Neutral and plasma shielding model for pellet ablation

Houlberg, W. A.; Milora, S.L.; Attenberger, S.E.

doi:10.1088/0029-5515/28/4/006

Cited by 105 publications

(119 citation statements)

References 22 publications

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“…1 we show the measured density profiles before and after 2.7 mm pellet injection in an H-mode and an L-mode discharge. The expected deposition from the NGS ablation model [12] is also shown, which indicates the level of radial displacement of the pellet mass that is believed to occur. In the example of H mode deposition shown in Fig.…”

Section: Pellet Plasma Interactionmentioning

confidence: 80%

Fueling efficiency of pellet injection on DIII-D

Baylor¹,

Jernigan²,

Lasnier³

et al. 1999

Journal of Nuclear Materials

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'Lawrence Livemore National Laboratory, Livemore, CA, USA "General Atomics, P.O. Box 85608, San Diego, CA 92186-9784, USA Pellet injection has been used on the DIII-D tokamak to study density limits and particle transport in H-mode and inner wall limited L-mode plasmas. These experiments have provided a variety of conditions in which to examine the fueling efficiency of pellets injected into DIII-D plasmas. The fueling efficiency defined as the total increase in number of plasma electrons divided by the number of pellet fuel atoms, is determined by measurements of density profiles before and just after pellet injection. We have found that there is a decrease in the pellet fueling efficiency with increased neutral beam injection power. The pellet penetration depth also decreases with increased neutral beam injection power so that, in general, fueling efficiency increases with penetration depth. The fueling efficiency is generally 25% lower in ELMing H-mode discharges than in L-mode due to an expulsion of particles with a pellet triggered ELM. A comparison with fueling efficiency data from other tokamaks shows similar behavior. IntroductionPellet fueling is an important technique developed for fueling and density profile control in a fusion grade plasma [l]. Much effort has been devoted in past pellet fueling experiments to understand pellet ablation and pellet induced changes in plasma transport.These studies have yielded an extensive validation of the neutral gas shielding (NGS) scaling law for pellet penetration [2] and have shown the capability of pellet fueling to strongly modify the density profile shape. device (an elongated, diverted, tokamak plasma) under various operating conditions and compare these experiments to the overall database results. The fueling efficiency, which is defined as the total increase in number of plasma electrons divided by the number of fuel atoms in the pellet, is determined by Thomson scattering measurements of density profiles before and just after pellet injection in conjunction with the pellet mass measured in a microwave cavity. These density measurements have been made as close as 20 ps after completion of the pellet ablation process, but are more typically made 1 to 2 ms after injection of the pellet. Multiple measurements made starting 150 ps following ablation show only very modest changes in the density profile in the first 2 ms following injection [7] after the initial rapid response to the pellet deposited mass. This indicates that no fast particle transport effects occur during this time. Pellet plasma interactionThere are several effects that can reduce pellet fueling efficiency from the ideal 100% level. First, there is some ablation of the pellet in the scrape off layer (SOL) before it reaches the last closed flux surface or seperatrix of the plasma. This is due to energetic particles in the SOL that impinge on the pellet as it traverses the SOL. The magnitude of this ablation is rather small on DIII-D as measured by D, light emitted by the ablating ...

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Section: Pellet Plasma Interactionmentioning

confidence: 80%

Fueling efficiency of pellet injection on DIII-D

Baylor¹,

Jernigan²,

Lasnier³

et al. 1999

Journal of Nuclear Materials

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“…-The detailed calculation of the energy depletion of the background electrons in the pellet cloud with account for the energy distribution of the electrons [19,39]. Since the collisional cross-section decreases with increasing particle energy, the high-energy electrons penetrate furhter in the shielding cloud.…”

Section: Wwwcpp-journalorgmentioning

confidence: 99%

Pellet – Plasma Interaction: an Analysis of Pellet Injection Experiments by Means of a Multi‐Dimensional MHD Pellet Code

Lengyel

Schneider

Kardaun

et al. 2008

Contrib. Plasma Phys.

