The Cherenkov light based spectrometer will measure the acceleration the beam experiences due to the plasma wake fields.
A spectrometer magnet bends the particles according to their energy before they exit the vacuum chamber and enters air (from the pipe on the left of the picture).
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.