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Plasma Characterisation Using Combined Mach/Triple Probe Techniques

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. What Is A Mach Probe?.

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Plasma Characterisation Using Combined Mach/Triple Probe Techniques

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  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, W. M. Solomon, M. G. Shats

  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. W. M. Solomon, M. G. Shats

  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. W. M. Solomon, M. G. Shats

  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 W. M. Solomon, M. G. Shats

  6. 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. W. M. Solomon, M. G. Shats

  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) W. M. Solomon, M. G. Shats

  8. TMT Solution Algorithm Described • Then, with some arbitrary initial choice of Ti , compute • Compute ne and then the flux W. M. Solomon, M. G. Shats

  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. W. M. Solomon, M. G. Shats

  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??? W. M. Solomon, M. G. Shats

  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 Poisson’s equation… • … time-resolved measurements of Er will help answer this question. W. M. Solomon, M. G. Shats

  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 W. M. Solomon, M. G. Shats

  13. Combining Measurable Signals And Solving For The Rest • Summarising our unknowns as functions of Ti . • Combining them into Poisson’s equation • In the above equation, the remaining unknown is . Then solutions take the form • We can “choose” so as to satisfy W. M. Solomon, M. G. Shats

  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 W. M. Solomon, M. G. Shats

  15. The Results W. M. Solomon, M. G. Shats

  16. The Results W. M. Solomon, M. G. Shats

  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. W. M. Solomon, M. G. Shats

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