The Cherenkov light based spectrometer will measure the acceleration the beam experiences due to the plasma wake fields.
It is situated on the optical table after the FACET beam pipe ends and before the FACET dump.
A spectrometer magnet bends the particles according to their energy before they exit the vacuum chamber and enter air.
As the ultra-relativistic electrons travel through air, faster than the local speed of light, they emit Cherenkov radiation in the optical spectrum.
The radiation is reflected on mirrors and is registered on a digital camera, rendering a precise picture of the beam distribution and of the energy gain in the plasma.
This diagnostic is available for other experimenters to use in support of their studies. Please note for anything downstream of the plasma chamber, it is not really possible to get imaging conditions in both planes without lowering the beam energy. It is possible to image in the y plane with nominal beam energy.
Please see the paper:
Cherenkov light-based beam profiling for ultrarelativistic electron beams
(E. Adli, S.J. Gessner, S. Corde, M.J. Hogan, H.H. Bjerke)