MHD properties of low-aspect ratio RFP in RELAX

The low-aspect-ratio (A) reversed field pinch (RFP) offers attractive properties such as enhanced bootstrap current and simpler MHD mode dynamics. The RELAX (REversed field pinch of Low-Aspect ratio eXperiment) machine with the world's lowest A of 2 (R/a = 0.5 m/0.25 m) has been constructed to explore the RFP properties in low-A regime. In flat-topped low-A RFP discharges in RELAX, plasma current of ~50 kA has been attained with discharge duration of ~2 ms. In round-topped discharges with plasma current of ~70 kA, quasi-periodic growth of a single helical mode has been observed. When the dominant m = 1/n = 4 mode grows, the toroidal mode spectrum looks like that of the quasi-single helicity (QSH) RFP state with higher amplitude. MHD equilibrium analyses using a reconstruction code have shown that the bootstrap current fraction is lower than ~5% in the present RELAX plasmas, and it will be ~25% if we could achieve the plasma density of 4 × 10 ¹⁹ m ^-3 and electron temperature of 300 eV at plasma current of ~100 kA.