Optimization of the LaB6 Cathode for Divertor Plasma Simulator (DiPS)

A steady-state high density LaB₆ dc plasma source is developed for the divertor plasma simulator (DiPS), which is a linear device for the studies on various electric probes, plasma-wall interactions, atomic processes, and for the development of plasma diagnostics. Since the LaB₆ is very weak against the thermal shock, which is possibly caused by the overflow of the discharge current in a flash, and leads to cracking the LaB₆ plate, the source is to be designed lower the thermal stress for stable and longer operation. Thermal shock can be mitigated by optimizing the configurations of the magnetic and electric fields with floating electrode. Magnetic cusp is formed between the LaB₆ cathode and anode in order for ion bombardment to the LaB₆ plate to be uniform over the surface and in order for electrons to follow the field lines and to wet the surface of the anode uniformly. Electric field is generated such that the maximum outflow of electrons toward the anode can be maximized by adjusting the direction of the field and by locating a floating electrode between LaB₆ cathode and anode. The LaB₆ source is composed of a cylindrical LaB₆ plate with 4 inches in diameter, graphite heater and Ta heat shield. From the different electric and magnetic field configurations, the LaB ₆ dc plasma source is optimized as n_e=10¹⁴ cm^-3 (Ar), 10¹³ cm^-3 (He), T_e=2 - 3 eV (Ar), 5 - 10 eV (He) and T_i les 0.2 eV (Ar)