Measuring the transverse RMS emittance and RMS pulse length of a short pulse, photoinjector produced electron beam with the second moment of its image charge

Radio frequency, photo-cathode injectors are a recent development in the electron accelerator community. They work by placing a small, photo-emissive surface inside a radio frequency accelerating cavity. Electrons are stripped from the photo-cathode with a pulsed laser and immediately accelerated by the cavity fields. These photoinjectors enable the creation of high charge, short pulse length beams. However, they also create problems for the electron beam diagnostics. A photoinjector accelerates the electrons to relativistic velocities very quickly. As a result, the beam does not have time to come to equilibrium. Its spatial distribution will be unknown and cannot be well approximated by a Gaussian. Therefore, diagnostic techniques can make no assumptions about the beam's spatial distribution. A class of diagnostics that fulfills this requirement look at the image charge "wake" generated in the metal walls of the beam pipe as the electron beam passes. These devices are generically known as beam position monitors and are normally used to measure the first moment of the image charge signal, thereby determining the position of the beam's center. However, coupled with a good knowledge of the beam line, they are also capable of determining the rms emittance of the beam by measuring the second moment of the image charge signal. In addition, when used in tandem with a deflecting cavity, beam position monitors can also be used to perform a measurement of the beam pulse length. Both of these measurements are independent of the beam's spatial distribution, making them ideal for photoinjectors. Described here is their theory and implementation.