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This work is aimed at the analysis of pellet injection experiments by means of a magnetohydrodynamic code that computes the processes of pellet ablation, expansion, ionization, and magnetic confinement of the ablated substance. In particular, experimental pellet injection scenarios stemming from the tokamak ASDEX-Upgrade (D2 pellets) and the stellarator W7-AS (carbon pellets) are analyzed by means of this time-dependent quasithree-dimensional MHD pellet code. Pellet penetration depths measured in ASDEX-Upgrade at LFS (lowfield-side) and HFS (high-field-side) pellet injection, are compared with computational results.It was found that the penetration depths of deuterium pellets observed in about twenty randomly selected LFS and HFS injection scenarios could accurately be reproduced by a single code in which drift effects were intentionally ignored. This may be understood as an indication of the negligible effect drift exerts on the ablation process. At the same time, the distribution of the ablated and partially ionized substance may very well be affected by the drift of the ionized particles in the direction of the decreasing magnetic field.With the help of the Dα radiation patterns observed, the magnitudes of the heat fluxes affecting the pellet and the ablation cloud were estimated. The calculations show that the thermal heat fluxes (electron and ion) acting along the magnetic field lines are not sufficient to explain the size of the radiating filaments observed.The effect of extra-thermal electrons on the ablation history of pellets injected in W7-AS is demonstrated: the particle end energy fluxes causing measured pellet ablation profiles are determined.Based on a statistically well-designed computational data set, an empirical scaling of the penetration depths in terms of the background plasma parameters was attempted. It is shown that the form of the temperature profile of the background plasma cannot be ignored while deriving scaling laws; the specification of the central or the bulk temperature alone is insufficient.Quantitative data are provided for the magnitudes of various additional processes such as the anomalous thermal conduction across the field lines or the grad(B)-induced drift of the shielding cloud surrounding the pellet.The modelling of supersonic gas jets, a fuelling method that appears to be an alternative to pellet injection, is also considered. Since the massive stream of a cold gas represents a disturbance for the recipient plasma similar to that of a pellet, its dynamics can be described by the same means as the evolution of pellet clouds.

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“…where || σ is the parallel conductivity, and The particle and energy flux depletion of hot background carriers (electrons and ions) inside the blob due to collisional interaction with the cold particles is modeled similarly to [20]. The distribution functions of the incident energy carriers (electrons and ions) are assumed to be Maxwellian and are modeled by 5 monoenergetic groups, carrying the equal energy flux.…”

Section: Jet Motion Outside the Plasmamentioning

confidence: 99%

Penetration of supersonic gas jets into a tokamak

Rozhansky¹,

Senichenkov²,

Veselova³

et al. 2006

Nucl. Fusion

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The injection of high-pressure supersonic jets into the tokamak plasma is considered as a promising way of future thermonuclear reactor fueling and as a tool for disruption mitigation. Successful experiments were performed on Tore Supra and DIII-D, correspondingly. In the present paper the evolution of such a jet is analyzed. The jet expansion, deceleration of the ambient electrons and ions by the jet, self-consistent electric field, elementary processes, radiation and adiabatic cooling of the ambient plasma are taken into account. The jet is simulated by a MHD code, which is similar to the code previously used for pellets. It is demonstrated that the ionization degree of the jet strongly depends on the jet parameters. Several simulations were performed for the range of parameters typical for DIII-D. The jet of initial density remains almost neutral, and only the outer regions are ionized. When the initial jet density is reduced by a factor of 2 or more the main part of the jet becomes ionized rather fast. It is demonstrated that ionization at the jet edge in poloidal (perpendicular to the magnetic field) direction of the jet is sufficient to stop poloidal expansion of the jet by -3 24 m 10 4 ⋅ B j r r × force. The final poloidal size of the jet remains of the order of its initial poloidal dimension (of the order of ten centimetres). The jet motion in the radial direction (direction of the injection) is provided by the polarization poloidal electric field and the correspoding B E r r × drift. In the paper two mechanisms of polarization reduction are considered: Alfvén conductivity of the ambient plasma and the B ∇ -induced drift. It is shown that almost neutral jet can penetrate deep into the tokamak while a modest ionization degree should prevent its penetration for the case of low field side (LFS) injection.

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Neutral and plasma shielding model for pellet ablation

Cited by 105 publications

References 22 publications

Fueling efficiency of pellet injection on DIII-D

Fueling efficiency of pellet injection on DIII-D

Pellet – Plasma Interaction: an Analysis of Pellet Injection Experiments by Means of a Multi‐Dimensional MHD Pellet Code

Penetration of supersonic gas jets into a tokamak

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