Identification of Ballooning-Type Pressure Driven Instability by Tomographic Reconstruction

This paper deals with the experimental identification of the mode structure for the CDC by the tomographic reconstruction. Theoretically, it was predicted the CDC is triggered by the high-n ballooning mode. In the experiment, the localized density fluctuations at the outboard side are observed at the onset, and this is evidence that the ballooning-like instability triggers the CDC. Still, it was not well confirmed because the mode structure is not experimentally understood yet. To understand the mode structure, a new tomographic reconstruction is developed by coupling the Fourier-Bessel series expansion method and sparse modeling with L1 regularization. Using this, the ballooning-like mode structure of the density fluctuations is successfully reconstructed at the precursor.