An ionizer for forming a gas-cluster ion beam is disclosed including inlet and outlet ends partially defining an ionization region traversed by a gas-cluster jet and one or more plasma electron source(s) for providing electrons to the ionizing region for ionizing at least a portion of the gas-clusters to form a gas-cluster ion beam. One or more sets of substantially linear rod electrodes may be disposed substantially parallel to and in one or more corresponding partial, substantially cylindrical pattern(s) about the gas-cluster jet axis, wherein some sets are arranged in substantially concentric patterns with differing radii. In certain embodiments, the ionizer includes one or more substantially linear thermionic filaments disposed substantially parallel to the gas-cluster jet axis, heating means, electrical biasing means to judiciously bias sets of the linear rod electrodes with respect to the thermionic filaments to achieve electron repulsion.