Ionizer for cluster ion accelerators

The design and performance of a cluster ionizer of the Bayard--Alpert type which is intended to be used in cluster accelerators is described. Positive cluster ions are formed from neutral hydrogen clusters with an efficiency of up to 80%. The mean size of clusters can be reduced by an order of magnitude by multiple ionization followed by splitting into charged fragments due to the repulsion between the individual charges. The fraction of the electrical current carried by cluster ions exceeds 90% of the total ion current. At low particle density two modes of operation are found. The ''normal'' mode A with high efficiency is characterized by space charge compensation of the primary electrons. In mode B this compensation is missing and the efficiency drops to less than 20% of that of mode A