Techniques and mechanisms applied in electron cyclotron resonance sources for highly charged ions

Abstract

Electron cyclotron resonance ion sources are delivering beams of highly charged ions for a wide range of applications in many laboratories. For more than two decades, the development of these ion sources has been to a large extent an intuitive and experimental enterprise. Much effort has been spent in theoretical work, but a consistent description still is not available. From experimental activities, scaling rules have been formulated, which have successfully been used for the construction of more powerful devices. Special techniques like the coating of the plasma chamber walls, usage of secondary electron emission materials, installation of a biased probe or disk, and mixing the supply gas with other gas species, are generally being incorporated for improving the output of highly charged ions. Various ideas to understand these mechanisms have been brought up, again without consistent description. In experiments, the effect of the techniques with respect to physical parameters, i.e., reducing the plasma potential and/or lowering the ion temperature, has been demonstrated. In a recent study, the requirement of charge neutrality in the fluxes from the plasma to the walls of the plasma chamber has been evaluated; this shows that the occurrence of Simon currents in the conducting walls plays an important role in determining the value of the self-adjusting plasma potential. Most of the special techniques do affect the Simon currents, and therefore the plasma potential, thus the confinement. The effect of the gas mixing technique is mainly (but not exclusively) to decrease the ion temperature, and by that to increase the confinement. The present state of understanding the various techniques will be reviewed.