View source for TJ-II:Large pellet injection into NBI-heated plasmas of tJ-II

== Experimental campaign ==
2019 Autumn

== Proposal title ==
'''Large Pellet Injection into TJ-II NBI plasmas'''

== Name and affiliation of proponent ==
K. J. McCarthy, Ciemat

== Details of contact person at LNF ==
K. J. McCarthy, Ciemat

== Description of the activity ==
Since initial pellet injections into TJ-II, the majority of successfully deposited small and medium-sized pellets (0.42, 0.6 & 0.7 mm) have been injected into ECRH plasmas while a smaller, but significant, number of similar pellets have been injected into NBI heated plasmas. Although these injection resulted in increased electron densities, fuelling efficiency (deposited particles/injected particles) was always less than ~40% in ECRH plasmas and ≤80% for NBI-heated plasmas.<ref> K. J. McCarthy, et al, Nucl. Fusion 57 (2017) 056039 </ref> It was found that rapid outwards radial drifting of the cloud attached to an ablating pellet leads to large particle losses.<ref>N. Panadero et al., Nucl. Fusion 58 (2018) 026025 </ref> Since these losses are most severe for particles ablated close to the plasma outer edge (injection side), the deeper penetration achieved for the lower temperature NBI-heated plasmas resulted in improved efficiency.

In contrast, only a few of the largest available pellets (1 mm) have been injected into TJ-II, these being injected into NBI-heated plasmas during injector commissioning. An analysis of Thomson Scattering profiles recorded just after these injections shows that core densities, with rather steep profiles, and temperatures reached values of ~5x10^19 m^-3 and 200 eV, respectively. Such plasmas are reminiscent of Super Dense Core plasmas  (high central densities, improved confinement and high β) achieved in the LHD after pellet injection.<ref> J.H. Harris et al, Stability of Super Dense Core Plasmas in LHD, Proceedings of ITC/ISHW2007 </ref> It is intended here to explore the density limit with pellet injection in NBI-heated plasmas of TJ-II. For this 1 mm diameter pellets will be injected into plasmas having peaked density profiles with line-averaged densities above 1.5x10^19 m^-3 and flat central temperature profiles with Te(0) ≥250 eV.  


== International or National funding project or entity ==
MICINN: FIS2019-89326-R
EUROfusion: WP1

== Description of required resources ==
Required resources:
Diagnostics: TS, microwave interferometer, plasma current measurements. Fast-frame camera with fibre-optic bundle.
Plasma Density: >1.5x1019 m-3
Configurations: Configurations about 100_44_64.
Heating: NBI heating.
Plasma Duration: ≥200 ms.
PI Checks: We will require some time at 9 am to check out and condition the PI. 
Other considerations: Frequent access to the TJ-II experimental hall. 
Days: 2.
Dates when unavailable: K. J. McCarthy will be unavailable from 4th to 15th of October (NIFS) and from 25rd to 29th of November (LATINfusion).
N. Pandero will be unavailable from 28th Oct to 8th Nov & 18th Nov to 29th Nov.

== Preferred dates and degree of flexibility ==
Preferred dates: (03-12-2019 to 19-12-2019)

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== References ==
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