Proto-I axial focusing experiments

The time-integrated axial (z) focus of the 4.5cm-radius Proto I (1.5 MV, 500 kA) radial proton diode is presently limited to ∼3 mm FWHM. This result is obtained with current neutralized beam transport in a gas cell with 6 Torr argon. If the vertical local divergence were the same (1° or less) as the horizotal divergence, the local divergence alone would produce a 1.5 mm FWHM focus. The axial focal size is evidently limited by time-dependent effects. These are studied by observing the beam incident upon various targets with two time-resolved pinhole cameras. The first camera observes Rutherford-scattered protons from gold targets with an array of 11 silicon PIN detectors. The second camera observes Kα-florescence from aluminum targets with 4 independently-gated microchannel plates imaging tubes.