Edge localized mode control with an edge resonant magnetic perturbation

Moyer, R. A.; Evans, T.E.; Osborne, T.H.; Thomas, Paul R.; Bécoulet, M.; Harris, J. H.; Finken, K. H.; Boedo, J.A.; Doyle, E. J.; Fenstermacher, M.E.; Gohil, P.; Groebner, R. J.; Groth, M.; Jackson, G.L.; Haye, R.J. La; Lasnier, C.J.; Leonard, A. W.; McKee, G. R.; Reimerdes, H.; Rhodes, T. L.; Rudakov, D.L.; Schaffer, M. J.; Snyder, P. B.; Wade, M. R.; Wang, G.; Watkins, J. G.; West, W. P.; Zeng, Long

doi:10.1063/1.1888705

Cited by 113 publications

(95 citation statements)

References 28 publications

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“…Of particular interest in this context are the observations in the QH-mode [27], which in the frame of our model should correspond to a particular class of plasma profiles (requiring for their attainment also special external measures) where the development into a nonlinearly strongly unstable state is prevented, ripple [40], edge ergodisation [41,42] or fast plasma motions [43,44]. And finally, the impact of pellet pacing on edge profiles and its self-consistent feedback achieving steady state conditions can provide valuable information on the trigger process and its limitations.…”

Section: The Quiescent H-modementioning

confidence: 99%

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

et al. 2008

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To get deeper insight into the MHD activity triggered by pellets we extended our previous analyses of standard type-I ELMs to pellets injected into discharge phases of the following types: Ohmic, L-mode, type-III ELMy H-mode, ELM-free, radiative edge scenarios with type-I ELMs, the quiescent H (QH)-mode regime. It turns out that pellet injection generally creates a strong, local perturbation of the MHD equilibrium in the ablation region and even beyond. Regarding the triggering of ELMs, this initial perturbation can damp out, indicating that the plasma is stable in the corresponding regime even for finite-size perturbations. This behaviour is observed not only in Ohmic and L-mode phases but also in the QH-mode where the edge harmonic oscillations (EHO) appear to keep the edge within or at the boundary of a stable regime. In case the plasma is prone to ELM growth, the large amplitude of the pellet perturbation can trigger the event even in situations where modes appear to be still linearly stable.The non-linear character of the ELM trigger process is highlighted also by the subsequent explosive growth of these events. For edge plasma conditions characterised by higher resistivity the growth time of spontaneously occurring ELMs increases when the plasma changes from the type-I into the type-III regime. Pellet-triggered ELMs, however, maintain the fast rise times otherwise typical for the hot edge type-I regime. In the discussion section we attempt to relate these observations also to core mode activity like neoclassical tearing modes or snakes. Data were taken from ASDEX Upgrade and JET.Pellet triggering of mode activity can be shown to be a quite universal phenomenon, which, however, only for the case of ELMs can be unambiguously attributed to prompt direct excitation by the pellet.

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Section: The Quiescent H-modementioning

confidence: 99%

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

et al. 2008

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“…For example the reduction of the energy flux to the wall by establishing a radiative edge [4,5] or by suppressing ELMs at all by edge ergodisation [6] was demonstrated. Another option is to establish ELM control by the pacing concept.…”

Section: Introductionmentioning

confidence: 99%

ELM triggering by local pellet perturbations in type-I ELMy H-mode plasma at JET

et al. 2007

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The pellet pacing ELM control concept developed at the mid sized tokamak ASDEX Upgrade is considered for the larger machines JET and ITER as well. By driving up the ELM frequency, the ELM size can be reduced and eventually suppressed below a dangerous level. An according pellet injection systems for JET is under construction, the ITER design includes one as well. However, it has still to be proven the concept will work also at bigger machine sizes. A step forward in this mission was achieved by re-analysing previous JET experiments dedicated to pellet particle fuelling. Although the experiments were performed in a parameter regime unsuitable for pellet ELM pacing both with respect to pellet frequency and size they demonstrate prompt triggering of ELMs takes place at JET as well. Prompt triggering, an indispensable premise for useful pacing, obviously can be achieved already with a pellet size sufficiently small to avoid unbearable side-effects by particle fuelling. An ELM released by the local perturbation imposed by the ablating pellet in the plasma edge region is detected when only a minor fraction (less than 1%) of the fuelling size pellets (containing about 4 × 10 21 D) was ablated and deposited yet. It is thus very likely JET will allow for investigations on the operational technique and the underlying physics of pellet pacing once the new system becomes operational.1

