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Equilibrium and Stability of RELAX

Equilibrium and Stability of RELAX. S.Masamune Kyoto Institute of Technology, Kyoto 606-8585, Japan.

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Equilibrium and Stability of RELAX

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  1. Equilibrium and Stability of RELAX S.Masamune Kyoto Institute of Technology, Kyoto 606-8585, Japan A.Sanpei, R.Ikezoe, T. Onchi, K.Oki, T.Yamashita, Y.Konishi, M.Nakamura,M.Sugihara, A.Higashi, H.Motoi, H. Himura, N.Nishino1), R.Paccagnella2) , A.Ejiri3), T.Akiyama4), K.Nagasaki5), J.K.Anderson6), D.J. Den Hartog6), H.Koguchi7) Kyoto Institute of technology, Kyoto, Japan 1)Hiroshima University, Higashi-hiroshima, Japan 2)Consorzio RFX, Padova, Italy 3)University of Tokyo, Kashiwa, Japan 4) National Institute for Fusion Science, Toki, Japan 5)Kyoto Univerisyt, Uji, Japan 6)University of Wisconsin, Madison, USA 7) National Institute for Advanced Industrial Science and Technology (AIST), Tsukuba, Japan

  2. REversed field pinch of Low-Aspect-ratio eXperiment • R/a = A = 2 (51 cm/25 cm) • Optimization in progress Kyoto Institute of Technology

  3. more space in the core region Lower A means lower n for dominant m = 1 modes

  4. Goal of RELAX experiment • Experimental study on advantages of low-A RFP configuration - Improved confinement with QSH forachieving high beta - Experimental identification of bootstrap current (targer parameters: Te~300eV, ne~4×1019m-3 at Ip~100kA)

  5. Outline - Descharge regimes in Θ-F space in RELAX Extremely deep reversal regime Shallow reversal regime • Helical State Helical hot core - Magnetic boundary control -- status

  6. RFP plasmas in RELAX • Low-aspect-ratio RFP plasmas: • Ip < 100 kA，ne = 1018～ 2 x 1019 m-3， • Te < 100 eV，τD～2 ms • MHD properties have been studied in discharges during flat-topped phase for 0.5ms with40kA < Ip < 100kA

  7. PPCD& IHTM BFM F and Θ keep some relation over wide range of parameters • In shallow reversal region, • Periodic QSH or Helical Ohmic RFP state tends to be realized • In deep reversal, high-Θ region, • Amplitudes of resonant modes are suppressed significantly • SXR emission increases, indicating improved plasma performance Medium to high aspect ratio RFP

  8. F dependence of amplitudes of magnetic fluctuationshows strong F dependence of resonant modes Trend: m=1 fluctuation amplitudes decrease with deepening field reversal Amplitudes of resonsnt modes are more sensitive to F non-resonant core-resonant edge-resonant

  9. Extremely deep reversal discharge Extremely deep reversal, high-Θ region is accessible without sawtooth crash or discrete dynamo event. Rapid rotation of the resonant MHD modes may be related with the low edge magnetic fluctuation amplitudes. MHD Stability in extremely deep reversal regime may be revisited in toroidal geometry.

  10. q profiles and toroidal mode spectrum of m=1 modes （-0.1 < F < 0） （-0.8 < F < -0.6） • Shallow reversal region: Dominant modes are core resonant m=1/n=4,5,6. Toroidal mode spectrum can be accounted for by q profile. • Deep reversal region: Amplitudes of m=1 modes are reduced to ~1/4. Spectrum becomes broader • Increased magnetic shear may contribute to the lower fluctuation amplitudes in deep reversal case. • No evidence of externally resonant mode? q profiles from equilibrium reconsytuction code RELAXFit

  11. Dependence of Soft-X ray emission intensity on F SXR emission: a measure of plasma performance • SXR emission tends to increase by deepeneng the field reversal • two distinctive regions, shallow-F and deep-F • density and temperature measurements are required for estimate of plasma performance

  12. m = 1 amplitude (a.u.) Toroidal mode number Characteristic phenomena in shallow-reversal region- Helical structure observed with high-speed camera - Visible light image n = 4 NOTE: Recent observation shows toroidal rotation of the simple helix

  13. Characteristic phenomena in shallow-reversal region- SXR images suggest helical hot core in QSH - <during multi-helicity state> filament SXR pin-hole camera Experimantal result 5-μs exposure < during m/n=1/4 QSH> トロイダルモード スペクトル Experimental result

