Transverse Beam Emittance Measurement by Undulator Radiation Power Noise

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.

Figure 1

(a) Layout of IOTA; electrons circulate clockwise. (b) Light path from the undulator to the detector (not to scale). (c) Block diagram of the detection circuit; $i$ is the IOTA revolution number. The delay in the comb filter equals exactly one IOTA revolution, $133.3\mathrm{n}\mathrm{s}$. Hence, its $\mathrm{\Delta }$ channel provides a signal difference between two consecutive turns.

Figure 2

Panels (a) and (b) show the measured fluctuations for the round and flat beams, respectively. The statistical error of each point is $2.7\times {10}^6$ (not shown). (c) The round-beam mode emittance $\epsilon$, determined via SLMs, via undulator radiation fluctuations, and via Touschek lifetime, assuming the effective momentum acceptance $2.0\times {10}^{-3}$. (d) The flat-beam horizontal emittance measurement via SLMs (left-hand scale), the vertical emittance measurement via fluctuations and via Touschek lifetime (right-hand scale). The SLMs had a monitor-to-monitor spread of $\pm 8\mathrm{n}\mathrm{m}$ (round beam) and $\pm 50\mathrm{n}\mathrm{m}$ (horizontal emittance of flat beam); these error bars are not shown. All emittances are rms, unnormalized.