MHD-mode stabilization by plasma rotation in TEXTOR

Vries, P. C. de; Waidmann, G.; Donné, A. J. H.; Schüller, F. C.

doi:10.1088/0741-3335/38/4/002

Cited by 40 publications

(47 citation statements)

References 12 publications

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“…As the poloidal harmonic of this exponentially growing n = 1 field is unknown, in principle it is possible that the 2/1 island is unchanging, while an m = 1 or m = 3 instability is appearing and growing. Coupling of the 2/1 and 3/1 modes has been observed on ASDEX-Upgrade [44] and investigated in numerical studies [45], while coupling of the 2/1 and m/n = 1/1 has been observed on TEXTOR [46], the RTP Tokamak [14], and studied in simulations [41]. Although m = 4 might be a candidate for this growth in some shots, it will be shown in section 8 that more than half of the disruptive discharges have q 95 < 4, so it could only explain a minority of cases.…”

Section: Distributions Of Irlm Toroidal Phase At Lockingmentioning

confidence: 99%

Statistical analysis of m/n = 2/1 locked and quasi-stationary modes with rotating precursors at DIII-D

Sweeney¹,

Choi²,

Haye³

et al. 2016

Nucl. Fusion

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Abstract.A database has been developed to study the evolution, the nonlinear effects on equilibria, and the disruptivity of locked and quasi-stationary modes with poloidal and toroidal mode numbers m = 2 and n = 1 at DIII-D. The analysis of 22,500 discharges shows that more than 18% of disruptions are due to locked or quasistationary modes with rotating precursors (not including born locked modes). A parameter formulated by the plasma internal inductance l i divided by the safety factor at 95% of the poloidal flux, q 95 , is found to exhibit predictive capability over whether a locked mode will cause a disruption or not, and does so up to hundreds of milliseconds before the disruption. Within 20 ms of the disruption, the shortest distance between the island separatrix and the unperturbed last closed flux surface, referred to as d edge , performs comparably to l i /q 95 in its ability to discriminate disruptive locked modes. Out of all parameters considered, d edge also correlates best with the duration of the locked mode. Disruptivity following a m/n = 2/1 locked mode as a function of the normalized beta, β N , is observed to peak at an intermediate value, and decrease for high values. The decrease is attributed to the correlation between β N and q 95 in the DIII-D operational space. Within 50 ms of a locked mode disruption, average behavior includes exponential growth of the n = 1 perturbed field, which might be due to the 2/1 locked mode. Surprisingly, even assuming the aforementioned 2/1 growth, disruptivity following a locked mode shows little dependence on island width up to 20 ms before the disruption. Separately, greater deceleration of the rotating precursor is observed when the wall torque is large. At locking, modes are often observed to align at a particular phase, which is likely related to a residual error field. Timescales associated with the mode evolution are also studied and dictate the response times necessary for disruption avoidance and mitigation. Observations of the evolution of β N during a locked mode, the effects of poloidal beta on the saturated width, and the reduction in Shafranov shift during locking are also presented.

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Section: Distributions Of Irlm Toroidal Phase At Lockingmentioning

confidence: 99%

Statistical analysis of m/n = 2/1 locked and quasi-stationary modes with rotating precursors at DIII-D

Sweeney¹,

Choi²,

Haye³

et al. 2016

Nucl. Fusion

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“…Plasma rotation is one of the central topics for toroidal plasma confinement and it is considered to play a key role in tokamak plasma performance. Plasma rotation can have a strong impact on plasma stability and confinement of fusion plasmas, through its ability to stabilize MHD modes [1], affect the strength of transport barriers [2], affect turbulence stabilization due to E Â B shear [3], and reduce the profile stiffness in the presence of anomalous transport [4]. In present day experiments, rotation is often driven by the torques of neutral beam injection (NBI), while in an alpha-heated reactor the momentum input is expected to be small [5].…”

mentioning

confidence: 99%

Influence of Magnetic Field Ripple on the Intrinsic Rotation of Tokamak Plasmas

Nave¹,

Johnson

Eriksson

et al. 2010

Phys. Rev. Lett.

