Beam Electron Microprobe A charge density monitor based on sensing a small deflection of a probe beam by electric field of the charge distribution has been tested. Results of beam runs using stored and electron-cooled protons in the Indiana Cooler ring in the period March-June 1998 have demonstrated the excellent spatial resolution of this non-destructive profile, position and intensity monitor. The electron beam has an rms width of 0.2 mm, sufficient to measure the width of well-cooled protons beams at a tight waist.
The following figure (22k jpg file) shows the transverse distribution of charge density of a 198 Mev, 308 uA coasting proton beam. The horizontal and vertical scales are in mm while the charge density is in uC/m^3. The rms width of the proton distribution in the vertical plane is 0.4 mm. The elongation in the ring plane may be caused in part by a low energy tail in combination with the 4 m. momentum dispersion at the point of observation.
In addition to the wide raster scan mode used to obtain the above figure, the probe beam may be scanned rapidly over a narrow range within the central density peak. Lock-in detection methods then yield a continuous record of density changes with time. This feature has been used to study time evolution of the central density from cooling following a kick-induced emittance growth.
This monitoring method has possible future application to radial distributions of non-neutral plasmas.
A short report on the properties of this diagnostic instrument has been submitted to Nuclear Instruments and Methods. A more complete exposition will be found in a PhD dissertation now in preparation by D. Stoller