The L–H transition in a helical-axis heliotron, Heliotron J, is investigated. For electron cyclotron heating (ECH), neutral beam injection (NBI) heating and ECH + NBI combination heating plasmas, the confinement quality of the H-mode is examined with an emphasis on its magnetic configuration dependence. The vacuum edge rotational transform, ι(a)/2π, is chosen as a label for the magnetic configuration where ι/2π is the rotational transform and a is the average plasma minor radius in metres. The experimental ι(a)/2π dependence of the enhancement factor over the L-mode confinement reveals that specific configurations exist where high-quality H-modes (1.3 < HISS95 < 1.8) are attained. is the experimental global energy confinement time and is the confinement time scaling from the international stellarator database given as . R is the plasma major radius in metres, is the line-averaged plasma density in 1019 m−3, PL is the power loss in megawatts that accounts for the time derivative of the total plasma energy content and Bt is the toroidal magnetic field strength in tesla (Stroth U. et al 1996 Nucl. Fusion 36 1063). The ι (a)/2π ranges for these configurations are near values that are slightly less than those of the major natural resonances of Heliotron J, i.e. n/m = 4/8, 4/7 and 12/22. To better understand this configuration dependence, the geometrical poloidal viscous damping rate coefficient, Cp, is calculated for different values of ι(a)/2π and compared with the experimental results. The threshold line-averaged density of the H-mode, which depends on the configuration, is in the region of 0.7–2.0 × 1019 m−3 in ECH (0.29 MW) + NBI (0.57 MW) operation. As for the edge plasma characteristics, Langmuir probe measurements have shown a reduced fluctuation-induced transport in the region that begins inside the last closed flux surface (LCFS) and extends into the scrape-off layer. In addition, a negative radial electric field Er (or Er-shear) is simultaneously formed near the LCFS at the transition.
Non-inductive currents of electron cyclotron heated plasmas have been examined in the helical-axis heliotron device, Heliotron J. The bootstrap and EC currents were separated by comparing experiments with positive and negative magnetic field. The estimated bootstrap current was found to be affected by the magnetic field configuration. It increases with an increase in the bumpy component of the magnetic field spectrum, which agrees well with a neoclassical prediction calculated using the SPBSC code. The EC current driven by oblique launch with respect to the magnetic field strongly depends on the field configuration and the location of the EC power deposition. The EC current is enhanced when the EC power is deposited on the magnetic axis. The maximum EC current is I EC = −4.6 kA and the current drive efficiency is η = n e RI p/P EC = 8.4 × 1016 A W−1 m−2. The flow direction of the EC current depends on the magnetic field ripple structure where the EC power is deposited.
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.
Neutral beam injection (NBI) plasmas have been initiated with the assistance of 5 kW, 2.45 GHz microwaves in a medium-sized heliotron, Heliotron J plasmas with a line-averaged electron density of over 1 × 1019 m−3 are generated with 1 MW NBI using a magnetic field between 0.63 and 1.25 T within 20 ms after turning on NBI. This technique does not require the electron cyclotron resonance layer for the plasma startup, which enables us to enlarge the operational space for the magnetic field strength. Electron cyclotron emission measured with a radiometer reveals that the condition of initial plasmas generated by 2.45 GHz microwaves is critical for reliable startup.
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