Transport of energetic ions due to sawteeth, Alfvén eigenmodes and microturbulence

Abstract

Utilizing an array of new diagnostics and simulation/modelling techniques, recent DIII-D experiments have elucidated a variety of energetic ion transport behaviour in the presence of instabilities ranging from large-scale sawteeth to fine spatial scale microturbulence. Important new insights include sawteeth, such as those of the ITER baseline scenario, causing major redistribution of the energetic ion population; high levels of transport induced by low-amplitude Alfvén eigenmodes can be caused by the integrated effect of a large number of simultaneous modes; and microturbulence can contribute to the removal of alpha ash while having little effect on fusion alphas. This paper provides an overview of recent and upcoming results from the DIII-D Energetic Particles research programme.

Energetic ion transport by microturbulence is insignificant in tokamaks

“…While FIDA was not available during those experiments, a system has since been commissioned 113 and observes classical energetic ion profiles during cases of P NB ¼ 5 MW in both on-axis and off-axis beam injection. 114 Results from DIII-D claim to demonstrate a measurable energetic ion transport by microturbulence in both offaxis 44,45,48 and on-axis 48 scenarios. The crucial aspect of these experiments is that the plasmas are MHD-quiescent.…”

“…Specifically, tearing modes are well known 115 to reduce NBCD. Additional analysis of these high-power shots is presented in other publications, 44,48 but still without quantifying the effects of coherent fluctuation activity. Another challenge for these experiments is that the off-axis NBCD is achieved by vertically shifting the plasmas.…”

“…An on-axis beam injection example is presented in Ref. 48, where decreasing beam energy is reported to enhance the energetic ion transport by microturbulence because the injected beam ions feature smaller gyroradii. In this example, additional characterization of the beam injection and deposition may be necessary.…”

“…44 For context, empirical studies at DIII-D found 46 that a constant D EI ¼ 0:3 m 2 =s is typically sufficient to account for transport of energetic ions due to Alfvén eigenmodes, while recent experiments 47 showed that neutron rates asymptote to the D EI ¼ 0 calculation as AE activity decays away. A separate case investigated the transport enhancement by using on-axis beam injection at reduced energy, E b ¼ 58 keV, along with spectroscopic techniques to show 48 that beam ion diffusion was larger than neoclassically expected for E b . 30 keV (plasma temperatures were not reported for that plasma).…”

Combination of fast-ion diagnostics in velocity-space tomographies

“…Velocityspace tomography is less developed [13,33,34] but could be particularly useful in studies of selective ejection or redistribution in velocity space. Several types of modes affect ions in only part of velocity space, for example sawteeth [35][36][37][38], Alfvén eigenmodes [6,[39][40][41][42][43] and neoclassical tearing modes [4,5]. Turbulent transport of fast ions also depends on the ion energy [44][45][46][47].…”