Overview and Issues of Experimental Observation of Microbunching Instabilities

The generation of the ultra-bright beams required by modern accelerators and drivers of free-electron lasers (FELs) has generally relied on chicane-based bunch compressions that often result in the microbunching instability. Following compression, spectral enhancements extend even into the visible wavelengths through the longitudinal space charge (LSC) impedances. Optical transition radiation (OTR) screens have been extensively used for transverse electron beam size measurements for the bright beams, but the presence of such longitudinal microstructures (microbunching) in the electron beam or the leading edge spikes can result in strong, localized coherent enhancements (COTR) that mask the actual beam profile. Generally, we have observed effects in rf photocathode (PC) injected linacs with chicane compressions since an R56 term is needed. In the past COTR had been only reported in S-band and L-band photoinjected based linacs with single or double bunch compression. Drive laser modulations and charge shot noise have been suspected of contributing to the cause. We now have evidence for the effects in both rf PC-gun injected linacs and thermionic- cathode (TC)-gun injected linacs (the latter do not involve a drive laser). Since the first observations, significant efforts have been made to characterize, model, and mitigate COTR effects on beam diagnostics. An update on the state-of-the-art for diagnosing these effects will be given as illustrated by examples at the Linac Coherent Light Source (LCLS), Spring-8 Compact SASE Source (SCSS), Spring-8 Angstrom Compact Free Electron LAser (SACLA), Advanced Photon Source (APS), and the Next Linear Collider Test Accelerator (NLCTA). These observations continue to be of interest to the accelerator community.