Density and temperature maxima at specific? and B

Transcription

1 Density and temperature maxima at specific? and B Matthew M. Balkey, Earl E. Scime, John L. Kline, Paul Keiter, and Robert Boivin 11/15/ Slide 1

2 Abstract 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. 11/15/ Slide 2

3 Helix in the foreground connected to Leia.

4 Measurements taken at different axial locations with distinct magnetic field strengths. Typical magnetic field strength versus position in Helix and Leia. Note field varies by factor of 7 between the antenna and Langmuir probe Magnetic field (Gauss) Antenna Magnetic Probe Array Laser Induced Fluorescence HELIX LEIA Langmuir Probe Position (cm) 11/15/ Slide 4

5 Helicon source connected to the large space simulation vacuum chamber Three sets of electromagnets are shown in diagram: the coils around the source, around the space chamber, and around the bellows connecting the two chambers. The current in the second electromagnet from the right end of the source is directed opposite to the other coils to create a field null 11/15/ Slide 5

6 Four distinct antenna geometries are studied Starting at the top left and proceeding clockwise are the 30 cm m = +1 antenna (NSH30), 19 cm Nagoya III antenna, 19 cm double-saddle antenna, and the 19 cm m = +1 antenna with 2.54 cm wide straps. The 19 cm m = +1 antenna with 1.8 cm straps is not shown (WSH19). 11/15/ Slide 6

7 Ion Temperature when driving frequency is greater than? LH, but density peaks when driving frequency is less than? LH T i (ev) N e (10 12 cm -3 ) T e (ev) The gas pressure was 3.6 mtorr and the rf power was 750 Watts for all of the measurements. The color bar for each plot has been individually adjusted to provide maximum contrast. There is no evidence of increased electron temperature at the parameters for which the ion temperatures are greatest Magnetic Fi eld (G) w > w LH WSH19 NSH19 NSH30 Nagoya III /15/ RF Frequency (MHz) Slide 7

8 Density using a Nagoya III antenna showed clear evidence of monotonically increasing plasma density with decreasing rf frequency 4 Below 4-5 mtorr, the frequency dependence of density on inverse rf frequency evident in plot was not observed. Simplified dispersion relation: 2 2 ω pe N. ωω cosθ ce [Storey, 1953] N e (10 10 cm -3 ) Inverse RF Frequency (1/MHz) Pressure of 6 mtorr, B = 800 G, RF power = 500W 11/15/ Slide 8

9 Density relation to frequency is not clear at other operating conditions. Downstream plasma density (solid circles) perpendicular ion temperature in the source (open squares) versus RF frequency. For these experiments, the magnetic field was 800 G, the RF power was 500 W, and the pressure was 3.6 mtorr N e (10 10 cm -3 ) N e (10 10 cm -3 ) N e (10 10 cm -3 ) N e (10 10 cm -3 ) (a) Saddle 19 cm double saddle (b) (c) (d) 19 cm Nagoya III nsh - 19 cm m=1 wsh - 19 cm m=1 Nagoya III NSH19 WSH T i (ev) T i (ev) T i (ev) T i (ev) Frequency (MHz) 11/15/ N e (10 10 cm -3 ) (e) nsh - 30 cm m=1 NSH T i (ev) Slide 9

10 Perpendicular ion temperature, downstream electron density, and downstream electron temperature 1200 T i (ev) N e (10 12 cm -3 ) T e (ev) The highest density values lay in the range 0.5 <? LH <? < <? LH The color bar for each plot has been individually adjusted to provide maximum contrast. The gas pressure was 3.6 mtorr and the RF power was 750 Watts for all of the measurements. Magnetic Field (G) WSH19 NSH19 NSH30 Nagoya III /15/ ω LH /ω Slide 10

11 The magnetic sense coil and the transformer circuit The magnetic sense coil and the transformer circuit. The transformer has a gain of 1:1 and the arrow indicates the cable to the digitizer which has an input impedence of 50 Ω cm 5 turns of 24-AWG wire on a delrin form Center tapped transformer with a turn ratio of 1:1 11/15/ Slide 11

12 The magnetic sense coil array and the transformer circuit Coil to measure db z dt Coil to measure db θ dt Coil to measure db r dt 11/15/ Slide 12

13 Wavenumber as a function of Helix magnetic field Driving frequency of 8.5 MHz and a fill pressure of 3.6 mtorr. Clockwise the top left are wavenumber measurements for the WSH19, the NSH19, the NSH30, and the Nagoya III antennas. Parallel Wavenumber (cm -1 ) Parallel Wavenumber (cm -1 ) NSH19 NSH Magnetic Field (G) WSH19 Nagoya III Magnetic Field (G) 11/15/ Slide 13

