DPB Dissertation Award (2022): Statistical Properties of Undulator Radiation: Classical and Quantum Effects

Two experiments were carried out to study the statistical properties of undulator radiation in the Integrable Optics Test Accelerator (IOTA) storage ring at Fermilab. The first experiment studied the turn-to-turn fluctuations in the power of the radiation generated by an electron bunch (1--3 billion electrons). 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 of the total emitted energy per pass. Generally, these turn-to-turn fluctuations depend on the full 6D phase-space distribution of the electron bunch. Therefore, the measured fluctuations can be used to infer some electron bunch parameters. Bunch lengths had been measured by this method previously. Our experiment in IOTA revealed the possibility to measure transverse emittances of electron bunches. This non-invasive diagnostic technique may be particularly beneficial for the existing and next-generation low-emittance high-brightness ultraviolet and x-ray synchrotron light sources. The second experiment studied the photon statistics of the undulator radiation generated by a single electron circulating in the ring. In this regime, any classical interference-related collective effects were eliminated, and the quantum fluctuations could be studied in detail. On average, there was only one photocount per several hundred revolutions in IOTA. The collected data were analyzed to find possible deviations from the expected Poisson process exhibiting uncorrelated detection events. In addition, the arrival times of the photocounts were used to track the longitudinal motion of a single electron and to compare it with simulations. This allowed us to determine several dynamical parameters of the storage ring such as the phase jitter of the radiofrequency cavity and the dependence of the synchrotron motion period on amplitude.