Experimental-Observations Of Beam Transport In Twisted Quadrupole Fields

In the course of phase space matching an electron beam from guide field focussing into a twisted quadrupole (stellar- ator) strong-focussing channel, we have developed Cher- cnkov imaging techniques to measure the profile and density distribution of the beam. We have imaged a 200 A, 850 keV electron beam (average radius - 4 mm) as it propagates through the transition from guide field focussing into a stel- larator channel. With no matching lenses, the maximum ex- cursion in major radius of the beam in the stellarator channel was - 40%. First attempts at matching reduced this excur- sion to the 24% level. Significant damping of these mis- matched oscillations has been seen over the 2.5 meter length of the stellarator magnet channel. Emittance and profile data will be compared for the beam at the end of the channel with and without detailed matching.