Electromagnetic stabilization of tokamak microturbulence in a high-βregime

Abstract

The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-β JET L-mode discharge illustrates the relation between ITG stabilization, and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-β JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant, and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-β generalization of comparable observations found at low-β at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

Electromagnetic turbulence suppression by energetic particle driven modes

“…Any insight on possible reductions of the underlying micro-instabilities which are driven by the steep density and temperature profiles, and/or on modifications of their nonlinear saturation mechanisms can be considered crucial on the way to self-sustained plasma burning and corresponding fusion power plants. A particularly interesting example is the recent experimental and numerical evidence suggesting a link between the presence of fast ions and substantial improvement of energy confinement in predominantly ITG (ion-temperature-gradient) driven turbulence [1][2][3][4][5]. Dedicated theoretical studies have already identified a number of possible energetic ion effects on plasma turbulence like dilution of the main ion species [1], Shafranov shift stabilization [6] and resonance interaction with bulk species micro-instabilities in certain plasma regimes [7].…”

“…Similarly, the fast-ion pressure may drive energetic particle (EP) modes if certain thresholds are exceeded. Although a possible relevance of the proximity to the onset of these modes has been noted [4,8], their role was not investigated in more detail. In any case, all of these effects are mainly linear, i.e., alter the growth of the underlying micro-instability.…”

Validation metrics for turbulent plasma transport

“…66,[82][83][84][85][86][87][88][89][90][91][92][93][94] Many other validation studies have assessed the sensitivity of 16 inputs but not explicitly presented the results in the form shown in Fig. 5.…”

Effect of the tangential NBI current drive on the stability of pressure and energetic particle driven MHD modes in LHD plasma

“…The main motivation of performing the analysis with the gyro-fluid code FAR3d is the computational efficiency; this is due to its reduction of selected kinetic effects to a set of 3D fluid-like equations rather than more complex approaches, for example initial value gyrokinetic codes as EUTERPE [76], GEM [77], GYRO [78], GTC [79], ORB5 [80] and GENE [107] or kinetic-MHD hybrid codes as MEGA [81]. FAR3d can be used for rapid parameter/profile scans in order to perform optimization/design studies where rapidly evaluated physics target functions are required.…”