Knowledge Bank

The Fourier transform spectrometer (FTS) is most commonly used to obtain frequency domain information about a source or interposed media. We have used a FTS as a submillimeter-wave autocorrelator, measuring the field associated with the mode locked electron beam spectra of a free electron laser (FEL) and other relativistic picosecond bunched electron beam (RPBEB) systems. Because the field associated with the RPBEB is directly related to the electron bunch duration and shape, the FTS measurements yield a non-destructive means of characterizing bunch parameters with sub-picosecond resolution. In this work, the FTS is of conventional design, however, because the electron bunches are produced in trains (individual bunches are spaced at the RF accelerator period of $\sim 350$ ps and trains are of microsecond length) we use a fast bolometer to resolve bunch differences at either end of the trains. Also we have measured real-time changes of the FEL electron bunches due to RPBEB system adjustments and long term changes due to system modifications. With the current FTS system we routinely characterize electron bunches of 1.3 ps duration.