H. Kitagawa

H-mode confinement of Heliotron J

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.

Dependence of the confinement of fast ions generated by ICRF heating on the field configuration in Heliotron J

A formation and confinement experiment for fast ions is performed using the ion cyclotron range of frequencies (ICRF) minority heating scheme with a proton minority and a deuteron majority in Heliotron J, a low-shear helical-axis heliotron. The effect of the magnetic configuration on the fast-ion confinement is one of the most important issues in helical devices. In this paper, the effect of bumpiness, one of the Fourier components of the field strength, on the trapped fast-ion confinement is clarified by using ICRF minority heating. The role of the bumpiness is a key issue for the design principle of the magnetic field of Heliotron J, where the particle confinement is controlled by the bumpiness. Here, the bumpiness or the bumpy ripple is defined by the Fourier harmonic ratio eb ≡ B04/B00 in (Mizuuchi T et al 2006 Fusion Sci. Technol. 50 352), where B04 is the bumpy component and B00 is the averaged magnetic field strength. The proper bumpiness causes deeply trapped particles to be confined in the small grad-B region. High-energy ions are produced up to 10 keV by injecting an ICRF pulse into an electron cyclotron heating target plasma where ion temperature at the centre Ti(0) = 0.2 keV, electron temperature at the centre Te(0) = 0.8 keV and line-averaged electron density . For the study of the configuration dependence of the fast particle confinement, three configurations are selected; the bumpy ripples are 0.01, 0.06 and 0.15 at the normalized minor radius ρ = 0.67. The measured tail temperatures by using a charge-exchange neutral energy analyser are 1.10 keV, 0.88 keV and 0.50 keV for the ripples of 0.15, 0.06 and 0.01, respectively. The heating efficiency of the bulk ion is also better in the high bumpy case. A Monte-Carlo analysis also indicates good confinement of the high energy ions in the high bumpy case although the difference is not very large compared with the experiment.

Formation and confinement of high-energy ions in heliotron J

Abstract A fast-ion formation and confinement experiment is performed using the ion cyclotron range of frequencies (ICRF) minority heating scheme in Heliotron J. In particular, the role of one of the Fourier components, the bumpiness, is an important issue for the design principle of the magnetic field of Heliotron J, where the particle confinement is controlled by the bumpiness. We study the dependence of the fast-ion confinement on the bumpiness using fast ions produced by the ICRF heating. High-energy ions are produced up to 10 keV by injecting an ICRF pulse into electron cyclotron heating target plasmas. Moreover, ions up to 36 keV are observed in the combination heating of ICRF and neutral beam injection (NBI), where the NBI energy is 28 keV. To clarify the role of the bumpy component for the high-energy ions, three configurations with various bumpy components are selected. The tail temperature is highest in the high bumpy case. It is considered that bumpy control is effective for the fast-ion confinement in Heliotron J. An increase of the bulk-ion temperature from 0.2 to 0.4 keV is observed during the ICRF pulse. The heating efficiency also depends on the bumpy component.

Configuration Control for the Confinement Improvement in Heliotron J

Abstract In the helical-axis heliotron configuration, bumpiness of the Fourier components in Boozer coordinates is introduced to control the neoclassical transport. The bumpiness helps not only to align the mod-Bmin contours with the magnetic flux surfaces but also to control the balance of bootstrap currents due to helical and toroidal ripples. Effects of bumpiness control on the plasma performance (noninductive currents, fast-ion behavior, and global energy confinement) have been investigated in Heliotron J by selecting three configurations with different bumpiness ([curly epsilon]b = B04/B00 = 0.01, 0.06, and 0.15 at ρ = 2/3) but almost the same edge rotational transform and plasma volume. The dependence of noninductive toroidal currents is qualitatively consistent with the neoclassical prediction for the bootstrap current. The high-bumpiness configuration seems to be preferable for the confinement of fast ions. However, the longer global energy confinement time is not observed in the highest-bumpiness configuration ([curly epsilon]b = 0.15). When the dependence of the effective ripple modulation amplitude in International Stellarator Scaling 04 scaling is examined, the experimental results show that the normalized global energy confinement time seems long in the configuration with the minimum effective ripple modulation amplitude, where [curly epsilon]b is 0.06.

Dependence of toroidal current on bumpy field component in heliotron J

Abstract Toroidal current has been studied in electron cyclotron heating (ECH) and ECH + neutral beam injection (NBI) plasmas on Heliotron J by controlling the bumpy field component. In the ECH plasma with high density, the toroidal current increases from 0.3 to 1.5 kA when B04/B00 is increased from 0.01 to 0.15, where B04 and B00 are the bumpy field and the uniform field components, respectively. The observed toroidal current is qualitatively in good agreement with neoclassical calculation results without radial electric field except for a low bumpy configuration case at low density. If the radial electric field is responsible for the deviation in the low bumpiness case, the central electric potential is estimated to be ~3 to 5 kV. The dependence of the net toroidal current on the bumpiness has also been observed in the ECH + NBI plasma. An estimation of the Ohkawa current has been attempted.

Assessment of Flow Drive by Use of Ion Bernstein Wave on Heliotron J and EAST Devices

The possibility of driving poloidal flows by use of ion Bernstein wave is assessed for Heliotron J and EAST devices by means of ray tracing analysis. Sheared poloidal flow is expected to suppress plasma turbulences due to the decorrelation of the waves. In Heliotron J and EAST plasma, the rays of Ion Bernstein Wave travel into the central region with oscillations along the magnetic lines of force and their power is absorbed by ions at the cyclotron resonance layers. The momentum input has been estimated by calculating the momentum change of rays and the poloidal flow has been estimated using neoclassical viscosities. The wave momentum changes its sign as it propagates inward, depositing sheared momentum to the plasma, and therefore causes sheared poloidal flows.