TJ-II: Potential asymmetries at low magnetic field

2017 Spring

Proposal title

Potential asymmetries at low magnetic field

Name and affiliation of proponent

José M García Regaña1, José Luis Velasco1, J. A. Alonso 1, I. Calvo 1, C. Hidalgo1, Bing Liu1, ...

Details of contact person at LNF (if applicable)

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

Background: Recent measurements in TJ-II have shown a qualitative good agreement between numerically predicted and measured potential variations in NBI ion-root conditions [1]. Experiments and simulations have shown similar scaling with the electron temperature $T_{e}$ and comparable phase difference between the measured and calculated values of the potential at two distant positions of the same flux surface. Heuristically, these variations can be shown to scale independently on $B$ since the magnetic and $E_r\times B$ drifts scale both with $B^{-1}$, see e.g. [2], unless that $E_{r}$ varies noticeably with $B$. Here $E_{r}$ and $B$ are the ambipolar neoclassical electric field and magnetic field modulus respectively.

Objective: the present experiment, which aims at assessing the scaling of $\Phi_1$ with $B$ (or the lack of it) and compare with theory, follows in essence the approach and techniques reported in ref. [1]. Hence, similar NBI plasma parameters as those in the past experiments scanning the electron temperature at the outer region are required.

If applicable, International or National funding project or entity

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Description of required resources

The signals and systems required to carry out the analysis are:

• The time evolution of the plasma floating potential $\phi_f(t)$ at the outer core region ($r/a\sim 0.9$), measured with the double Langmuir probe system.
• The application of biasing potential at the outer core region is necessitated to quantify $\Delta\phi_f$.
• The time evolution of the line-averaged density $\left<n_e(t)\right>$ 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 ion temperature in the core and in an outer radial position $T_i(r/a\sim 0.2,t)$ and $T_i(r/a\sim 0.8,t)$ with the Neutral Particle Analyzer (NPA).
• The time evolution of the radial electric field at the mid-outer region ($r/a\sim 0.5-0.8$) with reflectometry, if possible.
• The time evolution of the electrostatic potential in the mid-outer region $\Phi(r/a\sim0.7,t)$ with the double Heavy Ion Beam Probe (HIBP).

Other requirements:

• One successful day of operation.
• Background standard configuration with magnetic field rescaled to $B\sim 0.6$ T.
• NBI heating. No ECRH heating.

Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy)

References

1. B Liu et al. Direct experimental evidence of potential asymmetry in magnetic flux surfaces in stellarators to be submitted (2017)
2. D. D.-Ho and R. M. Kulsrud Neoclassical Transport in Stellarators Phys. Fluids 30 (2) 1987