Non-Invasive Bunch Length Monitor

It is extremely important to understand the temporal characteristics of electron beams at accelerator photoinjectors. Measuring and setting the electron bunchlength is critical to the operation of the accelerator, and can ultimately determine if physics experiments at user-based accelerators can be successfully performed. Many parameters affect the temporal characteristics of electron bunches at the photoinjector including the drive laser optical pulse, the photogun bias voltage, beam current and bunch charge, the field strength of focusing magnets and phase and field strength of RF bunching and accelerating cavities. Optimizing these parameters can be a time consuming process, particularly during accelerator commissioning. Electrodynamic has been investigating a novel concept for a compact, non-invasive beam monitor. A prototype was recently developed and installed on the experimental beamline at the Thomas Jefferson National Laboratory. The staff of the center for injectors and sources (CIS) used the monitor to non- invasively characterize the electron bunches produced from thin and thick GaAs photocathodes inside a DC high voltage photogun. The photogun was illuminated with 35ps (FWHM) laser pulses at 1500 MHz. The photoguns average current was varied from 10uA to 500uA and accelerating voltage was varied from 75 kV to 195 kV. The monitor exhibited bandwidth sufficient to resolve detail in 57 ps electron bunches as well as bunches that were hundreds of picoseconds in duration. The bunch length monitor was connected directly to a sampling oscilloscope and provided signal within the normal operating range of a sampling head for the entire experiment. The monitor clearly distinguished electron beams with varying degrees of space-charge induced bunchlength growth and different tail signatures. The objective is to develop this concept into an economical beam-monitoring product to provide to the accelerator community that increases the detection resolution, hardware and software development, and side by side comparison with well accepted but invasive deflecting cavity technology.