oxygen (O$^{-}$ or O$_{2}^{-})$ focused ion beam for trace element analysis of materials. The primary advantage of SIMS over other mass spectrometric techniques is the ability to provide spatial information in addition to high detection sensitivity. However, the scanned ion image resolution of these instruments is limited by the performance of the ion source employed to create the focused primary ion beam. To date, the duoplasmatron ion source has been the most suitable source of negatively charged oxygen ions, with an energy normalized brightness ($\beta _{r})$ for O$^{-}$ of $\sim $40 Am$^{-2}$sr$^{-1}$V$^{-1}$ and an energy spread ($\Delta $E) of $\sim $10-15eV. This produces imaging resolution in the range of 1-30$\mu $m for the ASI SHRIMP and Cameca 1280 tools and an impressive 150nm spot size for the NanoSIMS 50. Here we describe a newly developed inductively coupled plasma source with negative ion extraction. This source has $\beta _{r }$= 600Am$^{-2}$sr$^{-1}$V$^{-1}$ and $\Delta $E of only 3.5eV for O$^{-}$ ion extraction. This translates to a nominal 100x gain in current density in the focused beam, when operated for high resolution imaging. [Preview Abstract] |
Monday, October 22, 2012 10:15AM - 10:30AM |
AM2.00006: Electron temperature and plasma density distribution measurement along magnetic barrier in the PEGASES thruster Jerome Bredin, Ane Aanesland, Pascal Chabert, Valery Godyak The basic plasma parameter, electron temperature and plasma density were found as corresponding integrals of the measured EEDF in the PEGASES thruster. The measurements were carried out along the axis and off-axis of the magnetic barrier created with permanent magnets having their magnetic field lines normal to the plasma expansion. The plasma was generated with an induction coil on one end of the thruster, and diffused across magnetic field to the exit of the thruster. The experiments in argon gas were carried out for various parameters of magnetic field (strength, position and gradient). Previously, we showed that the electron temperature can be controlled by the magnetic field, and the degree of the temperature control depends on the gas pressure. In this study we measured the temperature off-axis to understand the influence of the wall conductivity on the electron transport across and along magnetic field. The off-axis probe measurements showed fine structures in the electron temperature and plasma density spatial distributions. A possible mechanism of the structures in the plasma density and the electron temperature distributions are discussed in this presentation. [Preview Abstract] |
Monday, October 22, 2012 10:30AM - 11:00AM |
AM2.00007: BREAK |
Monday, October 22, 2012 11:00AM - 11:30AM |
AM2.00008: Cross-field diffusion in Hall thrusters and other plasma thrusters J.P. Boeuf Understanding and quantifying electron transport perpendicular to the magnetic field is a challenge in many low temperature plasma applications. Hall effect thrusters (HETs) provide an excellent example of cross-field transport. The HET is a very successful concept that can be considered both as a gridless ion source and an electromagnetic thruster. In HETs, the electric field $E$ accelerating the ions is a consequence of the Lorentz force due to an external magnetic field $B$ acting on the $E\times B$ Hall electron current. An essential aspect of HETs is that the $E\times B$ drift is closed, i.e. is in the azimuthal direction of a cylindrical channel. In the first part of this presentation we will discuss the physics of cross-field electron transport in HETs, and the current understanding (or non-understanding) of the possible role of turbulence and wall collisions on cross-field diffusion. We will also briefly comment on alternative designs of ion sources based on the same principles as the conventional HET (Anode Layer Thruster, Diverging Cusp Field Thrusters, End-Hall ion sources). In a second part of the presentation we show that the Lorentz force acting on diamagnetic currents (associated with the $\nabla P_e \times B$ term in the electron momentum equation) can also provide thrust. This is the case for example in helicon thrusters where the plasma expands in a magnetic nozzle. We will report and discuss recent work on helicon thrusters and other devices where the diamagnetic current is dominant (with some examples where the $\nabla P_e \times B$ current is not closed and is directed toward a wall!). [Preview Abstract] |
Monday, October 22, 2012 11:30AM - 11:45AM |
