Transition to helical RFP states and associated confinement improvement in a spherical RFP

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It has been confirmed from experimental results in RELAX that the quasi-single helicity (QSH) state can be achieved easily in low-aspect-ratio machine. Magnetic measurements and soft-X ray imaging diagnostics have revealed dense or high-temperature helical core with steep gradient in the SXR emissivity profile at the helical boundary which may suggest the electron thermal transport barrier in the QSH state. Feedback stabilization of the m/n=1/2 kink mode (RWM) has resulted in improved discharge duration by ~50% (from 2 ms to 3 ms). The Thomson scattering measurement has revealed that the central electron temperature reaches 200eV at Ip~120kA, corresponding to the electron beta of ~10%. The electron parameters have reached the designed values in the low-A RFP machine RELAX.