Transverse beam emittance measurement by undulator radiation power noise

Abstract

Generally, turn-to-turn power fluctuations of incoherent spontaneous synchrotron radiation in a storage ring depend on the 6D phase-space distribution of the electron bunch. In some cases, if only one parameter of the distribution is unknown, this parameter can be determined from the measured magnitude of these power fluctuations. In this Letter, we report an absolute measurement (no free parameters or calibration) of a small vertical emittance (5-15 nm rms) of a flat beam by this method, under conditions, when it is unresolvable by a conventional synchrotron light beam size monitor.

Measurements of undulator radiation power noise and comparison with $\textit{ab initio}$ calculations

“…This is why we use Eq. ( 2) in our Letter [24], focused on emittance measurements via fluctuations, as opposed to [5, Eq. ( 49)], which is simpler, but neglects beam divergence.…”

“…In this case, the systematic error of the y measurement via the fluctuations, related to the uncertainty on the beam energy, would be negligible. Moreover, it can be shown [24] that if one places a narrow vertical slit in front of the detector, then the magnitude of the fluctuations would only depend on y , i.e., the systematic error of the y measurement related to the uncertainty of x , measured by the SLMs, would be minimized, too. Finally, for a brighter undulator, the statistical error on the measured value of the fluctuations would be lower as well.…”

“…Finally, we proposed a non-invasive technique to measure the small unknown vertical emittance of a flat beam via the fluctuations. This new technique is described in detail in a separate publication [24].…”

“…Taking beam divergence into account is critical in this specific measurement. For more results regarding beam diagnostics via fluctuations in IOTA, we refer the reader to our separate publication [24].…”