Ion Heating Arising from the Damping of Short Wavelength Fluctuations at the Edge of a Helicon Plasma Source

Transcription

1 Ion Heating Arising from the Damping of Short Wavelength Fluctuations at the Edge of a Helicon Plasma Source Division of Plasma Physics American Physical Society October 2012 Providence, RI Earl Scime, Richard Magee, Matthew Galante, and Jerry Carr. Jr Department of Physics, West Virginia University

2 Key Points Previous experiments indicated ion heating enhanced for driving frequencies matching the lower hybrid frequency in the edge of a helicon source. Edge ion heating results in a hollow ion temperature profile. Microwave scattering and electrostatic probe measurements indicate peak in fluctuations at plasma edge for same conditions that lead to enhanced ion heating. Time resolved ion temperature measurements show evolution from peaked ion temperature profile to hollow in the first few milliseconds of the plasma pulse. Electrostatic fluctuation measurements are dramatically different between edge and core regions. Edge measurements show well defined peak ~ 250 khz. Parametric decay in core.

3 Ions are heated to ~ 1 ev in a helicon source Ion temperatures measured by laser induced fluorescence in HELIX for a neutral pressure of 6.7 mtorr and an rf power of 750 W. The white line denotes where the source frequency matches the calculated lower hybrid frequency along the axis of the source LH ce ci pi ci At the plasma edge, the lower hybrid frequency curve shifts towards the lower right corner of the figure. The maximum ion temperature occurs for those source frequencies and magnetic fields at which the lower hybrid frequency matches the source frequency in the plasma edge.

4 Only slow wave has a resonance in cold plasma theory The cold plasma dispersion relation for helicon parameters is where 4 2 kc ^ N N 1 N N and c is the speed of light, is the wave frequency, and e 1, e 2, and e 3 are the cold plasma dielectric tensor components. Only two solutions, fast (helicon) and slow (TG) wave. 1 The cold plasma dispersion relation n = 5 x10 12 cm -3, k = 0.26 rad/cm, f = 9 MHz, and no collisions. (a) absolute value of the real k^ for the slow wave (b) absolute value of the imaginary k^ for the slow wave (c) absolute value of the real k^ for the fast wave (d) absolute value of the imaginary k^ for the fast wave

5 Experiments performed in the HELIX source Radial distance (cm) Hot helicon experiment [HELIX] Large Experiment on Instabilities and Anisotropies [LEIA] 60 c) B L =14 Gauss 40 B L =70 Gauss Axial distance (cm)

6 Short wavelength (< 1 mm) fluctuations measured with coherent microwave scattering and probes The mm wave system and the optical path through a cross section of the helicon source (at z = 85 cm): (S) is the mm-wave source, (D) is the detector, (M) are mirrors, (VM) is the adjustable vacuum mirror, (BS1) and (BS2) are the beam splitters. The cross sectional view includes the source chamber and additional structures that house that injection lens and collection mirror assembly.

7 Probe measurements in CW plasmas indicate a peak in short wavelength fluctuations at radial location of ~ 4 cm rf = 9.5 MHz rf = 11.5 MHz rf = 13.5 MHz 650 G 800 G 950 G 1100 G Integrated over a 55 khz window centered at 340 khz and over the wave number range of 10 to 40 rad/cm.

8 Non-perturbing mm-wave scattering shows peak at same radial location in CW plasmas Integrated spectral power for the electrostatic double probe (squares) and the CTS diagnostic (circles) for a driving frequency of 9.5 MHz, a magnetic field strength of 800 G, and a neutral pressure of 8 mtorr. The electrostatic spectral power is integrated over 55 khz centered at 340 khz The CTS spectral power is integrated over the range khz.

9 The ion temperature profile in pulsed helicon plasmas shows a clear evidence of edge ion heating Profile begins centrally peaked and quickly switches to hollow. Profile switches from peaked to hollow between 2 and 6 ms.

10 The ion temperature profile in pulsed helicon plasmas shows a clear evidence of edge ion heating t = 14 ms t = 2.7 ms

11 Continuous wavelet transform analysis indicates modest modulation of driving RF wave at r = 0 cm.

12 As seen in the CW Plasmas, edge electrostatic fluctuations ~ 250 khz appear at edge of pulsed plasmas. Quickly ramp up in first 2 ms. The first 5 ms of the electrostatic fluctuations measured at a radial location of 3.5 cm. The narrow spectral feature at 9.5 MHz is the primary rf wave for the source. Expanded view of the lower frequency portion of the spectrum shown in The amplitude of the peak at ~ 245 khz increases rapidly over the first 2 ms of the discharge. The amplitude of the wavelet power spectrum at 245 khz for radial locations of 0 cm (light gray) and 3.5 cm (black).

13 Sideband (slow mode) waves at and MHz evident in averaged (over 5 ms) FFT 12.5 ms into discharge.

14 Review Ion temperature profile begins centrally peaked and quickly evolves to a hollow profile. Over the same initial 2 ms, well defined electrostatic waves appear at the edge and grow in amplitude. In the core, the plasma is free of low-frequency, electrostatic waves and no ion heating is observed. The wavenumber of the fluctuations is k ~ 90 cm -1 and the wave dispersion is consistent with a parametrically driven ion acoustic wave. Measurements consistent with short wavelength fluctuations coupling to ions and heating them in the edge of the plasma. Still need to determine if short wavelength fluctuations appear at the driving frequency (TG modes).

15 di/dv Retarding Field Energy Analyzer [RFEA] Discriminator Voltage (V) Conventional Analysis ne ed Eb Tb b I( ) T e E erfc e E T 2m D b b D b b di( ) d n e 1 2m T e D D b b 2 ed Eb Tb 4 TE D b b