Carbonization in tokamaks

Abstract

Carbonization is here defined as the plasmachemical in situ deposition of homogeneous, thin, carbonaceous films on the entire first wall of a fusion device. It has led to a significant improvement of the plasma performance in TEXTOR: suppression of metal impurities ( n me n e ⩽ 1 × 10 −5 ), Z eff close to 1, density limit n e (0) × R/ B T = 4.7 × 10 19 m −2 T −1 for ohmic discharges, improved power coupling and low steady state impurity levels during ICR-heating in the megawatt range. The deposition technique developed at TEXTOR uses radiofrequency assisted DC-glow (RG) discharges in a flow of H 2(D 2)/CH 4(CD 4) gases; the process is monitored via residual gas analysis. The carbon-films are amorphous, semi-trans-parent, homogeneous without macroporosity and incorporate a large fraction of hydrogen ( H C ∼ 0.4 ). When we use a 300 monolayer thick coating, the reappearance of metals in the plasma occurs after about 3 working days of TEXTOR ( ∼100 discharges, ∼ 3 s each) when varying plasma conditions are used. The metals appear much later at high densities ( n e ⩾3 × 10 13 cm −3), where C redeposition on the limiters is important. The recycling properties of hydrogen during a tokamak discharge differ strongly from those of an all-metal machine. Isotope change-over experiments which illustrate these properties are presented and discussed in detail. The paper reviews the experience gained in TEXTOR with carbonized metal walls and limiters. It also covers in part, recent experience from operation with graphite limiters and wall carbonization. Results achieved in JET, ASDEX and JIPP T-IIU after carbonization are summarized. The applicability of carbonization to future large devices and the impact on tritium related problems is addressed.