Visible Light Tomography Considering Reflection Light in a Small Tokamak Device PHiX

Abstract

In a small tokamak, the visible light emission is observed and used to investigate plasmas' behavior with a fast visible camera. However, the reflected light causes a systemic error in measuring visible light emitted from the plasma. In this paper, we managed to overcome the reflection effect with the ray-tracing technique which is utilized in a synthetic diagnostic platform of the small tokamak device PHiX at Tokyo Institute of Technology using Raysect and CHERAB python libraries. We successfully evaluated the amount of reflected light and obtained tomographic reconstruction images from simulated and experimental data with the Tikhonov-Phillips regularization and the L-curve method to choose an optimal regularization parameter. We also proposed to project the contour of a reconstruction image onto a camera image to validate tomography results.

Stabilization of vertical plasma position in the PHiX tokamak with saddle coils

“…It is consistent with the vertical position calculated by MCCP method shown in figure 5(b). In addition to the above two types of measurements, visible light images of plasmas were taken with a high-speed camera of 10 000 fps, and the positions and shapes of the plasmas were measured [19]. Figure 6 shows an example of discharge time evolution.…”

Assessing global beryllium erosion via tomographic reconstruction of 3D beryllium emission profiles in ITER

“…The presence of stray light in the measured intensities is the reason why the tomography must be performed to assess Be influx, which could otherwise be estimated directly using the S/XB method [19]. Ray-tracing-aided tomography has been used for some time in fusion devices to mitigate the impact of stray light on optical diagnostics [20][21][22][23][24][25][26]. The performance of the method for the optical diagnostics of ITER was first demonstrated in [24], where the stray light was successfully filtered out in synthetic DIM signals.…”

“…While in [26], the method was applied to a combination of HA&VS, WAVS and DIM detectors, and the dependence of the accuracy of reconstruction of the H α emissivity profile in the entire SOL region on the edge plasma conditions and the uncertainty of the FW reflective properties was obtained. In [20][21][22][23][24][25][26] the problem was formulated and solved for axially symmetric (2D) emissivity profiles (H α radiation in [20][21][22][24][25][26] and unfiltered visible light in [23]). However the Be emissivity profiles are essentially 3D, although they show a certain periodicity in the toroidal direction due to the periodicity in the geometry and arrangement of the FW panels.…”