Effect of transonic flow in the ablation cloud on the lifetime of a solid hydrogen pellet in a plasma

Parks, P.B.; Turnbull, R. J.

doi:10.1063/1.862088

Cited by 265 publications

(347 citation statements)

References 9 publications

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“…Accordingly a spherically expanding neutral cloud forms around the pellet [31], which gradually becomes ionized. In this latter stage the cross field expansion of the cloud is stopped and the plasmoid is stretching out along the field lines [32,33].…”

Section: The Quiescent H-modementioning

confidence: 99%

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

et al. 2008

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To get deeper insight into the MHD activity triggered by pellets we extended our previous analyses of standard type-I ELMs to pellets injected into discharge phases of the following types: Ohmic, L-mode, type-III ELMy H-mode, ELM-free, radiative edge scenarios with type-I ELMs, the quiescent H (QH)-mode regime. It turns out that pellet injection generally creates a strong, local perturbation of the MHD equilibrium in the ablation region and even beyond. Regarding the triggering of ELMs, this initial perturbation can damp out, indicating that the plasma is stable in the corresponding regime even for finite-size perturbations. This behaviour is observed not only in Ohmic and L-mode phases but also in the QH-mode where the edge harmonic oscillations (EHO) appear to keep the edge within or at the boundary of a stable regime. In case the plasma is prone to ELM growth, the large amplitude of the pellet perturbation can trigger the event even in situations where modes appear to be still linearly stable.The non-linear character of the ELM trigger process is highlighted also by the subsequent explosive growth of these events. For edge plasma conditions characterised by higher resistivity the growth time of spontaneously occurring ELMs increases when the plasma changes from the type-I into the type-III regime. Pellet-triggered ELMs, however, maintain the fast rise times otherwise typical for the hot edge type-I regime. In the discussion section we attempt to relate these observations also to core mode activity like neoclassical tearing modes or snakes. Data were taken from ASDEX Upgrade and JET.Pellet triggering of mode activity can be shown to be a quite universal phenomenon, which, however, only for the case of ELMs can be unambiguously attributed to prompt direct excitation by the pellet.

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Section: The Quiescent H-modementioning

confidence: 99%

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

et al. 2008

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“…Anyhow a reduction of pellet mass causes a reduction of the pellet ablation rate dN P dt , which may reduce the perturbation below the triggering threshold. As the ablation rate depends on the pellet mass as dN P dt ∼ m 4/9 P [18,19], reducing the pellet mass to 0.01 times the initial value would cause a reduction to about 0.1 times the initial ablation rate.…”

Section: Enhancement Potential By Mass Adaptationmentioning

confidence: 99%

ELM triggering by local pellet perturbations in type-I ELMy H-mode plasma at JET

et al. 2007

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The pellet pacing ELM control concept developed at the mid sized tokamak ASDEX Upgrade is considered for the larger machines JET and ITER as well. By driving up the ELM frequency, the ELM size can be reduced and eventually suppressed below a dangerous level. An according pellet injection systems for JET is under construction, the ITER design includes one as well. However, it has still to be proven the concept will work also at bigger machine sizes. A step forward in this mission was achieved by re-analysing previous JET experiments dedicated to pellet particle fuelling. Although the experiments were performed in a parameter regime unsuitable for pellet ELM pacing both with respect to pellet frequency and size they demonstrate prompt triggering of ELMs takes place at JET as well. Prompt triggering, an indispensable premise for useful pacing, obviously can be achieved already with a pellet size sufficiently small to avoid unbearable side-effects by particle fuelling. An ELM released by the local perturbation imposed by the ablating pellet in the plasma edge region is detected when only a minor fraction (less than 1%) of the fuelling size pellets (containing about 4 × 10 21 D) was ablated and deposited yet. It is thus very likely JET will allow for investigations on the operational technique and the underlying physics of pellet pacing once the new system becomes operational.1

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“…We used here the most popular one: the neutral gas shielding (NGS) model [11]. This ablation rate by the scaling is described as …”

Section: Pellet Injectionmentioning

confidence: 99%

Internal Transport Barrier Analysis Including Impurities in Tokamak and Helical Reactor Plasmas

Hori

Yamazaki

Oishi

et al. 2011

Plasma and Fusion Research

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The operation with Internal Transport Barrier (ITB) is expected as a high performance operation. ITB is utilized to improve core plasma confinement in the reversed magnetic shear. It is considered that the changes of core plasma profile by the ITB cause changes of impurity transport. In a large fusion reactor, high-Z materials will be used as plasma facing components because high loads of heat and particles concentrate there. However, high-Z impurities from these components cause large radiation loss and dilute the fuel even if the amount of impurities is small. Therefore, in this study, firstly, the ITB formation which includes the effects of the magnetic shear and perturbed profiles by the pellet injection was simulated using the Toroidal Transport Analysis Linkage code TOTAL. Secondly, we analyzed transport of the tungsten impurities using an impurity model in TOTAL code, and compared the impurity profile in the case with ITB to the one without ITB in the tokamak reactor. The impurities decreased in the ITB formation region when ITB was formed, and the outward flux of total impurity density was observed there. It can be expected that outward flux of impurities is generated by the temperature and the density gradients.

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Effect of transonic flow in the ablation cloud on the lifetime of a solid hydrogen pellet in a plasma

Cited by 265 publications

References 9 publications

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

ELM triggering by local pellet perturbations in type-I ELMy H-mode plasma at JET

Internal Transport Barrier Analysis Including Impurities in Tokamak and Helical Reactor Plasmas

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