Repetitive fueling pellet injection in large helical device

Yamada, H.; Sakamoto, R.; Viniar, I.; Oda, Yasushi; Kikuchi, K.; Lukin, A. Ya.; Skoblikov, S. V.; Umov, A. P.; Takaura, K.; Onozuka, M.; Katō, Susumu; Sudo, S.

doi:10.1016/s0920-3796(03)00225-4

Cited by 22 publications

(10 citation statements)

References 8 publications

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“…The size of the repetitive pellets is 2.5 mm diameter and the volume averaged density increases by 1:6 10 19 m ÿ3 with one pellet. The speed of the pellet is 450-500 m=s and the pellets penetrate as far as from half of the plasma minor radius to the plasma center depending on the density [17]. The time evolution of the electron temperature (T e ) and the density (n e ) are measured with a 27 channel electron cyclotron emission (ECE) radiometer [18] and 14 channel FIR CO 2 laser interferometer [19].…”

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confidence: 99%

Slow Transition of Energy Transport in High-Temperature Plasmas

et al. 2006

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A new slow transition process for energy transport in magnetically confined plasmas is reported. The slow transition is characterized by the change between two metastable transport conditions characterized by a weak and a strong electron temperature (T e ) dependence of normalized heat flux. These two branches are found to merge at the critical T e gradient. In metastable transport, the derivative of normalized heat flux to the T e gradient, @ Q e =n e =@ ÿrT e , is positive, while it becomes negative during the transition phase. The time for the transition increases as the normalized T e gradient is increased and exceeds the transport time scale characterized by the global energy confinement time. High-temperature plasmas, which are seen in nature or confined in laboratory experimental devices, are very often far from equilibrium. One of the noticeable features is that the observed temperature profiles are realized as a balance between the flux of energy and turbulence-driven transport. A sudden and distinctive jump in the profiles has been found, e.g., the H-mode phenomena in tokamak plasmas [1]. The concept of a profile transition has been employed in order to understand the formation and dynamics of a self-sustained radial profile in inhomogeneous plasmas [2]. The transitions in the profiles in H-mode plasmas are caused by the S-curve property (cusp-type catastrophe) in the gradient-flux relation, showing a feature analogous to the first-order phase transition [3,4]. As the second-order phase transition exists in terrestrial matter, it is possible that the other type of transition exists in the process of transport barrier formation in plasmas [4]. We here report the discovery of a new, slowly evolving transition between two transport branches that have different electron temperature (T e ) dependences, in toroidal plasmas.Heat transport in a magnetically confined toroidal plasma is strongly governed by turbulence. The mechanism of the H-mode transition has been studied both experimentally [5,6] and theoretically [7][8][9][10]. The bifurcation of the radial electric field and the associated suppression of turbulence are the key parameters to explain the fast transition between L mode and H mode [2,[7][8][9]. This transition mechanism appears as various phenomena such as the electric pulsation observed in the toroidal helical plasma and the internal transport barrier [11]. In parallel with these rapidly evolving phenomena, slow evolutions into the state of improved plasma confinement have also been observed in a slowly developing edge transport barrier [12] and in other confinement improved modes. Improved Ohmic confinement mode [13], counter-neutral beam injection improved mode [14], pellet enhanced performance mode [15], radiatively improved mode [16] associated with the change of density profiles without a clear fast transition. Although these improved modes associated with the slow transition are as important as the improved mode with a fast transition, the mechanism of the slow transition has been studie...

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confidence: 99%

Slow Transition of Energy Transport in High-Temperature Plasmas

et al. 2006

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“…On the pipe-gun pellet injector, solid hydrogen pellets are produced in 8 barrels, and the pellet velocity and size are 1,000 to 1,200 m/s, with each cylinder being 3.0 mm in diameter and 3.0 mm in length (∼1 × 10 21 atoms) [14]. On the repetitive pellet injector, pellets are continuously formed and injected at a maximum repetition frequency of 11 Hz, and the pellet velocity and size are 300 to 700 m/s, with each cylinder being 2.5 mm in diameter and 2.5 mm in length (∼6 × 10 20 atoms) [15]. From both injectors, pellets are injected through the outer port of LHD into the plasmas by means of the pneumatic pipe-gun method using high pressure He gas.…”

Section: Methodsmentioning

confidence: 99%

“…This is in contrast to the fact that major tokamaks utilize NBI with an energy of at most 100 keV. Since pellet injection is routinely available in LHD [14,15], an experimental study of the penetration depth of pellets in NBI heated plasmas in LHD can make a unique contribution to our understanding of the pellet ablation mechanism, in particular the effect of energetic ions over a wide energy range. In this article, we describe experimental observations of pellet penetration depth, reflecting information regarding pellet ablation in plasmas which are rich in energetic ions.…”

Section: Introductionmentioning

confidence: 99%

Observation of the Effect of Energetic Ions on Pellet Ablation in the Large Helical Device

Hoshino

Sakamoto

Yamada

et al. 2006

Plasma and Fusion Research

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Ablation of a solid hydrogen pellet in hot plasmas was investigated in the Large Helical Device (LHD). The penetration depth of the injected pellets in the experiment was compared to a theoretical model employing ablation due to the heat flux of fast ions as well as thermal electrons. Shallower penetration than that predicted based on the model considering only electrons was observed in LHD plasmas in the presence of highly energetic (up to 180 keV) fast ions due to neutral beam injection (NBI). This discrepancy can be quantified by the contribution of fast ions in terms of the stored energy of fast ions. The ablation model calculation taking into consideration the effect of thermal electrons and fast ions agrees with the experimental results obtained by LHD. Scaling which includes the fast-ion effect on penetration depth in LHD was compared with the wider multi-experiment database (IPADBASE) [L.R. Baylor et al., Nucl. Fusion 37, 445 (1997)].

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“…Two fueling hydrogen ice pellets [30][31][32] are injected to "Serpens mode" and "Attached plasma" discharges shown in Fig. 1.…”

Section: Typical Waveformsmentioning

confidence: 99%

Density Regimes of Complete Detachment and Serpens Mode in LHD

Miyazawa

Masuzaki

Goto

et al. 2006

Plasma and Fusion Research

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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.

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Repetitive fueling pellet injection in large helical device

Cited by 22 publications

References 8 publications

Slow Transition of Energy Transport in High-Temperature Plasmas

Slow Transition of Energy Transport in High-Temperature Plasmas

Observation of the Effect of Energetic Ions on Pellet Ablation in the Large Helical Device

Density Regimes of Complete Detachment and Serpens Mode in LHD

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