Studies of Space Charge Neutralisation in Intense 500 keV Proton Beams

Some aspects of the transport of intense 500 keV proton beams have been investigated in the CERN 3 MeV experimental linac. Results are presented for the current transmitted through a known set of restricting apertures. These currents are considerably higher than the theoretical values for a uniform density zero emittance beam. To explain these results in regions where the pressure is too low (10-6 torr) for the wellknown gas neutralisation mechanism, we propose that the beam is partially neutralised by secondary electrons produced from metallic surfaces in the beam pipe. Using a specially designed apparatus which enables electrons to be removed from the beam over a substantial fraction of its path we find that the measured currents are more consistent with an unneutralised beam. In addition this apparatus has enabled us to determine a very high value for the secondary electron emission coefficient for proton impact near glancing incidence. Finally the possible perturbing effect of secondary electrons on emittance measurements in our low energy drift space is discussed.