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

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

The Langmuir probe UCLA Compensation Electrode (CE)

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.

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

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

RF amplitude inside discharge UCLA

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

Importance of a large C.E. UCLA

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

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

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

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

Title here UCLA