Observation of Magnetohydrodynamic Instabilities in Heliotron J Plasmas

Improved design for Heliotron J soft X-ray diagnostic for tomographic reconstruction studies

Identifications of the plasma boundary shape and mode structure are important problems for the studies of the magnetohydrodynamics equilibrium and stability studies. Imaging diagnostics have the possibility to measure the plasma boundary shape and mode structure directly. The soft X-ray (SX) tomography is an imaging diagnostic for offering these direct measurements. However, SX tomography is by nature a limited angle tomography and provides a challenge as an ill-condition inverse problem. The reconstructed profiles depend heavily on the quality of measurements along with the employed inversion algorithm. This paper provides a detailed description of tomographic reconstruction of simulated SX measurements from the current SX diagnostic design of Heliotron J (H-J), which is a medium size heliotron device. Heliotron J has a unique magnetic geometry, and the quantitative identification of the plasma boundary and mode structure is a critical problem. The Phillip-Tikhonov regularization is employed to address the inversion problem. Finally, a new design for Heliotron J soft X-ray diagnostic is discussed which holds enhanced capabilities for tomographic reconstructions especially for the high poloidal number perturbations.

Highly time-resolved evaluation technique of instantaneous amplitude and phase difference using analytic signals for multi-channel diagnostics

A fluctuation analysis technique using analytic signals is proposed. Analytic signals are suitable to characterize a single mode with time-dependent amplitude and frequency, such as an MHD mode observed in fusion plasmas since the technique can evaluate amplitude and frequency at a specific moment without limitations of temporal and frequency resolutions, which is problematic in Fourier-based analyses. Moreover, a concept of instantaneous phase difference is newly introduced, and error of the evaluated phase difference and its error reduction techniques using conditional/ensemble averaging are discussed. These techniques are applied to experimental data of the beam emission spectroscopic measurement in the Heliotron J device, which demonstrates that the technique can describe nonlinear evolution of MHD instabilities. This technique is widely applicable to other diagnostics having necessity to evaluate phase difference.

Density fluctuation measurements using beam emission spectroscopy on Heliotron J

This paper describes the measurement of the density fluctuation using beam emission spectroscopy in Heliotron J, having the non-symmetrical helical-magnetic-axis configuration. In order to optimize the sightlines, the numerical calculations are carried out to estimate the spatial resolution and the observation location. When a tangential neutral beam is used as diagnostic one, suitable sightlines from the newly installed diagnostic port are selected whose spatial resolution Δρ is less than ± 0.07 over the entire plasma region. Modification of the interference filter and the detection systems enables us to measure the radial profile of the density fluctuation. Each of the three coherent modes due to the fast-ion-driven magnetohydrodynamic instabilities has different radial structure of the density fluctuation.