Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session GM5: Mini-conference on Optimizing Helicon Source Performance-Absorption and Heating Mechanisms |
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Chair: Earl Scime, West Virginia University Room: Rosen Centre Hotel Salon 11/12 |
Tuesday, November 13, 2007 9:30AM - 9:50AM |
GM5.00001: Helicon Sources: Why they work Rod Boswell A helicon source is a cylindrical ceramic tube containing a gas in the milliTorr region immersed in an axial magnet field surrounded at some region by an antenna fed with rf around 10 MHz. As the power is increased it will show a variety of modes characterised by changes in the plasma density. Low power produces a spatially uniform capacitive discharge with the electric field heating the electrons: at higher power the discharge can show a jump (with hysteresis) into an inductive mode when the skin depth enters the plasma that5 produces an annular plasma: powers around a kilowatt produce centrally peaked plasmas of densities $\sim $10$^{12}$ cm$^{-3}$ where optical measurements show bursts of electrons traveling with the helicon velocity, consistent with acceleration by the E$_{z}$ fields of the m=1 mode. At higher densities, when the plasma enters the ``blue mode'', the coulomb mean free path becomes sufficiently small that the plasma becomes resistive and the helicon damps linearly. Other electron heating proposals are discussed and compared to experiment. [Preview Abstract] |
Tuesday, November 13, 2007 9:50AM - 10:10AM |
GM5.00002: Magnetic Field Dependencies in the Mini-RFTF Light Ion Helicon Plasma Source R.H. Goulding, F.W. Baity, D.A. Rasmussen, D.O. Sparks, M.D. Carter, M. Yoshitaka For several years hydrogen plasmas have been produced in the Mini-RFTF Light Ion Helicon device with densities ($\leq 2.5\times 10^{19} m^{-3}$) comparable to those commonly observed in helicon devices using heavier ion species for comparable input powers (1-5 kW). The use of light ions including hydrogen and helium has allowed the continuous range of regimes $\omega < \omega_{LH}$ to $\omega > \omega_{LH}$, where $\omega_{LH}$ is the lower hybrid frequency, to be carefully explored at modest magnetic field strength. A detailed set of electron density and rf $\tilde{B}$ measurements with widely varying |$B$| at the antenna strongly suggest that in the case of this device, collisional damping of the fast (helicon) wave, with the electric field strength enhanced by the presence of eigenmodes, is responsible for the efficient power coupling to the plasma. The lower hybrid frequency has been shown not to play an important role, at least during equilibrium operation. The extensive evidence in support of these findings will be reviewed [Preview Abstract] |
Tuesday, November 13, 2007 10:10AM - 10:30AM |
GM5.00003: Density and temperature maxima at specific w and B Matthew Balkey, Earl E. Scime, John L. Kline, Paul Keiter, Robert Boivin We report measurements of electron density and perpendicular ion temperatures as a function of driving frequency and magnetic field strength in an argon helicon plasma for five different RF antennas: a Nagoya type III antenna, a ``Boswell'' saddle coil antenna, a 19 cm long m=+1 helical antenna, a 30 cm long m=+1 helical antenna, and a 19 cm long m=+1 helical antenna with narrow straps. The experimental results clearly indicate that for all antennas, the electron density is maximized at a significantly different RF frequency than the frequency, which yields the maximum ion temperature. Ion temperatures in excess of 1 eV for 750 W of input power are observed. These results suggest that the mechanisms responsible for coupling energy into the ions and electrons are distinct and therefore helicon sources can be configured to maximize electron density without simultaneously maximizing the perpendicular ion temperature. [Preview Abstract] |
Tuesday, November 13, 2007 10:30AM - 10:50AM |
GM5.00004: Radially localized helicon mode and power deposition in a helicon source Guangye Chen, Alexey Arefiev, Roger Bengtson, Boris Breizman, Charles Lee, Laxminarayan Raja Radially localized helicon (RLH) modes [1] arise in a magnetized plasma with a density gradient across the confining magnetic field. The density gradient modifies the dispersion relation of conventional helicon waves, so that the resulting eigenfrequency of an RLH mode is much lower than that of a conventional helicon wave in an elongated plasma under similar conditions. This work presents evidence that RLH waves play a significant role in helicon plasma sources. Plasma density profile was measured in an argon helicon discharge driven by a 1 kW power supply at 13.56 MHz through a half-turn helical antenna. The experimentally measured density profile was then used to calculate the rf field structure. It is found that RLH waves with an azimuthal wave number m=1 form a standing wave structure in the axial direction and that the frequency of the RLH eigenmode is close to the driving frequency of the rf antenna. The calculated resonant power absorption, associated with the RLH eigenmode, accounts for most of the rf power deposited into the plasma in the experiment. The power deposited via TG modes does not exceed 10\% of the total power absorption. \newline [1] B. N. Breizman and A. V. Arefiev, Phys. Rev. Lett. 84, 3863 (2000). [Preview Abstract] |
Tuesday, November 13, 2007 10:50AM - 11:10AM |
GM5.00005: Helicon mode formation and rf power deposition in a helicon source Michael Kraemer, Kari Niemi The nonlinear nature of the rf absorption in a helicon-produced plasma was investigated on the helicon device HE-L [1] with the aid of a double pulse technique providing high and low amplitude helicon propagation under nearly identical conditions. Time- and space-resolved (2D) measurements of the rf magnetic field (amplitude and phase of all components) were carried out by means of a B-dot probe array. For high rf power, a small narrow peak arises on top of the density profile close to the axis leading to focusing of the rf field energy and the rf power deposition. Nevertheless, in accordance with the linear helicon theory for a non-uniform plasma, the axial wavenumber remains nearly the same as for low power. The rf power deposition in the core of the helicon discharge deduced from the energy flux balance was compared with that obtained from the rf field distribution assuming collisional absorption. It turns out that collisions are by far not sufficient to account for the absorption of helicon modes, particularly for high rf power. Nonlinear processes, most likely associated with the parametric excitation of electrostatic fluctuations [2], are thus involved.- This work was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 591, Project A7).- [1] M.~Kr\"{a}mer, B.~Lorenz, B.~Clarenbach, Plasma Sources Sci. Technol. 11A (2002) 120. [2] B. Lorenz, M. Kr\"{a}mer, V.L. Selenin, Yu.M. Aliev, Plasma Sources Sci. Technol. 14, 623 (2005). [Preview Abstract] |
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