Radiation enhanced sublimation

The limiting temperature for graphite use in fusion systems is defined by tliermal sublimation (--1500-2000°C). However, a process which is very similar to thermal sublimation (in cause and in effect) appears to define the current temperature limit. This phenomenon, which is known as radiation enhanced sublimation (RES), is not clearly understood but dominates above a temperature of about 1000°C and increases exponentially with increasing temperatme. 

W. Eckstein and V. Philipps, Physical Sputtering and Radiation-Enhanced Sublimation. In Physical Processes of the Interaction of Fusion Plasmas with Solids, W.O. Hofer and J. Roth, Eds., 1996, Academic Press, pp. 93-134. 

In ion beam experiments, carbon materials show an unexpected additional erosion mechanism which dominates the carbon erosion in the temperature range 1200 K 2200 K, called radiation enhanced sublimation (RES) [35,36]. There are doubts whether this effect also exists with very high particle flux densities as they are typical for tokamaks. Test limiter experiments in TEXTOR have demonstrated that RES is not important under such conditions. However, other devices reported a carbon influx by RES and recent measurements indicate also an enhanced erosion of metals at high temperatures under low energy particle impact [34]. Further R D is needed to clarify these issues. 

The conditions where net erosion turns into net deposition for carbon irradiation may further be complicated by effects of surface temperature, influencing the self-sputtering yield by radiation-enhanced sublimation (RES) as well as the diffusion and surface segregation of implanted impurities. An example is introduced in Fig. 9.4b, where the erosion of W due to C+ bombardment... 

W. Eckstein, J.A. Stephens, R.E.H. Clark, J.W. Davis, A.A. Haasz, E. Vietzke, Y. Hirooka, Particle induced erosion of Be, C and W in fusion plasmas. Part B Physical sputtering and radiation enhanced sublimation. In Atomic and Plasma-Material Interaction Data for Fusion, Vol. 7B, (IAEA, Vienna, 1999) to appear... 

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