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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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slide1

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

slide2

The permanent-magnet helicon source

The discharge tube is 5 cm in diam and 5 cm high

UCLA

slide3

The Langmuir probe

Compensation Electrode (CE)

UCLA

slide4

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.

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slide5

The simple Langmuir formula is valid!

This gives Te and VS after subtracting ion current fit

This gives n without knowing Te

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slide6

The art of ion subtraction

Electron distribution functions cannot be trusted.

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slide8

False Te’s without Compensation Electrode

T1 = 8.22 eV

T2 = 4.65 eV

T3 = 2.97 eV

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slide10

Sample data

Density scan along axis

Pressure scan of n and Te

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slide11

Density saturation inside discharge

Power scan at center of discharge

Power scan 17 cm below discharge

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slide12

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

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slide13

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