The influence of hole-coupling on the transverse structure of intra-cavity light pulses

The important issue addressed is the spatial structure of the light pulse inside the cavity of a hole-coupled resonator and the fraction of power lost through the aperture. The radial structure of the radiation is determined by the competition between power loss and scattering at the aperture and by the gain due to the interaction with the electrons. The additional effect of slippage couples the transverse and longitudinal profiles of the light pulse. It is shown that, at the back of the pulse, gain and gain-focussing are dominant. The front of the pulse propagates mainly in vacuum. The on-axis field intensity in this region is reduced by scattering. The influence of these mechanisms on the intra-cavity field structure is analyzed using a model that takes the time-dependent spatial dependence of the optical pulse into account. The phenomena are demonstrated numerically for FEL parameters close to those of FELIX.