Coherent Transition Radiation Interferometry

A now standard way of measuring short beam pulses is by the analysis of the spectrum of some coherent radiation, with coherent transition radiation (CTR) is now the most used. This is not a single-shot measurement, and a beam profile can hardly be obtained (autocorrelations are not invertible), but the technique is well established and can be more robust and accurate as one makes the bunch shorter. Coherent transition radiation (CTR)-based measurements have shown sub-ps resolution, as illustrated by the data from Neptune Laboratory and PLEIADES velocity bunching data shown below. These experiments are at the limit of resolution of the Martin-Puplett polarizing interferometer, which is dictated in scale by the spacing of the polarizing wire grid (100 microns in this case). UCLA PBPL is a leader in these types of measurements; we are known for developing a time-domain fitting procedure which allows the missing (diffracted) long wavelengths components of the CTR to be accounted for

Autocorrelation scan from CTR emitted in UCLA Neptune lab velocity bunching experiment, using short, high-gradient standing wave linac as buncher.

Autocorrelation scan from CTR emitted in LLNL/UCLA PLEIADES velocity bunching experiment, using long, low-gradient traveling wave linac sections.

In order to improve upon this resolution, as is needed at ongoing UCLA Neptune, UCLA/LLNL, and UCLA/BNL ATF experiments, a new design for the interferometer has been adopted, as shown below. This new design has been developed by UCLA and its collaborator Prof. Uwe Happek of Univ. Georgia. It is a simple, compact Michelson, using partially reflective, <10 micron thick beam splitters. It should have a resolution one order of magnitude smaller than that of the polarizing scheme. In addition, it has a much smaller footprint, and is commensurately less expensive, because of size, fewer optics, and less cost in the beam splitters.

Compact CTR Michelson interferometer for high resolution (sub-10 micron) bunch length measurements (courtesy U. Happek).