Particle optics for a plasma-based beam focusing and transport system

The ion optical properties of plasma-based final focusing and transport for a heavy ion ICF reactor are discussed. The advantages of plasma-based final focusing are mainly the structure of the field, the beam charge- and current neutralization and the reduction of beam rigidity due to stripping of electrons in the plasma. Two possible focusing modes are considered: the first one, more traditional, is the so-called coherent mode; it is based on the symmetric strong focusing which can be obtained with a cylindrical, or eventually shape optimized, “short” lens. The optical properties are in this case favorable compared to quadrupole lenses. The lens is, in particular, highly insensitive to chromatic aberrations. The disadvantage of this focusing scheme, in an ICF scenario, is the relatively short focal length, which requires a design with the lens integrated in the reactor chamber. A very interesting alternative is the use of a transport plasma channel, which can provide the required standoff distance from the pellet. In this case one can exploit the incoherent focusing of an adiabatic lens. The adiabatic lens has an extremely large acceptance both of beam emittance and momentum spread. Two examples of final focusing and transport systems adapted to different accelerator and target designs are presented in detail.