An insulator for an ion source is positioned between the apertured ground electrode and apertured suppression electrode. The insulator has an elongate body having a first end and a second end, where one or more features are defined in the elongate body and increase a gas conductance path along a surface of the elongate body from the first end to the second end. One or more of the features is an undercut extending generally axially or at a non-zero angle from an axis of the elongate body into the elongate body. One of the features can be a rib extending from a radius of the elongate body.