The ultimate goal of this project is to experimentally demonstrate the applicability of photonic band gap (PBG) accelerator cells for wakefield suppression room-temperature high-energy accelerators of the future. A PBG structure or simply, photonic crystal, represents a periodic lattice of macroscopic components (e.g., rods), metallic, dielectric or both. One can design and construct photonic crystals with photonic band gaps, preventing light of certain frequencies from propagating in certain directions. A "PBG cavity" is formed by removing a rod in the periodic structure. The mode with a frequency in the band gap cannot propagate out transversely through the bulk of the PBG structure and will thus be localized inside the PBG cavity around the defect.

The field pattern of the mode can be made to closely resemble the field pattern of the TM01 accelerating mode of a pillbox cavity, and the cavity can be employed as an accelerator cavity. However, the modes with frequencies outside of the band gaps (such as the higher order wakefield modes in accelerator cavities) will be free to propagate through the PBG structure. In this regard, the PBG resonator intrinsically acts as an extremely efficient higher order mode (HOM) coupler.

The ways to fabricate PBG accelerating structures (Figure 1), their performance at high gradients, and demonstration of the wakefield suppression are currently being investigated by the scientists at Los Alamos National Laboratory.

Technical Progress

LANL collaborates with a major US superconducting radio-frequency (SRF) structures manufacturer, Niowave, Inc. to develop procedures for fabrication of superconducting photonic band gap structure cells. The cells are being designed at LANL. Niowave manufactures resonators according to LANL's specifications. Upon fabrication, the resonators (Figure 2) are being shipped to LANL for high power testing. A unique SRF laboratory is being operated by the AOT division at LANL and allows vertical testing of the superconducting rf structures up to 48 inches in diameter and the temperatures of 4 Kelvin and below. Gradients as high as 18 MV/m have been recently demonstrated in SRF PBG resonators by the LANL team. Horizontal testing of PBG structures with a high current electron beam is in the near term plans.

LANL also collaborates with the Argonne National Laboratory (ANL) to conduct the tests of the room-temperature copper photonic band gap resonators at X-band (11 GHz). LANL designs the resonators at the frequency of 11.700 GHz. Once manufactured, the resonators are undergoing an extensive tuning procedure at LANL's MST division. Once tuned and tested, the structure is on the way to be shipped to ANL to be tested at their Argonne Wakefield Accelerator (AWA) DOE user test facility. A very high charge 100 nC electron bunch will be passed through the structure and the level of wakefields will be recorded and expected to be very low compared to conventional copper resonators.