Electron Thermal Transport Within Magnetic Islands in the RFP

Transcription

1 Electron Thermal Transport Within Magnetic Islands in the RFP Hillary Stephens University of Wisconsin Madison APS-DPP Meeting November 3, 2009

2 J.R. Amubel, M.T. Borchardt, D.J. Den Hartog, C.C. Hegna, D.J. Holly, A.F. Falkowski, E. Parke, J.A. Reusch, P. E. Robl, Y.M. Yang, and MST Team University of Wisconsin-Madison Center for Magnetic Self-Organization This work was supported by the U. S. Department of Energy, National Science Foundation, and Fusion Energy Sciences Fellowship.

3 Burst mode Thomson scattering enables investigations into tearing modes and electron thermal transport. Thomson scattering lasers Burst mode capability enables study of fast dynamics Able to see structures in Te profile Implications for electron thermal transport Flattening Helical Structure

4 Outline New Thomson Scattering Diagnostic Capabilities The Madison Symmetric Torus Tearing Mode and Temperature Correlation Experimental Results Temperature profile flattening with remnant islands Peaked helical temperature profile with mode resonant near axis Low fluctuations with inductive current profile control Summary

5 New Thomson scattering diagnostic capabilities open up new opportunities to study Te dynamics. Basic capabilities Electron temperature (not calibrated for density) 21 spatial locations (from core to edge) < 2 cm resolution Temperature range: 10 ev to 10 kev NEW capabilities Up to 30 time points per shot 2 khz continuous operation Burst mode operation: 6 pulses at 12.5 khz every 1 ms

6 The Madison Symmetric Torus (MST) is a Reversed Field Pinch (RFP) Major radius: R = 1.5 m Minor radius: r = 0.5 m Plasma currents: 400 ka Electron density ~1 x 1013 cm-3

7 Many magnetic tearing modes are resonant in MST. Magnetic field lines are twisted in MST. Tearing mode resonant k B=0 (m/r)b p +(n/r)b t =0 q(r)=m/n Tearing modes result in magnetic islands.

8 Tearing mode growth is dynamic and quasi-periodic. Peaked parallel current is a free energy source for tearing modes. Non-linear tearing mode growth Leads to abrupt changes: sawtooth crash Focus on n=5 and n=6 Magnetic Fluctuations Wb

9 As tearing modes grow and overlap the magnetic field becomes stochastic. Heat is transported outward across magnetic islands. Simulated field line trajectories - Show remnant n=6 island r(m)

10 Electron temperature can be mapped to magnetic perturbations. Modes rotate toroidally and poloidally around MST (~10 khz). Magnetic mode position measured with coil arrays. Data (measured) Position of magnetic mode (measured) Te fluctuation amplitude for a given mode (unknown) Average Te for a given r/a (unknown)

11 Isothermal islands lead to temperature perturbations with a phase flip across the resonant surface. With Te constant inside island: Te is lower than circle average at O-point for inner circle. Te is constant for middle circle (resonant surface). Te is higher than circle average at O-point for outer circle.

12 Islands with a temperature peak have no phase flip. With a temperature peak inside island structure: Fluctuations are highest for circle sampling hot spot. Fluctuations are in phase (same sign) for all flux surfaces.

13 Between sawteeth a temperature flattening is consistent with remnant island structure. Te flattening across m=1, n=6 island Resonant surface is clearly defined Fluctuation levels ~10-20 ev in a background of ~300 ev

14 Fluctuations associated with tearing modes are not present at the sawtooth crash, consistent with a fully stochastic field. Temperature fluctuations show a strong sawtooth dependence. As magnetic field becomes stochastic n=6 structures disappear. Reinforces the observation that the field is not stochastic before the crash.

15 After the sawtooth crash a helical peaked Te profile appears. m=1, n=5 mode appears after a sawtooth crash: q(0) ~ 0.2 A strikingly different temperature characteristic Fluctuation does not flip sign - temperature peaking

16 Electron thermal diffusion is low inside m=1, n=5 structure. Correlated (m=1, n=5) temperature structure appears ~0.5 ms after crash Fluctuations peak between 1-2 ms after crash Local thermal diffusion decreases inside structure temperature peaks

17 The reason for different n=5 and n=6 Te structures is unclear. m=1, n=6 is always resonant - removed from axis m=1, n=5 mode is resonant close to axis OR Proper island structure cannot form m=1, n=5 mode is NOT resonant m=1, n=5 mode is an ideal kink

18 3D resistive simulations show a different magnetic field structure for the n=5 and n=6 modes. n=6 Flux surfaces inside island have been washed out r(m) Before sawtooth 0.5 n=5 Intact flux surfaces inside island r(m) After sawtooth 0.5

19 Inductive parallel current profile control reduces magnetic tearing modes. Parallel current profile can be controlled Free energy source for tearing modes is reduced Magnetic fluctuations are small Standard J II control Standard J II control

20 Electron temperature fluctuations associated with tearing modes are reduced with inductive current profile control. Temperature fluctuations are low ~ 2% of mean (compared to 10-30% in Standard Plasmas). 400 ka J II control

21 Summary New Thomson scattering diagnostic capabilities enable studies of fast electron dynamics Between sawteeth, data suggests: remnant m=1, n=6 island structure field is not fully stochastic After a sawtooth m=1, n=5 helical temperature structure low thermal diffusion in structure With inductive parallel current profile control no discernable electron temperature structure associated with tearing modes **Physics progress of Reversed Field Pinch magnetic confinement** - John Sarff, 8 am Wednesday ** MST Poster Session Thursday Morning**