Speaker
Description
Experimental investigation of impurity transport in the scrape-off-layer (SOL) and divertor region of tokamaks is crucial for developing accurate impurity transport models. Since various divertor configurations are proposed for future fusion reactor, flexibility in magnetic field configuration is required for experimental devices. Some tokamaks provide various divertor configurations such as double-null, negative-triangularity, snowflake, super-X. Using smaller tokamaks will accelerate experimental research on impurity transport by improving diagnostic accessibility. However, stable formation of divertor configuration usually requires feedback control of plasma position and shape, which is technically difficult in smaller tokamaks due to the shorter timescale.
In this presentation, a smaller toroidal device for divertor configuration experiment is proposed. An internal coil, installed along the magnetic axis instead of the plasma current, generates poloidal flux. Then divertor configuration is achieved without feedback control of plasma. A scrape-off-layer experiment device using an internal coil, SOLEIL, has been designed and constructed in Nagoya University. Major and minor radii of an internal coil are $R = 0.18$ m and $a = 0.03$ m. The internal coil is suspended by insulator covered stainless steel rods from a center post. The radii of surface of the center post and inner wall of cylindrical vacuum vessel are $R = 0.075$ m and 0.3 m, respectively. Flat divertor plates are installed at $Z = \pm 0.18$ m. Triangularity $-0.4 < \delta < +0.4$ and elongation $\kappa < 1.8$ are roughly achievable shape parameters the limits imposed by electromagnetic forces and thermal constraints.
Plasma was produced by electron cyclotron resonance using a 2.45 GHz microwave. Langmuir probe measurements were conducted. The electron temperature is about 10 eV. In the horizontal plane, the electron density exhibited a radially decaying profile within the SOL.
Development on spectroscopic measurement system is also progressing. Argon ions are planned for impurity; expected charge states and wavelength of those line emissions are evaluated using collisional radiative models. Then, up to doubly charged argon ion ($Ar^{2+}$) will be distributed in SOL for above mentioned initial experiment. Required plasma parameter for higher charge states is also discussed.