Observation of Striation in Collapsing Plasma

Abstract

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-

Observation of Marfes in the Wendelstein 7-X stellarator with inboard limiters

“…A much better studied radiation belt is the so-called Serpent in the heliotron LHD [15] which forms the plasma operational limit in the so-called Serpens mode. The Serpent is also a high-density plasma structure with densities in the range from 2 to 5 * 10 20 m −3 [16,17] but has a more complex geometry than the axisymmetric Marfe in a tokamak. We focus here on measurements in W7-X and a first analysis of the radiation belts in order to understand the physical nature of the phenomenon and the relation to Marfes in tokamaks and Serpents in LHD.…”

“…A much better studied radiation belt is the so-called Serpent in the heliotron LHD [15] which forms the plasma operational limit in the so-called Serpens mode. The Serpent is also a high-density plasma structure with densities in the range from 2 to 5 × 10 20 m −3 [16,17] but has a more complex geometry than the axisymmetric Marfe in a tokamak.…”

Abrupt Flushing of High-Density Core in Internal Diffusion Barrier Plasmas and its Suppression by Plasma Shape Control in LHD

“…A discovery of internal diffusion barrier (IDB) plasmas in the Large Helical Device (LHD) [3] makes it possible to realize a high central plasma pressure of 1.3 atm [4,5]. IDB plasmas are produced by hydrogen ice pellet injection and characterized by a strongly peaked denauthor's e-mail: miyazawa@LHD.nifs.ac.jp sity profile with relatively low density in the edge region, which is called "mantle."…”

“…Then, the density in the core region is flushed into the mantle region within < 1 ms [7]. We call this event "core density collapse (CDC)" [4,5]. CDC must be suppressed since it prevents further increase of the central pressure and the fusion triple product.…”

Ultrahigh neutral pressures in the sub-divertor of the Large Helical Device

“…Such a rapid transition is reminiscent of an instability of the thermal front as analyzed in [11]. Furthermore, no thermal instability of the Marfe type was observed which occurs in the LHD at low edge temperatures of about 30 eV [12]. Figure 4(a) shows that the edge temperatures were always above the Marfe threshold.…”