In contrast to disruptions, ELMs occur during nor­mal operation in the H-mode and are characterized as instabilities caused by the steep temperature and density gradients at the plasma edge, which deposit a significant amount of energy at a high repetition rate.181’182 In particular’ it is the expected high repe­tition rate for ELMs during the lifetime of the PFC (>1 million of events at a frequency of 1-25 Hz183) that, although yet unexplored, will impose high demands on the PFMs.

While it is the desire of plasma physicists to oper­ate in H-mode regimes with high-energy ELM depo­sition (> 1 MJ m~2) the response of bulk tungsten’ tungsten coatings, and tungsten alloys to such loading conditions, that is, surface melting, melt motion, material erosion, and vaporization, is

detrimental. To obtain further insight into material behavior under these conditions’ modeling of experi­mental conditions was carried out.9’167’168’190-195 It has been shown that with regard to melt motion/ erosion, the results of the different facilities cannot be directly compared196 and none of the testing facil­ities used provides identical conditions to those that will occur in a tokamak. However’ mitigation techni­ques have been explored for reducing the applied ELM energy, which, in general, can only be done at the expense of a higher repetition rate.183 The extent to which the ELMs have to be mitigated depends on the melt formation at tile edges due to the shallow plasma impact, which was experimentally found to be between 0.4 and 0.6 MJ m~2 for pure forged tung­sten.189’197 On the other hand’ the effect of crack formation during ELMs on the lifetime behavior of the PFCs has to be taken into account. As mentioned before, this behavior is yet unexplored at high repe­tition rates.

Typical investigations on various grades of W82’187’189’198 coatings21’54’186’199 and alloys146’157’187 were in the range of 10-100 repetitions. In a few cases up to 1000 repetitions and in single experiments even on the order of tens of thousands of repetitions have been obtained depending on the testing facility used. As the repetition rate is still rather low compared to the expected millions of events the main interest of

(a)

(b) these investigations was the qualification of different W grades and alloys (see Section 4.17.3.3) with regard to their damage and cracking thresholds. The characterization was done as a function of the main parameters described in Section 4.17.4.1, that is, microstructure, power density, and base temperature.

The results obtained so far showed that crack formation20 vanishes above a certain base tem­perature (see Figure 6).82,157,198 This temperature decreases with increasing material ductility, indicat­ing that the use of W alloys or fine-grained W is preferred. In the case of an anisotropic microstructure, this effect strongly depends on the material’s orienta­tion. Better results are obtained for grain orientations parallel to the loaded surface (see Section 4.17.4.1), yielding differences in the threshold temperature compared to the orthogonal direction of up to several hundred K (cf. Figure 6(a) and 6(b)). Recrystallization leads to a slight homogenization of the material’s
microstructure and therefore the mechanical proper­ties; however, there is no full convergency of the orientation-dependent thresholds.82

Despite the fact that for the currently limited number of applied pulses no crack formation was observed above a material and orientation-dependent temperature, the material is still damaged by plastic deformation and surface roughening. The evolution of this plastic deformation and of the related material hardening as a function of the applied number of loads is still unclear and has to be investigated. However, there are also heat load levels (at least up to Tbase < 800 °C), at which no visual material degradation could be determined and the future goal will be to investigate if these damage thresholds are still valid for high repetition rates, at higher base temperatures, and particularly in combination with neutron irradiation (see Section 4.17.4.3) and plasma wall interaction (see Section 4.17.4.4).

All the information given above on the effect of ELMs is also directly transferable to the short tran­sient events expected for inertial fusion applications and has been verified by IFE-related tests on dif­ferent W-based materials.201-204 There are coating parameters of high interest besides those mentioned above; these include the manufacturing-induced residual stresses at the surface, which are dependent on the used substrate, and the coating thickness. As mentioned in Section 4.17.4.1.1, the applied loading conditions and therefore the pulse length determine the heat penetration depth.163 As a result, the tem­perature and stress gradient induced under IFE applications should be similar to those in X-ray anodes (see Section 4.17.2). In case of thin coatings, residual and induced stresses might affect the coating to substrate interface and could lead to interfacial crack formation and delamination. This leads to minimum requirements for coating thicknesses that depend on the applied loading conditions.54 For example, in industrially produced X-ray anodes, W-Re coatings are typically used with a thickness of 200-700 pm205,206 to provide better mechanical and thermal-shock properties compared to pure W.204 However, the first experience on the influence of ELMs on coatings under real plasma operational con­ditions will be gained in the ITER-like wall project in JET, which involves testing relatively thin PVD- tungsten coatings (14-20 pm) on a CFC substrate that provides a strong and anisotropic CTE difference.19,142

The behavior of this material under the above outlined transient heat loads is of course a key factor for the lifetime assessment of PFCs. However, the
results obtained for pure thermal shock testing might underestimate the material damage and by this over­estimate its lifetime. Only a combination of thermal shock, thermal fatigue (see Section 4.17.4.2), neu­tron irradiation (see Section 4.17.4.3), and plasma wall interaction (see Section 4.17.4.4) will be able to give appropriate answers for the selection of suitable grades of W.