Radio frequency siliconization: An approach to the coating for the future large superconducting fusion devices

Radio frequency (rf) siliconization has been carried out on the HT-7 superc onducting tokamak in the presence of a high magnetic field, which is a try on superconducting tokamaks. Three different procedures of rf siliconizatio n have been tested and a very promising method to produce high quality sili con films was found after comparing the film properties and plasma performa nce produced by these three different procedures. The Si/C films are amorph ous, semitransparent, and homogeneous throughout the layer and adhere firml y to all the substrates. The advantages of silicon atoms as a powerful radi ator and a good oxygen getter have been proved. An outstanding merit of rf siliconization to superconducting devices is its fast recovery after a seri ous degradation of the condition due to the leakage of air to good wall con ditions. A wider stable operation region has been obtained and plasma perfo rmance is improved immediately after each siliconization due to significant reduction of impurities. Energy confinement time increases more than 50% a nd particle confinement time increases by a factor of 2. The lifetime of th e silicon film is more than 400 standard ohmic heated plasma discharges. Si mulation shows that the confinement improvement is due to the reduction of the electron thermal diffusivity in the outer region of the plasma. (C) 200 1 American Vacuum Society.