Measuring the RMS length of short electron pulses with an RF cavity and a beam position monitor

The Sub-picosecond Accelerator at Los Alamos National Laboratory is a 1300 MHz, 8 MeV photoinjector. Its beam can be bunched to sub-picosecond lengths using a magnetic chicane. To observe this, we use an RF cavity operating in a TM₁₁₀ mode to generate a transverse magnetic dipole field that “streaks” each bunch as it passes. Inserting a screen downstream, we determine the pulse length by measuring the increase in the beam spot width. To achieve sufficient resolution two things must happen: first, the unstreaked beam must be well focused at the screen, and, second, the drift after the cavity must be long enough. At high charge and short pulse lengths, these goals become mutually exclusive. To eliminate this problem, we propose using a beam position monitor instead of a screen. The second moment of the beam position monitor signals determines the difference between the x and y rms widths of the beam. With a properly orientated cavity, the resulting change in this quantity also gives the pulse length. However, the BPM does not require a well focused beam at its position, eliminating that constraint on resolution