EVIDENCE OF HOT-SPOT FORMATION ON CARBON LIMITERS DUE TO THERMAL ELECTRON-EMISSION

Carbon test limiters have been exposed in TEXTOR to high heat loads up to about 30 MW/m(2). The evolutions of the surface temperature distri bution and of the carbon release have been observed by means of local diagnostics. A sudden acceleration of the rise of the surface temperat ure has been found at a critical temperature of approximate to 2400 de grees C. The increase of the rate of the temperature rise is consisten t with an enhancement of the power loading by a factor of 2.5-3. Follo wing the temperature jump (hot spot), a quasi-equilibrium temperature establishes at approximate to 2700 degrees C. The development of the h ot spot is explained by an increase of the local power loading due to the breakdown of the sheath potential by thermal emission of electrons from the carbon surface. Simultaneously with the appearance of the ho t spot, the carbon release from the surface increases sharply. This in crease can be explained by normal thermal sublimation. Sublimation coo ling contributes to the establishment of the quasi-equilibrium tempera ture at about 2700 degrees C.