Alfvenic Ion Temperature Gradient Activities in a Weak Magnetic Shear Plasma

TL;DR

This paper presents the first experimental detection of Alfvenic ion temperature gradient modes in HL-2A plasmas, linking these instabilities to plasma pressure gradients and weak magnetic shear, with implications for transport barriers and H-mode physics.

Contribution

It provides the first experimental evidence of AITG modes in HL-2A plasmas and offers theoretical analysis explaining their frequency scaling and stability conditions.

Findings

Detection of AITG modes in high-density Ohmic plasmas.

Modes are excited by pressure gradients in weak magnetic shear.

AITG modes may influence internal transport barriers and H-mode pedestal physics.

Abstract

We report the first experimental evidence of Alfvenic ion temperature gradient (AITG) modes in HL-2A Ohmic plasmas. A group of oscillations with $f=15-40$ kHz and $n=3-6$ is detected by various diagnostics in high-density Ohmic regimes. They appear in the plasmas with peaked density profiles and weak magnetic shear, which indicates that corresponding instabilities are excited by pressure gradients. The time trace of the fluctuation spectrogram can be either a frequency staircase, with different modes excited at different times or multiple modes may simultaneously coexist. Theoretical analyses by the extended generalized fishbone-like dispersion relation (GFLDR-E) reveal that mode frequencies scale with ion diamagnetic drift frequency and $\eta_i$, and they lie in KBM-AITG-BAE frequency ranges. AITG modes are most unstable when the magnetic shear is small in low pressure gradient regions. Numerical solutions of the AITG/KBM equation also illuminate why AITG modes can be unstable for weak shear and low pressure gradients. It is worth emphasizing that these instabilities may be linked to the internal transport barrier (ITB) and H-mode pedestal physics for weak magnetic shear.