Resistive wall modes in a reversed field pinch with interchangeable shells

Greene, Philip; Barrick, Greg; Robertson, Scott

doi:10.1063/1.860107

Cited by 11 publications

(7 citation statements)

References 19 publications

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“…Numerical three-dimensional, non-linear resistive MHD simulations of the RFP dynamo process have shown that thin shell operation would be characterized by enhanced TM fluctuation levels and higher, anomalous toroidal loop voltages, mainly associated with surface magnetic helicity losses due to the finite radial magnetic fields at the plasma boundary [3][4][5]. Initial experimental results from thin shell RFPs were in qualitative agreement with the numerical simulations, showing growing modes and increased toroidal loop voltages [6,7]. Since the RFP plasma normally rotates, the resonant TMs can be convected by the plasma fluid at the mode rational surfaces and if the TM rotation frequencies are high compared with the inverse shell time, the associated perturbed radial fields near the shell remain small, and the surface magnetic helicity losses associated with the TMs are not present.…”

Section: Introductionsupporting

confidence: 56%

Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R

et al. 2006

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Discharges in the thin shell reversed field pinch (RFP) device EXTRAP T2R without active feedback control are characterized by growth of non-resonant m = 1 unstable resistive wall modes (RWMs) in agreement with linear MHD theory. Resonant m = 1 tearing modes (TMs) exhibit initially fast rotation and the associated perturbed radial fields at the shell are small, but eventually TMs wall-lock and give rise to a growing radial field. The increase in the radial field at the wall due to growing RWMs and wall-locked TMs is correlated with an increase in the toroidal loop voltage, which leads to discharge termination after 3-4 wall times. An active magnetic feedback control system has been installed in EXTRAP T2R. A two-dimensional array of 128 active saddle coils (pair-connected into 64 independent m = 1 coils) is used with intelligent shell feedback control to suppress the m = 1 radial field at the shell. With feedback control, active stabilization of the full toroidal spectrum of 16 unstable m = 1 non-resonant RWMs is achieved, and TM wall locking is avoided. A three-fold extension of the pulse length, up to the power supply limit, is observed. Intelligent shell feedback control is able to maintain the plasma equilibrium for 10 wall times, with plasma confinement parameters sustained at values comparable to those obtained in thick shell devices of similar size.

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Section: Introductionsupporting

confidence: 56%

Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R

et al. 2006

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“…Alternatively, an active feedback control would be required to suppress the dominant MHD activities. The importance of the study of the effects of the electromagnetic boundary conditions on the RFP plasma characteristics has long been recognized, and many experimental [4][5][6][7][8][9] and theoretical [10][11][12][13][14][15] studies have been conducted so far. All of the previous experiments were conducted under extreme boundary conditions using a resistive shell for which the time required for penetration by an external vertical field is much smaller than the expected pulse duration time.…”

Section: Introductionmentioning

confidence: 99%

“…The experimental studies on the electromagnetic boundary conditions of the RFP plasmas have been conducted mainly under conditions of extreme resistivity or thickness of the shell. There have been several resistive/thin shell experiments [4][5][6][7][8][9] where the resistivity and the geometrical parameters were selected so that τ s was comparable to or smaller than the pulse duration time, τ d . Table 1 summarizes these resistive/thin shell experiments.…”

Section: Introductionmentioning

confidence: 99%

Effects of shell configuration on the performance of reversed field pinch plasmas

Yagi¹,

Maejima²,

Zollino³

et al. 2000

Nucl. Fusion

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The experimental results of three different reversed field pinch machines at the Electrotechnical Laboratory in Tsukuba are compared. In particular, the results are summarized in terms of the difference in the configuration of the conductive shell system. The three configurations correspond to TPE-1RM15 (b/a = 1.18), TPE-1RM20 (b/a = 1.08) and TPE-1RM20mod (b/a = 1.12), where b/a is a measure of the shell proximity, and a and b are the minor radii of the plasma and the inner surface of the shell closest to the plasma, respectively. The magnetic fluctuations, global confinement properties and mode locking events induced by the external field error at the thick shell gap are studied. It is shown that the magnetic fluctuation amplitude increases with b/a, as expected from the linear growth rate of the dominant MHD modes, and that a multilayered, close fitting shell system improves the performance under extreme operating conditions in the high pinch parameter region and when external field errors are considered, while the performance under normal operating conditions is comparable within the shot to shot deviations of the data.

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“…3 For RFP devices using a thin shell with a time constant shorter than the plasma pulse length a range of unstable RWMs are expected. [6][7][8] Since the growth time typically is of the order of milliseconds, active feedback stabilization of the modes using external magnetic field coils is feasible. [6][7][8] Since the growth time typically is of the order of milliseconds, active feedback stabilization of the modes using external magnetic field coils is feasible.…”

Section: Introductionmentioning

confidence: 99%

Resistive wall mode feedback control in EXTRAP T2R with improved steady-state error and transient response

et al. 2007

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Experiments in the EXTRAP T2R reversed field pinch [P. R. Brunsell, H. Bergsåker, M. Cecconello et al., Plasma Phys. Control. Fusion 43, 1457 (2001)] on feedback control of m=1 resistive wall modes (RWMs) are compared with simulations using the cylindrical linear magnetohydrodynamic model, including the dynamics of the active coils and power amplifiers. Stabilization of the main RWMs (n=−11,−10,−9,−8,+5,+6) is shown using modest loop gains of the order G∼1. However, other marginally unstable RWMs (n=−2,−1,+1,+2) driven by external field errors are only partially canceled at these gains. The experimental system stability limit is confirmed by simulations showing that the latency of the digital controller ∼50μs is degrading the system gain margin. The transient response is improved with a proportional-plus-derivative controller, and steady-state error is improved with a proportional-plus-integral controller. Suppression of all modes is obtained at high gain G∼10 using a proportional-plus-integral-plus-derivative controller.

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Resistive wall modes in a reversed field pinch with interchangeable shells

Cited by 11 publications

References 19 publications

Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R

Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R

Effects of shell configuration on the performance of reversed field pinch plasmas

Resistive wall mode feedback control in EXTRAP T2R with improved steady-state error and transient response

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