Experimental observations of beam transport in twisted quadrupole fields

The authors have developed Cerenkov imaging techniques to measure the profile and density distribution of the beam. They have imaged a 200-A, 850-keV electron beam (average radius approximately 4 mm) as it propagates through the transition from guide field focusing into a stellarator channel. With no matching lenses, the maximum excursion in major radius of the beam in the stellarator channel was approximately 40%. First attempts at matching reduced this excursion to the 24% level. Significant damping of these mismatched oscillations has been seen over the 2.5-m length of the stellarator magnet channel. Emittance and profile data are compared for the beam at the end of the channel with and without detailed matching.<>