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UCLA

LANGMUIR PROBES IN THE INTENSE RF ENVIRONMENT INSIDE A HELICON DISCHARGE. Francis F. Chen, UCLA. Gaseous Electronics Conference, Austin TX, Tuesday, October 23, 2012. UCLA. The permanent-magnet helicon source. The discharge tube is 5 cm in diam and 5 cm high. UCLA. The Langmuir probe.

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UCLA

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  1. LANGMUIR PROBES IN THE INTENSE RF ENVIRONMENT INSIDE A HELICON DISCHARGE Francis F. Chen, UCLA Gaseous Electronics Conference, Austin TX, Tuesday, October 23, 2012 UCLA

  2. The permanent-magnet helicon source The discharge tube is 5 cm in diam and 5 cm high UCLA

  3. The Langmuir probe Compensation Electrode (CE) UCLA

  4. Distortion caused by RF pickup Electron current is greatly distorted. This is new: residence time at cos(wt) ~ 0 is taken into account. Saturation ion current is not affected. UCLA

  5. The simple Langmuir formula is valid! This gives Te and VS after subtracting ion current fit This gives n without knowing Te UCLA

  6. The art of ion subtraction Electron distribution functions cannot be trusted. UCLA

  7. RF amplitude inside discharge UCLA

  8. False Te’s without Compensation Electrode T1 = 8.22 eV T2 = 4.65 eV T3 = 2.97 eV UCLA

  9. Importance of a large C.E. UCLA

  10. Sample data Density scan along axis Pressure scan of n and Te UCLA

  11. Density saturation inside discharge Power scan at center of discharge Power scan 17 cm below discharge UCLA

  12. Electron emission at high Vp Same data, w. Vmax=70 point +100V Emission adds to ion current in subsequent pulses -100V Hiden ESPion Scan Average SA = 4 here 25 msec/div UCLA

  13. Conclusions • Probes can be used even under the antenna • The compensation electrode has to be large enough • Spuriously high KTe otherwise • KTe is Maxwellian if ion current is subtracted right • Non-Maxwellian EEDFs cannot be trusted • Fast sweeps are needed to avoid electron emission UCLA

  14. Title here UCLA

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