AM2.00009: Cross-Field Transport of Electrons in Hall Plasma Accelerators Mark A. Cappelli The transport of electrons across the magnetic field within the channel of a Hall discharge plasma accelerator is still the subject of much research. Experiments [1] have shown that the electron mobility is greater than can be accounted for by elastic scattering with neutrals. A number of models have been proposed but the mechanism for this anomalous transport is still heavily debated. In this presentation, we will review the measurements and discuss the mechanisms proposed, including collisions with channel walls and instabilities. We will also present past attempts at incorporating transport models into numerical simulations of Hall plasma thrusters used in space propulsion. \\[4pt] [1] Meezan et al., Phys. Rev. E 63, 026410 (2001) [Preview Abstract] |
Monday, October 22, 2012 11:45AM - 12:00PM |
AM2.00010: Effect of Secondary Electron Emission on Electron Cross-Field Current in \textit{E$\times$B} Discharges Yevgeny Raitses, Igor D. Kaganovich, Alex V. Khrabrov, Michael D. Campanell, Erinc Tokluoglu, Dmytro Sydorenko, Andrei Smolyakov This paper reviews recent experimental, theoretical, and numerical studies of plasma-wall interaction in a weakly collisional magnetized plasma bounded with channel walls made from different materials [1-3]. A low-pressure \textit{E $\times $ B} plasma discharge of the Hall thruster was used to characterize the electron current across the magnetic field and its dependence on the applied voltage and the electron-induced secondary electron emission (SEE) from the channel wall [1]. The presence of a depleted anisotropic electron energy distribution function with beams of secondary electrons was predicted to explain the enhancement of the electron cross-field current observed in experiments. Without the SEE, the electron cross-field transport can be reduced from anomalously high to nearly classical collisional level. The suppression of the SEE was achieved using an engineered carbon-velvet material for the channel walls [3]. Both theoretically and experimentally, it is shown that the electron emission from the walls can limit the maximum achievable electric field in the magnetized plasma. \\[4pt] [1] Y. Raitses, et al, IEEE Trans. on Plasma Scie. \textbf{39}, 995 (2011). \\[0pt] [2] M. D. Campanell, et al, Phys. Rev. Lett. \textbf{108}, 235001 (2012). \\[0pt] [3] Y. Raitses, et al, \textit{J.Appl. Phys.} \textbf{99}, 036103 (2006). [Preview Abstract] |
Monday, October 22, 2012 12:00PM - 12:15PM |
AM2.00011: ABSTRACT WITHDRAWN |
Monday, October 22, 2012 12:15PM - 12:45PM |
AM2.00012: Bull Ring Discussion |
Monday, October 22, 2012 12:45PM - 2:00PM |
AM2.00013: BREAK |
Monday, October 22, 2012 2:00PM - 2:10PM |
AM2.00014: Reminder of Rules |
Monday, October 22, 2012 2:10PM - 2:25PM |
AM2.00015: Modification of turbulent transport with continuous variation of flow shear in the Large Plasma Device Troy Carter, David Schaffner, Giovanni Rossi, Daniel Guice, Jim Maggs, Stephen Vincena, Brett Friedman The LArge Plasma Device (LAPD) at UCLA is a 17~m long, 60~cm diameter magnetized plasma column with typical plasma parameters $n_e \sim 1\times 10^{12}$cm$^{-3}$, $T_e \sim 10$eV, and $B \sim 1$kG. Broadband, fully-developed turbulence is observed in the edge of the LAPD plasma along with spontaneously driven azimuthal flows. Recently, the capability to continuously vary the edge flow and flow shear has been developed in LAPD using biasing of an annular limiter. Spontaneous flow is observed in the ion diamagnetic direction (IDD), biasing tends to drive flow in the opposite direction, allowing a continuous variation of flow from the IDD to the electron diamagnetic direction, with a near-zero flow and flow shear state achieved along the way. Enhanced confinement and density profile steepening is observed with increasing shearing rate; degraded confinement is observed when spontaneous flow is nulled-out and near-zero shear is acheived. Particle flux and radial correlation length are observed to decrease with increasing shear. The decrease occurs with shearing rates which are comparable to the inverse turbulent autocorrelation time in the zero flow state. [Preview Abstract] |
Monday, October 22, 2012 2:25PM - 2:45PM |
