Beam Emittance Measurements with a Dispersion-Matched Magnetic Spectrograph

The technique of dispersion-matching a beam to the split pole magnetic spectrograph at the MSU Cyclotron Laboratory was employed to eliminate the broadening of the beam spot size due to the coherent energy variations which come mainly from fluctuations of the amplitude of the cyclotron dee voltage. The incoherent emittance can therefore be measured directly. This was accomplished by observing the spot size in the focal plane of the spectrograph with a high magnification television system and by measuring the beam divergence angle with a simple intercepting probe placed upstream. The emittance for proton beams was in agreement with previous indirect measurements. The luminosity of a proton beam was approximately equal to that inferred from previous d.c. test stand measurements. Measurements for 3He2+ and 12C4+ beams are also presented. The contribution to the divergence angle by the coherent energy spread was found to be negligible, in agreement with calculation. The correlation of observed line width with ion source slit width suggests that the complete optical system from ion source to the spectrograph is linear, and that an image of the ion source slit was observed in the spectograph focal plane.