Global gyrokinetic turbulence simulations of MAST plasmas

Saarelma, S.; Hill, P.; Bottino, A.; Colyer, Greg; Field, A. R.; McMillan, B. F.; Peeters, A. G.; Roach, C. M.

doi:10.1088/0741-3335/54/8/085012

Cited by 12 publications

(36 citation statements)

References 25 publications

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“…The fluctuation level δn e /n e exhibits similar behaviour. The underprediction in the core is consistent with the previous linear studies [21], which showed the region whereŝ ≤ 0 to be linearly stable. The effect of including equilibrium flow shear (III) in the simulations with kinetic electrons is not dramatic, with a slight decrease in Q sim i compared to case II in the core where the flow shear is strongest, but an increase in the peripheral region, where Q sim i approaches the experimental level -the corresponding changes in δn e /n e are rather slight.…”

Section: Turbulence Amplitudes and Ion Heat Fluxessupporting

confidence: 91%

“…Results of runs incorporating electron collisions, which had been shown to reduce the linear drive due to trapped electrons [34], were not reported in Ref. [21] (simulations with collisions have however been peformed for the more recent equilibrium discussed in §2.2.2).…”

Section: 2mentioning

confidence: 99%

“…This equilibrium configuration was also used as the basis for the earlier global, nonlinear simulations presented in Ref. [21]. An L-mode plasma is ideal for these studies because, in the peripheral region, the equilibrium flow shear is too weak to stabilise the ITG turbulence fully.…”

Section: Introductionmentioning

confidence: 99%

“…Therefore, as discussed in Ref. [21], for current-generation ST plasmas in which ρ * ∼ 1/50, global codes may be required to perform meaningful non-linear simulations of the ion-scale turbulence.…”

Section: Introductionmentioning

confidence: 99%

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Comparison of BES measurements of ion-scale turbulence with direct gyro-kinetic simulations of MAST L-mode plasmas

Field

Dunai

Ghim

et al. 2014

Plasma Phys. Control. Fusion

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Abstract. Observations of ion-scale (k y ρ i ≤ 1) density turbulence of relative amplitude 0.2% are available on the Mega Amp Spherical Tokamak (MAST) using a 2D (8 radial × 4 poloidal channel) imaging Beam Emission Spectroscopy (BES) diagnostic. Spatial and temporal characteristics of this turbulence, i.e., amplitudes, correlation times, radial and perpendicular correlation lengths and apparent phase velocities of the density contours, are determined by means of correlation analysis. For a low-density, L-mode discharge with strong equilibrium flow shear exhibiting an internal transport barrier (ITB) in the ion channel, the observed turbulence characteristics are compared with synthetic density turbulence data generated from global, non-linear, gyro-kinetic simulations using the particle-in-cell (PIC) code NEMORB. This validation exercise highlights the need to include increasingly sophisticated physics, e.g., kinetic treatment of trapped electrons, equilibrium flow shear and collisions, to reproduce most of the characteristics of the observed turbulence. Even so, significant discrepancies remain: an underprediction by the simulations of the turbulence amplituide and heat flux at plasma periphery and the finding that the correlation times of the numerically simulated turbulence are typically two orders of magnitude longer than those measured in MAST. Comparison of these correlation times with various linear timescales suggests that, while the measured turbulence is strong and may be 'critically balanced', the simulated turbulence is weak.

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Section: Turbulence Amplitudes and Ion Heat Fluxessupporting

confidence: 91%

Section: 2mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Comparison of BES measurements of ion-scale turbulence with direct gyro-kinetic simulations of MAST L-mode plasmas

Field

Dunai

Ghim

et al. 2014

Plasma Phys. Control. Fusion

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“…The turbulence predicted by our simulations is, therefore, only representative of the turbulence at a single flux surface, even though our box sizes can be the size of MAST (several such formally overlapping simulations can be then used to model transport across the entire radial extent of the machine). For the MAST discharge and radial location studied in this work, one finds ρ i /a ∼ 1/100, where ρ i ≈ 6×10 −3 m and a ≈ 0.6 m. While this is a reasonably small number, previous studies of simpler geometries have suggested that non-local effects can reduce the level of turbulent transport by 50% at values similar to 1/100 [63,64]. A scan of different values of ρ * using a global gyrokinetic code would be required to test whether non-local effects change the level of turbulence for the MAST turbulence studied in this paper.…”

