At some locations (mostly at the upper inboard region and the lower region of the first wall), the Be armored PFCs must withstand a certain number of ‘slow’ thermal transients resulting from loss-of — control of plasma position during VDEs. Typical parameters for these events are 60 MJ m~2 over 0.3 s. In contrast to thermal quench disruptions, VDEs lead not only to significant erosion or melting, but also to high heat fluxes and a subsequent temper­ature increase at the armor/heat sink interfaces that can result in a failure of the armor/heat sink joints.217 As a matter of fact, because of their short duration (<10ms), ELMs and the thermal-quench phase of a disruption have no significant thermal effects on structural materials and coolant channels. In con­trast, plasma instabilities such as VDEs (duration 100-300 ms), and runaway electron impact, in addi­tion to causing severe surface melting and erosion, can result in substantial bulk damage to these com­ponents. Elevated temperatures and high thermal stresses in the structure can seriously degrade the integrity of the interface bonding and burnout the coolant channels. Runaway electrons (up to many megaelectron volt) penetrate many centimeters of beryllium and directly heat underlying metal struc­tures, potentially damaging coolant channels.218-220

The erosion due to VDEs in a device like ITER has been modeled by various authors,2 1 whereas the results of analysis carried out to quantify the effects on PFCs resulting from runaway electrons can be found in Raffray et al222 As an example, Figure 23 shows the surface temperature of a 5 mm copper

Copper substrate

(Carbon, beryllium, or tungsten coating on
copper structure)

Figure 23 Interface copper surface temperature rise during a vertical displacement event for different surface coating materials. Reproduced with permission from Federici, G.; Skinner, C. H.; Brooks, J. N.; etal. Plasma-material interactions in current tokamaks and their implications for next-step fusion reactors. Nucl. Fusion 2001,41,1967-2137 (review special issue), with permission from IAEA.

substrate at its interface with a tungsten, beryllium or carbon tile of 10 mm thickness during a typical VDE releasing about 60 MJ m~2 to the surface in 300 ms.7 Tungsten and carbon armors of similar thickness usually result in a similar and higher cop­per surface temperature than that of beryllium armor of the same thickness. This is because most of the incident plasma energy is removed by the beryllium’s higher surface vaporization rate, which leaves little energy to be conducted through the structural material. In order to reduce the tempera­ture at the copper interface, thicker tiles would be required. Only beryllium tiles of reasonable thickness (<5-10 mm) or very thick carbon or W tiles (>20mm) can withstand the acceptable temperature rise in the copper structure for the conditions shown. The coolant flux and, conse­quently, the Be/Cu interface temperature increase with decreasing Be thickness. The evaporated and melting thickness and temperature at the Be/Cu alloy interface during each VDE is shown in Table 5 for Be tiles (5 and 10 mm thick); for two values of the VDE energy density (30 and MJ m~ ) and for two VDE durations (10 and 100 ms).