Electromagnetic turbulence suppression by energetic particle driven modes

Abstract

In recent years, a strong reduction of plasma turbulence in the presence of energetic particles has been reported in a number of magnetic confinement experiments and corresponding gyrokinetic simulations. While highly relevant to performance predictions for burning plasmas, an explanation for this primarily nonlinear effect has remained elusive so far. A thorough analysis finds that linearly marginally stable energetic particle driven modes are excited nonlinearly, depleting the energy content of the turbulence and acting as an additional catalyst for energy transfer to zonal modes (the dominant turbulence saturation channel). Respective signatures are found in a number of simulations for different JET and ASDEX Upgrade discharges with reduced transport levels attributed to energetic ion effects.PACS numbers: 52.65.Tt Introduction.Being an almost ubiquitous phenomenon, turbulence with its highly stochastic and nonlinear character is a subject of active research in various fields. In magnetically confined plasma physics, it is of particular interest since it largely determines the radial heat and particle transport and thus the overall confinement. 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]. They furthermore contribute to the total plasma pressure and increase the kinetic-tomagnetic pressure ratio, β, which is a measure for the relevance of electromagnetic fluctuations, known to stabilize ITG modes. Such behaviour could indeed be confirmed in simulations [4,8,9] of JET hybrid discharges [10,11] with substantial fast ion effects that, however, also identified an upper limit for this beneficial fast-ion-pressure effect. If the total plasma pressure exceeds a critical value, kinetic ballooning or Alfvénic ITG modes with smaller toroidal mode numbers and frequencies higher than the ITG modes are destabilized which increase particle/heat fluxes [12]. 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

Heat transport driven by the ion temperature gradient and electron temperature gradient instabilities in ASDEX Upgrade H-modes

“…Attempted non-linear simulations did not reach a convincingly long steady saturated state and this hypothesis is not excluded. Furthermore, it seems that this investigation and the assessment of the results should probably also include both the role of nonlinearly excited energetic particle driven modes [35] and an values, which is a general issue, see e.g. [32,31].…”

Turbulence Suppression by Energetic Particle Effects in Modern Optimized Stellarators

“…Plasma quasineutrality is always fulfilled. Moreover, we consider a=L Tf ¼ 18, which is consistent with the realistic values observed in ICRF H-minority heating in tokamak experiments [11,20]. The hydrogen minority is modeled with an equivalent Maxwellian background.…”

Progress and challenges in understanding core transport in tokamaks in support to ITER operations

“…The physics mechanisms behind the stabilization have been investigated in detail with GENE for the JET case. Part of the stabilization is due to an ES resonant linear effect associated to the ICRH fast ions [66], and part is due to a nonlinear EM effect caused by non-linear coupling of ITG, marginally stable Toroidal Alfven Eigenmodes and Zonal Flows [67]. This second mechanism is analogous to the non-linear EM stabilization of ITGs by the thermal plasma β (i.e.…”