The design of narrow-band Compton scattering sources for specific applications using nuclear resonance fluorescence (NRF) is presented. NRF lines are extremely narrow (ÁE=E $ 10 À6 ) and require spectrally narrow sources to be excited selectively and efficiently. This paper focuses on the theory of spectral broadening mechanisms involved during Compton scattering of laser photons from relativistic electron beams. It is shown that in addition to the electron beam emittance, energy spread, and the laser parameters, nonlinear processes during the laser-electron interaction can have a detrimental effect on the gamma-ray source bandwidth, including a newly identified weakly nonlinear phase shift accumulated over the effective interaction duration. Finally, a design taking these mechanisms into consideration is outlined.

In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at X band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of 65 MV=m yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above 110 MV=m was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of 100 MV=m and a surface magnetic field of 890 kA=m. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electric field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about 14 MV=m, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.

Frequency-locked Smith-Purcell radiation (FL-SPR), generated by a train of electron bunches traveling above a grating, is characterized by a broad range of frequencies which are locked to the train frequency in a discrete comb and are spatially dispersed in space. We report absolute-scale power measurement of FL-SPR in the millimeter wave range. A 50 ns long train of 170 m electron bunches was produced by a 15 MeV, 17 GHz accelerator with 80 mA of average current. The grating had 20 periods spaced by 2.54 mm. The experimental results were compared, on an absolute scale, with the electric-field integral equation model which takes into consideration the finite length of the grating. Very good agreement was obtained. The present results should be useful in planning SPR applications such as diagnostics of electron bunch length on the femtosecond scale and coherent THz radiation sources.