A liquid-helium-free superconducting electron cooler at the storage ring TARN II

A superconducting electron cooler with an adiabatic expansion factor of 100 was designed for high-precision experiments and high-speed cooling. The gun solenoid is a liquid-helium-free superconducting magnet with a 20-cm room-temperature bore, which can produce a magnetic field of up to 3.5 T. An electron beam is expanded from a diameter of 5 to 50 mm in a gradually decreasing solenoid field from 3.5 T to 35 mT. With this cooler, a transverse electron temperature on the order of 1 meV was attained. The longitudinal cooling force was measured with an induction accelerator as functions of the ion and electron currents, and the expansion factor. The longitudinal cooling force does not depend much on the expansion factor.