In addition to the temperature-dependent chemical sputtering of beryllium when exposed to deuterium plasma, another temperature-dependent loss term is present in beryllium exposed to plasma bombard­ment at elevated temperature. The classical picture of the temperature dependence of erosion from chemically inert surfaces exposed to energetic parti­cle bombardment is composed of the superposition of a constant physical sputtering yield with an expo­nentially varying thermal sublimation curve. The classical picture is contradicted, however, by experi­ments that show an exponential increase in erosion at lower temperature that cannot be explained by classical thermodynamic sublimation. First observed by Nelson75 for a variety of metal surfaces, similar results have been measured for Be,76,77 W78 and C79,80 surfaces. In the case of carbon, this mechanism has been called RES.

In the case of beryllium, two explanations have been proposed and both rely on the large flux of ions incident during plasma bombardment to modify the

plasma-facing material surface. In the first, the inci­dent plasma ions, in addition to creating sputtered atoms from the surface, also create a population of surface adatoms. An adatom is an atom from a lattice site on the surface that has gained sufficient energy to leave its lattice location, yet does not have sufficient energy to escape from the surface as a sputtered atom. The atom then occupies a site on top of the regular lattice sites. Because an adatom does not have the same number of adjacent atoms as those in the lattice, it is less strongly bound to the surface and can therefore sublime at a lower temperature than one associates with equilibrium thermodynamic sublima­tion. In the second explanation, incident plasma ions that have thermalized somewhere below the surface of the lattice exert a stress on the surface atoms of the target again resulting in a lower binding energy of the surface atoms to the bulk of the material.

Measurements show atoms are being lost from the surface at thermal energies,77 rather than the energy associated with sputtered particles (i. e., on the order of electron volts). This seems to verify the loss mech­anism that occurs because of the thermal release of an ensemble of particles with a lower surface binding energy than that of bulk atoms of that element. Addi­tional measurements at elevated temperature have documented the variation in Be atom surface loss rate with changes of the incident flux of energetic particles.81 The larger the incident flux, the lower the onset temperature for the enhanced erosion. The implication of this enhanced loss rate at ele­vated temperature is a reduction of the permissible operating temperature of any plasma-facing material, or alternatively that the lifetime of a component operating at extreme temperature may be less than that expected based on the predictions from classical surface loss terms.