In the past, plasma spraying was considered as a high deposition rate coating method, which could offer the potential for in situ repair of eroded or damaged Be surfaces. Development work was launched during the early phase of the ITER R&D Program in the mid-1990s.136 In the plasma spray process, a powder of the material to be depos­ited is fed into a small arc-driven plasma jet, and the resulting molten droplets are sprayed onto the target surface. Upon impact, the droplets flow out and quickly solidify to form the coating. With recent process improvements, high quality beryllium coat­ings ranging up to more than 1 cm in thickness have been successfully produced. Beryllium deposition rates up to 450 gh-1 have been demonstrated with 98% of the theoretical density in the as-deposited material. Several papers on the subject have been published.136-138 A summary of the main achieve­ments can be found in Table 4.

However, based on the results available, the initial idea of using plasma-sprayed beryllium for in situ (in tokamak) repair was abandoned for several rea­sons. First was the complexity of the process and requirements to control a large number of para­meters, which affect the quality ofthe plasma sprayed
coatings. Some of the most important parameters include plasma spray parameters such as (1) power, gas composition, gas flow-rate, nozzle geometry, feed, and spray distance; (2) characteristics of the feedstock materials, namely, particle size distribution, morphol­ogy, and flow characteristics; (3) deposit formation dynamics, that is, wetting and spreading behavior, cooling and solidification rates, heat transfer coeffi­cient, and degree of undercooling; (4) substrate conditions, where parameters such as roughness, temperature and thermal conductivity, and cleanli­ness play a strong role; (5) microstructure and properties of the deposit, namely, splat characteris­tics, grain morphology and texture, porosity, phase distribution, adhesion/cohesion, and physical and mechanical properties; and (6) process control, that is, particle velocity, gas velocity, particle and gas temperatures, and particle trajectories. Second, plasma-sprayed beryllium needs (1) inert gas pres­sure, (2) reclamation of the oversprayed powder (more than 10%), and (3) strict control of the sub­strate temperature. The higher the temperature the higher the quality of the plasma-sprayed coating, but unfortunately, an easy and reliable method to heat the first wall to allow in situ deposition was not found. Finally, tools to reliably measure the quality of the coating and its thickness are not available today and a strict control of the coating parameters is difficult to achieve.

Thus, it was concluded that plasma-sprayed beryl­lium for in situ repair is too speculative for ITER without further significant developments. Neverthe­less, this method still remains attractive and could be used for refurbishment of damaged components in

Table 4 Main achievements of ITER-relevant plasma-sprayed technology (summary of best results, not always achieved together)

Value/results Comments

Residual porosity (%) Thermal conductivity (WmK-1) Bond strength (MPa) Substrate temperature (°C)	~2 Up to 160 at RT 100-200 >450	Could be more than 5% Depends on temperature of substrate, maximum achieved at T~ 600-800 °C with addition of H Reasonable Very important for good strength, adhesion, and thermal conductivity. Keep in mind that CuCrZr temperature should not be higher than 500 °C for several hours due to overageing of CuCrZr Needed, but very difficult to do in situ Reasonable Reasonable It means that more than 10% of powder will be lost in chamber For first-wall conditions tested
Substrate preparation Deposition rate (kg IT1) Thickness (mm) Deposition efficiency (%) Thermal fatigue (MW m~2/ number of cycles)

hot cell, albeit it may be cheaper to replace a dam­aged component with a new one.