2D and 1D surface photonic band gap structures for accelerator applications

First measurements of microwave radiation from a coaxial Free-Electron Maser (FEM), based on a two-mirror cavity, formed by 2D and 1D Surface Photonic Band Gap (SPBG) structures (input and output mirrors) with an intermediate coaxial waveguide are presented. The input mirror provides two-dimensional distributed feedback and ensures mode selection over the wave azimuthal index. The use of a 1D Bragg structure as an output mirror reduces the cavity Q-factor, improves the RF field profiles inside the cavity and increases the output power compared to FEMs based solely on 2D SPBG structures. The FEM has been driven by an oversized, high-current (1.4kA), thin annular electron beam of 200ns pulse duration. An output power of ∼60MW corresponding to an efficiency of ~10% was measured. The directional mode pattern of the microwave radiation launched from the output horn was also measured. Using cut-off filters the location of the operating frequency was found to lie between 35 GHz and 39 GHz. The generation of high power microwave pulses in the Ka-band can be adapted to test the breakdown strength of advanced accelerator structures such as CLIC. The possibility to "condition" and to compress the pulse using active 1D SPBG structures is presented. The results of preliminary experiments show that the band gap location can be manipulated by adjustment of the phase between the periodic perturbations of an active 1D SPBG mirror.