Compact noninvasive electron bunch-length monitor

A compact rf cavity was constructed that simultaneously resonates at many harmonic modes when excited by a repetitive bunched electron beam passing through its bore. The excitation of these modes provides a Fourier description of the temporal characteristics of the bunch train. The cavity was used to noninvasively characterize electron bunches produced from thin and thick GaAs photocathodes inside a DC high voltage photogun illuminated with 37 ps (full width half maximum, FWHM) laser pulses at repetition rates near 1500 MHz, at average beam current from 5 to $500\mu \mathrm{A}$, and at beam energy from 75 to 195 keV. The cavity bunch-length monitor could detect electron bunches as short as 57 ps (FWHM) when connected directly to a sampling oscilloscope, and could clearly distinguish bunches with varying degrees of space-charge induced growth and with different tail signatures. Efforts are under way to detect shorter bunches by designing cavities with increased bandwidth. This demonstration lends credibility to the idea that these cavities could also be used for other applications, including bunching and shaping, when driven with external rf.

Figure 1

The TM modes of the cavity bunch-length monitor from ${\mathrm{TM}}_{010}$ through ${\mathrm{TM}}_{080}$.

Figure 2

The cavity bunch-length monitor during construction.

Figure 3

The DC high voltage load-locked GaAs photogun and diagnostic beam line. The cavity bunch-length monitor was positioned near the beam dump. Besides the labeled components, the beam line consisted of valves, vacuum pumps, view screens, beam position monitors, a wire scanner, steering and focusing magnets.

Figure 4

Oscilloscope traces of (a) the drive laser optical pulse train, (b) the cavity bunch-length monitor, and (c) the FFT of signal from the cavity bunch-length monitor together with an overlaid Gaussian fit.

Figure 5

(a) The spacing of the positive and negative peaks as a function of beam current and photogun bias voltage. (b) The sensitivity of the cavity bunch-length monitor as a function of average beam current and photogun bias voltage.

Figure 6

Electron bunches from thin (100 nm) strained-layer superlattice GaAs/GaAsP photocathodes. The left side images show oscilloscope traces and the right side images show postprocessed FFTs, for 75 kV bias voltage (b) and 175 kV bias voltage (d) at three currents.

Figure 7

Comparison of bunches from thick GaAs (blue traces) and thin strained-layer superlattice GaAs/GaAsP photocathodes (red traces) at 175 kV bias voltage and at three currents: (a) $25\mu \mathrm{A}$, (b) $250\mu \mathrm{A}$, and (c) $500\mu \mathrm{A}$.