Third PRC-US Magnetic Fusion Collaboration Workshop 18-19 May 2006

1. Third PRC-US Magnetic Fusion Collaboration Workshop 18-19 May 2006 Plans and Results of the Texas Collaboration with ASIPP K.W. Gentle Fusion Research Center University of Texas

2. Plans and Results of the Texas Collaboration with ASIPP • Long history of collaboration between the Fusion Research Center, Texas and the Institute of Plasma Physics, Hefei • Plans for HT-7 and EAST • ECE -- Electron Cyclotron Emission radiometer for Te • CXRS -- Charge Exchange Recombination    Spectroscopy for Ti and rotation • Expanded divertor • Results of Helimak project

3. Spatial Coverage of ECE System on HT-7

4. Schematic of the ECE diagnostic on HT-7

5. ECE Data with position shift to obtain a relative calibration.

6. Te Profile (ECE) • Relative calibration from shift is position position • Absolute calibration form Thomson Scattering (central temperature)

7. ECE Temperature Profile • Shot 81535:

8. EAST ECE System

9. Proposed ECE Antenna for EAST • Diffraction limited spatial resolution • Integrated hot calibration source • Possible test of ITER prototype calibration source

10. CXRS on HT-7 and EAST W. L. Rowan,1 Yuejiang Shi,2 June Huang,2Huang He1, and B. N. Wan2 1Fusion Research Center, The University of Texas at Austin 2Institute of Plasma Physics, Chinese Academy of Sciences • DNB transferred to ASIPP and brought back into operation through common effort • CXRS spectrometer and optics installed • Plans • Develop CXRS analysis codes • Conduct transport experiments on HT-7 • Transfer DNB to EAST • Transfer CXRS to EAST

11. CXRS view range DNB, Component Mix, andCXRS Viewing Range • The beam has operated for one campaign with an useful density component mix E:E/2:E/3:E/18 = 10:26:49:15 • The CXRS diagnostic is installed for the current campaign and is expected to provide Ti, v over the LFS of the plasma HT-7 DNB

12. Divertor Projections M. Kotschenreuther, P. M. Valanju, S. M. Mahajan, J. C. Wiley, M. Pekker Sherwood Fusion Theory Conference, April, 2006 • Although ITER divertor may handle heat loads adequately, the divertor heat loads for the next-step reactor will exceed material limits: This is a show-stopper • Other divertor configurations including radiating mantle and swept divertor will not scale to ITER or to a reactor • Need an expanded divertor or other configuration

13. Expanded Divertor for EAST • A new configuration to reduce the heat load on the divertor plates • Axisymmetric coils near the divertor plates expand the footprint of the intersection of the field lines with the divertor plates • Divertor coil currents are comparable to other PF coil currents • The first test of this idea is proposed for EAST. Use reduced plasma current and pulsed divertor coils as a proof of concept • A concept could be presented in August at ASIPP

14. An Experiment for EAST • Energize coils in blue to yield flux expansion • To prove the concept, use a set of coils with pulsed current just large enough to observe the expansion effect easily

15. An Experiment for EAST Flux Expansion Versus Divertor Coil Current I = 0 kA expansion = 2.2 I = 40 kA expansion = 4.3 I = 80 kA expansion = 10.3 • Energize coils in blue to expand the green flux at the divertor plate (in the circle)

16. Helimak Collaboration • Unique concept for a basic plasma experiment • Simple sheared cylindrical slab geometry • Device large compared with all scale lengths • Designed, engineered, and built by ASIPP • Operating successfully at Texas Helimak

17. Helimak Objectives • Dimensionless test of drift-wave turbulence • Simple, but physical geometry (curvature) • Open field lines, but long ( up to ~1 km) • Test of flow shear stabilization of turbulence • Dimensionless model of SOL Helimak

18. Helimak Probe connections Vacuum Vessel Toroidal field coils Vertical field coils Microwave feed Magnetron Amplifiers and A/D

