Experimental Study of Trapped-Electron-Mode Properties in Tokamaks: Threshold and Stabilization by Collisions

Ryter, F.; Angioni, C.; Peeters, A. G.; Leuterer, F.; Fahrbach, H.‐U.; Suttrop, W.

doi:10.1103/physrevlett.95.085001

Cited by 119 publications

(131 citation statements)

References 24 publications

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“…Numerical simulations of these plasmas using gyrofluid and gyrokinetic codes predict that TEM turbulence should become linearly stable at high densities/collisionalities while ITG turbulence remains unstable [3,30]. There are also experimental observations, e.g.…”

Section: Turbulence Phase Velocitymentioning

confidence: 99%

Observations on core turbulence transitions in ASDEX Upgrade using Doppler reflectometry

et al. 2006

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Abstract. In Doppler reflectometry the antenna tilt angle θ t induces a Doppler frequency shift f D = u ⊥ 2 sin θ t /λ o in the measured spectrum which is directly proportional to the rotation velocity u ⊥ = v E×B + v ph of the turbulence moving in the plasma. Measurements in ohmic ASDEX Upgrade core plasmas show u ⊥ of the order of 1 − 2 km s −1 and reversing direction with increasing collisionality. Numerical simulations of the turbulence phase velocity v ph using the GS2 linear gyrokinetic code reveal a change in the dominant core turbulence from ITG to TEM with decreasing collisionality. The transition coincides with the u ⊥ reversal. Extracting the E × B velocity from the measured u ⊥ and the simulation v ph indicates that the core radial electric field reverses sign with the turbulence. Using the radial force balance equation v E×B = v ⊥ − v * and measured diamagnetic velocities v * gives a perpendicular fluid velocity v ⊥i reversing from ∼ 5 to 7.5 km s −1 (ion direction) at low collisionality to −0.7 km s −1 (electron direction) at high collisionality. Neoclassical predictions using the neoart and nclass codes give poloidal Deuterium fluid velocities too small by factor of ten. Toroidal ion fluid velocities would need to be significant (> 30 km s −1 ) at low ν * to account for the difference. A clear ITG to TEM transition in the core turbulence has also been demonstrated using on-axis electron cyclotron heating to perturb the collisionality.

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Section: Turbulence Phase Velocitymentioning

confidence: 99%

Observations on core turbulence transitions in ASDEX Upgrade using Doppler reflectometry

et al. 2006

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“…This model is based on the assumption that electron heat transport is driven by the TEM instability [16,17]. It is of the gyro-Bohm type and one assumes that the electron heat diffusivity increases above a threshold in R/L T e , labelled R/L T e,crit , whose existence, which is predicted by theory [18] [19], has been evidenced experimentally [20]. Below the threshold the transport is very low.…”

Section: Critical Gradient Modelmentioning

confidence: 99%

Investigation of transport models in ASDEX Upgrade current ramps

et al. 2013

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Abstract.Understanding the physics of transport during current ramps in tokamaks is essential in order to prepare adequate ramp-up and ramp-down scenarios for ITER, where constraints on the transformer flux and the internal inductance will be more stringent than in present-day devices. In this paper five transport models (CoppiTang, Neo-Alcator, Bohm/gyro-Bohm, critical gradient model and H98/2 scalingbased model) are used to reproduce the experimental data during the current ramps of ASDEX Upgrade with its metallic wall. The calculated temperature profiles are compared to the experimental temperature profiles under different ramp-up conditions. Our study reveals important differences between boronised and non boronised wall which are reproduced with variable success by the models. We also investigated rampup phases heated centrally by electron cyclotron heating which creates experimental conditions with peaked electron temperature profiles, which are very different from the usual ohmic heated cases. Our study reveals that the models react very differently to this additional heating.

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“…16 Experimental evidence for profile stiffness and critical gradients is also found in several tokamaks. [17][18][19][20][21][22][23] As a consequence of stiff transport in the plasma core, the edge temperature provides the key boundary condition dictating overall plasma performance. 10,14,24 This means that in very stiff core plasmas, like many H-modes, small decreases/increases in local temperature gradients can lead to large decreases/increases in local diffusivity.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear gyrokinetic simulations of the I-mode high confinement regime and comparisons with experimenta)

et al. 2015

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For the first time, nonlinear gyrokinetic simulations of I-mode plasmas are performed and compared with experiment. I-mode is a high confinement regime, featuring energy confinement similar to H-mode, but without enhanced particle and impurity particle confinement [D. G. Whyte et al., Nucl. Fusion 50, 105005 (2010)]. As a consequence of the separation between heat and particle transport, I-mode exhibits several favorable characteristics compared to H-mode. The nonlinear gyrokinetic code GYRO [J. Candy and R. E. Waltz, J Comput. Phys. 186, 545 (2003)] is used to explore the effects of E Â B shear and profile stiffness in I-mode and compare with L-mode. The nonlinear GYRO simulations show that I-mode core ion temperature and electron temperature profiles are more stiff than L-mode core plasmas. Scans of the input E Â B shear in GYRO simulations show that E Â B shearing of turbulence is a stronger effect in the core of I-mode than L-mode. The nonlinear simulations match the observed reductions in long wavelength density fluctuation levels across the L-I transition but underestimate the reduction of long wavelength electron temperature fluctuation levels. The comparisons between experiment and gyrokinetic simulations for I-mode suggest that increased E Â B shearing of turbulence combined with increased profile stiffness are responsible for the reductions in core turbulence observed in the experiment, and that I-mode resembles H-mode plasmas more than L-mode plasmas with regards to marginal stability and temperature profile stiffness. V C 2015 AIP Publishing LLC.

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Experimental Study of Trapped-Electron-Mode Properties in Tokamaks: Threshold and Stabilization by Collisions

Cited by 119 publications

References 24 publications

Observations on core turbulence transitions in ASDEX Upgrade using Doppler reflectometry

Observations on core turbulence transitions in ASDEX Upgrade using Doppler reflectometry

Investigation of transport models in ASDEX Upgrade current ramps

Nonlinear gyrokinetic simulations of the I-mode high confinement regime and comparisons with experimenta)

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