Alfvén limit in fast ignition.
Fast ignition inertial confinement fusion relies on rapidly heating the compressed fuel to ignition using a laser-generated electron beam. The current required far exceeds the Alfvén limit, so it can only propagate while the plasma provides a nearly coincident return current. The resistive decay of the return current is shown to be too rapid for the originally proposed scheme to work. Possible solutions to this problem are to increase the mean energy of the beam, to heat the fuel to a higher temperature by lowering the beam radius and duration, to use multiple beams, and to use an annular beam. Considering the laser wavelength required shows that increasing the mean energy and number of beams are the most practical solutions.