Abstract
The instability that was previously identified (Gorelenkov 2009 Phys. Plasmas 16 056107) as a fast-ion driven beta-induced Alfvén-acoustic eigenmode (BAAE) in DIII-D was misidentified. In a dedicated experiment, low frequency modes (LFMs) with characteristic 'Christmas light' patterns of brief instability linked to the safety factor evolution occur in plasmas with electron temperature T e ⪆ 2.1 keV but modest beta. To isolate the importance of different driving gradients on these modes, the electron cyclotron heating (ECH) power and 80 keV, sub-Alfvénic neutral beams are altered for 50-100 ms durations in reproducible discharges. Although beta-induced Alfvén eigenmodes and reversed-shear Alfvén eigenmodes stabilize when beam injection ceases (as expected for a fast-ion driven instability), the LFMs that were called BAAEs persist. Data mining reveals that characteristic LFM instabilities can occur in discharges with no beam heating but strong ECH. A large database of over 1000 discharges shows that LFMs are only unstable in plasmas with hot electrons but modest overall beta. The experimental LFMs have low frequencies (comparable to diamagnetic drift frequencies) in the plasma frame, occur near the minimum of the safety factor q min, and appear when q min is close to rational values. Theoretical analysis suggests that the LFMs are a low frequency reactive instability of predominately Alfvénic polarization.
| Original language | English |
|---|---|
| Article number | 016029 |
| Journal | Nuclear Fusion |
| Volume | 61 |
| Issue number | 1 |
| DOIs | |
| State | Published - Jan 2021 |
Funding
Helpful discussions with Xiaodi Du, Nate Ferraro, Nikolai Gorelenkov, Craig Petty, and Fulvio Zonca are gratefully acknowledged, as is the vital assistance of the DIII-D team. This work is supported by US Department of Energy award DE-SC0020337 and DE-FC02-04ER54698. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. This work has been partially carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training program 2014 2018 and 2019 2020 under Grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
