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

Abstract

Generally, turn-to-turn fluctuations of synchrotron radiation power in a storage ring depend on the 6D phase-space distribution of the electron bunch. This effect is related to the interference of fields radiated by different electrons. Changes in the relative electron positions and velocities inside the bunch result in fluctuations in the total emitted energy per pass in a synchrotron radiation source. This effect has been previously described assuming constant and equal electron velocities before entering the synchrotron radiation source. In this paper, we present a generalized formula for the fluctuations with a non-negligible beam divergence. Further, we corroborate this formula in a dedicated experiment with undulator radiation in the Integrable Optics Test Accelerator (IOTA) storage ring at Fermilab. Lastly, possible applications in beam instrumentation are discussed.

Transverse beam emittance measurement by undulator radiation power noise

“…Primary sources of this noise were the integrator's op-amp and the oscilloscope's pre-amp. In [21], we showed that this noise level was independent of N via measurements with an independent test light source. Therefore, it can be simply subtracted.…”

“…where k is the magnitude of the wave vector, θ x and θ y represent the horizontal and vertical angles of the direction of the radiation in the paraxial approximation, C is a constant. Then [10,21]…”

“…The simulated average number of detected photons per pass, per one electron of the electron bunch was 9.1 × 10 −3 . The empirical value was 8.8 × 10 −3 [21]. There were no free adjustable parameters in this calculation.…”

“…where χ = 2.08 × 10 7 photoelectrons/V, with a 5 % uncertainty, as per the characteristics of our integrator and the photodiode [21]. The op-amp was capable of driving the 50-Ω input load of a fast digitizing scope, located ≈ 100 m away.…”

“…Therefore, it can be simply subtracted. Reference [21] also describes the details of the photocurrent integrator and the comb filter.…”