A Nondestructive Method for Measuring the RMS Length of Charge Bunches Using the Wake Field Radiation Spectrum

Abstract

We report progress in the development of a nondestructive technique to measure bunch rms‐length in the psec range and below, and eventually in the fsec range, by measuring the high‐frequency spectrum of wake field radiation which is caused by the passage of a relativistic electron bunch through a channel surrounded by a dielectric. We demonstrate both experimentally and numerically that the generated spectrum is determined by the bunch rms‐length, while the choice of the axial and longitudinal charge distribution is not important. Measurement of the millimeter‐wave spectrum will determine the bunch rms‐length in the psec range. This has been done using a series of calibrated mesh filters and the charge bunches produced by the 50MeV rf linac system at ATF, Brookhaven. We have developed the analysis of the factors crucial for achieving good accuracy in this measurement, and find the experimental data are fully understood by the theory. We point out that this technique also may be used for measuring fsec bunch lengths, using a prepared planar wake field microstructure.