1. 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

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

3. The Langmuir probe UCLA Compensation Electrode (CE)

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

5. The simple Langmuir formula is valid! UCLA This gives n without knowing T e This gives T e and V S after subtracting ion current fit

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

7. RF amplitude inside discharge UCLA

8. False T e ’s without Compensation Electrode UCLA T 1 = 8.22 eV T 2 = 4.65 eV T 3 = 2.97 eV

9. Importance of a large C.E. UCLA

10. Sample data UCLA Density scan along axisPressure scan of n and T e

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

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

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

14. Title here UCLA