Multifaceted asymmetric radiation from the edge-like asymmetric radiative collapse of density limited plasmas in the Large Helical Device

Peterson, B.J.; Xu, Yuhong; Sudo, S.; Tokuzawa, T.; Tanaka, K.; Osakabe, M.; Morita, S.; Goto, M.; Sakakibara, S.; Miyazawa, J.; Kawahata, K.; Ohyabu, N.; Yamada, H.; Kaneko, O.; Komori, A.

doi:10.1063/1.1390330

Cited by 26 publications

(21 citation statements)

References 20 publications

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“…Finally, we note that the current approach allows one to speculate about the nature of phenomena observed in Large Helical Device ͑LHD͒ close to the density limit, 56,57 where the development of a MARFE-like structure was preceded by a poloidally symmetric cooling of the edge plasma. The poloidal width of unstable perturbations is governed by the factor p ͓see Eqs.…”

Section: Discussion Of Resultsmentioning

confidence: 99%

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

Tokar

Kelly

2003

Physics of Plasmas

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Plasma-wall interaction leads to the release of impurities and neutrals of the working gas, which contribute significantly to the energy losses from the plasma edge, and therefore, crucially affects the development of thermal instabilities in fusion devices. An analytical model for impurity radiation is proposed, which takes into account the erosion mechanisms of wall material and the motion of impurity particles across magnetic surfaces. The temperature dependence of radiation losses is found to be very different from that predicted by the coronal approximation often used in considering thermal instabilities. The consequences for the development of poloidally symmetric detachment and multi-faceted asymmetric radiation from the edge ͑MARFE͒ are analyzed. It is demonstrated that the MARFE threshold principally depends on the mechanism by which working gas neutrals are released from the wall and on the neutral's properties, e.g., their ionization rate.

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Section: Discussion Of Resultsmentioning

confidence: 99%

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

Tokar

Kelly

2003

Physics of Plasmas

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“…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.…”

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confidence: 99%

Observation of a Rotating Radiation Belt in LHD

Masuzaki

Sakamoto

Miyazawa

et al. 2005

J. Plasma Fusion Res.

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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...

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“…However, when the higher deposited power was considered the previously derived scaling law was still obeyed [7]. Initial investigations into the density limit on the Large Helical Device (LHD) [8] have indicated that the density in LHD is limited by a radiative thermal instability which results in the collapse of the plasma at a limit which is greater than 1.4 times the Sudo limit. This collapse is characterized by a poloidal asymmetry in the radiation and density with the high density and high radiation region on the inboard side.…”

Section: Introductionmentioning

confidence: 99%

Characteristics of Radiating Collapse at the Density Limit in the Large Helical Device

Peterson

Miyazawa

Nishimura

et al. 2006

Plasma and Fusion Research

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Steady state densities of up to 1.6 × 10 20 m −3 have been sustained using gas puff fuelling and NBI heating up to 11 MW in the Large Helical Device (LHD). The density limit in LHD is observed to be greater than 1.6 times the Sudo limit. The density is ultimately limited by a radiating collapse which is attributed to the onset of a radiative thermal instability of the light impurities in the edge region of the plasma based on several recent observations in LHD. First of all the onset of the radiative thermal instability is tied to a certain edge temperature threshold. Secondly, the onset of the thermal instability occurs first in oxygen and then carbon as expected from their cooling rate temperature dependencies. Finally, radiation profiles show that as the temperature drops and the plasma collapses the radiating zone broadens and moves inward. In addition, comparison of impurity lines with the total radiated power behaviour suggests that carbon is the dominant radiator. Two dimensional tomographic inversions of Absolute eXtreme UltraViolet Diode (AXUVD) array data and comparison of modelling with images of radiation brightness from imaging bolometers indicate that the poloidal asymmetry which accompanies the radiating collapse is roughly toroidally symmetric.

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Multifaceted asymmetric radiation from the edge-like asymmetric radiative collapse of density limited plasmas in the Large Helical Device

Cited by 26 publications

References 20 publications

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

Observation of a Rotating Radiation Belt in LHD

Characteristics of Radiating Collapse at the Density Limit in the Large Helical Device

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