Thermal quench of a full-performance ITER plasma, with ~-350 MJ of thermal energy will result in signif­icant transient heat loads causing vaporization and melting especially of divertor material. The erosion lifetime due to these events will depend on mitigating effects resulting from vapor shielding, redeposition of the eroded materials, and melt layer behavior. Dis­ruptions could also result in significant Be erosion due to vaporization and possible loss of the melt layer. The evaporated and melt layer thicknesses are of the order of ~10 mm and ^50 mm, respectively, for the radiation energy density of 1 MJ m~ over 1 ms expected for the first wall under the assumption of no vapor shielding.214 Figure 22 compares the pre­dicted melted layer thickness for a thermal quench time of 0.1 and 1 ms for beryllium and tungsten. For a disruption energy density of 1 MJ m~ , we see that about 50 mm of Be are melted, as compared to 60 mm of tungsten. This result occurs even though beryllium melts at 1283 °C, whereas the melting point of tung­sten is 3410 °C. The explanation for this result is that, under very intense energy deposition, a nearly instantaneous thermal balance is established between

the energy deposited by the plasma and cooling by vaporization of beryllium. The vaporization temper­ature of beryllium is variously reported as 2480­2979 °C, as compared to over 5630 °C for tungsten. Similarly, the latent heats of melting and vaporization of Be are also lower than the corresponding tungsten values. This explanation is consistent with the results in Figure 22(b), which shows the amount of material that is vaporized for a thermal quench time of (1) 0.1 ms and (2) 1 ms. At an energy density of 1 MJ m~2 and for a time of 0.1 ms the thickness of vaporized material is 10 pm for beryllium and 2.5 pm for tungsten. It must be noticed that vapor shielding is not included in these calculations and that the results therefore should be considered conservative.

High-pressure noble-gas-jet injection, for exam­ple, of neon and argon, has shown to be a simple and robust method to mitigate the deleterious effects of disruptions in tokamaks.215 The gas jet penetrates the central plasma at its sonic velocity. The deposited species dissipate >95% of the plasma energy by radiation and substantially reduce mechanical stress on the vessel caused by poloidal halo currents. Nevertheless, there remains some concern that even mitigated disruptions could damage the Be wall
in ITER. Preliminary calculations show that even during a mitigated disruption in which the plasma energy is intentionally dissipated by radiation in ^1 ms by disruption mitigation techniques, the entire first wall of beryllium can melt to a depth of roughly 20-50 mm.212,216 The fate of this melted layer is uncertain. If the melt layer resolidifies, it provides a means of removing the oxide layer and creating a clean Be layer for oxygen gettering. On the other hand, if significant j xB forces associated with the plasma termination mobilize the melt layer within the vessel, it will likely lead to operational difficulties.

Another area of possible concern is the small surface cracks that form when molten metals resolid­ify. These resolidification cracks could serve as ther­mal fatigue crack initiation sites and accelerate this type of damage. While this effect has not been exten­sively studied because of the difficulty of simulating disruptions in the laboratory, it may not be a critical issue as thermal fatigue cracks form after a few hundred cycles in most materials and they grow to depths only where the thermal stress level is above the yield stress.