The variable current gun: the parameter tests and the results of the first electron cooling experiments at LEAR

The electron cooling time (or equivalently the cooling force) is inversely proportional to the electron beam current Ib, aimed to cool hot low-energy ions [1–3]. On the other hand, highly cooled ions tend to become unstable. This implies that once the ion beam upper density limit is attained, the electron current intensity has to be reduced to a level which prevents the ion beam instability and maintains low emittances. An adiabatic-type gun [4–6] has been constructed, which provides low-temperature electrons of large, but variable densities. The electron current density is adjusted through the voltage control, Ug, of a so-called grid electrode. Its main drawback is the storage of secondary electrons when the grid potential is larger than the anode potential, thus inducing a reduction of the nominal electron current intensity. A detailed analysis of the storage process is presented, and the way to cure this drawback is explained. Finally, experimental results are given.