AM2.00016: Radial transport in bounded cylinders and physics of ``universal'' profiles Francis F. Chen Even strong magnetic fields cannot confine electrons radially in a cylinder if the cylinder length is defined by endplates that are not far apart. The sheaths on the endplates will adjust themselves to allow the electron density $n$ to ``follow'' the radial electric field, even though the electrons are actually lost to the ends, not to the sidewall. This nanosecond mechanism allows electrons to follow the Boltzmann relation, even across the B-field. If the plasma is ionized near the wall, the ions will be driven inwards by the resulting E- field, which is scaled to $T_{e}$. Thus, the (weakly magnetized) ions will reach the center faster than at their thermal velocities. When equilibrium is reached, the field is reversed to push the ions outward to the sidewall, their closest escape path. Under these conditions, $n$ is always peaked on axis. A detailed treatment\footnote{D. Curreli and F.F. Chen, Phys. Plasmas \textbf{18}, 113501 (2011).} of this problem yields the surprising fact that the density and potential profiles in equilibrium are independent of neutral pressure and cylinder radius, varying only with $T_{e}$. A simple physical argument shows why this has to be true. [Preview Abstract] |
Monday, October 22, 2012 2:45PM - 3:00PM |
AM2.00017: Turbulence-induced anomalous electron diffusion in the plume of the VASIMR VX-200 Christopher Olsen, Maxwell Ballenger, Jared Squire, Benjamin Longmier, Mark Carter, Tim Glover The separation of electrons from magnetic nozzles is critical to the function of the VASIMR engine and is of general importance to the field of electric propulsion. Separation of electrons by means of anomalous cross field diffusion is considered. Plume measurements using spectral analysis of custom high frequency probes characterizes the nature of oscillating electric fields in the expanding magnetic nozzle. The oscillating electric field results in frequency dependent density variations that can lead to anomalously high transport in the absence of collisions mimicking collisional transport. The spatial structure of the fluctuating fields is consistent with turbulence caused by separation of energetic ($>$ 100 eV) non-magnetized ions and low energy magnetized electrons via the modified two-stream instability (MTSI) and generalized lower hybrid drift instability (GLHDI). Electric fields as high as 300 V/m are observed at frequencies up to an order of magnitude above the lower hybrid frequency. The electric field fluctuations dissipate with increasing axial distance consistent with changes in ion flux streamlines as plasma detachment occurs. [Preview Abstract] |
Monday, October 22, 2012 3:00PM - 3:15PM |
AM2.00018: Particle-in-cell simulations of ambipolar and nonambipolar diffusion in magnetized plasmas Rod Boswell, Trevor Lafleur Using a two-dimensional particle-in-cell simulation, we investigate cross-field diffusion in lowpressure magnetized plasmas both in the presence and absence of conducting axial boundaries. With no axial boundary, the cross-field diffusion is observed to be ambipolar, as expected. However, when axial boundaries are added, the diffusion becomes distinctly nonambipolar. Electrons are prevented from escaping to the transverse walls and are preferentially removed from the discharge along the magnetic field lines, thus allowing quasi-neutrality to be maintained via a short-circuit effect at the axial boundaries. [Preview Abstract] |
Monday, October 22, 2012 3:15PM - 3:30PM |
AM2.00019: Cross Field Workshop Wendell Horton |
Monday, October 22, 2012 3:30PM - 4:00PM |
AM2.00020: BREAK |
Monday, October 22, 2012 4:00PM - 4:15PM |
AM2.00021: Stability of spontaneously appearing ion beams in expanding plasmas Earl Scime, Jerry Carr Jr., Robert VanDervort, Njal Gulbrandsen We present time resolved measurements of ion beam formation in the expansion region of a pulsed helicon plasma. The ion beams are identified in ion velocity distribution function measurements obtained through laser induced fluorescence and retarding field energy analyzer diagnostics. As the plasma discharge forms, an ion beam appears and then vanishes coincident with the appearance of large amplitude electrostatic fluctuations. For a strong mirror ratio, the correlation between fluctuations in the ion population and the electrostatic fluctuations are strong. For a weak mirror ratio, the beam is less correlated with the electrostatic fluctuations than the bulk ion population and the beam persists throughout the discharge. The fluctuation measurements are analyzed using a time-resolved spectral method that provides a measurement of the fluctuation spectrum throughout a single discharge pulse. Combined with the time-resolved ion distribution function measurements, it is possible to identify which portions of the ion velocity distribution most strongly interact with the electrostatic fluctuations. [Preview Abstract] |
Monday, October 22, 2012 4:15PM - 4:30PM |
