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. W. M. Solomon, M. G. Shats2 What Is A Mach Probe? Two identical collectors separated by a ceramic insulator Two identical collectors separated by a ceramic insulator The insulator makes the Mach probe sensitive to plasma drifts. Generally, The insulator makes the Mach probe sensitive to plasma drifts. Generally,

3. W. M. Solomon, M. G. Shats3 Evidence That Mach Probes Are Sensitive to Fluctuations Probes often used to study density fluctuations,. Probes often used to study density fluctuations,. Observe Observe If probe was primarily sensitive to, then would not expect this. If probe was primarily sensitive to, then would not expect this.

4. W. M. Solomon, M. G. Shats4 Far from the probe sheath, the ions have an average velocity dependent on their thermal velocity and their drift. Far from the probe sheath, the ions have an average velocity dependent on their thermal velocity and their drift. Bohm Theory Revised: Mach Probe Saturation Currents And Drift Velocity Ions arrive at the probe sheath with the ion acoustic velocity Ions arrive at the probe sheath with the ion acoustic velocity

5. W. M. Solomon, M. G. Shats5 Bohm Theory Revised: Mach Probe Saturation Currents And Drift Velocity Using conservation of energy Using conservation of energy …and assuming a Boltzmann distribution for the density …and assuming a Boltzmann distribution for the density The saturation current takes the form The saturation current takes the form We can then determine drift velocity by taking the ratio of the upstream/downstream currents where We can then determine drift velocity by taking the ratio of the upstream/downstream currents where

6. W. M. Solomon, M. G. Shats6 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. 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. Two triple probes surround the radial Mach probe – all are aligned to the same flux surface by electron gun.

7. W. M. Solomon, M. G. Shats7 Likewise for and (Row 4) Likewise for and (Row 4) TMT Solution Algorithm Described Row 2 shows signals readily determined from the probes. Row 2 shows signals readily determined from the probes. T e and  p (Row 3) readily determined by the triple probe T e and  p (Row 3) readily determined by the triple probe

8. W. M. Solomon, M. G. Shats8 TMT Solution Algorithm Described Then, with some arbitrary initial choice of T i, compute Then, with some arbitrary initial choice of T i, compute Compute n e and then the flux Compute n e and then the flux

9. W. M. Solomon, M. G. Shats9 Practically, minimise by modifying T i Practically, minimise by modifying T i TMT Solution Algorithm Described Invoke the condition of ambipolarity of the fluctuation driven fluxes Invoke the condition of ambipolarity of the fluctuation driven fluxes Output of algorithm is then time-resolved measurements of T i, n e, and V ri, with fluctuations properly accounted for. Output of algorithm is then time-resolved measurements of T i, n e, and V ri, with fluctuations properly accounted for.

10. W. M. Solomon, M. G. Shats10 Why Do Ion Temperature Fluctuations Appear High? As large (or larger) than ! As large (or larger) than ! Observe Observe If have high levels for then is also higher from If have high levels for then is also higher from But is it real??? But is it real???

11. W. M. Solomon, M. G. Shats11 More Probe Measurements! Testing The Condition Of Ambipolarity… What if ? What if ? Total fluxes must be still be equal in steady state, but fluctuations may drive non-ambipolar fluxes. Total fluxes must be still be equal in steady state, but fluctuations may drive non-ambipolar fluxes. From Poisson’s equation… From Poisson’s equation… … time-resolved measurements of E r will help answer this question. … time-resolved measurements of E r will help answer this question.

12. W. M. Solomon, M. G. Shats12 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. 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 Measure

13. W. M. Solomon, M. G. Shats13 Combining Measurable Signals And Solving For The Rest Summarising our unknowns as functions of T i. Summarising our unknowns as functions of T i. Combining them into Poisson’s equation Combining them into Poisson’s equation In the above equation, the remaining unknown is. Then solutions take the form In the above equation, the remaining unknown is. Then solutions take the form We can “choose” so as to satisfy We can “choose” so as to satisfy

14. W. M. Solomon, M. G. Shats14 Estimating The Total Flux To proceed, we need an estimate of the total flux, To proceed, we need an estimate of the total flux, Use Ionisation rate and approximate profiles for n e and n n (neutral density) to estimate flux. In steady state Use Ionisation rate and approximate profiles for n e and n n (neutral density) to estimate flux. In steady state

15. W. M. Solomon, M. G. Shats15 The Results

16. W. M. Solomon, M. G. Shats16 The Results

17. W. M. Solomon, M. G. Shats17 Conclusion: Fluctuations Can Drive Non-Ambipolar Fluxes The complex of probes allows local time- resolved measurements of key plasma parameters The complex of probes allows local time- resolved measurements of key plasma parameters Electron density Electron density Electron and Ion Temperature Electron and Ion Temperature Electron and Ion particle Fluxes Electron and Ion particle Fluxes Fluctuations fluxes in H-1 are indeed non- ambipolar in L-mode. 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. In fact, fluctuations seem to drive only electron transport, as in the regions of maximum fluctuations.