Fast-ion stabilization of tokamak plasma turbulence

Abstract

A significant reduction of the turbulenceinduced anomalous heat transport has been observed in recent studies of magnetically confined plasmas in the presence of a significant fastion fractions. Therefore, the control of fastion populations with external heating might open the way to more optimistic scenarios for future fusion devices. However, little is known about the parameter range of relevance of these fastion effects which are often only highlighted in correlation with substantial electromagnetic fluctuations. Here, a significant fast ion induced stabilization is also found in both linear and nonlinear electrostatic gyrokinetic simulations which cannot be explained with the conventional assumptions based on pressure profile and dilution effects. Strong wavefast particle resonant interactions are observed for realistic parameters where the fast particle trace approximation clearly failed and explained with the help of a reduced Vlasov model. In contrast to previous interpretations, fast particles can actively modify the Poisson field equation-even at low fast particle densities where dilution tends to be negligible and at relatively high temperatures, i.e. T < 30T e . Further key parameters controlling the role of the fast ions are identified in the following and various ways of further optimizing their beneficial impact are explored. Finally, possible extensions into the electromagnetic regime are briefly discussed and the relevance of these findings for ITER standard scenarios is highlighted.

Electromagnetic turbulence suppression by energetic particle driven modes

“…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.…”

“…Our study is based on the JET-like scenario with deuterium, electrons and externally injected neutral beam deuterium described in detail in Ref. [7,16], with a reduced safety factor of q = 1. and the minimum k y ρ s = 0.025 with thermal gyroradius ρ s = (T e /m i ) 1/2 /Ω i . Here, Ω i = m i c/qB 0 denotes the gyro-frequency, T i the main ion temperature, m i the main ion mass, q the charge, B 0 the magnetic field on axis and c is the speed of light.…”

Implementation of energy transfer technique in ORB5 to study collisionless wave-particle interactions in phase-space

“…It reads as The same AUG shot has been simulated in GENE in case with adiabatic electrons (one can see also Ref. 47 ), using the flux-tube version of the code at s = 0.5. In Fig.…”

“…From the energy relation of Eq. 111 it is to compute the more unstable linear growth rate γ through the time variation of the potential energy, as shown in details in Ref47,56,58,59 , by removing the sum over all the species and studying each term separately. Positive (negative) values of ∂E k,s /∂t indicate that the plasma species considered is giving (taking) energy to (from) the electrostatic field component with a consequent growth (damping) of the mode.…”

Turbulent transport stabilization by ICRH minority fast ions in low rotating JET ILW L-mode plasmas

“…Recent results [9] indicate that an electrostatic mechanism, related related to a wave-fast particle resonant interaction [10,11], can also play an important role in the stabilization of ITG modes in some conditions. The study in ref.…”