ANL, Lemont, Illinois, USA
RadiaBeam, Santa Monica, California, USA
UCLA, Los Angeles, California, USA
Private Address, Silver Spring, USA
One of the advantages of relativistic electron beams with microbunching at UV to visible wavelengths is the potential to generate coherent optical transition radiation (COTR) at a metal foil for diagnostics purposes. A significant microbunching fraction of at least 10% is expected for the case of a seed laser at 257.5 nm copropagating with a 343-MeV electron beam through a modulator undulator (3.2 cm period) at the Linac Extension Area (LEA) at Argonne National Laboratory. Diagnostic plans have been made for the COTR based-characterization of the microbunched beam size (~100 microns), divergence (sub-mrad), microbunching fraction, spectrum, and bunch length (sub-ps), as well as coalignment of the laser pulse and electron beam as previously described**. For that case, COTR enhancements over OTR of more than seven million were calculated, and we expect a similar enhancement of coherent optical diffraction radiation (CODR). Thus, we propose the modification of the microbunching diagnostics station to support initial (CODR) experiments with beam transit through an aperture in a metal screen or near a metal edge at the second screen position of the interferometer. We would explore whether the coherence function for CODR provides a complementary beam size monitor and whether COTR and CODR interferences from the two interferometer screens provide divergence and pointing information.
**A.H. Lumpkin and D.W. Rule, Proceedings of FEL19, WEP041, Hamburg, Germany, Aug. 2019, https://JaCoW.org.
Fermilab, Batavia, Illinois, USA
One of the predicted signatures of successful optical stochastic cooling (OSC) in the Fermilab Integrable Optics Test Accelerator (IOTA) ring is reduction of the bunch length. The IOTA OSC experiment is designed for a low nominal beam current (~0.1 microAmps of 100-MeV electrons) to reduce intrabeam scattering (IBS), and during cooling, OSC is expected to reduce the bunch length from ~200 ps to ~130 ps*. These equilibrium bunch lengths can be measured using a streak camera and the optical synchrotron radiation (OSR) generated in a ring dipole by the circulating beam. A similar measurement was previously performed at the Advanced Photon Source with a Hamamatsu C5680 synchroscan streak camera operating at 117.3 MHz**. In this case, synchronous summing of OSR resulted in a bunch length measurement of 276 ± 30 ps using only 57 electrons circulating at 425 MeV. At IOTA, an existing streak camera has been modified to operate at the 11th harmonic of the ring’s revolution frequency of 7.50 MHz and has been installed on an OSR port in support of the OSC experiments. The integrated system will have sufficient sensitivity and resolution for measuring the evolution and equilibrium values of the bunch length during OSC experiments.
*V. Lebedev, J. Jarvis, H. Piekarz, A. Romanov, J. Ruan, M. Andorf, https://arxiv.org/abs/2012.09967
** A.H. Lumpkin and K. P. Wootton, TUPP18, Proc.of IBIC20 in https://JACoW.org.