ELECTRON COOLING - THEORY, EXPERIMENT, APPLICATION

As a consequence of Liouville's theorem, the momentum spread and the emittance in charged-particle beams cannot be reduced by means of ion optics. This limits the number of ions that can be accelerated or decelerated in a circular machine and, in turn, the intensity and luminosity availab le for experiments. Electron cooling can overcome this obstacle. The mechanism underlying electron cooling is equivalent to that of temperature relaxation in a plasma consisting of a hot and a cold component. Initially electron cooling was proposed for the accumulation of antiprotons, but today its application is found mainly in the improvement of light- and heavy-ion beams to be used for precision experiments in atomic and nuclear physics. Electron cooling reduces the spread in the longitudinal and transverse ion velocities, which means a decrease in the momentum spread, in the diameter, and in the divergence of the ion beam. The time that elapses until equilibrium is reached is usually of the order of seconds. Under favourable conditions, thermal equilibrium between electrons and ions can be obtained, yielding very high phase-space densities. Consequently, more intense and brilliant beams of a wide range of ions can be delivered to atomic, nuclear and intermediate-energy physics experiments. The cooling beam provides also a clean, free-electron target, and this is especially interesting for atomic physics. The understanding of electron cooling - both theoretically and experimentally - has now reached such a level that there are only a few questions still to be answered and a few doubts to be removed. This article attempts to give a comprehensive coverage of the subject and summarizes the present knowledge. Possible future developments and refinements of the method are discussed, as well as the application of the merged parallel-beam arrangement for atomic physics. © 1990.