|Fig. 1: A cross-section view of the tokamak core. Plasma-material interactions occur in the edge region as well as the divertors where material is injected or removed from the plasma. (After Tillack et al. )|
While research into using nuclear fusion for energy generation has been ongoing for over 50 years, there continues to exist many challenges that must be overcome before fusion can be used economically and sustainably as a source of energy. Since the conception of the idea of harnessing fusion energy, many reactor designs have been proposed and studied to varying degrees of success. Arguably, currently the most mature technology for power generation from fusion is the tokamak, which uses powerful magnetic fields to trap hot plasma in a circular trajectory inside a toroid reactor, where fusion takes place.  A cross section view of a tokamak is shown in Fig. 1. The well-known International Thermonuclear Experimental Reactor (ITER), currently being built in France, uses this concept.  While there are many advantages and disadvantages to the tokamak concept and many design constraints and trade-offs in designing such a reactor, we will restrict our focus to a particularly challenging aspect of tokamak design, that of managing the interactions between the hot plasma and the walls of the reactor (referred to as plasma-material interactions, or PMI).
Plasma facing components (PFCs) are the parts of the reactor that line the inside walls of the reactor chamber. PFCs are subjected to extreme conditions under operation since it is in direct contact with plasma inside the reactor. The many requirements on materials used in PFCs include the ability to 
Efficiently conduct heat from the reactor
Handle large heat fluxes during transient states
Handle large electromagnetic loads
Withstand neutron damage
Limit the retention of fuel, especially tritium
Heat removal from the reactor is crucial for energy generation as well as to maintain the structural integrity of the reactor itself. In Deuterium-Tritium (D-T) reactors such as the ITER, around 80% of the fusion power is carried by fast moving neutrons, so that large shielding blocks (SB) are needed to absorb the neutron energy.  In the ITER, this will be accomplished by stainless steel blocks 50 cm thick!  Fig. 2 shows a model of the cross section of the ITER, where the steel shield blocks are clearly seen. The shield blocks form a "blanket" layer for neutron shielding and energy absorption and are protected from direct contact with the plasma by an inner layer, known as the "first wall". The first wall must withstand very large heat fluxes (10 MW/m2) due to transient states in the plasma.  These transient states include the startup phase, Edge-localized modes (ELMs), and disruptions/vertical displacement events (VDE). [2,4] ELMs and VDEs are instabilities that have been observed in the plasma. During these transient states, hot plasma can come in direct contact with the wall. This will cause the wall material to melt (even if the wall material has a high melting temperature) and go into the plasma, which causes contamination of the plasma and disrupt the fusion process. Finally, some fuel may be thrown from the plasma and become trapped in the wall, resulting in radioactivity in the wall and unnecessary depletion of fusion fuel. This phenomenon was seen in the Joint European Torus (JET) reactor, where 33% of the tritium fuel was deposited on the graphite reactor wall. 
|Fig. 2: A cross-section model of the ITER. (Source: Wikimedia Commons)|
The materials proposed for PFCs typically include light-weight elements such as carbon or beryllium, or heavy elements such as tungsten.  Carbon was used in the JET reactor and saw significant amounts of tritium retention, which can pose an environmental hazard in case of a leak or accident.  During transient states of the reactor, some of the wall material will inevitably be melted and swept into the plasma. In the case of light elements such as carbon and beryllium, the contamination is thought to be not too detrimental to the plasma integrity. However, this is not the case with heavy elements like tungsten. Plasma contamination, along with PFC degradation can lead to frequent downtime in the reactor, which may severely impact the economics of power generation.  The best trade-offs are not yet known and research is ongoing in terms of finding suitable materials and modelling the plasma-material interactions.
Several potential solutions are also being investigated, including using a liquid metal layer for PFCs. This would allow the surface of the PFC to be renewed constantly, without shutting down the reactor.  However, this technology has not yet been proven and many challenges have yet been resolved. Another option for extending the life of PFCs is to control the size of the ELMs. Some proposed solutions include operating the reactor with reduced energy confinement, using resonant magnetic perturbations, fuel control via pellet pacing, and operating in "ELM-free regimes", which was observed to occur within a narrow operational range. 
In conclusion, the engineering challenges of building and operating fusion power plants are numerous, and many design challenges remain unresolved. The design of PFCs is only one of many challenges we need to solve. Once completed, the ITER will be able to answer many questions about the viability of fusion power generation, but until then it remains unclear if or when sustainable, safe, and cost effective fusion power can be achieved.
© Rick Zhang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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