A new high intensity cesium ion source and sample changer has been built and installed at PRIME Lab. Relative to the old ion source, all species tested to date have substantially greater currents. Although the ion source is based on the LLNL CAMS high-intensity design the PRIME Lab source differs in key aspects. The PRIME Lab source uses an oversized spherical ionizer (37% larger diameter than the size nominally used at most facilities). The immersion lens design is also different, necessitated by the difference in cathode transport. The extraction and focussing optics consist of an octopole steerer, two Einzel lenses, an x-y steerer placed between the Einzel lenses, and a pre-acceleration stage before the injector magnet. Tests performed on a “test-stand” validated the ion optics but indicated several inadequacies in the initial design. In particular, cycling cathodes with the cesium on resulted in greater degradation of the vacuum inside the source than tests done with the cesium off. Evidently, atmospheric volatiles adhering to the cesium on the rod were slowly outgassing into the vacuum chamber. To ameliorate this effect a gas box was added to isolate the rod from atmospheric gases. After installation of the ion source on the injector numerous other modifications have been made. These include redesigning the cesium transfer line, enlarging the exit hole in the ionizer, and altering the geometry of the cesium exit holes in the ionizer shroud. The sample changer is based on a Chalk River design. The goals in implementing this particular design were to minimize the effective memory of the source by only having one sample in the source at a time, and to provide a convenient means of changing samples without having to let portions of the source up to air for sample changes. Based on data taken during Cl runs it appears that the memory effects in the new source are substantially lower than in the old source. In the near future we anticipate other changes, including improvements to the vacuum and bar-coding of cathodes.
See more of Ion Sourcery (optics, modeling, performance, etc)
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)