A Tokamak Reactor with Lithium Walls

Background: A recently discovered mechanism of magnetic propulsion of a liquid metal in the toroidal magnetic eld of tokamaks (December of 1998) enables intense stationary liquid lithium (Li) streams covering the plasma facing surface of the vacuum chamber. There are several attractive properties of such a regime. These are presented, together with the possible issues, experimental foundations and the necessary R & D. Li streams: The streams are about 1 cm in thickness and are held to the surface by the Ipol Btor electromagnetic force, where Ipol represents the externally driven electric current through the streams. In the strong tokamak magnetic eld this force can be so large (3 orders of magnitude larger than in the momentum or gravity driven liquid walls) that the liquid Li is capable of withstanding the possible electromagnetic forces during potential disruptions. Electro-magnetic propulsion: The intrinsic inhomogeneity of the Ipol Btor force drives the liquid Li along the walls (see Fig. 1) from the inner, strong magnetic eld, side to the outer side of the torus, where the toroidal eld is smallest. The velocity of the Li streams due to magnetic propulsion is of order 10-20 m/sec and should easily compensate all associated viscous losses. The magnetic Reynolds number for such a streams should be about 0:2 0:4 in order to satisfy the power absorption requirements. The diamagnetic drag force is negligible given the small thickness of the streams. For a tokamak-reactor with R ' 5 m and a = 1:5 m with the modest cost of about 20 MW of electric power for driving the streams, the magnetic propulsion allows the propulsion of more than 2 T=sec of Li. This ammount is capable of absorbing 1 GW of power and will also enable its transmission out of the device, while keeping the temperature of the Li in the range of 300 350 C. On one hand, the relatively small temperature of the Li stream, does not allow eÆcient conversion of the energy coming from the -particles to electricity i.e., 20% of the total fusion power. On the other hand, the low temperature of the Li streams with a continuously refreshed plasma facing surface allows us to utilize the unique properties of Li to absorb the D and T particles (by producing Li hydrate molecules). With such an absorption environment at the plasma edge, the discharge may go into a new, non-recycling regime in tokamaks, which overcompensates the unavoidable loss of the 20% of the fusion power.