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TJ-II:Understanding an often observed transient rise in core electron temperature during pellet injection into TJ-II plasmas: Difference between revisions
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TJ-II:Understanding an often observed transient rise in core electron temperature during pellet injection into TJ-II plasmas (view source)
Revision as of 17:45, 19 February 2018
, 19 February 2018→Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)
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== Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)== | == Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)== | ||
A short-lived (≤200 μs) transient rise of core electron temperature has often been observed before an injected pellet is completely ablated by the TJ-II plasma.<ref>K. J. McCarthy, et al, Proc. 43rd EPS Conference, Leuven, Belgica (2016)</ref> It is detected by both the Thomson Scattering and Electron Cyclotron Emission diagnostic systems and when observed, this core temperature rise begins within ~100 μs after a pellet enters the plasma through the last-closed magnetic flux surface, as it is approaches the plasma core. Such behaviour occurs in plasmas maintained by either on- or off-axis electron cyclotron resonance heating. It is postulated that a steepening of the radial temperature gradient leads to a more positive radial electric field in the core so that the plasma moves deeper in Core Electron Root Confinement. The resultant improved confinement of injected heating power then leads to the raised core temperature. Conversely, it is observed that, when a pellet is injected into plasma with a peaked core electron temperature profile, the recovery time for core temperatures is significantly longer than for edge temperatures. Given the possible implications for pellet penetration and particle deposition, it is intended to make a systematic study by injecting pellets in plasmas in which such a core temperature rise is observed and using the TS diagnostic to understand the influence of the temperature gradient. | Pellet injection is a widely used tool in magnetically confined plasma devices for core fueling and for studying impurity transport, as well as other for other objectives.<ref>KJ McCarthy et al, Nucl Fusion 57 (2017) 056039</ref> While much of the physics of pellet ablation and of the subsequent particle deposition and transport processes is understood, significant research is still needed to achieve complete knowledge. For instance, phenomena such as cold waves that travel ahead of ablating pellets have been detected in some devices or increased core electron temperatures following pellet injections are reported in the literature.<ref>P. Mantica, et al. Phys. Rev. Lett., 82 (1999) 5048</ref> However, satisfactory explanations for such occurrences are still required. Moreover, it is known that while pellet penetration into a plasma is dependent on plasma electron density, Ne, as well as on pellet mass and velocity, it is most sensitive to plasma electron temperature, Te.<ref>L. R. Baylor, et al, Nucl. Fusion, 37 (1997) 445</ref> Hence, given that particle deposition is dependent on the pellet ablation profile and penetration depth, it is imperative to evaluate and comprehend any phenomenon that may modify these as well as to understand the plasma response to injections.<ref>K. J. McCarthy, et al,Europhys lett 120 (2017) 25001</ref> | ||
A short-lived (≤200 μs) transient rise of core electron temperature has often been observed before an injected pellet is completely ablated by the TJ-II plasma.<ref>K. J. McCarthy, et al, Proc. 43rd EPS Conference, Leuven, Belgica (2016)</ref> It is detected by both the Thomson Scattering and Electron Cyclotron Emission diagnostic systems and when observed, this core temperature rise begins within ~100 μs after a pellet enters the plasma through the last-closed magnetic flux surface, as it is approaches the plasma core. Such behaviour occurs in plasmas maintained by either on- or off-axis electron cyclotron resonance heating. It is postulated that a steepening of the radial temperature gradient leads to a more positive radial electric field in the core so that the plasma moves deeper in Core Electron Root Confinement. The resultant improved confinement of injected heating power then leads to the raised core temperature. Conversely, it is observed that, when a pellet is injected into plasma with a peaked core electron temperature profile, the recovery time for core temperatures is significantly longer than for edge temperatures. Given the possible implications for pellet penetration and particle deposition, it is intended to make a systematic study by injecting pellets in plasmas in which such a core temperature rise is observed and using the TS diagnostic to understand the influence of the temperature gradient. | |||
== If applicable, International or National funding project or entity == | == If applicable, International or National funding project or entity == | ||