Experiments on the Injection and Containment of Electron Clouds in a Toroidal Apparatus

Injection and containment of unneutralized clouds of electrons has been accomplished with an azimuthally symmetric, toroidal, magnetic field. The confining magnetic field is produced within a conducting toroidal chamber. The induction of this magnetic field has been used to inject the electron cloud (inductive charging). Average electron densities of 4 × 109 cm−3 and peak electrostatic well depths of ≈ 400 kV have been achieved. Semiempirical correlations are given which show the inductive charging scheme to be governed by an electron energy restriction and finally limited by the occurrence of anomalous crossed field beam noise. Stable equilibria, without the necessity of a rotational transform have been observed for times in excess of 60 μsec. Correlation of the containment time observations with the predictions of a theoretical model for an ion-diocotron wave instability shows that the containment time is governed by the rate of ionization of the residual neutral gas (p > 10−7 Torr) in the apparatus. The correlation of theory and experiment appear to confirm the theoretical prediction that a significant degree of charge neutralization (in the range of 10-20%) can be stably contained. Interpretation of these results suggests that significant improvement in both electron cloud density and containment time should be possible.