Emittance growth in a low energy proton beam

We have measured emittances in a low energy proton beam at energies between 19 and 45 keV and currents between 9 and 39 mA. The RMS emittance of the space charge dominated proton beam, as measured by a moving-slit emittance probe grew by an average amount of 60% in a propagation distance of 2.5 cm. An Abel inversion procedure was applied to the measured transverse charge distribution of the proton beam in order to calculate the electrostatic field energy, which is the driving quantity for emittance growth. We have found that all of the emittance growth is due to a halo containing ≃10% of the beam particles