Non-invasive energy spread monitoring for the JLAB experimental program via synchrotron light interferometers

The hypernuclear physics program at Jefferson Lab [JLAB] requires a tight upper limit on the RMS beam energy spread of σE/E<3×10-5. The energy spread is determined by measuring the beam width at a dispersive location (D∼4m) in the transport line to the experimental halls. Ignoring the intrinsic beam size, this low energy spread corresponds to an upper bound on the beam width of σbeam<120μm. Such small beam sizes cannot be measured using direct imaging of the synchrotron light due to diffraction limitations. Using interferometry of the synchrotron light the resolution of the optical system can be made very high. The non-invasive nature of this measurement is also very advantageous as it allows continuous energy spread monitoring. Two synchrotron light interferometers have been built and installed at Jefferson Lab, one each in the Hall-A and Hall-C transport lines. The two devices operate over a beam current range from 10 to 120μA and have a spatial resolution better than 10μm. The structure of the interferometer, the experience gained during its installation, beam measurements and energy spread stability are presented.