A Simple Variable Focus Lens for Field-Emitter Cathodes

We present the design and initial test results for a simple, variable-focus solenoidal lens with integrated emittance filtering. The design was developed as a first-iteration injection optics solution for transport of a beam from a field-emitter cathode into a dielectric laser accelerator structure. The design is easy to fabricate and, while based on permanent magnets, can be readily modified to allow for remote control of the focal length. The emittance is controlled via a selection of collimating irises. The focal length can be changed by altering the spacing between the two permanent ring magnets. This allowed us to focus a 1.6 μA beam to a 10 μm spot size.