TJ-II:Effect of pellet injection on the radial electric field profile of stellarators

Experimental campaign

2017 Spring

Proposal title

Effect of pellet injection on the electron temperature profile and the radial electric field profile of stellarators

Name and affiliation of proponent

Davide Silvagni, Kieran McCarthy, José L. Velasco, the HIBP team the pellet team, et al.

Details of contact person at LNF (if applicable)

José L. Velasco

Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)

Motivation. Pellet injection modifies in a very abrupt manner the density and electron temperature profiles, see e.g. , and this is known to affect radial transport (at least transiently) in such a way that can be described by neoclassical theory, see e.g. . In particular, through modification of the density and electron temperature gradients, it can affect the radial electric field profile, which is set by ambipolarity of the neoclassical fluxes. Positive radial electric fields, connected to hollow density profiles, have been found in the Large Helical Device after injection of a pellet close to the edge . This would be a manifestation of the well-known positive ion-root feature . The evolution of the temperature profile is itself relevant, as pre-cooling with a pellet is thought to be a possible strategy for increasing the fuelling efficiency of pellets. We will characterise it and we will try to model it with neoclassical simulations.

Objectives. In this experiment we plan to inject pellets into steady-state plasmas and characterize the variation of the radial gradients of:

• Electron density.
• Electron temperature.
• Electrostatic potential.

The goal is to check whether the time evolution of the profiles is compliant with neoclassical predictions.

If applicable, International or National funding project or entity

EUROfusion WP17.S1.A2, WP17.S1.A3,ENE2015-70142-P

Description of required resources

Required resources:

• Number of plasma discharges or days of operation: 1 day.
• Essential diagnostic systems:

We will measure:

- The time evolution of the line-averaged density $<n_{e}(t)>$ with interferometry.

- The radial profiles of electron density $n_{e}(r,t_{0})$ and temperature $T_{e}(r,t_{0})$ at one time instant $t_{0}$ with Thomson Scattering (TS).

- The time evolution of the electron temperature profile $T_{e}(r,t)$ with Electron Cyclotron Emission (ECE), when available, calibrated with TS.

- The time evolution of the ion temperature in the core and in an outer radial position, $T_{i}(r/a=0.2,t)$ and $T_{i}(r/a~=0.6,t)$, with the Neutral Particle Analyzer (NPA).

- The time evolution of the radial electric field in the gradient region $E_{r}(r/a~=0.65,t)$, with reflectometry.

- The time evolution of the electrostatic potential in the core region $\Phi (r<0.5,t)$ with the double Heavy Ion Beam Prove (HIBP). For this:

o one of the HIBPs will be scanning from $r/a=0.2$ to $r/a=0.5$.

o the other HIBP will be kept fixed at $r/a~0.2$.

- The ablation process will be detected with $H_{alpha}$ monitors.NC simulations will be performed with analytical methods in the plateau limit and with DKES.

In principle, a positive electric field should lead outward convection for impurities, but this was not the case of W7-AS plasmas with core electron root and positive ion root at intermediate radial positions . We will nevertheless keep an eye on the radiation signals from the bolometers as they can provide (when normalized by the electron density) an indication of the impurity content of the plasma.

• Type of plasmas (heating configuration): NBI plasmas with approximately constant line-averaged density.
• Specific requirements on wall conditioning if any: recent lithium-coating for good density control.
• External users: need a local computer account for data access: yes/no
• Any external equipment to be integrated? Provide description and integration needs:

Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy): no earlier than 01-03-2017 and no later than 01-04-2017.

References

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