Application of the Tikhonov tomography method for the gas electron multiplier (GEM) system on experimental advanced superconducting tokamak

Abstract

A novel GEM (Gas Electron Multiplier) system has been installed on experimental advanced superconducting tokamak (EAST) which is used for collecting the line integral of the soft X-ray radiation (SXR) through a pinhole-collimated Beryllium window. The sightline of the 2-D GEM system is tangential to the toroidal field. To obtain the local SXR emission, the Tikhonov algorithm is applied for the imaging of the poloidal cross section emission in the vacuum vessel. In the meanwhile, the L-curve method is used to find an optimized solution of the regularization parameters. The tomography reliability has been tested with a known emission function where the error is also discussed. The tomography method has been coded as a graphic user interface for the fast analysis of GEM experimental data. The typical tomography results have been shown for the EAST shot (#79282) in this paper.

Observation of neoclassical tearing-mode excitation in EAST low-β p H-mode plasmas using a gas electron multiplier-based camera

“…The projecting cross-section is defined by a coordinate (R A , Z A ) which usually covers the core and depends on the image distance between the pinhole and the GEM. The GEM camera is highly sensitive to photons in the energy range of 3.5 ∼ 20 keV and can be made basically noise free by setting a threshold voltage [24,25]. Its signal-to-noise ratio is much higher than those of conventional semiconductor photoelectric diodes; as a result, it is much more applicable to these experiments because the RMP usually causes high-density pumping out and induces a low soft x-ray radiation intensity.…”

“…Although a sight line of the GEM camera is straight in the 3D space of the vacuum chamber, it passes through the same magnetic surface twice and has different (R, Z) coordinates. Many interactions naturally form in the R-Z plane, which is the basis of tomography imaging using the GEM camera [24,25]. In later descriptions, both the diagnostic space coordinate and the relative position to the q = 1 surface are illustrated.…”

“…The sawtooth period varies, and partial crashing is present. The GEM signal is proportional to the line integral of the local emissivity, thus we can get the local emissivity by a tomography method [24]. Figure 11(b) displays the tomography results with the sawtooth oscillation.…”

“…Plasma responses to the penetration of resonant or non-RMPs have been widely studied in tokamaks [23]; however, these are beyond the scope of this paper. Instead, we are interested in the NTM onset and would like to provide data or enlightening observations since we recently installed a 2D gas electron multiplier (GEM) camera at EAST [24,25]. The RMP coils are preset with an n = 1 fixed phase difference, the same current ramping rate, and the same switch-on and switch-off times [26].…”

Tomography of emissivity for Doppler coherence imaging spectroscopy diagnostic in HL-2A

“…From a mathematical point of view, the tomography problem is an estimation of the quantity of interest from the measured line-integral signals [7]. This is a classical ill-posed problem, for which there is no unique solution because of the finite sampling, and a differential operator in the inversion amplifies noise present in the input data [8].…”