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Advanced Plasma Control Techniques for Magnetic Chaos Suppression

Explore innovative strategies in plasma control to reduce magnetic chaos and enhance self-organization processes in the RFP system, focusing on MHD tearing and relaxation adjustments. Techniques include current profile tuning, helicity injection, and mean-field adjustment.

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Advanced Plasma Control Techniques for Magnetic Chaos Suppression

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  1. Control of Magnetic Chaos & Self-Organization John Sarff for MST Group CMSO General Meeting • Madison, WI • August 4-6, 2004

  2. Plasma control permits adjustment of magnetic reconnection and self-organization processes in the RFP. Example: Reduce tearing fluctuations and magnetic chaos by current profile control. Adjust Current Drive Toroidal, f Toroidal, f r/a r/a

  3. Outline. Control MHD tearing and consequent relaxation processes by: • Adjustments to inductive current drive • Reduce tearing by matching E(r) to more stable J(r) • AC helicity injection (oscillating loop voltages) • Adjustment of mean-field B(r) to include/exclude resonant surfaces • Tuning for empirically different resonant mode spectra, e.g., quasi-single-helicity (QSH) Other control techniques used previously: – electrostatic probe biasing (edge current drive & rotation control) – helical magnetic perturbations from external coils

  4. Outline. Control MHD tearing and consequent relaxation processes by: • Adjustments to inductive current drive • Reduce tearing by matching E(r) to more stable J(r) • AC helicity injection (oscillating loop voltages) • Adjustment of mean-field B(r) to include/exclude resonant surfaces • Tuning for empirically different resonant mode spectra, e.g., quasi-single-helicity (QSH) Other control techniques used previously: – electrostatic probe biasing (edge current drive & rotation control) – helical magnetic perturbations from external coils

  5. Magnetic reconnection (resonant tearing) occurs at many radii in the RFP’s sheared magnetic field. Tearing resonance: RFP Magnetic Geometry

  6. “Standard” induction produces a peaked current profile, unstable to MHD tearing (free energy rJ||/B ). Standard RFP Ohm’s law imbalance characteristicof steady induction in the RFP

  7. “Standard” induction produces a peaked current profile, unstable to MHD tearing (free energy rJ||/B ). Standard RFP Ohm’s law imbalance characteristicof steady induction in the RFP multiple dynamo-like effects possible(several observed)

  8. Poloidal inductive current drive targeted to outer region reduces MHD tearing instability. “Pulsed Poloidal Current Drive” (PPCD) Measured E(r) Profiles @15 ms

  9. Magnetic fluctuations reduced at all scales & frequencies. • Long wavelength amplitude spectrum • Frequency power spectrum Standard (T2/Hz) PPCD Toroidal Mode, n Frequency (kHz)

  10. Dynamo essentially absent with PPCD. Standard RFP PPCD Simple Ohm’s law satisfied strong dynamo weak dynamo (simple Ohm’s law satisfied)

  11. Electron Te and energy confinement increase. PPCD PPCD

  12. Stochastic magnetic diffusivity and heat transport reduced 30-fold in core. Standard PPCD field line tracing predicted “Rechester-Rosenbluth” cR-R measured measured (m2/s) where magnetic chaos is strong (several overlapping islands) cR-R r/a r/a

  13. Anomalous ion heating probably reduced. • Standard: Pe-i < PCX and Ti / Te ~ 1  anomalous ion heating must occur • PPCD: Pe-i ≥ PCX and Ti / Te ~ 0.5  collisional ion heating only?? Ti(r) Profiles PPCD

  14. Anomalous ion heating probably reduced. • Standard: Pe-i < PCX and Ti / Te ~ 1  anomalous ion heating must occur • PPCD: Pe-i ≥ PCX and Ti / Te ~ 0.5  collisional ion heating only?? Ti(r) Profiles PPCD

  15. Outline. Control MHD tearing and consequent relaxation processes by: • Adjustments to inductive current drive • Reduce tearing by matching E(r) to more stable J(r) • AC helicity injection (oscillating loop voltages) • Adjustment of mean-field B(r) to include/exclude resonant surfaces • Tuning for empirically different resonant mode spectra, e.g., quasi-single-helicity (QSH) Other control techniques used previously: – electrostatic probe biasing (edge current drive & rotation control) – helical magnetic perturbations from external coils

  16. Nonlinear mode coupling appears important for anomalous momentum transport. • Nonlinear torque: (plasma rotation) force on n=6

  17. Adjusting B(r) to exclude m = 0 resonance greatly reduces momentum loss & ion heating during relaxation events. Shift q > 0 to removem = 0 resonance

  18. Adjusting B(r) to exclude m = 0 resonance greatly reduces momentum loss & ion heating during relaxation events. No sudden rotation loss with small m = 0

  19. Outline. Control MHD tearing and consequent relaxation processes by: • Adjustments to inductive current drive • Reduce tearing by matching E(r) to more stable J(r) • AC helicity injection (oscillating loop voltages) • Adjustment of mean-field B(r) to include/exclude resonant surfaces • Tuning for empirically different resonant mode spectra, e.g., quasi-single-helicity (QSH) Other control techniques used previously: – electrostatic probe biasing (edge current drive & rotation control) – helical magnetic perturbations from external coils

  20. Under come conditions, the tearing spectrum is dominated by one mode. Spontaneous “Quasi-Single Helicity” (QSH) Soft x-ray image RFX MST

  21. Magnetic & velocity fluctuations are single-mode dominated. QSH Standard (mT) QSH Standard (km/s)

  22. MHD dynamo is single-mode dominated in QSH. QSH Standard (V/m)

  23. Outline. Control MHD tearing and consequent relaxation processes by: • Adjustments to inductive current drive • Reduce tearing by matching E(r) to more stable J(r) • AC helicity injection (oscillating loop voltages) • Adjustment of mean-field B(r) to include/exclude resonant surfaces • Tuning for empirically different resonant mode spectra, e.g., quasi-single-helicity (QSH) Other control techniques used previously: – electrostatic probe biasing (edge current drive & rotation control) – helical magnetic perturbations from external coils

  24. AC helicity injection using oscillating loop voltages. • Magnetic helicity balance evolution: apply oscillatingVf , F : (Standard RFP: Vf , F = constant)

  25. AC helicity injection using oscillating loop voltages. • Magnetic helicity balance evolution: apply oscillatingVf , F : (Standard RFP: Vf , F = constant)

  26. MHD behavior is altered when AC loop voltage applied. relaxation events entrained AC volts on increasebetween crash (V) m = 0 (G) m = 1 (G) Time (ms)

  27. Between-crash heating should help identify anomalous ion heating mechanism. sawtooth crash smaller heating atapplied frequency

  28. Summary. • Several methods to control and adjust MHD tearing-reconnection have been developed for the RFP. • Characteristics and strength of consequent relaxation processes are adjustable. • MST’s CMSO plans systematically include “PPCD”, “q > 0”, “OFCD”, etc. as tools to expose underlying physics.

  29. Tearing occurs spontaneously, both from linear instability and nonlinear mode coupling. Core-resonant m=1 modes are largest, calculated to be linearly unstable from . Edge-resonant m=0 modes grow from nonlinear coupling to the unstable m=1 modes.

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