Soft x-ray tomography using L1 regularization for MHD modes with limited sight lines in JT-60SA

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Tomography

Background:

• Soft x-ray (SX) tomography is crucial for diagnosing fusion plasmas.
• JT-60SA, a large tokamak, will have limited sight lines for its SX diagnostic, common in future devices.
• Limited sight lines introduce artifacts in SX tomography, hindering magnetohydrodynamics (MHD) mode analysis.

Purpose of the Study:

• Investigate L1 regularization to reduce artifacts in SX tomography with limited sight lines.
• Evaluate alternative basis functions for analyzing MHD modes in elongated tokamak plasmas.

Main Methods:

• Applied L1 regularization alongside traditional L2 regularization for SX tomography.
• Utilized Fourier-Bessel series (FBS) and introduced Saito's Laplacian eigenfunction (LEF) as basis functions.
• Simulated a high beta scenario in JT-60SA prone to MHD modes.

Main Results:

• L1 regularization significantly reduces artifacts in SX tomography compared to L2 regularization alone.
• Saito's LEF offers an advantage over FBS as it does not require interior plasma measurements.
• Both FBS and LEF, when combined with L1 regularization, improve the reconstruction of plasma spatial structures.

Conclusions:

• L1 regularization is effective in mitigating artifacts in SX tomography with limited sight lines.
• Saito's LEF is a viable and practical basis function for analyzing MHD modes in tokamaks.
• The developed methods show promise for improving tomographic analysis in current and future fusion devices.