Effects of energetic particles on the density-gradient-driven collisionless trapped electron mode instability in tokamak plasmas

Abstract

We investigate analytically the effects of energetic particles (EPs) on the instability of the density-gradient-driven collisionless trapped electron mode (CTEM) through linear gyrokinetic theory and bounce kinetic theory in tokamak plasmas. The effects of EPs, including fusion-born alpha particles and neutral-beam-injection-driven beam ions, on the CTEM instability are compared for the dynamic model with slowing-down (SD) and equivalent Maxwellian (EM) equilibrium EP distribution functions and dilution model. It is found that the density-gradient-driven CTEM instability in the long wavelength regime can be further destabilized by EPs mainly due to the downshift in the real frequency of the mode by dilution effects. This is attributed to more resonant electrons around the smaller phase velocity of the drift wave and the consequent stronger excitation of CTEM instability. The growth rate is slightly higher for the dilution model as compared to that for the dynamic model since the Landau damping effects of EPs are neglected in the dilution model. Moreover, there is no significant difference in the growth rate between the cases of SD and EM equilibrium EP distribution functions, except for the case of alpha particles and with relatively higher electron temperature.

Effects of alpha particles on the transport of helium ash driven by collisionless trapped electron mode turbulence

“…For α particles, their equilibrium distribution functions are described by the SD distribution function as in [54] in the limit of shearless slab geometry. The total perturbed number density of α particles has been derived in [35] in detail, and we just use it as…”

“…where the concentrations of species j and α particles are expressed as f j = n 0 j /n 0e and f α = n 0α /n 0e , respectively. For simplicity of calculation, it should be stressed that the higher order term proportional to ω * e ω r (1 + η e ) in the non-resonant part of trapped electron perturbed density can be dropped in order to solve the linear dispersion rather than quadratic dispersion equation as used in [35,55]. Then, the normalized real frequency ωr = ω r / v thH /R and growth rate γk = γ k / v thH /R are given as…”

“…α particles are also fast ions, thus are quite possible to affect the turbulent transport of helium ash. It has been indicated that fast ions can either stabilize or destabilize CTEM turbulence [33][34][35], depending on the type of free energy source and the regime of wavelength. However, the effects of α particles on the transport of helium ash driven by CTEM turbulence have not been further considered.…”

“…One of the objectives of studying helium ash transport is exploring favorable parameter regimes for expelling more helium ash. The presence of α particles and their density gradient can affect CTEM instability [35], and thus might further influence helium ash removal. Through varying hydrogenic isotope mass, transport properties are also changed by isotopic effects [42][43][44][45][46].…”

Multi-scale turbulence simulation suggesting improvement of electron heated plasma confinement

“…It has been reported that TEM drives low-level transport of alpha particles 39 . The resonant interactions between energetic particles and micro-instability will be significant for ITG modes 40 , 41 but not for TEM and ETG modes because the propagation directions are opposite 42 . Therefore, interactions between energetic particles and TEM/ETG turbulence are expected to be weak.…”