AM2.00022: Electron diamagnetic effect in a magnetic nozzle on a helicon plasma thruster performance Kazunori Takahashi, Trevor Lafleur, Christine Charles, Peter Alexander, Rod Boswell The axial force, which is called thrust sometimes, imparted from a magnetically expanding helicon plasma thruster is directly measured and the results are compared with a two-dimensional fluid theory. The force component solely transmitted to the expanding field is directly measured and identified as an axial force produced by the azimuthal current due to an electron diamagnetic drift and the radial component of the applied magnetic field. In this type of configuration, plasma diffusion in magnetic field affects a spatial profile of the plasma density and the resultant axial force onto the magnetic field. It is observed that the force component onto the magnetic field increases with an increase in the magnetic field strength, simultaneously with an increase in the plasma density downstream of the source exit, which could be due to suppression of the cross field diffusion in the magnetic nozzle. [Preview Abstract] |
Monday, October 22, 2012 4:30PM - 4:45PM |
AM2.00023: Characteristics of High-Density Helicon Plasma Sources and Their Application to Electrodeless Electric Propulsion S. Shinohara, H. Nishida, T. Nakamura, A. Mishio, H. Ishii, N. Teshigahara, H. Fujitsuka, S. Waseda, T. Tanikawa, T. Hada, F. Otsuka, I. Funaki, T. Matsuoka, K. Shamrai, T. Rudenko High-density but low temperature helicon plasmas have been proved to be very useful for fundamental research as well as for various applications. First, we introduce our very large helicon sources [1] with a diameter up to 74 cm. For the industrial and propulsion applications, we have reduced the aspect ratio (axial length-to-diameter) down to 0.075, and examined the discharge performance and wave characteristics. Then, we discuss our small helicon sources [1] for developing new electrodeless acceleration schemes. Some experimental and theoretical results [2] by applying the rotating magnetic (or electric) fields to the helicon plasma under the divergent magnetic field will be presented, along with other propulsion schemes. In addition, an initial plasma production experiment with very small diameter will be described.\\[4pt] [1] S. Shinohara \textit{et al}., Jpn. J. Appl. Phys. \textbf{35} (1996) 4503; Rev. Sci. Instrum. \textbf{75} (2004) 1941; Phys. Plasmas \textbf{16} (2009) 057104.\\[0pt] [2] S. Shinohara \textit{et al}., \textit{32th Int. Electric Propul. Conf.}, IEPC-2011-056, 2011. [Preview Abstract] |
Monday, October 22, 2012 4:45PM - 5:00PM |
AM2.00024: Plume detachment from a magnetized plasma thruster Roger Bengtson High-powered electric propulsion thrusters utilizing a magnetized plasma require that plasma exhaust detach from the onboard magnetic coils in order to produce thrust. We present experimental and theoretical results demonstrating that a sufficiently powerful plasma flow does indeed detach from a magnetic nozzle. Measurements of ion flux show a low-beta plasma plume which follows applied magnetic field until the magnetic pressure falls below the plasma energy density. The plasma flow becomes super-Alfvenic at that point and it continues ballistically downstream. Several magnetic configurations were tested including a reversed field nozzle configuration. Despite the dramatic change in magnetic field profile, the reversed field configuration yielded little measurable change in plume trajectory, demonstrating the plume is detached. Numerical simulations yield density profiles in agreement with the experimental results. [Preview Abstract] |
Monday, October 22, 2012 5:00PM - 5:15PM |
AM2.00025: Evidence for anomalous resistivity in a helicon plasma source Boris Breizman Measurements and modeling of the rf field structure in a helicon plasma source at UT have demonstrated that these fields represent radially localized helicon waves that propagate in the axial direction. A good agreement in the absolute amplitude and phase of the fields between measurement and simulation could only be reached by enhancing the electron Coulomb collision frequency by a factor of 30 in the simulation. We attribute the enhanced collision frequency to the excitation of an ion-acoustic instability, as the electron azimuthal diamagnetic drift exceeds the ion sound speed under the experimental conditions. This interpretation is also consistent with the observed suppression of the electron heat transport along the magnetic field lines. [Preview Abstract] |
Monday, October 22, 2012 5:15PM - 6:00PM |
AM2.00026: Running of the Bulls Yevgeny Raitses |
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