Plasma Confinement Characteristics in Heliotron J—Spontaneous Change of Plasma Confinement State

Mizuuchi, T.; Okada, H.; Nagasaki, K.; Kobayashi, S.; Yamamoto, Satoshi; Sano, F.; Kondo, K.; Hanatani, K.; Nakasuga, M.; Nakamura, Yuji; Suzuki, Y.; Manabe, Yukio; Satō, Hiroyuki; Kawazome, H.; Kaneko, Masashi; Nishioka, Yuichiro; Ohno, Yoichi; Takamiya, T.; Yukimoto, Hidetoshi; Fukagawa, Y.; Morita, Yoshiro; Nakazawa, Shingo; Nishio, S.; Takahashi, Koichi; Tuboi, S; Yamada, Masaki; Ohashi, Keisuke; Ijiri, Y.; Senju, T.; Yaguchi, K.; Sakamoto, K.; Tohshi, K.; Shibano, Masaya; Obiki, T.

doi:10.1088/1009-0630/6/4/005

Cited by 2 publications

(6 citation statements)

References 5 publications

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“…A similar spontaneous transition to an improved confi nement mode was also observed for NBI (H 0 beam of ~ 28 keV) plasmas [10,11]. Figure 5 shows an example of the time history of NBI-only discharge in the standard confi guration (ι(a)/2π ≈ 0.560).…”

Section: Nbi-only Plasmassupporting

confidence: 68%

“…After the change of the launching position of the microwaves, similar transition phenomena were observed again for second-harmonic ECH plasmas [11].…”

Section: Characteristics Of the Spontaneous Transition To The Improvesupporting

confidence: 66%

“…During the low-Hα /Dα phase, it was observed that the increasing rate of SX photo-diode array signals was larger than that at other timings [11]. This tendency is clear for the edge chord.…”

Section: Nbi-only Plasmasmentioning

confidence: 88%

“…These observations are similar to the characteristics of the H-mode in tokamaks. There are some reports of these kinds of observations in Heliotron J for ECH-only [7][8][9], NBI-only and ECH+NBI plasmas [10,11]. Recently, the confi guration effects on the transition were studied by changing the edge rotational transform ι(a)/2π as a label of the confi guration [12].…”

Section: Experimental Conditions For Improved Confi Nement Modes In Hmentioning

confidence: 99%

“…These observations suggest that the change of confi nement occurs in the edge region. Figure 6 shows an example of the time history of ECH+NBI combination heating discharge for the standard confi guration (ι(a)/2π ≈ 0.560) [11]. The initial plasma was produced and heated by 70 GHz microwaves (second-harmonic ECH, P ECH ~ 0.3 MW).…”

Section: Nbi-only Plasmasmentioning

confidence: 99%

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Experimental Conditions for Improved Confinement Modes in Heliotron J

Mizuuchi

Sano

Kondo

et al. 2005

J. Plasma Fusion Res.

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ECH/NBI experiments in Heliotron J have revealed the existence of a spontaneous transition to improved confi nement modes. Based on the experimental database obtained up to now, the characteristics of the transition and the density and power thresholds are discussed paying attention to the confi guration effects. For ECH+NBI discharges with injection powers of P ECH ~ 0.29 MW and P NBI ~ 0.57 MW, transition phenomena were observed in almost all ι(a)/2π confi gurations. The global plasma confi nements before and after the transition are affected by the value of the edge rotational transform ι(a)/2π. During the improved mode, the ι(a)/2π-dependence of the plasma stored energy is mitigated compared to the pre-transition phase. The database indicates the existence of a critical value in the lineaveraged electron density under which the transition cannot be observed. This threshold line-averaged density is in the range of 1.1-2.0×10 19 m -3 in most confi gurations, indicating that it is not a strong function of the injection power and/or the heating method. The existence of an ι(a)/2π value where the transition was not observed for ECH-only discharges but observed for NBI-only or ECH+NBI discharges suggests the existence of a confi guration effect and/or heating-method effect on the threshold power.

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Section: Nbi-only Plasmassupporting

confidence: 68%

“…After the change of the launching position of the microwaves, similar transition phenomena were observed again for second-harmonic ECH plasmas [11].…”

Section: Characteristics Of the Spontaneous Transition To The Improvesupporting

confidence: 66%

Section: Nbi-only Plasmasmentioning

confidence: 88%

Section: Experimental Conditions For Improved Confi Nement Modes In Hmentioning

confidence: 99%

Section: Nbi-only Plasmasmentioning

confidence: 99%

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Experimental Conditions for Improved Confinement Modes in Heliotron J

Mizuuchi

Sano

Kondo

et al. 2005

J. Plasma Fusion Res.

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H-mode confinement of Heliotron J

et al. 2005

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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.

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Plasma Confinement Characteristics in Heliotron J—Spontaneous Change of Plasma Confinement State

Cited by 2 publications

References 5 publications

Experimental Conditions for Improved Confinement Modes in Heliotron J

Experimental Conditions for Improved Confinement Modes in Heliotron J

H-mode confinement of Heliotron J

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