The physics of sawtooth stabilization

Chapman, I. T.; Pinches, S. D.; Graves, J. P.; Akers, R.; Appel, L. C.; Budny, R.; Coda, S.; Conway, N. J.; Bock, Maarten De; Eriksson, L.‐G.; Hastie, R. J.; Hender, T. C.; Huysmans, G.; Johnson, T.; Koslowski, H. R.; Krämer‐Flecken, A.; Lennholm, M.; Liang, Y.; Saarelma, S.; Sharapov, S. E.; Voitsekhovitch, I.; Teams, Textor; Contributors, Jet

doi:10.1088/0741-3335/49/12b/s35

Cited by 58 publications

(79 citation statements)

References 37 publications

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“…This is not only because of the impact on plasma confinement due to turbulence suppression by sheared flows [44,45], which give rise to internal transport barriers (ITB), but also due to the impact of flow on MHD stability [46,47]. Plasmas in NBI heated spherical tokamaks show a fast toroidal rotation with thermal Mach numbers measured on MAST of up to M th = v φ /v th 0.8 [48].…”

Section: Momentum Confinementmentioning

confidence: 99%

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Overview of physics results from MAST

Raman¹,

Ahn²,

Allain³

et al. 2011

Nucl. Fusion

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Several improvements to the MAST plant and diagnostics have facilitated new studies advancing the physics basis for ITER and DEMO, as well as for future spherical tokamaks. Using the increased heating capabilities P NBI ≤ 3.8 MW H-mode at I p = 1.2 MA was accessed showing that the energy confinement on MAST scales more weakly with I p and more strongly with B t than in the ITER IPB98(y,2) scaling. Measurements of the fuel retention of shallow pellets extrapolate to an ITER particle throughput of 70% of its original design value. The anomalous momentum diffusion, χ φ , is linked to the ion diffusion, χ i , with a Prandtl number close to P φ ≈ χ φ /χ i ≈ 1, although χ i approaches neoclassical values. New high spatially resolved measurements of the edge radial electric field, E r , show that the position of steepest gradients in electron pressure and E r are coincident, but their magnitudes are not linked. The T e pedestal width on MAST scales with the β pol rather than ρ pol . The ELM frequency for type-IV ELMs, new in MAST, was almost doubled using n = 2 resonant magnetic perturbations from a set of 4 external coils (n = 1, 2). A new internal 12 coil set (n ≤ 3) has been commissioned. The filaments in the inter-ELM and L-mode phase are different from ELM filaments, and the characteristics in L-mode agree well with turbulence calculations. A variety of fast particle driven instabilities were studied from 10 kHz saturated fishbone like activity up to 3.8 MHz compressional Alfvén eigenmodes (CAE). The damping rate of ellipticityinduced AE was measured to be 4% using the new internal coils as antennae. Fast particle instabilities also affect the off-axis NBI current drive and lead to fast ion diffusion of the order of 0.5 m 2 /s and reduce the driven current fraction from 40% to 30%. EBW current drive start-up is demonstrated for the first time in a spherical tokamak generating plasma currents up to 55 kA. Many of these studies contributed to the physics basis of a planned upgrade to MAST. Introduction: MAST [1]is one of the two leading tight aspect ratio (A = ε −1 = R/a = 0.85 m/0.65 m ∼ 1.3, I p ≤ 1.5 MA) tokamaks in the world. The hot T ≤ 3 keV, dense n e = (0.1 − 1) × 10 20 m −3 and highly shaped (δ ≤ 0.5, 1.6 ≤ κ ≤ 2.5) plasmas are accessed at moderate toroidal field B t (R = 0.7 m) ≤ 0.62 T and show many similarities to conventional aspect ratio tokamaks. Detailed physics studies using the extensive array of state of the art diagnostics and access to different physics regimes help to consolidate the physics basis for ITER and DEMO [2,3], and explore the viability of future devices based on the spherical tokamak (ST) concept such as a component test facility (CTF) [4] or an advanced power plant [5]. The challenge for today's experiments is to find an integrated scenario that extrapolates to these future devices, in particular to develop plasmas with reduced power load on plasma facing components, notably from edge localised modes (ELM), but high confinement facilitated by internal or edge transport ba...

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Section: Momentum Confinementmentioning

confidence: 99%

“…Furthermore, low order rational surfaces have to be avoided by operating with q min > 1.3, because of the detrimental impact of low n MHD such as infernal modes [46], NTMs and "monster" sawteeth [47].…”

Section: Current Drive and Start-upmentioning

confidence: 99%

Overview of physics results from MAST

Raman¹,

Ahn²,

Allain³

et al. 2011

Nucl. Fusion

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“…However, minority populations of super thermal ions can delay the onset of these instabilities, thereby improving confinement properties of tokamaks and allowing steeper pressure and current gradients to develop. The presence of fusion-born alpha particles in ITER is predicted to significantly lengthen the time between consecutive sawtooth crash events [1][2][3][4]. This means that when the sawtooth crash occurs in the presence of stabilizing fast ions it is often more violent and more likely to trigger neoclassical tearing modes (NTMs), leading to a degradation in pressure and thus in fusion performance.…”

Section: Introductionmentioning

confidence: 99%

Sawtooth control using electron cyclotron current drive in ITER demonstration plasmas in DIII-D

et al. 2012

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Sawtooth control using electron cyclotron current drive (ECCD) has been demonstrated in ITER-like plasmas with a large fast ion fraction, wide q = 1 radius and long uncontrolled sawtooth period in DIII-D. The sawtooth period is minimized when the ECCD resonance is just inside the q = 1 surface. Sawtooth destabilization using driven current inside q = 1 avoids the triggering of performance-degrading neoclassical tearing modes (NTMs), even at much higher pressure than required in the ITER baseline scenario. Operation at β N = 3 without 3/2 or 2/1 NTMs has been achieved in ITER demonstration plasmas when sawtooth control is applied using only modest ECCD power. Numerical modelling qualitatively confirms that the achieved driven current changes the local magnetic shear sufficiently to compensate for the stabilizing influence of the energetic particles in the plasma core.

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“…Such redistribution is observed with collective Thomson scattering (CTS) diagnostic in TEXTOR [5]. mode and increases the sawteeth period [6]. The fast particle distribution is peaked in the plasma centre (inside q=1 surface) for central ICRH heated discharges in ASDEX Upgrade [7].…”

Section: Introductionmentioning

confidence: 99%

Destabilization of fast particle stabilized sawteeth in ASDEX Upgrade with electron cyclotron current drive

Igochine

Chapman

Bobkov

et al. 2011

Plasma Phys. Control. Fusion

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It is often observed that large sawteeth trigger the neoclassical tearing mode (NTM) well below the usual threshold for this instability. At the same time, fast particles in the plasma core stabilize sawteeth and provide these large crashes. The paper presents results of first experiments in ASDEX Upgrade for destabilization of fast particle stabilized sawteeth with electron cyclotron current drive (ECCD). It is shown that moderate ECCD from a single gyrotron is able to destabilize the fast particle stabilized sawteeth. A reduction in sawtooth period by about 40% was achieved in first experiments. These results show that ECCD can be used as a tool for control of sawteeth also in presence of fast particles.

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The physics of sawtooth stabilization

Cited by 58 publications

References 37 publications

Overview of physics results from MAST

Overview of physics results from MAST

Sawtooth control using electron cyclotron current drive in ITER demonstration plasmas in DIII-D

Destabilization of fast particle stabilized sawteeth in ASDEX Upgrade with electron cyclotron current drive

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