‘BAAE’ instabilities observed without fast ion drive

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.

Core localized alpha-channeling via low frequency Alfven mode generation in reversed shear scenarios

“…For the effective transfer of alpha particle energy to core ions, ω B O(v i /(qR 0 )), and thus, |ω B | ≪ |ω 0 |, |ω 1 | and k ,B ≃ 0. Thus, the q surface where secondary Ω B locates, also corresponds to the rational surface of ω B , i.e., Ω B is the LFAM in the reversed shear configuration, as investigated experimentally [51] and theoretically [27]. We then have, ω 0 ≃ ω 1 and k ,0 ≃ k ,1 .…”

Linear kinetic effects of core plasma on low frequency Alfven and acoustic eigenmodes in tokamaks

“…The GFLDR contains all the elements of the kinetic thermal ion gap necessary to identify and study low frequency modes and their polarization, including the background plasma MHD potential, resonant and non-resonant energetic particle drive, as well as thermal ion and electron dynamics. Hence, the equation as roots has both the reactive low frequency instability [6], as well as several different modes in this frequency range like BAEs, EPMs, KBMs, BAAEs and MHD modes [14]. For modes located at the vanishing shear, i.e.…”

“…Additionally, non-resonant effects of the background (or hot) plasma might provide enough drive to destabilize low frequency modes, in which case the mode frequency is not determined by particle resonance, but by the thermal plasma diamagnetic frequency ω * pi [6]. The modes previously recognized as BAAEs in experiments are much more likely to be excited in this way, and hence should be identified as reactive modes.…”

“…The modes previously recognized as BAAEs in experiments are much more likely to be excited in this way, and hence should be identified as reactive modes. A significant number of experiments on DIII-D tokamak [6] have shown this type of modes appearing in a "Christmas lights" pattern due to the time evolution of the q-profile which pases through several rational numbers q = m/n, with m and n being the poloidal and toroidal numbers, respectively. Common feature of these modes, labeled Low Frequency Modes (LFMs) is that they appear near q min of a reversed shear profile, and they are driven by high electron temperature T e , but modest β i .…”

How Zonal Flow Affects Trapped Electron Driven Turbulence in Tokamaks

“…[13]. We note that while this work analyzes CTEMs in tokamaks, the theoretical approach presented here could conceivably be applicable toward clarifying the role of ZF in other plasma turbulence; e.g., shear Alfvén waves excited by either energetic [15] or thermal [16] particles, where wave-particle interactions of different particle species occur on distinctively separated spatial scales.…”