Charge Breeding and Production of Multiply Charged Ions in EBIS and ECRIS

Doctoral thesis, 2001

The REXEBIS is an Electron Beam Ion Source (EBIS) developed for charge breeding of the exotic and sometimes short-lived isotopes that are produced at ISOLDE for the REX-ISOLDE post accelerator. Bunches of singly charged radioactive ions are injected into the EBIS and charge bred to a charge-to-mass ratio of approximately ¼, and thereafter extracted and injected into a short 3-stage LINAC for acceleration to a few MeV/u. This novel concept, employing a Penning trap to bunch and cool the ions from an on-line mass separator in combination with a charge breeding EBIS, should result in an efficient and compact system. The REXEBIS is based on a 0.5 A electron beam produced in the fringe field of a magnetic solenoid, and compressed to a current density of >200 A/cm2. The 2 T magnetic field is provided by a warm-bore superconducting solenoid, thus giving easy accessibility but no cryogenic pumping. The EBIS is switched between 60 kV (ion injection) and ~20 kV (ion extraction). This thesis presents the design and construction of the REXEBIS, together with initial commissioning results. A complete ion injection, breeding and extraction cycle for an EBIS has been simulated to certify high injection and extraction efficiencies. The entire EBIS was modelled using an ion-tracing program, SIMION 3D. Acceptance and emittance phase-spaces were determined for different source settings and ions types. Beam optics parameters such as: lens positions and voltages; accepted beam tilt and displacement tolerances at the focal points were settled. These simulations verified an analytically derived acceptance formula. General conclusions on acceptance, emittance and energy spread of an EBIS were drawn. The correlation between the two transverse emittance phase-spaces was shown to be small. Various experiments have been performed at an Electron Cyclotron Resonance Ion Source (ECRIS) in order to improve the ion yield. The source is operated in pulsed afterglow mode, and provides Pb27+ ions to the CERN Heavy Ion Facility on an operational basis. With the aim to reach higher beam intensities, the effect of a pulsed biased disk was investigated with different pulse structure and voltage settings. Various plasma electrode geometries were tested, including operating the source without a plasma electrode. The use of CF4 as mixing gas was explored, and high secondary electron emission materials were inserted inside the plasma chamber in an attempt to increase the cold electron density. No proof of absolute higher intensities was seen for any of these modifications. Nevertheless, the yield from a poorly tuned/low-performing source could readily be improved with bias voltage applied and the extracted pulse became less noisy. The fast response to the bias implies that increases/decreases are not due to ionisation processes. Finally, a design proposal utilising an EBIS for the ion pre-injector for the Large Hadron Collider (LHC) is presented. The LHCEBIS would produce lead ions, as well as lighter ions, that are directly injected into and further accelerated in an RFQ/LINAC arrangement. The source would operate with a repetition rate of 0.8 Hz, and the extracted yield is estimated to 1.6·109 Pb54+ per pulse. Using fast extraction, the extraction time can be less than 10 µs, possibly allowing single-turn injection into the PS Booster.