Section: Local Gyrokinetic Descriptionmentioning

confidence: 48%

Ion-scale turbulence in MAST: anomalous transport, subcritical transitions, and comparison to BES measurements

Wyk

Highcock

Field

et al. 2017

Plasma Phys. Control. Fusion

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We investigate the effect of varying the ion temperature gradient (ITG) and toroidal equilibrium scale sheared flow on ion-scale turbulence in the outer core of MAST by means of local gyrokinetic simulations. We show that nonlinear simulations reproduce the experimental ion heat flux and that the experimentally measured values of the ITG and the flow shear lie close to the turbulence threshold. We demonstrate that the system is subcritical in the presence of flow shear, i.e., the system is formally stable to small perturbations, but transitions to a turbulent state given a large enough initial perturbation. We propose that the transition to subcritical turbulence occurs via an intermediate state dominated by low number of coherent long-lived structures, close to threshold, which increase in number as the system is taken away from the threshold into the more strongly turbulent regime, until they fill the domain and a more conventional turbulence emerges. We show that the properties of turbulence are effectively functions of the distance to threshold, as quantified by the ion heat flux. We make quantitative comparisons of correlation lengths, times, and amplitudes between our simulations and experimental measurements using the MAST BES diagnostic. We find reasonable agreement of the correlation properties, most notably of the correlation time, for which significant discrepancies were found in previous numerical studies of MAST turbulence. * ferdinand.vanwyk@physics.ox.ac.uk † highcock@chalmers.se ‡ alex.schekochihin@physics.ox.ac.uk arXiv:1704.02830v2 [physics.plasm-ph] 1 Aug 2017 2 V 0 dV B · ∇φ is the toroidal magnetic flux, V is the volume enclosed by the flux surface, B is the magnetic field, φ is the toroidal angle, and ψ tor,LCFS is the toroidal flux enclosed by the last closed flux surface [see figure 1(b)], ψ pol = (1/2π) 2 V 0 dV B · ∇θ is the poloidal magnetic flux, θ is the poloidal angle, and ψ pol,LCFS is the poloidal flux enclosed by the LCFS. 4 http://w3.pppl.gov/transp/ c , including two cases that match the experimental level of heat flux. This is a considerable improvement over previous nonlinear gyrokinetic simulations of this MAST discharge [51], which overpredicted τ SYNTH c by two orders of magnitude. Examining figure 24(d), we see that (δn i /n i ) SYNTH rms increases with increasing κ T or de-

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Plasma rotation and transport in MAST spherical tokamak

et al. 2011

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The formation of internal transport barriers (ITBs) is investigated in MAST spherical tokamak (ST) plasmas. The roles of E×B flow shear, q-profile (magnetic shear) and MHD activity in their formation and evolution are studied using data from high-resolution kinetic-and q-profile diagnostics. In L-mode plasmas, with co-current directed NBI heating, ITBs in the momentum and ion thermal channels form in the negative shear region just inside q min. In the ITB region the anomalous ion thermal transport is suppressed, with ion thermal transport close to the neo-classical level, although the electron transport remains anomalous. Linear stability analysis with the gyro-kinetic code GS2 shows that all electrostatic micro-instabilities are stable in the negative magnetic shear region in the core, both with and without flow shear. Outside the ITB, in the region of positive magnetic shear and relatively weak flow shear, electrostatic micro-instabilities become unstable over a wide range of wave-numbers. At ITG length scales, flow shear reduces linear growth rates and narrows the spectrum of unstable modes, but flow shear suppression of ITG modes is incomplete. Flow shear has little impact on growth rates at ETG scales. This is consistent with the observed anomalous electron and ion transport in this region. With counter-NBI ITBs of greater radial extent form outside q min due to the broader profile of E×B flow shear produced by the greater prompt fast-ion loss torque.

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Global gyrokinetic turbulence simulations of MAST plasmas

Cited by 12 publications

References 25 publications

Comparison of BES measurements of ion-scale turbulence with direct gyro-kinetic simulations of MAST L-mode plasmas

Comparison of BES measurements of ion-scale turbulence with direct gyro-kinetic simulations of MAST L-mode plasmas

Ion-scale turbulence in MAST: anomalous transport, subcritical transitions, and comparison to BES measurements

Plasma rotation and transport in MAST spherical tokamak

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