Measurement of 3-D Mode Structure of the Edge Harmonic Oscillations in CHS using Beam Emission Spectroscopy

Abstract

The 3-D spatial structure-radial locality and poloidal/toroidal mode numbers-of the magnetohydrodynamic fluctuation called "edge harmonic oscillation (EHO)" in the compact helical system (CHS) was investigated using beam emission spectroscopy (BES) as the diagnostic method of the local density fluctuations and the magnetic probe array. We found two groups of harmonic oscillations in CHS, one with a frequency of 4.0 kHz and a harmonic located in the edge region of the normalized minor radius ρ = 0.95 near the rotational transform ι = 1 surface, and the other with a frequency of 3.5 kHz and a harmonic located in the core region ρ = 0.53 near the ι = 0.5 surface. The magnetic probe signals showed that the poloidal/toroidal mode numbers of the edge mode and the core mode were −1/1 and −2/1, respectively. They were consistent with the rotational transform of the magnetic field at the locations of those modes.

Bispectral Analysis of Harmonic Oscillations Measured using Beam Emission Spectroscopy and Magnetic Probes in CHS

“…One is located in the core region of normalized minor radius ρ = 0.4 -0.5 near the ι = 0.5 rational surface (denoted as HO (core)), the other is located in the edge region of ρ = 0.9 -1.0 near ι = 1.0 rational surface (denoted as HO (edge)). All harmonic components of both HO (core) and HO (edge) have identical mode numbers, namely, m/n = −2/1 for HO (core) and m/n = −1/1 for HO (edge), where negative m indicates the electron diamagnetic direction and positive n indicates the toroidal field direction, even though the frequency of pth harmonic is p times larger than that of 1st harmonic for both HO (core) and HO (edge), where p is a positive integer [21].…”