Improving fast-ion confinement in high-performance discharges by suppressing Alfvén eigenmodes

Abstract

We show that the degradation of fast-ion confinement in steady-state DIII-D discharges is quantitatively consistent with predictions based on the effects of multiple unstable Alfvén eigenmodes on beam-ion transport. Simulation and experiment show that increasing the radius where the magnetic safety factor has its minimum is effective in minimizing beam-ion transport. This is favorable for achieving high performance steady-state operation in DIII-D and future reactors. A comparison between the experiments and a critical gradient model, in which only equilibrium profiles were used to predict the most unstable modes, show that in a number of cases this model reproduces the measured neutron rate well.

DIII-D Research to Prepare for Steady State Advanced Tokamak Power Plants

“…Experiments and modeling have shown a variety of options for improving fast ion confinement in DIII-D q min [ 2 discharges. Modeling predicts, and some experimental evidence confirms, that further broadening of the q-profile can reduce AE-induced fast ion loss [63]. Specifically, increasing q 0 is predicted to eliminate Toroidal AEs by closing off the frequency gap in which they can exist, and increasing the radius of q min is predicted to lower the impact of Reverse Shear AEs on fast ion confinement by pushing the modes to a region of lower fast ion density.…”

“…Comparison of cases with high and low fast ion transport has suggested ways to improve the q min * 2 scenario [63] that the DIII-D EP program has begun to explore experimentally. For example, if the negative magnetic shear region can be expanded so that the q min radius is moved outwards where there are fewer fast-ions, then the drive of RSAEs (and perhaps other EP modes) should be greatly reduced, as shown in Fig.…”

“…b Time-averaged divergence of modulated flux, i.e.,transport, inferred from the neutron emission. Adapted from [62] [ 63,140]. Also, the drive for AEs can be altered by varying the radial gradient in the fast ion pressure profile grad-b fast [129,136] to keep it below a critical threshold [139] through variation of density and NBI power as well as NBI The solid white lines denote the TAE frequency at the magnetic axis.…”

“…Adapted from data presented in[37] Graphical depiction of High-Betap scenario and improved resilience to AE induced EP transport. Adapted from data presented in[63] Core TAEs occur inside of q min where the fast-ion profile is flat or hollow with off-axis injection, RSAEs occur near q min where grad (b fast ) is small during off-axis injection, and global TAEs lie in a region where the gradient is insensitive to changes between on-axis and off-axis injection. The illustrated fast-ion profiles are from classical calculations performed prior to the experiment.…”

Machine Learning Characterization of Alfvénic and Sub-Alfvénic Chirping and Correlation With Fast-Ion Loss at NSTX

“…Furthermore, experiments and simulations in the literature 2,15,22,23 have shown that rapid long range frequency chirping across a wide range of frequency chirps (mode 'avalanching') is correlated with high amounts of fast ion loss. Other work has shown that mode-mode destabilisation may play a role in Alfvénic frequency chirping -activity in the KTF frequency range (∼ 1 kHz to 30 kHz) may instigate Alfvénic activity, and vice-versa.…”