Abstract. Second harmonic electron cyclotron current drive (ECCD) has been applied in Heliotron J to stabilize magnetohydrodynamic (MHD) modes in Heliotron J. Localized EC current driven at central region modifies the rotational transform profile, , making a high magnetic shear. An energetic-ion-driven MHD mode of 80 kHz has been fully stabilized by co-ECCD, and another mode of 90 kHz has been stabilized by counter-ECCD when the EC current of a few kA is driven. Both co-and counter-ECCD is effective for the energetic-ion-driven MHD modes. An experiment of scanning the EC driven current shows that there is a threshold in magnetic shear to stabilize the energetic-ion-driven MHD mode.

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A new laser timing controller for the high time-resolution Nd:YAG Thomson scattering system with two Nd:YAG lasers has been developed to study improved confinement physics in Heliotron J. A PIC-based timing controller synchronizes the timings of laser oscillations with plasma discharges and enables the measurement of plasma profiles with a precision of <1 μs. The timing controller is used for the "soft start" of the system, which protects the optical components against initial unstable laser oscillations. The timing controller is designed to precisely control the delay time of the laser pulse from one laser to another, and to investigate the profile change of electron temperature and density within a short time span (> 80 ns), which is crucial for transport physics studies including spontaneous transitions.

The effect of the magnetic configuration on fast-ion confinement is one of the most important topics for helical devices. Fast-ion velocity distributions have been investigated using ion cyclotron range of frequencies (ICRF) minority heating in Heliotron J with special emphasis on the effect of the toroidal ripple (bumpiness) of the magnetic field strength. In measurements of the fast-ion tail generated by ICRF minority heating, a high bumpiness configuration is found to be preferable for tail formation. However, the measurement area based on the line of sight of the fast-ion detector was restricted in this experiment. Due to the complexity of the magnetic field in Heliotron J, three-dimensional analysis is required to interpret the experimental results. Monte-Carlo simulations were performed. The calculation results agree well with the experimental results for high-energy tail formation. The effective temperature of minority protons was estimated.

This paper reviews recent progress on plasma control studies to improve plasma performance in Heliotron J. The SMBI fueling is successfully applied to Heliotron J plasma. A supersonic H 2 -beam is effective to increase fueling efficiency and make a peaked density profile.Local fueling with a short pulse by SMBI can increase the core plasma density avoiding the degradation due to the edge cooling. Second harmonic ECCD experiments have been performed by injecting a focused Gaussian beam with a parallel refractive index of -0.05  N ||  0.6. The experimental results show that the electron cyclotron (EC) driven current is determined not only by N || but also by local magnetic field (B) structure where the EC power is deposited. The detailed analysis of the observed N || and B dependences is in progress with a ray-tracing simulation using TRAVIS code. Fast ion velocity distribution has been investigated using fast protons generated by ICRF minority heating. In the standard configuration in Heliotron J, CX-NPA measurements show the higher effective temperature of fast minority protons in the on-axis resonance case than that in the HFS (high field side) off-axis resonance case. However, the increase of the bulk ion temperature in the HFS resonance case is larger than that in the on-axis resonance.