Nonlinear subpicosecond electron-bunch compressor

The minimum bunch length achievable using a magnetic bunch compression system is related to the nonlinear dispersion of the compressor and the nonlinear energy-phase correlation of the bunchʹs longitudinal phase space. For electron bunches with medium or high charge (on the order of 1 nC, or higher), the longitudinal wakefields in the accelerating structure cause a large nonlinearity in the initial energy-phase correlation. Specific nonlinearities in the dispersion of the bunch compressor must then be introduced in order to maximally compress the bunch length, which is done by a chicane-style compressor by increasing the bend angle. This nonlinear dispersion actually decreases the rms longitudinal-beam emittance, by compensating for the nonlinearities in the beamʹs energy-phase correlation. In this paper, we report on the compression theory, design, and operation of such a compressor. For this compressor, introducing nonlinear dispersion reduced the minimum compressed bunch length by about a factor of 5, resulting in measured FWHM bunch lengths of 23ps for bunch charges as large as 1 nC.