Mechanism for Collective Energy Transfer from Neutral Beam-Injected Ions to Fusion-Born Alpha Particles on Cyclotron Timescales in a Plasma.

Helium ash alpha particles at ∼100 keV in magnetically confined fusion plasmas may have the same Larmor radius, as well as cyclotron frequency, as the energetic beam-injected deuterons that heat the plasma. While the velocity-space distribution of the helium ash is monotonically decreasing, that of the energetic deuterons is a delta function in the edge plasma. Here we identify, by means of first principles particle-in-cell computations, a new physical process by which Larmor radius matching enables collective gyroresonant energy transfer between these two colocated minority energetic ion populations, embedded in majority thermal plasma. This newly identified nonlinear phenomenon rests on similar underlying physics to widely observed ion cyclotron emission from suprathermal minority ion populations.

Comparing pedestal structure in JET-ILW H-mode plasmas with a model for stiff ETG turbulent heat transport.

A predictive model for the electron temperature profile of the H-mode pedestal is described, and its results are compared with the pedestal structure of JET-ILW plasmas. The model is based on a scaling for the gyro-Bohm normalized, turbulent electron heat flux [Formula: see text] resulting from electron temperature gradient (ETG) turbulence, derived from results of nonlinear gyrokinetic (GK) calculations for the steep gradient region. By using the local temperature gradient scale length [Formula: see text] in the normalization, the dependence of [Formula: see text] on the normalized gradients [Formula: see text] and [Formula: see text] can be represented by a unified scaling with the parameter [Formula: see text], to which the linear stability of ETG turbulence is sensitive when the density gradient is sufficiently steep. For a prescribed density profile, the value of [Formula: see text] determined from this scaling, required to maintain a constant electron heat flux [Formula: see text] across the pedestal, is used to calculate the temperature profile. Reasonable agreement with measurements is found for different cases, the model providing an explanation of the relative widths and shifts of the [Formula: see text] and [Formula: see text] profiles, as well as highlighting the importance of the separatrix boundary conditions. Other cases showing disagreement indicate conditions where other branches of turbulence might dominate. This article is part of a discussion meeting issue 'H-mode transition and pedestal studies in fusion plasmas'.