MHD stability with strongly reversed magnetic shear in JET

Hender, T. C.; Hennequin, Pascale; Alper, B.; Hellsten, T.; Howell, D. F.; Huysmans, G.; Joffrin, E.; Maget, P.; Manickam, J.; Nave, M. F. F.; Pochelon, A.; Sharapov, S. E.; workprogramme, contributors to Efda-Jet

doi:10.1088/0741-3335/44/7/306

Cited by 35 publications

(36 citation statements)

References 24 publications

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“…Moreover, the fast T e decrease (a few ms) which terminates several GOs has been identified as a multiresonant large scale tearing mode (double or possibly triple) localized on the q 2 surface. This fast termination scheme, which gives birth systematically to another giant temperature increase, has the same phenomenology as the ''q 2 sawteeth'' observed in reversed shear configuration in TFTR [10] and JET [11]. Nevertheless, the GO is not a pure MHD phenomenon: the giant T e increase above the O regime level has to be related to non-MHD transport phenomena.…”

supporting

confidence: 55%

Giant Oscillations of Electron Temperature during Steady-State Operation on Tore Supra

et al. 2006

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During fully noninductively driven discharges in the Tore Supra tokamak, large spontaneous oscillations of the core electron temperature (DeltaTe/Te>50%) have been observed for the first time. They occurred during the standard O regime, which is itself characterized by periodic oscillations of much smaller amplitude. The "giant" oscillations appear to involve distinct mechanisms with respect to the O regime and provide a spectacular example of the complex nonlinear interactions between energy confinement, noninductive current sources, and MHD that may occur in a tokamak plasma during steady-state operation.

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supporting

confidence: 55%

Giant Oscillations of Electron Temperature during Steady-State Operation on Tore Supra

et al. 2006

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“…The axisymmetric sawteeth are distinct from the high-q sawteeth shown on the T e traces in Fig. 1, which are believed to be n=1 double-tearing modes [7]. …”

Section: Simulation Of Current Hole Evolutionmentioning

confidence: 75%

“…The LHCD prelude starts at 1.5 s. It is followed by a short pre-heat phase of low-power neutral beam and ICRH injection and then a longer high-power main heating phase. The T e time evolution shows 'high-q' sawteeth [7] that are always seen during the LHCD prelude in JET discharges with extreme shear reversal. The pitch angle profile measured by the MSE diagnostic (tanγ m ) at 3.78 s is shown as the points in Fig.…”

Section: Experimental Scenariomentioning

confidence: 83%

Equilibria and Stability of JET Discharges with Zero Core Current Density

Stratton¹,

Hawkes²,

Huysmans³

et al. 2002

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ABSTRACT. Injection of Lower Hybrid Heating and Current Drive (LHCD) into the current ramp-up phase of JET discharges can produce extremely reversed q-profiles characterized by a core region of near zero current density (within Motional Stark Effect diagnostic measurement errors). Non-inductive, off-axis co-current drive induces a back electromotive force inside the non-inductive current radius that drives a negative current in the plasma core. The core current density does not go negative, although current diffusion calculations indicate that there is sufficient LHCD to cause this. The clamping of the core current density near zero is consistent with n=0 reconnection events redistributing the core current soon after it goes negative. This is seen in reduced MHD simulations and in nonlinear resistive MHD simulations which predict that these discharges undergo n=0 reconnection events that clamp the core current near zero.

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“…Higher elongation is beneficial to all modes, while the combination of triangularity and outer squareness has a varying effect [267]. Localized minor collapses were observed near the transport barrier in JT-60U and TFTR [247] and q > 1 sawtooth-like events [258] in JET. The barrier localized mode (BLM) [244] was first observed in the high-β p mode discharges in JT-60U, and the BLM was also observed in reversed shear discharges which was identified as an ideal MHD n = 1 instability [247].…”

Section: Mhd Stability In Plasmas With Strong Negative Magnetic Shearmentioning

confidence: 99%

Chapter 3: MHD stability, operational limits and disruptions

Hender¹,

Wesley²,

Bialek³

et al. 2007

Nucl. Fusion

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947

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Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document Nucl. Fusion 39 2137-2664, is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER baseline ELMy H-mode scenario, but the tools exist to avoid or control them through localized current drive or fast ion generation. Active control of other MHD instabilities will most likely be also required in ITER. Extrapolation from existing experiments indicates that stabilization of neoclassical tearing modes by highly localized feedback-controlled current drive should be possible in ITER. Resistive wall modes are a key issue for S128 Chapter 3: MHD stability, operational limits and disruptions advanced scenarios, but again, existing experiments indicate that these modes can be stabilized by a combination of plasma rotation and direct feedback control with non-axisymmetric coils. Reduction of error fields is a requirement for avoiding non-rotating magnetic island formation and for maintaining plasma rotation to help stabilize resistive wall modes. Recent experiments have shown the feasibility of reducing error fields to an acceptable level by means of non-axisymmetric coils, possibly controlled by feedback. The MHD stability limits associated with advanced scenarios are becoming well understood theoretically, and can be extended by tailoring of the pressure and current density profiles as well as by other techniques mentioned here. There have been significant advances also in the control of disruptions, most notably by injection of massive quantities of gas, leading to reduced halo current fractions and a larger fraction of the total thermal and magnetic energy dissipated by radiation. These advances in disruption control are supported by the development of means to predict impending disruption, most notably using neural networks. In addition to these advances in means to control or ameliorate the consequences of MHD instabilities, there has been significant progress in improving physics understanding and modelling. This progress has been in areas including the mechanisms governing NTM growth and seeding, in understanding the damping controlling RWM stability and in modelling RWM feedback schemes. For disruptions there has been continued progress on the instability mechanisms that underlie various classes of disruption, on the detailed modelling of halo currents and forces and in refining predictions of quench rates and disruption power loads. Overall the studies reviewed in this chapter demonstrate that MHD instabilities can be controlled, avoided or ameliorated to the extent that they should not compromise ITER operation, though they will necessarily impose a range of constraints.

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MHD stability with strongly reversed magnetic shear in JET

Cited by 35 publications

References 24 publications

Giant Oscillations of Electron Temperature during Steady-State Operation on Tore Supra

Giant Oscillations of Electron Temperature during Steady-State Operation on Tore Supra

Equilibria and Stability of JET Discharges with Zero Core Current Density

Chapter 3: MHD stability, operational limits and disruptions

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