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“…Based on vacuum field line modeling, these resonances resulted in small, well isolated, magnetic islands with a narrow stochastic layer located at the foot of the pedestal [13]. Experimentally they had little or no effect on the pedestal profiles, H-mode transport barrier, the depth or location for the edge E r well or on the overall quality of the plasma confinement although the toroidal rotation across the entire plasma was significantly reduced after a 200-300 ms decay time [13,14]. It is interesting to note that in these high e,neo * experiments the effects of the I-coil perturbation were maximized at the same resonance condition, q 95 = 11 3, as in low e,neo * lower single null plasmas and that the odd parity I-coil configuration produces a minimum in the spectrum amplitude across the pedestal [13] as opposed to the low e,neo * case with even parity which produces a maximum in the pedestal spectrum when q 95 = 11 3.…”

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confidence: 99%

“…It is interesting to note that in these high e,neo * experiments the effects of the I-coil perturbation were maximized at the same resonance condition, q 95 = 11 3, as in low e,neo * lower single null plasmas and that the odd parity I-coil configuration produces a minimum in the spectrum amplitude across the pedestal [13] as opposed to the low e,neo * case with even parity which produces a maximum in the pedestal spectrum when q 95 = 11 3. In the high e,neo * ELM control experiments, ELMs appear to be stabilized by increasing small, high frequency, edge fluctuations which are believed to inhibit the onset of the large Type-I ELMs [13,14].…”

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The physics of edge resonant magnetic perturbations in hot tokamak plasmas

et al. 2006

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Small edge resonant magnetic perturbations are used to control the pedestal transport and stability in low electron collisionality ( e * ), ITER relevant, poloidally diverted plasmas. The applied perturbations reduce the height of the density pedestal and increase its width while increasing the height of the electron pedestal temperature and its gradient.The effect of the perturbations on the pedestal gradients is controlled by the current in the perturbation coil, the poloidal mode spectrum of the coil, the neutral beam heating power and the divertor deuterium fueling rate. Large pedestal instabilities, referred to as edge as an array of higher frequency electromagnetic waves and turbulence [7]. Outside the plasma, sources such as field-errors from asymmetries in toroidal and poloidal magnetic field coils, magnetic materials, vacuum vessel image and return currents [8], external control coils used to stabilize plasma modes, and correction coils [9] used to minimize perturbations from known field-errors on low integer rational surfaces all contribute to the structure of the magnetic field in which the plasma resides. At the edge of the plasma where the safety factor [ q ( ) defined as the rate of change in toroidal magnetic flux with poloidal magnetic flux ] increases rapidly, all of these perturbations contribute to the creation of closely spaced resonant magnetic islands which may result in the formation of edge stochastic layers. In a poloidally diverted tokamak, the high magnetic shear q across the edge of the plasma results in an increased density of island states and a significantly higher probability of forming open stochastic layers that connect magnetic field lines to plasma facing material surfaces [10]. Thus, the edge plasma is immersed in a dynamically complex magnetic topology over the same region where substantial radial flows of mass and energy, driven by large gradients, compete with strong turbulent transport in highly sheared toroidal and poloidal plasma flow fields. Large radial gradients in the plasma pressure are often established by a strong reduction in turbulent 4 transport due to sheared plasma flow. The large edge gradients are a key factor in the establishment of good confinement levels that make the tokamak the leading candidate for fusion reactors. However, they also lead to the instabilities known as ELMs which drive impulsive energy losses that can be detrimental to plasma facing surfaces. It is easy to understand why the development of tools to control the pedestal transport and stability is a compelling issue for improving the performance and operational safety of high energy density tokamak based fusion confinement systems.A strong motivation for understanding the physics of edge stochastic layers is to enable the development of predictable and reliable tools for controlling key fusion plasma pedestal processes such as: the plasma temperature and pressure at the top of the pedestal, the size and frequency of ELMs, the energy and particle exhaust rate in steady-state cond...

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Edge localized mode control with an edge resonant magnetic perturbation

Cited by 113 publications

References 28 publications

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

ELM triggering by local pellet perturbations in type-I ELMy H-mode plasma at JET

The physics of edge resonant magnetic perturbations in hot tokamak plasmas

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