Plasma lenses

Summary Form only given, as follows. A charge-neutralized heavy ion beam can be focused on a target by the use of a plasma lens in which the magnetic field acts directly on the neutralizing electrons, which in turn provide electrostatic focusing for the ions. A simple analysis shows that such a lens can be effective at relatively low magnetic field strengths and a short focusing region. The lens is, however, subject to two aberrations that can increase the beam emittance and spread the focus: (1) the diamagnetism resulting from the high azimuthal electron current in the lens and (2) the relativistic increase in electron mass, which changes the electron current variation with radius. The two effects, however, act in opposite directions, the diamagnetism being most pronounced at small radii and the current reduction due to the relativistic mass increase being greatest at large radius. Both effects vary approximately quadratically with radius and cancel for a proper choice of parameters. As an example, for a beam of 10-GeV lead ions of charge 3, a particle current of 30 kA carried by many beams, an initial radius of 5 cm, and a magnetic field of 400 G, the two effects cancel for 58 beams, giving a perfect focus to this order of correction. For the focus to be less than 1 mm, the number of beams must be greater than 34.<>