  14. Detailed comparison with simulated imagesindicates helical hot core < m=1/n=4mode dominant case> Experimental image A model contours of SXR emissivity for a large island (a=3, w=15cm) Simulated image (a=3, w=15cm,rn=4=14cm) Horizontal intensity profiles Vertical intensity profile

  15. Characteristic phenomena in shallow-reversal region SXR emission is centrally peaked, with slight change in time ☆multi-chord SXR emission measurement with AXUV (Al 2mmt filter) AXUVarray p: impact parameter Time evolution of SXR emission profile Quasi-periodic change in SXR profile -with frequency of ~10kHz -change in peak may indicate rotation of helical structure in SXR emission? Change in gradient Change in peak

  16. Magnetic field profile in shallow-reversal regions is characterized by Helical Ohmic Equilibrium state Symbols: measured profiles with radial array of magnetic probes. Solid lines: Ohmic helical equilibrium solution (Paccagnella (2000)), details are in the next slide. Shafranov shift Δ/a ~ 0.2. The helical structure rotates at a frequency of ~10 kHz. Axisymmetric components m=1 helical components

  17. Helical Ohmic Equilibrium Solution - Helical Equilibrium and Ohm’s Law - Grad-Shafranov equation with helical symmetry Ohm’s law with helical symmetry ( ) Contours of helical flux function Theta=1.78, F=-0.06, beta=0.1 (courtesy of R.Paccagnella)

  18. Feedback control of magnetic boundary at the gap with saddle coils Block diagram for control system Poloidal gap with four saddle coils (in red) Four saddle coils →control of m=1 component saddle coil Current driver using IGBT

  19. RFP performance is improved using the feedback control at the gap w/o control with control threshold Reduction of gap flux (field errors) results in improved discharge →longer discharge duration horizontal field for further improvements Covering the vacuum vessel with4×16 (or 32) saddle coils Use of digital feedback controller for flexibility vertical field 0 0.5 1.0 1.5 2.0 t (ms)

  20. RWM is problematic for longer pulse operation in RELAX =>Ip starts decreasing Br (m=1/n=2) measured on the outer surface of the vacuum vessel 3-D nonlinear MHD simulation predicts RWM in RELAX (Paccagnella, 2008) Initial growth of m=1/n=-4 resonant mode, followed by growth of the non-resonant m=1/n=2 external kink mode with growth time of resistive wall time constant

  21. 32 (or 16) toroidally, 4 • poloidally separated saddle coils • for feedback control of RWM Magnetic boundary control plan in RELAX • Construction of the power supplies • in progress • Introduction of digital feedback • controllers under discussion

  22. Summary • RFP plasmas with MHD properties characteristic to low-A configuration have been attained in RELAX. • Two characteristic regimes for possible improved performance have been found. • Rotating helical structure have been observed. • Possible Helical Ohmic Equilibrium state with hot core has been demonstrated. • Feedback control of magnetic boundaries are in progress for further improvement of plasma performance.

  23. backup slides

  24. Characteristic phenomena in shallow-reversal region- Possibility of rotating Helical Ohmic Equilibrium state -A large-scale magnetic field profile change • Quasi-periodic oscillation between reversed and non-reversed states • Similar large-scale oscillatory behavior in Br and B B (mT) Ip (kA)

  25. Characteristic phenomena in deep-reversal region Phase locking are influenced by field reversal （-0.1 < F < 0.1，Pf = 1.6 mTorr） • In shallow reversal discharges, locked mode tends to occur frequently Phase dispersion <= Resonant surfaces are closer to the wall （-0.8 < F < -0.6，Pf = 0.1 mTorr） • In deep reversal discharges, locked mode tends to be unlocked, probaly due to the increased distances between the wall and resonant surfaces • Phase locking also tends to be unlocked, due to the lower mode amplitude?

  26. Digital feedback controller using BlackFin board Digital controller developed for ICC experiments ・clock frequency: 500MHz ・SDRAM 64MB ・8 programmable timers ・12bit 1MHz digitizer ・digital signal processor applicable 　 → feedback control scheme Courtesy of Prof. B. Nelson (U. Washington), Profs. Y.Kikuchi, M.Nagata (U. Hyogo)

  27. Characteristic phenomena in shallow-reversal region SXR emission is centrally peaked, with slight change in time ☆multi-chord SXR emission measurement with AXUV (Al 2mmt filter) AXUVarray Time evolution of SXR emission profile p: impact parameter Quasi-periodic change in SXR profile -with frequency of ~10kHz -change in peak may indicate rotation of helical structure in SXR emission? Change in gradient Change in peak

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