Optical Cerenkov measurements of emittance growth in the Advanced Test Accelerator beam under laser ion-guiding

A new beam profile diagnostic using optical Cernekov radiation from a quartz foil has been deployed at the Advanced Test Accelerator (ATA). Quartz performs better than previously tested foils (titanium and Kapton) in terms of damage threshold. The emission intensity from quartz is over an order of magnitude brighter than that from graphite foil. Data were taken at the beginning, middle, and end of ATA, i.e. at 3.8 MeV (cell 15), 22.5 MeV (cell 90), and 43 MeV (11 m beyond the end). Profile measurements of the ion-focused beam showed emittance growth through the pulse. Double-Gaussian fits to the radial profile were often superior to single-Gaussian fits and showed that a large beam dominated the second half of the pulse. This was observed for currents of 1, 3, and 7 kA at the accelerator output and at cell 15 for the 3-kA case. The dependence of emittance growth on several parameters was studied.<>