Observation of a Rotating Radiation Belt in LHD

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A high-luminosity striation, which rotates within a flux surface of the plasma boundary having helical structure, has been observed in the LHD plasmas by means of a fast camera. The striation appears when the plasma is shrinking due to excessive gas fueling despite the existence of neutral beam heating. In order to explore the operational space of the highdensity regime, massive gas puffing and/or pellet injection have been performed in the Large Helical Device (LHD). Under these experimental conditions, high density plasmas collapse when excessive gas fueling is applied. Through the use of a fast camera, we have revealed a rotating highluminosity striation in such collapsing plasma. This paper is the first to describe the three-dimensional structure of that striation. The fast camera is outfitted with bifurcated imaging fiber optics, and it can make observation from two different locations simultaneously. Since the fast camera optics system has been calibrated [1], the image can be related to a three-dimensional spatial point. Figure 1 (a) shows a vertical cross-section of the LHD plasma and the observation points of the fast camera. The fast camera views the plasma at a viewing angle of 15 degrees without an optical filter. The time resolution of the fast camera is 20 kHz. The last closed flux surface (LCFS) which is observed from field of view 1 (FOV-1) and FOV-2 is shown in Fig. 1 (b). Blue, red, green, and black lines indicate nearside field lines, far-side field lines, the magnetic axis, and a horizontally elongated poloidal cross-section, respectively. Typical striation images are shown in Fig. 1 (c). The fields of view are cut by inner wall of viewing ports; therefore, the interior image of the circle is valid. The striation appears under the condition that the boundary plasma density exceeds 1 × 10 20 m −3 and the high temperature plasma boundary shrinks into the LCFS due to excessive gas puffing despite the existence of neutral beam heating. The position of the striation oscillates at a frequency of around 200 Hz. A similar phenomenon has been observed by absolute extreme ultraviolet photodiode (AXUVD) fan arrays [2]. Assuming that the striation spreads along the magnetic field line, magnetic field line trace calculations have been performed by using the position and apparent tilt angle of the striation on the image as a fitting parameter. The striation images, which are viewed from the FOV-2, author's e-mail: sakamoto@LHD.nifs.ac.jp are shown in Fig. 2. These images employ pseudo-colormapping in order to emphasize the low intensity emission. The calculated magnetic flux surface, which appears to correspond to the striation since one of the field lines coincides with the striation (indicated by the thick-line in Fig. 2), is superimposed onto the striation images. We use the far-side magnetic field line to fit here because the tilt angle of this field line is more sensitive than the near-side one from a geometrical consideration and can be easily dis-

supporting

confidence: 55%

Observation of Striation in Collapsing Plasma

Sakamoto

Masuzaki

Miyazawa

et al. 2006

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“…During this stationary period, the particle flux on the divertor plate, which is measured with Langmuir probes, decreases considerably [6]. Simultaneous measurement with an array of absolute extreme-ultraviolet photodiodes (AXUVD) [8] indicates the emergence of a rotating radiation belt [6].…”

Section: Methodsmentioning

confidence: 99%

“…In the Large Helical Device (LHD), complete detachment of the divertor plasma has been obtained with strong puff of fuel gas (hydrogen) [6,7]. A salient feature of the detached plasma is that it is accompanied by emergence of a luminous radiation belt in the boundary region.…”

Section: Introductionmentioning

confidence: 99%

Ionization Balance in the Rotating Radiation Belt Accompanying Complete Divertor Detachment in LHD

Goto

Morita

2008

A spectroscopic measurement is performed for a plasma in a rotating radiation belt, which accompanies the complete divertor detachment recently realized in the Large Helical Device (LHD) [J. Miyazawa et al., Nucl. Fusion 46, 532 (2006)]. The population distribution over the exited levels is experimentally determined from the Balmer series line intensities and is compared with the result of the collisional-radiative model calculation to determine the electron temperature T e and density n e . No reasonable pair of T e and n e is found when either the ionizing plasma or the recombining plasma is assumed. A good fitting is obtained under an assumption of the ionization balance plasma with T e = 1.8 eV and n e = 2 × 10 20 m −3 . These parameters are confirmed through analysis of a high-wavelength-resolution measurement for the Balmer series lines which clearly exhibit the Stark broadening effect.

“…Fluctuations appear in I sat during the Serpens mode phase. This corresponds to the rotation of a radiation belt, named the serpent [25], which will be reviewed in the next subsection.…”

Section: Typical Waveformsmentioning

confidence: 99%

“…During the Serpens mode, the divertor flux (ion saturation current) normalized by the main plasma density decreases to ∼ 10 % of that during the attached phase [24]. A rotating helical radiation belt named the serpent [25] appears in the Serpens mode, which is asymmetric both poloidally and toroidally. The serpent causes fluctuations in edge plasma properties such as the divertor flux, H α and C III intensities.…”

Section: Introductionmentioning

confidence: 99%

Density Regimes of Complete Detachment and Serpens Mode in LHD

Miyazawa

Masuzaki

Goto

et al. 2006

Self Cite

In the Large Helical Device (LHD), the hot plasma column shrinks at the high-density regime and complete detachment takes place. Hydrogen volume recombination is observed at complete detachment. This phase is self-sustained under specific experimental conditions and called the Serpens mode (self-regulated plasma edge 'neath the last-closed-flux-surface). The Serpens mode is achieved after either rapid or slow density ramp up, and either by hydrogen or helium gas puffing. The threshold conditions for complete detachment and the Serpens mode are experimentally documented in the parameter space of heating power and density. The threshold density for the Serpens mode transition increases with ∼ 0.4 power of the heating power. The total radiation is shown to be not adequate to describe the threshold conditions, since it mainly includes the information of very edge region outside the hot plasma column. The operational density limit in LHD, which is sustainable in steady state, has been extended to 1.7 times as high as the Sudo density limit, by applying pellet injection to the Serpens plasmas.