To characterize the properties of the lens measurements using ions were performed. The conversion surface was replaced by a highly transparent grid. Because of the positive ions generated in the CASYMS ion source the lens voltages were reversed and the grid set to ground potential. O ions made out of CO were used as primary particles. O ions from 10eV to 450eV were used for the measurements. Using the x-table the entry slit of the CODIF was moved across the exit aperture of the extraction lens to investigate the primary energy focusing properties of the lens. Furthermore, the overall voltage over the lens was varied from 5kV to 15kV in order to find a combination of parameters with a high signal to noise ratio. Keeping the primary beam energy constant, the location of maximum intensity was shifted towards the low energy edge of the lens exit aperture when increasing the overall lens voltage. Two maxima were observed when varying the lens position (x-translation). The splitting into two maxima was probably caused by beam deflections at the grid. With 0.5mm spacing between the wires of the grid the path of a low energy ion (E < 100eV) might have been disturbed easily when getting close to the grid, resulting in particles leaving the grid on the inside at angles not representative of converted neutrals.
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland