Advances in time-resolved measurement of |B| and Te in low-magnetic-field plasmas

1. Advances in time-resolved measurement of |B| and Tein low-magnetic-field plasmas D. J. Den Hartog University of Wisconsin – Madison Open Magnetic Systems for Plasma Confinement Novosibirsk, Russia 8 July 2010

2. We are developing techniques for internal time-resolved measurement of |B| and Te in a low-field plasma. • Brief introduction to the MST Reversed-Field Pinch • Spectral motional Stark effect (MSE) for magnetic field • High-repetition-rate Thomson scattering for electron temperature Co-authors: J. R. Ambuel, M. T. Borchardt, K. Caspary, E. A. Den Hartog, A. F. Falkowski, W. S. Harris, J. Ko, N. A. Pablant, J. A. Reusch, P. E. Robl, H. D. Stephens, H. P. Summers, Y. M. Yang University of Wisconsin–Madison, University of California–San Diego, University of Strathclyde This work is supported by the U. S. Department of Energy and the National Science Foundation.

3. The Reversed-Field Pinch: toroidal confinement at low |B|. • Toroidal field BT is ~10X smaller in RFP than same-current tokamak • Equilibrium substantially determined by self-generated plasma currents • Internal |B| and Te measurements critical to equilibrium reconstruction

4. MST is a large RFP operated at moderate current. R = 1.5 m, a = 0.5 m, I ≤ 0.6 MA ne~ 1019 m-3, Te, Ti ≤ 2 keV

5. Spectral motional Stark effect

6. Two compact and reliable diagnostic neutral beams are installed on MST. • Two beams from the Budker Institute • 50 keV H beam for MSE (and CHERS) • 20 ms duration • 20 keV He beam for Rutherford scattering • 3 ms duration • Beams have relatively low divergence and small energy spread

7. Spectral MSE diagnostic measures magnetic field using the hydrogen neutral beam. • Beam of H atoms is excited and radiates as it traverses plasma plasma n = 3 • H0 electron energy levels are split by motional E = vbeam × B • Measure emitted radiation pattern, know vbeam, calculate B H n = 2 p+ s p-

8. The MSE diagnostic has an on-axis view and a new mid-radius view.

9. Spectral MSE technique measures B ≥ 0.2 T. • For low |B|, vbeam B is small (~1 MV/m) • Stark components overlap • Data is fit with sophisticated model of Stark spectrum • New mid-radius view gives both |B| and pitch angle γ Spectra from mid-radius views with orthogonal polarization 400 fit sigma |B| = 0.42 ± 0.07 T 300 g = 40.3° ± 9.4° amplitude (counts) pi 200 100 0 wavelength wavelength

10. The spectral MSE diagnostic enables precise time-resolved measurement of internal magnetic field. MSE on-axis |B|, event ensemble 0.42 0.40 |B| (T) 0.38 0.36 100 µsec shuttering 0.34 -2.0 -1.0 0 1.0 2.0 time relative to reconnection event (ms)

11. High-repetition-rate Thomson scattering

12. Standard commercial flashlamp-pumped Nd:YAG lasers have been upgraded to “pulse-burst” capability. • A single laser produces a burst of up to fifteen 2 J Q-switched pulses • repetition rates variable 1–12.5 kHz Burst of 15 pulses at 1 kHz repetition rate

13. The Thomson scattering diagnostic on the MST RFP uses two upgraded lasers. • Collect thirty Te profiles during a single MST discharge • at rates from 1–25 kHz (with two interleaved lasers) • each Te profile consists of twenty spatial points • detailed measurements in an overdense plasma

14. The laser head was a standard commercial flashlamp-pumped Nd:YAG. • “As manufactured” operation: • single flashlamp pump pulse • single Q-switch • reliably produced one 2 J, 9 ns laser pulse • First upgrade was to take direct control of Pockels cell Q-switching • optimize optical energy extraction from laser rod

15. Major upgrade was to take control of flashlamp pulsewidth and repetition rate. • Original flashlamp drive was a simple inductor/capacitor pulse-forming network • produced a single 100 µs pump pulse • New system based on solid-state switching of large electrolytic capacitor banks • produces multiple pulses of adjustable width at variable repetition rates

16. Most typical mode of operation is to drive flashlamps with a burst of fifteen 150 µs pulses at a repetition rate of 1 kHz. • applied voltage in black • flashlamp current in red • The Pockels cell is switched near the end of each flashlamp pulse • produces a train of fifteen ~2 J pulses from the laser

17. Flexible control of flashlamp drive and Q-switch enables new Thomson scattering capabilities. • For example, to measure fast Te dynamics: • drive the flashlamps with a burst of five wider pulses at 1 kHz rep rate • switch the Q-switch three times during each flashlamp pulse • produce three laser pulses (80 µs separation) from each flashlamp pulse Two lasers interleaved to produce six pulses at 25 kHz every ms

18. In MST, structure in the Te profile is strongly correlated with rapidly rotating magnetic island structure. Helical Structure Flattening • m = 1 helical island chains rotate toroidally at ~10 kHz • Correlate many Te profile measurements with phase of island • Able to reconstruct modulation of Te profile correlated with island position

19. Next step pulse-burst laser system for fast Thomson scattering is being commissioned. • Laser will produce a burst of ~1 J pulses at rep rates up to 250 kHz • low duty cycle (~2 min) for these “bursts”

20. Nd:YAG stages are complete, Nd:glass amplifier is being commissioned.

21. Summary • The spectral MSE diagnostic records the entire hydrogen Stark spectrum • precise time-resolved measurement of internal magnetic field • Two Nd:YAG lasers have been upgraded to “pulse-burst” capability • These lasers are used in the Thomson scattering system to • record the dynamic evolution of the Te profile • measure Te fluctuations associated with large-scale tearing modes