Performance of an expanding telescope used to obtain a low divergence H- beam

An expanding telescope constructed by Los Alamos National Laboratory was installed at Argonne National Laboratory to produce a low divergence, 50 MeV H- beam. The telescope with a twenty times magnification of a singlet eyepiece and triplet objective lens constructed from permanent quadrupole magnets. The output beam divergence was measured with a rms resolution near 1 μrad using a pinhole membrane and an optical imaging system. This system also provided a measure of the third order geometric aberrations of the telescope by the displacement of the centroids in the projected image of a rectilinear array of pinholes placed across the beam diameter. The geometric aberrations were found to be in agreement with computer calculations, having magnitudes on the order of a few μrad/cm3. Chromatic aberrations in the telescope were also found to be in agreeemnt with theory. These measurements showed that the telescope achieved a nearly parallel beam with a divergence of less than 25 μrad. (orig.)