19. A Cylindrical Slab Helimak

20. Helimak Dimensions and Parameters A Sheared Cylindrical Slab <R> = 1.1 m ∆R = 1 m h = 2 m BT = 0.1 T Bv ≤ 0.01 T Pulse ≤ 60 s Plasma source and heating: 6 kW ECH @ 2.45 GHz n ≤ 1011 cm-3 Te ~ 10 eV Argon, Helium cs = 3 x 104 m/s (Argon) Vdrift = 100 m/s Vdiamagnetic = 103 m/s drift-wave ~ 1 kHz Connection length: 10 m < L < 1000 m p (parallel loss) > 1 ms Probe arrays in end plates provide vertical and full radial profiles Isolated end plates may apply radial electric fields: Vp ≤ ±100 Volts Helimak

21. Density Profiles for various ECH Resonant Radii Helimak

22. Typical Density, Temperature, and Floating Potential Profiles

23. Radial Profiles of Fluctuation Amplitude ∆n/n (Various ECH Resonant radii) R

26. Helimak

27. Turbulence Bifurcation Helimak

28. Major Points • The Helimak provides a good example of a       turbulence bifurcation (shear stabilization) • The stabilization is caused by j (not E) • The transition is binary, not gradual -- no       intermediate states as threshold       approached from either direction Helimak

29. Cross-section • Field lines terminate on      isolated end plates • Biasing #2 plates with respect      to others imposes radial      electric field, current Helimak

30. Response to Negative Bias Probe n(t) across radial profile Bias Reduced ∆n Reduced ∆n; increased <n> Helimak

31. Response to Positive Bias Probe n(t) across radial profile Bias Reduced ∆n; increased <n> Increased <n> Reduced ∆n Helimak

32. Response to Negative Bias Probe n(t) across radial profile Helium Bias Increased <n> Reduced ∆n Helimak

33. Time History of a Bifurcation Negative Bias Positive Bias Isat(t) Bias Voltage Current

34. Profile Changes at Bifurcation -50 V

36. Frequency-Resolved Particle Transport

37. Phase Velocity Change with Bias High Field Side • Larger changes for positive bias • Equilibrium flow reversed by positive bias • Negative bias adds to equilibrium flow Low Field Side

38. Inferred Velocity Shear • Same |∂Vz/∂z| for ± bias ~104 s-1 • Equilibrium V from potential profile • ∆V with bias from ∆Vphase of turbulence Helimak

39. Velocity Shear vs.  Autocorrelation Velocity shear ~104 s-1 comparable with shortest turbulence autocorrelation time High field side c = 0.7 ms Density max c = 0.4 ms Low field side c = 0.14 ms Helimak

40. Drive for Velocity Shear: • E x B or j x B? • Plasma floating potentials and Erdecrease at bifurcation,     despite large bias • Threshold voltages for positive and negative bias different • Threshold currents for positive and negative bias similar Helimak

43. Biasing drives current from + plates into plasma along field lines, across the field lines, and back out along field lines to the - plates. For typical threshold currents, <jr> ~ 0.1 A/m2 j X B = dp/dt ~ p/p p = mnVz For p ~ 1 ms, Vzmax ~ 2 km/s Shear, ∂vz/∂r ~ 104 s-1

44. Drive for Velocity Shear: • E x B or j x B? • Plasma floating potentials and Erdecrease at bifurcation • Threshold voltages for positive and negative bias different • Threshold currents for positive and negative bias similar • Symmetric current flow essential to bifurcation; if one • plate isolated to stop current flow, transition absent. • Observations favor j • (Shear flow driven by radial shear in j x B) Helimak

45. Behavior Near Threshold Helimak

46. Normal, Fast Bifurcation • Jump between      two steady      states • Simultaneous      at all radii • No hysteresis;      bias directly      controls      instability Bias Isat(t) at various radii

47. Slow Sweep Through Threshold • Bias always near threshold • Jump between two steady states; sharp threshold, no graded transition • No hysteresis Bias Isat(t) at various radii

48. Helimak