TJ-II:Radiation asymmetries and potential variations

Experimental campaign

2018 Spring

Proposal title

Impurity dynamics, radiation asymmetries and potential variations

Name and affiliation of proponents

José M García Regaña¹, D. Carralero¹, M. A. Ochando¹, T. Estrada¹, F. Medina¹, José Luis Velasco¹, J. A. Alonso ¹ and TJ-II team.

Details of contact person at LNF (if applicable)

Enter contact person here or N/A

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

Neoclassical theory predicts a non-constant portion of the electrostatic potential over the flux surfaces , usually denoted by $\Phi _{1}=\Phi _{1}(\theta ,\phi )$, with $\theta$ and $\phi$ the poloidal and toroidal angular coordinates. When this is taken into account the equilibrium density of the different species a present in the plasma varies according to their adiabatic response and can be written as: Failed to parse (syntax error): {\displaystyle n_{a0}=\left<n\right>\exp\left(-Z_{a}e\Phi_1/T_{a}\right)} , with Failed to parse (syntax error): {\displaystyle \left<...\right>} the flux-surface-average. In TJ-II plasmas experiments and simulations have shown that $e\Phi _{1}/T_{a}$ can take values from $O(0.01)$ to $O(0.1)$. Variations are predicted to be larger at the outer radii than at the inner core, and stronger in ECRH plasmas than in NBI plasmas. Under conditions with large $\Phi _{1}$ the impurities of moderate to high $Z$ should experience strong variations of their densities over the flux surfaces, increasing with $Z$. These, in turn, should result in an anisotropic radiation over each flux surface and consequently a radially asymmetric radiation pattern should follow.

In the present experiment the analysis of the radial profiles and time evolution of the plasma emissivity using the TJ-II bolometry system after the inyection of some selected impurities by gas puffing is proposed. The experiment aims at studying the relation between the radially asymmetric emissivity and the measured and predicted $\Phi _{1}$ and compare this with the case where $\Phi _{1}$ is neglected . The measurement and evolution of $\Phi _{1}$ will be tracked during the discharges using the duplicated Langmuir probe system plasma floating potential measurements. Numerical calculations of $\Phi _{1}$ will be carried out with the neoclassical version of the code EUTERPE at different radial locations.

Description of required resources

The required signals to perform the analysis are:

• The time evolution of the plasma emissivity along the available lines of sight of the tomography camera systems.
• The time evolution of the plasma floating potential at the outer core region ($r/a\sim 0.9$).
• The time evolution of the line-averaged density Failed to parse (syntax error): {\displaystyle \left<n_e(t)\right>} with interferometry.
• The radial profiles of electron density $n_{e}(r,t_{0})$ and temperature at one time instant $t_{0}$ using 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 radial electric field at the mid-outer resion ($r/a\sim 0.7-0.8$) with reflectometry.
• The time evolution of the electrostatic potential in the mid-outer region $\Phi (r/a\sim 0.7,t)$ with the double Heavy Ion Beam Probe (HIBP).

Other constraints regarding the desired experimental conditions are:

• ECH is the main and only heating scheme that will be employed. Accessing the regimes detailed in the following line will be realized by the usage of different power and off-axis heating when necessary.
• A total of 20 discharges are estimated to be the minimum required.
• The injection of impurities by means of LBO or puffing is required, in order to track the impact and evolution of the bolometry radiation signals, and correlated with the measurement and predictions of the potential variations and radial electric field.
• Accessing different absolute values and regimes $E_{r}$ is essential. These regimes can be roughly referred to as "high ion root $E_{r}$", "low ion root $E_{r}$" and the same for electron root conditions. 2 discharges per regime are required in order to characterize $E_{r}$ at different positions over the same flux surface.
• Reproducing some of these regimes in two different configurations (high-iota and standard), where changes in the the sign of $E_{r}^{Left}-E_{r}^{Right}$ have been observed, is planned.
• Good stationarity of plasma parameters at the instant where the impurities are injected is required in order to extract the stationary background emissivity from that produced by the injected impurity. Hence the study shall preferably be perform in ECRH plasmas.

Preferred dates and degree of flexibility

Preferred dates: due to the availability on-site of the proponents the experimental days should fall out of the calender weeks 14, 17, 18 and 19.