# TJ-II:Validation of bootstrap predictions

2018 Spring

## Proposal title

Validation of bootstrap predictions.

## Name and affiliation of proponent

José Luis Velasco, Kieran McCarthy, Enrique Ascasíbar, Shinsuke Satake (NIFS) et al.

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

Motivation.

The bootstrap current is a neoclassical effect triggered by the radial gradients of the density and temperature in the presence of an inhomogeneous magnetic field. This current may perturb the desired magnetic configuration produced by the external coils, which is especially problematic for shearless devices, such as W7-X or TJ-II. Therefore, in order to design magnetic configurations and suitable operation scenarios, it is necessary to gain confidence on the available neoclassical predictions of bootstrap current (see e.g. [1] and references therein). While the ion parallel flow (the ion contribution to the bootstrap current) is considered to be validated, see e.g. [2], the electron contribution is not. Since no measurement of the electron parallel flow is available, what is left is to measure the total current or its effect on the magnetic configuration [3]. The proponents have participated in a recent experiment in the deuterium campaign of LHD [4].

Objectives.

In this experiment we will create ECH plasmas with constant line-density and electron temperature during all the discharge. The working gas will be alternatively Hydrogen and Helium, and we will scan (shot-to-shot) the radial position of ECH absortion. We will measure the time evolution of the current with the Rogowski coils and two components of the magnetic field with the Motional Stark Effect diagnostic [5]at the end of the discharge (measurements of the vacuum field will also be necessary). We will then compare the measurements with neoclassical simulations.

Off-axis heating should lead to lower electron temperatures, and use of Helium to larger effective charge. Both things should contribute to a faster equilibration of the total current and thus make the measurements easier. We will consider adding short gas puffs of impurities (e.g. N) in order to further accelerate the time evolution of the current.

## If applicable, International or National funding project or entity

EUROfusion WP18.S1.A2, ENE2015-70142-P

## Description of required resources

Required resources:

• Number of plasma discharges or days of operation: 2 day (1 H, 1 He)
• Essential diagnostic systems:

We will measure:

- The time evolution of the total current $<I_t(t)>$ with the Rogowski coils.

- Two components of the magnetic field at the end of the discharge (when $<I_t(t)>$ has converged) with the Motional Stark Effect

- 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) and the He beam.

- 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.

• Type of plasmas (heating configuration): ECH plasmas with constant line-averaged density and electron temperature as measured by ECE. We will scan the position of one of the gyrotrons in order to change the electron temperature profile.
• Specific requirements on wall conditioning if any: Lithium coating if considered necessary for stable plasmas
• External users: need a local computer account for data access: no
• Any external equipment to be integrated? Provide description and integration needs:

## Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy): J. L. Velasco and K. J. McCarthy will be unavailable from 10th to 12th of April (CWGM). K. J. McCarthy will be unavailable from 23rd to 27th of April (W7-X) and on Thursday 17th of May (Erasmus Laboratories).

## References

1. Velasco 2011 PPCF
2. Arevalo 2013 NF
3. Colchin 1990 PoF, Watanabe 1995 NF, Estrada 2002 PPCF, Schmitt 2015 PoP
4. Satake 2017 ISHW
5. McCarthy 2015 CPP