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

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Presentation transcript:

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

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

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

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

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

Standard induction produces a peaked current profile, unstable to MHD tearing (free energy r J || /B ). Standard RFP Ohms law imbalance characteristic of steady induction in the RFP

Standard induction produces a peaked current profile, unstable to MHD tearing (free energy r J || /B ). Standard RFP Ohms law imbalance characteristic of steady induction in the RFP multiple dynamo-like effects possible (several observed)

Poloidal inductive current drive targeted to outer region reduces MHD tearing instability. Measured E(r) Profiles Pulsed Poloidal Current Drive ms

Magnetic fluctuations reduced at all scales & frequencies. Long wavelength amplitude spectrum Frequency (kHz) (T 2 /Hz) Frequency power spectrum PPCD Standard Toroidal Mode, n

Dynamo essentially absent with PPCD. PPCDStandard RFP Simple Ohms law satisfied strong dynamoweak dynamo (simple Ohms law satisfied)

Electron T e and energy confinement increase. PPCD

Stochastic magnetic diffusivity and heat transport reduced 30-fold in core. (m 2 /s) r/a field line tracing where magnetic chaos is strong (several overlapping islands) measured predicted Rechester-Rosenbluth PPCD Standard R-R

Anomalous ion heating probably reduced. T i (r) Profiles Standard: P e-i < P CX and T i / T e ~ 1 anomalous ion heating must occur PPCD: P e-i P CX and T i / T e ~ 0.5 collisional ion heating only?? PPCD

Anomalous ion heating probably reduced. T i (r) Profiles Standard: P e-i < P CX and T i / T e ~ 1 anomalous ion heating must occur PPCD: P e-i P CX and T i / T e ~ 0.5 collisional ion heating only?? PPCD

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

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

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

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

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

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

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

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

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

AC helicity injection using oscillating loop voltages. apply oscillating V Magnetic helicity balance evolution: (Standard RFP: V, = constant)

AC helicity injection using oscillating loop voltages. apply oscillating V Magnetic helicity balance evolution: (Standard RFP: V, = constant)

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

Between-crash heating should help identify anomalous ion heating mechanism. sawtooth crash smaller heating at applied frequency

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. MSTs CMSO plans systematically include PPCD, q > 0, OFCD, etc. as tools to expose underlying physics.

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.