The SPES laser ion source: Time structure, laser enhancement and efficiency measurements with gallium at ISOLDE Offline 2

The SPES laser ion source has been tested at ISOLDE Offline 2, CERN. A two-step single resonance photoionization scheme has been used to ionize gallium atoms in the SPES tantalum hot -cavity ion source. The ion beam time structure, laser enhancement of ion yield, and ionization efficiency are investigated in relation to the ion source temperature and ion load. From the time structures, it is inferred that a significant fraction of the extracted ions are generated in the transfer line rather than just in the hot cavity. The effect of the electrostatic axial field on the movement of ions inside the ion source is discussed. Generally, there is an inverse relationship between total ion load and the laser enhancement factor. This dependency is enhanced at lower operating temperature of the ion source. This is explained by the influence of thermionic electron emission and ion density on the transverse laser -ion confinement, and therefore the survival of ions as they drift towards the extraction region of the ion source. At 2200 C, the nominal temperature for on-line operation of the ion source, the ratio of laser -ionized to surface -ionized gallium was stable around 55 during the measurement campaign, and independent of the total extracted ion current up to the measured value of 1.1 mu A. A resonance laser ionization efficiency value of 27.2% for gallium has been measured.