Focussing of an H[sup [minus]] beam by a gridded Einzel lens

The focussing of an H[sup [minus]] beam by a gridded Einzel lens has been studied at beam currents of 20--40 mA at 25 kV and 30 kV, with and without adding argon to the vacuum vessel as a neutralizing gas. The lens consisted of a central cylinder (6 cm long [times] 6 cm ID) sandwiched between two ground rings, each filled with a 92% transparent tungsten mesh. A negative high voltage was applied to the central cylinder to focus the beam. The beam diameter and divergence at the input to the lens were varied by changing the beam perveance in two ways: by varying the discharge power, and by varying the bias voltage on the plasma electrode of the source. Emittance and spot size were measured about 9.3 cm from the center of the lens. Emittance growth through the lens with and without the argon will be discussed. Introducing the argon produced a parallel beam core whose extent depended on the lens voltage.