Experiments and modelling of external kink mode control using modular internal feedback coils

Pedersen, T. S.; Maurer, D.A.; Bialek, J.; Katsuro-Hopkins, O.; Hanson, J.M.; Mauel, M. E.; James, R. W.; Klein, A.; Liu, Y.; Navratil, G.A.

doi:10.1088/0029-5515/47/9/028

Cited by 15 publications

(11 citation statements)

References 22 publications

(35 reference statements)

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“…Similarly, n ¼ 1 feedback experiments in HBT-EP have shown that when control coils at one out of the five toroidal positions are turned off, the reduction of RWM amplitude at that location is weaker than at the active locations. 178 This "non-rigid" behavior suggests coupling to other toroidal mode numbers. Coupling between toroidal modes can also be induced by non-axisymmetry of the resistive wall or other external conducting structures.…”

Section: E Feedback: Discussion and Challengesmentioning

confidence: 99%

Magnetic control of magnetohydrodynamic instabilities in tokamaks

Strait

2014

Physics of Plasmas

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Externally applied, non-axisymmetric magnetic fields form the basis of several relatively simple and direct methods to control magnetohydrodynamic (MHD) instabilities in a tokamak, and most present and planned tokamaks now include a set of non-axisymmetric control coils for application of fields with low toroidal mode numbers. Non-axisymmetric applied fields are routinely used to compensate small asymmetries (δB/B∼10−3 to 10−4) of the nominally axisymmetric field, which otherwise can lead to instabilities through braking of plasma rotation and through direct stimulus of tearing modes or kink modes. This compensation may be feedback-controlled, based on the magnetic response of the plasma to the external fields. Non-axisymmetric fields are used for direct magnetic stabilization of the resistive wall mode—a kink instability with a growth rate slow enough that feedback control is practical. Saturated magnetic islands are also manipulated directly with non-axisymmetric fields, in order to unlock them from the wall and spin them to aid stabilization, or position them for suppression by localized current drive. Several recent scientific advances form the foundation of these developments in the control of instabilities. Most fundamental is the understanding that stable kink modes play a crucial role in the coupling of non-axisymmetric fields to the plasma, determining which field configurations couple most strongly, how the coupling depends on plasma conditions, and whether external asymmetries are amplified by the plasma. A major advance for the physics of high-beta plasmas (β = plasma pressure/magnetic field pressure) has been the understanding that drift-kinetic resonances can stabilize the resistive wall mode at pressures well above the ideal-MHD stability limit, but also that such discharges can be very sensitive to external asymmetries. The common physics of stable kink modes has brought significant unification to the topics of static error fields at low beta and resistive wall modes at high beta. These and other scientific advances, and their application to control of MHD instabilities, will be reviewed with emphasis on the most recent results and their applicability to ITER.

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Section: E Feedback: Discussion and Challengesmentioning

confidence: 99%

Magnetic control of magnetohydrodynamic instabilities in tokamaks

Strait

2014

Physics of Plasmas

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“…Passive kink stability can be studied by moving the wall itself, instead of changing the location of the plasma with respect to a fixed wall. HBT-EP has also pioneered the use of active magnetic feedback to control the RWM [15][16][17][18][19]. RWMs have been suppressed or enhanced as a function of feedback parameters.…”

Section: Introductionmentioning

confidence: 99%

Multimode observations and 3D magnetic control of the boundary of a tokamak plasma

Levesque¹,

Rath²,

Shiraki³

et al. 2013

Nucl. Fusion

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We present high-resolution detection and control of the 3D magnetic boundary in the High Beta Tokamak-Extended Pulse (HBT-EP) device. Measurements of non-axisymmetric radial and poloidal fields are made using 216 magnetic sensors positioned near the plasma surface. Control of 3D fields is accomplished using 40 independent saddle coils attached to the passive stabilizing wall. The control coils are energized with high-power solid-state amplifiers, and massively parallel, high-throughput feedback control experiments are performed using low-latency connections between PCI Express analogue input and output modules and a graphics processing unit. The time evolution of unstable and saturated wall-stabilized external kink modes are studied with and without applying magnetic perturbations using the control coils. The 3D dynamic structure of the magnetic field surrounding the plasma is determined through biorthogonal decomposition using the full set of magnetic sensors without the need to fit either a Fourier or a model-based basis. Naturally occurring external kinks are composed of multiple independent helical modes. Smooth transitions between dominant poloidal mode numbers are observed for simultaneous n = 1 and n = 2 modes as the edge safety factor changes. Relative amplitudes of coexistent m/n = 3/1 and 6/2 modes depend on the plasma's major radius and edge safety factor. When stationary 3/1 magnetic perturbations are applied, the resonant response can be linear, saturated, or disruptive, depending upon the perturbation amplitude and the edge safety factor; increased plasma–wall interactions from the perturbed plasma are proposed as a saturation mechanism. Initial feedback experiments have used 40 sensors and 40 control coils, producing mode amplification or suppression, and acceleration or deceleration depending on the feedback phase angle.

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“…The control of magnetohydrodyamic (MHD) instabilities with active feedback is a promising technique for maintaining the plasma confinement in nuclear fusion systems with improved economic viability [1][2][3]. Development of advanced feedback algorithms has been shown to reduce the number of necessary sensors and actuators, as well as the required control power, without compromising feedback performance [4][5][6][7][8][9], which translates directly to simpler engineering and increased economic efficiency of any fusion plant.…”

Section: Introductionmentioning

confidence: 99%

Adaptive feedback control of rotating external kink modes in HBT-EP

et al. 2013

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An advanced adaptive control algorithm is used to study the control of external kink modes that are excited in the HBT-EP tokamak and have a natural toroidal rotation frequency near 8 kHz. The controller's system model is parametrized and the parameters are adjusted in real-time to match the measured plasma evolution. Depending upon a programmed phase angle, active feedback control is shown to either suppress or amplify the controlled amplitude by an amount comparable to the variations observed over a variety of plasma discharges. With negative feedback, the maximum amplitude is reduced to 40% of the uncontrolled value. With positive feedback, the amplitude increases to 155%. Intermediate feedback phases lead to an acceleration or deceleration of the mode rotation frequency in addition to changes in mode amplitude. As the feedback gain increases, the level of both mode suppression and mode amplification also increases. However, for sufficiently high gain, kink mode suppression becomes limited by the excitation of an additional, slowly rotating 1.4 kHz mode having the same helical structure as the uncontrolled kink. High gain amplification is limited by disruptive loss of plasma current. Experimental results are compared with numerical simulations based on a single helicity model, and qualitative agreement is found.

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Experiments and modelling of external kink mode control using modular internal feedback coils

Cited by 15 publications

References 22 publications

Magnetic control of magnetohydrodynamic instabilities in tokamaks

Magnetic control of magnetohydrodynamic instabilities in tokamaks

Multimode observations and 3D magnetic control of the boundary of a tokamak plasma

Adaptive feedback control of rotating external kink modes in HBT-EP

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