&Bullet; physics 14, p41

A new way to measure an important property of electron beams is helping researchers prepare for next-generation synchrotron light sources.

G. Stancari / Fermilab

Scientists planning future particle accelerators and synchrotron light sources are striving for tighter and stronger electron beams. But the narrower the beams get, the more difficult it becomes to measure properties like size and angular divergence – important information for predicting their performance and accurately interpreting experimental results. Now, Ihar Lobach of the University of Chicago and colleagues have demonstrated a new way to measure the vertical emittance of a beam – an important parameter that represents the size and spread of the beam – more accurately than existing methods [1, 2] .

The team tested their method on the Integrable Optics Test Accelerator (IOTA) at Fermilab. In this system, a packet of electrons that circles a particle storage ring passes an undulator – a sequence of alternating magnetic fields that “wobble” the electrons back and forth and cause them to emit a pulse of radiation. Although the number of electrons circulating in the ring remains constant, variations in the positions and directions of movement of the electrons lead to tiny fluctuations in the radiation intensity from one ring to another.

Lobach and colleagues measured these fluctuations in intensity with a circuit that splits the radiation pulses and delays a branch by exactly one lap. This delay allowed them to subtract one intensity reading from another made during the successive round. They fed the resulting fluctuation values ‚Äč‚Äčinto a theoretical model that relates the radiation intensity fluctuations to the vertical emittance of the electron beam (and several other parameters) to determine the vertical emittance of the electron beam.

While the IOTA undulator generates infrared radiation, the researchers say a key feature of their technique is that it can be used with synchrotron light sources of any wavelength. This property is important because synchrotrons are widely used as X-ray sources for research in a number of fields from biology to materials science.

–Erika K. Carlson

Erika K. Carlson is Corresponding Editor for physics based in New York City.


  1. I. Lobach et al., “Crossbeam emittance measurement by undulator radiated power noise”, Phys. Rev. Lett.126, 134802 (2021).
  2. I. Lobach et al., “Undulator Radiated Power Noise Measurements and Comparison to from the beginning Calculations “, Phys. Rev. Accel. bar24, 040701 (2021).

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