Design of the Lanthanum Hexaboride (LaB₆) based plasma source for the Large Plasma Device at UCLA.

The original LAPD cathode was BaO based. A uniform layer of a cathode mixture was sprayed on a Ni sheet and heated in vacuum to produce the coating. BaO is very sensitive of contamination if the machine is opened the cathode must be cleaned and recoated. LaB₆ , on the other hand has a much higher electron emissivity ( > 10 A/cm² ) and is capable of producing dense (n > 10¹³ cm³) plasmas. It is not very sensitive to contamination and may be reused after machine opening. The challenge that comes with this material is that it must be heated to 1800^o C. The cathode temperature must be highly uniform across its surface to produce a uniform plasma. As the LAPD runs 24/7 the cathode ( 38.5 cm dia) and heater design must be robust and withstand cracking and other failure modes under continuous heater power ( 77 kW) and JXB forces ( 12 kA operation at several kG). The heater is made from six parallel elements. The predicted temperature and deformation of the heater, cathode, reflectors and encasing oven was simulated using COMSOL for various designs. The LaB₆ was cut into 4 tiles in a tongue and groove configuration and held in place with specially designed carbon springs. The simulations results, machine drawings and photographs of the cathode at emission along with temperature measurements with a 2 color pyrometer will be presented. The emission as a function of heater power and parameters of the resulting plasma will be shown.