Disruptions still occur frequently in operating tokamaks, and therefore they are also expected for ITER with an anticipated occurrence in <10% of the ITER pulses (3000 pulses per expected compo­nent lifetime). During a disruption in which the plasma undergoes a partial or full thermal quench, most of the plasma thermal energy will be dumped on the divertor plates.166 Taking into account the resultant loading conditions (see Section 4.17.2), significant material loss from the tungsten plasma­facing surface should occur by melting and evapora­tion particularly in the dome area.167,168 In simulating these events, the amount of melting, the melt motion and subsequent roughening of the surface, the mate­rial erosion by droplet emission, the resolidification behavior, and finally, the crack formation occurring in the loaded area or at the boundary between melted and unmelted zone are the most important para­meters to be determined.

The underlying mechanisms for the above — mentioned material degradation are well described (see Figure 5).169 Thermal loading of tungsten and metals, in general, at ‘moderate’ energy densities (up to a few MJ m~ ) will result in a homogeneous, localized melting of the sample surface. When higher energy densities are applied, surface evaporation occurs; the momentum transfer due to evaporating atoms from the surface generates an effective pressure on the melt layer, which finally results in the formation of a melting ridge. Increasing the incident energy density even fur­ther, the material’s response is characterized by intense

I Incident beam

Л

Cracking Homogeneous

roughening melting

Increasing energy density

Figure 5 Performance of tungsten and metals in general under transient thermal loads.

boiling and convection of the melt layer resulting in droplet formation and ejection.170-172 Open pores in the recrystallized material have a strong impact on the thermophysical properties.

The melting threshold and subsequently the amount of melt formation depend on the material’s thermal conductivity, which is lower for porous materials such as plasma-sprayed tungsten, and for tungsten alloys. In particular, it has to be taken into account that dispersoids such as La2O3 (Tm = 2578 K) have a lower melting temperature than tungsten. This may result in early melting and increased evaporation causing the formation of a porous and depleted sur­face layer, which becomes even more important when applying loads below the melting threshold (see below and Section 4.17.4.1.2). On the other hand, the melting threshold is correlated with the base temperature of the PFM. When the base temperature increases, the melting threshold energy decreases and the amount of melt formation, the obtained cra­ter depth, and the evaporation losses for the same applied loading conditions increase significantly.169

As it cools, the material resolidifies in a recrystallized state providing a columnar grain structure typical of PVD or CVD coatings. With further cooling, depending on the base temperature of the material/component (see ‘Base Temperature’ in Section 4.17.4.1.3), brittle crack formation will not take place above a certain threshold temperature. However, with fast cooling after loading below this temperature, the material will undergo severe cracking with crack lengths that can reach the order of millimeters.169

When the qualification of different W grades and alloys108,141,147 is done in combination with thermal fatigue loading,90 materials with high thermal con­ductivity in combination with superior mechanical properties, that is, with high ductility, performed best with regard to melt material loss and crack formation. This comprises low-alloyed W materials with increased ductility such as W-Re or W-Ta, or fine-grained pure W or W alloys.

Disruption simulation experiments on bulk tungsten and tungsten coatings have also been described in the literature. These were performed not only to investigate the melting behavior but also for the purpose of characterizing the cracking behavior.26’42’60’101’122’130’131’162’165’173-176 Although

these experiments are more related to those on the characterization of ELM conditions (see Section 4.17.4.1.2) and were often performed only at RT) the results indicate that the use of highly ductile SC materials is preferred.90’177 Alternately in case of cheaper polycrystalline materials it is necessary for the material to have the proper microstructure orientation as described above’ that is’ the grain ori­entation perpendicular to the loaded surface. The reason for this is that crack formation occurs mainly along the grain boundaries and follows the orienta­tion ofthe deformed microstructure. The crack depth is in general related to the applied loading conditions and therefore the pulse length which determines the heat penetration depth and the temperature and stress gradient induced during loading. The temper­ature gradient also determines the recrystallization zone which is generated below the loaded area as a function of temperature (< Tm) and time.

However the quantification of the applied condi­tions and by this a comparison of the materials response is often not straight forward as each testing facility has its own characteristics. Most ofthe time the cited incident power density for example in Hirooka etal.’114 Linke etal.,178 and Makhankov etal.9 does not correspond to the absorbed power density. For exam­ple’ with an electron beam at 10keV’ Pabs ~ 0.62 Pinc179 — with the ratio slightly decreasing at higher acceleration voltages. In a plasma accelerator Pabs depends on incident angle and for a perpendicular impact might only reach 0.1 Pinc.1 0 For a rough esti­mate of the temperature impact, the given conditions can be compared to the heat flux parameter introduced above. For a base temperature of RT’ this amounts to a melting threshold of ^60 MW m~2 s~1/2 for pure and fully dense tungsten. Due to the fact that this parame — ter163 is also directly proportional to the thermal con­ductivity 1 the specific heat cf, and the density p: 2 jn1cPp a decrease in thermophysical properties consequently reduces the heat flux parameter and the melting threshold.

As all performed investigations indicated that melting will cause increased material degradation
and the continuous erosion of the PFM’ it will signif­icantly limit the lifetime of the PFCs. Therefore’ the safe and economic operation of a future fusion reac­tor requires that scenarios causing melt formation have to be limited to a minimum.