Abstract
We report 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 wakefield radiation which is caused by the passage of a relativistic electron bunch through a channel surrounded by a dielectric. We demonstrate numerically that the generated spectrum is determined by the rms bunch length, while the specific axial and longitudinal charge distribution is not important. Measurement of the millimeter-wave spectrum will determine the rms bunch length in the psec range. This has been done using a series of calibrated mesh filters and the charge bunches produced by the 50 MeV rf linac system at ATF (Accelerator Test Facility), 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 wakefield microstructure.
Highlights
We report 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 wakefield radiation which is caused by the passage of a relativistic electron bunch through a channel surrounded by a dielectric
There are a number of diagnostics which have been used to measure the axial length of relativistic electron bunches in the picosecond and sub-psec ranges, corresponding to lengths of 1000 m and less [1]
In this paper we describe a nondestructive diagnostic which can measure the rms bunch length using an observation of the frequency spectrum of wakefield radiation set up as the bunch passes through a vacuum channel in a hollow dielectric element [3]
Summary
There are a number of diagnostics which have been used to measure the axial length of relativistic electron bunches in the picosecond (psec) and sub-psec ranges, corresponding to lengths of 1000 m and less [1] Among these are coherent transition radiation, diffraction radiation, synchrotron radiation, energy modulation, spontaneous emission single-shot spectrum, and electro-optical detection. The dielectric can be made of alumina, which has low losses and good vacuum properties Inside this structure, the bunch emits coherent Cerenkov radiation, which can be extracted and detected externally. The power spectrum of these modes is a function that increases with frequency for radiation whose wavelengths exceed the bunch length, decreases with frequency for shorter wavelengths, and falls to negligible amplitude beyond a cutoff frequency This is a common feature for bunches radiating coherently, where the radiated power scales as Nb2, with Nb being the number of electrons in the bunch. We point out that a bunch-length diagnostic for laser accelerators, where the length is in the fsec range, would be useful too
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