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Using the unique capability of JET to monotonically change the amplitude of the magnetic field ripple, without modifying other relevant equilibrium conditions, the effect of the ripple on the angular rotation frequency of the plasma column was investigated under the conditions of no external momentum input. The ripple amplitude was varied from 0.08% to 1.5% in Ohmic and ion-cyclotron radio-frequency (ICRF) heated plasmas. In both cases the ripple causes counterrotation, indicating a strong torque due to nonambipolar transport of thermal ions and in the case of ICRF also fast ions. Plasma rotation is one of the central topics for toroidal plasma confinement and it is considered to play a key role in tokamak plasma performance. Plasma rotation can have a strong impact on plasma stability and confinement of fusion plasmas, through its ability to stabilize MHD modes [1], affect the strength of transport barriers [2], affect turbulence stabilization due to E Â B shear [3], and reduce the profile stiffness in the presence of anomalous transport [4]. In present day experiments, rotation is often driven by the torques of neutral beam injection (NBI), while in an alpha-heated reactor the momentum input is expected to be small [5]. In reactors with high NBI power, the torque is still expected to be small due to the high injection energy needed for the beam to penetrate deep into the plasma, which reduces the torque per MW of injected power. Thus, there has been a growing interest in the intrinsic rotation of the plasma, which is observed to occur in the absence of momentum sources such as NBI. Besides its relevance for fusion reactors, the residual plasma rotation in confinement configurations, without momentum injection, is an interesting, and not yet understood, problem from a basic physics point of view that has attracted great theoretical and experimental interest in recent years. The extrapolation from intrinsic plasma rotation data observed in several machines suggests that a substantial rotation in the cocurrent direction, i.e., in the direction parallel to the plasma current, will occur in ITER [6]. However, an effect that is still to be accounted for when extrapolating intrinsic rotation observations to ITER comes from the toroidal ripple in the magnetic field, which appears in tokamaks due to the finite number of toroidal magnetic field coils. ITER will have 18 toroidal magnetic field coils and, a ripple amplitude ¼ B=B of 0.5% to 1.2% at the edge (depending on the configuration of ferritic inserts) [7] where B is the averaged toroidal magnetic field and B is the Fourier amplitude of the toroidal ripple. Since the ripple breaks the toroidal symmetry, the motion of individual particles may lead to nonambipolar transport that can affect the plasma rotation through the neoclassical toroidal viscosity [8][9][10].The effect of ripple on plasma rotation during NBI has been reported from and JET [12]. Evidence that ripple transport of fast ions induces edge counterrotation in plasmas with no external momentum input wa...

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“…This motivates further investigation of up-down asymmetric devices because significant momentum transport has been shown to improve tokamak performance. Experiments on DIII-D [18,19] and TEXTOR [20] (among others) used plasma rotation to stabilize resistive wall modes, a dangerous MHD instability, and enable discharges with plasma beta that exceeded the Troyon limit [21]. Additionally, a gradient in rotation is theoretically expected to decorrelate turbulence and directly reduce energy transport [22].…”

Section: Introductionmentioning

confidence: 99%

Poloidal tilting symmetry of high order tokamak flux surface shaping in gyrokinetics

Ball

Parra

Barnes³

2016

Plasma Phys. Control. Fusion

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Abstract.A poloidal tilting symmetry of the local nonlinear δf gyrokinetic model is demonstrated analytically and verified numerically. This symmetry shows that poloidally rotating all the flux surface shaping effects with large poloidal mode number by a single tilt angle has an exponentially small effect on the transport properties of a tokamak. This is shown using a generalization of the Miller local equilibrium model to specify an arbitrary flux surface geometry. With this geometry specification we find that, when performing an expansion in large flux surface shaping mode number, the governing equations of gyrokinetics are symmetric in the poloidal tilt of the high order shaping effects. This allows us to take the fluxes from a single configuration and calculate the fluxes in any configuration that can be produced by tilting the large mode number shaping effects. This creates a distinction between tokamaks with mirror symmetric flux surfaces and tokamaks without mirror symmetry, which is expected to have important consequences for generating toroidal rotation using updown asymmetry.

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MHD-mode stabilization by plasma rotation in TEXTOR

Cited by 40 publications

References 12 publications

Statistical analysis of m/n = 2/1 locked and quasi-stationary modes with rotating precursors at DIII-D

Statistical analysis of m/n = 2/1 locked and quasi-stationary modes with rotating precursors at DIII-D

Influence of Magnetic Field Ripple on the Intrinsic Rotation of Tokamak Plasmas

Poloidal tilting symmetry of high order tokamak flux surface shaping in gyrokinetics

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