14 Power spectrum for the NSH19 antenna as a function of magnetic field strength RF power of 750 W G 1000 G 840 A 690 A 540 A 390 G Driving frequency of 8.5 MHz Pressure of 3.6 mtorr. Amplitude (G 2 ) Frequency (MHz) 11/15/ Slide 14

15 Summary These experiments have demonstrated a clear correlation between the lower hybrid resonance, electron density production, and ion temperature in helicon sources. Ion Temperature maximums do not correlate with either increased density or increased electron temperature The data also strongly supports the conclusion that collisional equilibration with electrons is not responsible for the ion heating observed in helicon sources Operating a helicon source at frequencies slightly above the lower hybrid frequency, maximum densities can be achieved at moderate ion temperatures. To further reduce the perpendicular ion temperature, the helicon source can be operated at higher frequencies, lower magnetic fields, or with an antenna that is less effective at coupling energy into the ions, e.g., a Nagoya III antenna. Wave spectrum analysis should be completed 11/15/ Slide 15

16 The experimental configuration for perpendicular and parallel LIF measurements. wavemeter fiber Coherent 899 Ring Dye Laser CW 6W Innova argon - ion pump laser linearly polarized perpendicular injection B beam dump collection optics PMT circularly polarized parallel injection SRS Lock - In amplifier 11/15/ Slide 16

17 Perpendicular and Parallel LIF data 1.2 LIF Signal Amplitude (arb) Frequency Shift (GHz) 11/15/ Slide 17

18 RF matching circuit 11/15/ Slide 18

19 Measurements taken at different axial locations with distinct magnetic field strengths HELIX Magnetic field (Gauss) Antenna Magnetic Probe Array Laser Induced Fluorescence LEIA Langmuir Probe Position (cm) 11/15/ Slide 19

20 Langmuir probe and measurement circuit The exposed graphite tip is 2 mm long and runs the length of the alumina tube into the boron-nitride cap. The capacitor between the graphite tip and the small secondary tip has a value of approximately 5 nf. Five inductors having values between 15 and 270 µh are placed in series behind the capacitor [Keiter, 1999]. probe tip assembly (see Figure 7 for details) KF 40 vacuum coupling double O-ring seal electrical feedthrough PC GPIB card Keithly 2400 sourcemeter 11/15/ Slide 20

21 Microwave density measured in Helix, scales with Langmuir probe measurements Comparison of the number of microwave interference fringes (open squares) obtained at startup for 33.6 GHz microwaves and the downstream density measured with a Langmuir probe (filled circles) during pulsed operation of the plasma source. A typical error bar representative of the statistical error in the number of fringes is shown for the 100 Gauss field data point. Number of Fringes Magnetic Field (G) Langmuir Probe Density (10 12 cm -3 ) 11/15/ Slide 21

22 Original Helix matching circuit and antenna. The connection between the matching circuit and the antenna is made with a RG8 coaxial cable. The cable shield is grounded at the box, forcing one lead to the antenna to be held at ground. The connection to the antenna is made in the center of the antenna. Figure 27. Modified matching circuit and antenna. The connection to the antenna is now made with copper bars and the connection to the antenna is at one end. The ground in the circuit has been moved so that neither end of the antenna is fixed at ground. The wires inside the matching box were replaced with solid copper sheets 11/15/ Slide 22

23 11/15/ Slide 23

24 Electron density versus Leia radius for the five new antennas and the original antenna and matching circuit. The RF power was 400W, the Helix magnetic field was 596G, the Leia magnetic field was 35G, and the neutral fill pressure was 3.6 mtorr. The driving frequency was 8.1 MHz for the 19 cm wide strap m = +1 and the double-saddle antennas and was 8.0 MHz for the other antennas. Density (10 12 cm -3 ) Original antenna and matching 19 cm wide strap m = +1 antenna 19 cm narrow strap m = +1 antenna 30 cm m = +1 Nagoya III Double-Saddle Radius (cm) 11/15/ Slide 24

25 Helix Operating Parameters Parameter Helicon Source (Ar) Plasma lifetime steady-state n > 1 x m -3 B 440 G G pressure 3.2 mtorr T e T i λ D ρ i ρ e L (chamber diameter) L (chamber length) ~ 5 ev 1 ev 5 x 10-6 m 1-4 x 10-2 m ~ 9 x 10-5 m 0.15 m 1.6 m ion β ~ 4 x 10-4 f ce f ci ν in ν ii f pe f pi GHz khz ~ 2.8 khz 26 MHz ~ 28 GHz 105 MHz 11/15/ Slide 25

26 G 540 G G 840 G 10-4 Amplitude (G 2 s) G 1000 G G 1150 G /15/ Wavenumber (cm -1 ) Slide 26