Tungsten as first wall material in fusion devices
Summary (2 min read)
1. Introduction
- In the case of tungsten, erosion by fast particles plays an important role.
- In connection with the pronounced neoclassical inward drift of high-Z ions, turbulent transport plays a beneficial role here.
- The plasma wall interaction, discussed in section 2, shows significant differences to carbon -mainly due to the higher atomic mass.
- The summary stresses the necessity for further investigations.
2. Plasma Wall Interaction with Tungsten
- Another source for erosion are fast particles from additional heating.
- Fast deuterium ions generated by Neutral Beam Injection can be lost due to drifts or MHD-effects, like ELMs.
- Variations of the field line angle with respect to the Faraday screen -which should lead to a strong variation of the electric potential -do not show a clear effect.
- The limited theoretical understanding poses a challenge for the combination of ICRH and a tungsten covered wall.
3. Edge and Core Transport
- In the H-mode, one has to subdivide the confined plasma into the pedestal and core regions.
- This suggests that ELMs are a necessary prerequisite for sufficiently low tungsten content in the plasma.
- In the simple picture, neoclassical theory predicts a diffusion coefficient which scales proportional to 1/Z 2 while the inward drift is proportional to 1/Z.
- If the heat flow in the core is sufficiently high then anomalous transport can easily exceed the neoclassical effects, especially of high-Z ions because of the 1/Z and 1/Z 2 scaling respectively.
- 2 ), while on the other hand anomalous transport can directly determine the tungsten flux.
4. Technological Developments
- On the technical side, the critical issues are the mechanical properties of tungsten.
- Its tensile elongation at room temperature is almost zero, making it brittle.
- In order to improve the brittleness several kinds of tungsten based alloys have been developed.
- In steady state devices, actively cooled components will be necessary and for high heat flux components, the bonding between the W armour and the heat sink is of special concern.
- In the case of ITER, copper alloys are proposed as the heat sink material.
4.2 Bulk Tungsten Components
- TEXTOR has performed experiments with bulk W limiters to explore their behaviour under fusion relevant particle and power loads.
- Particularly, the influence and performance of castellated structures were investigated [55] .
- JET plans to address the issue of melt layer erosion within the 'ITER-like wall project' with a W lamellae design, which was successfully tested by cyclic loading at ≈8 MW/m² [45] .
- The components in ITER will be actively cooled and the cooling structure will be made from CuCrZr.
- In a reactor, the technical concept will depend on the cooling medium, e.g. water or helium.
5. Summary
- So far, plasma experiments have demonstrated that in most scenarios the tungsten erosion of the surfaces and its concentration in the central plasma can be kept sufficiently low.
- In certain scenarios with high edge temperatures this may, however, not be the case.
- In addition, the high erosion in the neighbourhood of an ICRH antenna needs particular attention.
- Technological solutions for the highly loaded divertor targets in a fusion reactor are under development.
- Altogether tungsten as the first wall material looks promising, but several open questions still remain to be solved.
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Cites background from "Tungsten as first wall material in ..."
...Current technology employs W as a thin (1–3 mm) coating on graphite or carbon fiber composite tiles, but future PFCs might be composed simply of bulk W.(2,5) Tungsten retains strength at high temperatures and can sufficiently conduct heat away from the surface, although underlying Cu-based components will likely act as future heat sinks....
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...This is a combination of good and bad news as it relates to the efficacy of W PFCs....
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..., W) materials have been considered for use in PFCs, tungsten has emerged as one of the most promising materials for use in PFCs.(2,4,5) Tungsten has several properties that make it well suited for use as a PFM....
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...While both low-Z (e.g., C, Be) and high-Z (e.g., W) materials have been considered for use in PFCs, tungsten has emerged as one of the most promising materials for use in PFCs.2,4,5 Tungsten has several properties that make it well suited for use as a PFM. Current technology employs W as a thin (1–3 mm) coating on graphite or carbon fiber composite tiles, but future PFCs might be composed simply of bulk W.2,5 Tungsten retains strength at high temperatures and can sufficiently conduct heat away from the surface, although underlying Cu-based components will likely act as future heat sinks.2 Ingress, transport, and retention of radioactive tritium in PFMs is of great concern and the amount of hydrogen isotopes permanently retained in tungsten is low compared to carbon-based materials.6 This is one of the main justifications for using W as a PFM....
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References
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