Observation of a Rotating Radiation Belt in LHD

Abstract

A poloidally rotating radiation belt with helical structure was observed during the high density discharges with detachment by photodiode fan arrays and a fast camera in LHD. The peak of radiation rotates inside the last closed fl ux surface, and the direction and mode number of the poloidal rotation are electron diamagnetic and one, respectively. During the recombination phase after termination of the plasma heating, the rotation continues, and its rotating radius shrinks with shrinking of the plasma column. The poloidal rotating frequency depends on the heating power, and increases from the orders of several tens of Hz to several hundreds of Hz with shrinking of the rotation radius. The mechanism of the rotation remains uncertain. Keywords:LHD, rotating radiation belt, MARFE, Serpens-mode In experiments conducted on LHD in 2004, a poloidally rotating radiation belt was observed during the high density discharges with detachment by absolute extreme ultraviolet photodiode (AXUVD) fan arrays [1] and a fast camera (20,000 frames/s). The belt appeared similar to MARFE in tokamaks [2] and ARC (asymmetric radiative collapse) in LHD [3] with respect to the localized radiation volume, but there was a crucial difference in terms of whether or not the position of the radiation volume moved continuously. Figure 1 shows the time evolutions of plasma parameters during a discharge with the magnetic axis radius of 3.65 m in which the rotating radiation belt was observed. With gas puffi ng and injection of two hydrogen ice pellets, the line averaged density, n ebar , reaches about 1.6 × 10 20 m -3, and is kept only by recycling after the pellet injection. It should be noted that pellet injection is not essential in this paper. The total radiation, P rad , measured by a resistive bolometer with a wideangle view of the plasma [1] is about 40% of the input power during the high density phase. Usually in LHD, 30-40% of P rad -input power ratio is transiently observed just before the radiative collapse, that is, ARC. At that stage, the plasma column shrinks, and plasma is detached from the divertor. During the discharge in Fig. 1, shrunk column is stably maintained after the termination of gas puffi ng, and this state is called Serpens-mode [4]. The ion saturation current measured by the Langmuir probe arrays on the divertor plates, I sat , decreases, and intermittent spikes appear during Serpens-mode. Figure 2(b) shows the time evolution of the sight volume integrated radiation power, 〈P rad 〉, profi le during Serpens-mode, measured by an AXUVD fan array in a nearly horizontally elongated poloidal cross-section [1] (see Fig. 2(a)). This fi gure shows that the poloidal mode number of the rotation is one, and the rotating frequency is several tens of Hz, varying with input power. The turn-rounds of the 〈P rad 〉 peak are channel numbers 4 and 17 (see Fig. 2(a)). This indicates that the peak location of the rotating radiation volume is inside the LCFS. Figure 2(c) shows that the rotation continues in the recombination phase after...

Observation of Striation in Collapsing Plasma

“…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.…”

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

“…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].…”

“…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.…”

Density Regimes of Complete Detachment and Serpens Mode in LHD

“…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.…”

“…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.…”