Beam Profile Measurement at the Entrance of the Gas-Filled Solenoid GASSOL

Abstract A beam profilometer has been developed and tested for use at the entrance of the gas-filled solenoid GASSOL as it is put into operation and adjustment. The profilometer is made as a mosaic of cathodes. The current generated by the secondary emission during bombardment by accelerated ions is measured from each cathode. A corresponding number of current amplifier channels is used to measure the currents from the cathodes. A software program has been developed for visualizing the beam profile. During an experiment with an 40 Ar +16 ion beam at an energy of 4 MeV per nucleon, the lower sensitivity limit of the profilometer was found to be below 0.001 pnA. The upper sensitivity limit, determined with thermal calculations, was established to be 100 pnA. One critical factor is the risk of signal cable contact failures as a result of heating. The accuracy of determining the beam centre is ±1 mm, and this value lies within the specified tolerance limits for radial displacement of the beam axis relative to the solenoid magnetic axis. The use of two spatially separated profilometers makes it possible to control the alignment of the beam and solenoid axes. When operating in the gas medium, the profilometer can also function as an ionization chamber, in addition to operating via secondary emission. The simplicity of its design enables the fabrication of profilometers in a wide range of sizes and with various numbers of cathodes.