Plasma Characterisation Using Combined Mach/Triple Probe Techniques

1
Plasma Characterisation Using Combined
Mach/Triple Probe Techniques

• W. M. Solomon, M. G. Shats
• Plasma Research Laboratory
• Research School of Physical Sciences and
  Engineering
• Australian National University
• Canberra ACT 0200

2
What Is A Mach Probe?

• Two identical collectors separated by a ceramic
  insulator
• The insulator makes the Mach probe sensitive to
  plasma drifts. Generally,

3
Evidence That Mach Probes Are Sensitive to
Fluctuations

• Probes often used to study density fluctuations,
  .
• Observe
• If probe was primarily sensitive to , then
  would not expect this.

4
Bohm Theory Revised Mach Probe Saturation
Currents And Drift Velocity

• Ions arrive at the probe sheath with the ion
  acoustic velocity

• Far from the probe sheath, the ions have an
  average velocity dependent on their thermal
  velocity and their drift.

5
Bohm Theory Revised Mach Probe Saturation
Currents And Drift Velocity

• Using conservation of energy
• and assuming a Boltzmann distribution for the
  density
• The saturation current takes the form
• We can then determine drift velocity by taking
  the ratio of the upstream/downstream
  currents where

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Enter the TMT Probe

• Since the plasma is unmagnetised for ions, we may
  align the Mach probe so that it is sensitive to
  radial motions.
• Two triple probes surround the radial Mach probe
  all are aligned to the same flux surface by
  electron gun.

7
TMT Solution Algorithm Described

• Row 2 shows signals readily determined from the
  probes.

• Te and ?p (Row 3) readily determined by the
  triple probe

• Likewise for and
  (Row 4)

8
TMT Solution Algorithm Described

• Then, with some arbitrary initial choice of Ti ,
  compute

• Compute ne and then the flux

9
TMT Solution Algorithm Described

• Invoke the condition of ambipolarity of the
  fluctuation driven fluxes

• Practically, minimise by modifying Ti

• Output of algorithm is then time-resolved
  measurements of Ti , ne , and Vri , with
  fluctuations properly accounted for.

10
Why Do Ion Temperature Fluctuations Appear High?

• As large (or larger) than !
• Observe
• If have high levels for then is also higher
  from
• But is it real???

11
More Probe Measurements! Testing The Condition
Of Ambipolarity

• What if ?
• Total fluxes must be still be equal in steady
  state, but fluctuations may drive non-ambipolar
  fluxes.
• From Poissons equation
• time-resolved measurements of Er will help
  answer this question.

12
Ahhhh! More Probes Fork Probe Measures Radial
Electric Field

• A fork probe, consisting of two more triple
  probes radially separated (slight toroidal
  displacement) is added to the probe set, also
  aligned by electron gun.
• Measure

13
Combining Measurable Signals And Solving For The
Rest

• Summarising our unknowns as functions of Ti .
• Combining them into Poissons equation
• In the above equation, the remaining unknown is
  . Then solutions take the form
• We can choose so as to satisfy

14
Estimating The Total Flux

• To proceed, we need an estimate of the total
  flux,
• Use Ionisation rate andapproximate profiles
  forne and nn (neutraldensity) to estimateflux.
  In steady state

15
The Results
16
The Results
17
Conclusion Fluctuations Can Drive Non-Ambipolar
Fluxes

• The complex of probes allows local time-resolved
  measurements of key plasma parameters
• Electron density
• Electron and Ion Temperature
• Electron and Ion particle Fluxes
• Fluctuations fluxes in H-1 are indeed
  non-ambipolar in L-mode.
• In fact, fluctuations seem to drive only electron
  transport, as in the regions of
  maximum fluctuations.