The french laser megajoule (LMJ) is designed to produce, in laboratory, fusion energy with a significant gain. Such an energy could be achieved by imploding a small capsule filled with a DT mixture. Fusion experiments produce a large emission of neutrons, x-rays, laser scattered light and debris which impose a first wall protection for the laser target chamber made of a low Z and refractory material. As boron carbide appeared to be a good candidate, among others, it was decided to evaluate the potentiallity of plasma sprayed B4C coatings for this application. This paper deals with the optimization of plasma spraying conditions to built up coatings which satisfie specifications required for the first wall. Coating general properties are presented as well as outgassing performances. Specific x-ray and laser tests were performed to evaluate coating behavior close to real LMJ working conditions.