Extraction of an ion beam from a laser ion source

• Main Author: Scrivens, R. M.
• Published: Swansea University 1999
• Subjects: 535
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638785

The CERN Laser Ion Source (LIS) aims to provide a short pulse (˜5 μs), high current (˜ 10mA) and high charge state heavy ion beam (Pb<SUP>25+</SUP>) for acceleration by a LINAC and injection into the Proton Synchrotron Booster (PSB). The laser beam time profile was measured using detectors with time response of the order of 1 ns. Cross correlating of the laser pulse form and the ion beam current one fails to find any significant relation. The laser spatial profile was measured in the focal plane by a Spiricon pyroelectric camera in conjunction with a long focal length lens. In the Master Oscillator and Power Amplifier laser configuration, the beam was found to be astigmatic and exhibit a large pointing instability. The free-running laser produced a beam in good agreement with the simulation of beam propagation along the 30 m path extension and was also astigmatic. As for the time profile, no firm correlation of the laser spatial profile fluctuation and the ion beam instability could be found. Within the framework of this thesis, a critical study has been performed on whether shot-to-shot instabilities are being caused by fluctuation in the laser beam parameters (time profile, spatial distribution and energy) as well as the extraction of the ions from the expanded laser plasma to form an ion beam. In addition, a technique for the calculation of absolute ion numbers was derived. The extraction of the ions from the laser ion source plasma was experimentally studied using Faraday cup collectors, and a compact single shot emittance measurement system. The extraction of the ions was found to be correctly modelled by the Child-Langmuir equations for charge extraction, with some modification necessary to account for the initial significant ion velocity (˜10<SUP>5</SUP> ms<SUP>-1</SUP>) found after plasma expansion. The equivalent proton perveance applicable to the extraction geometry used, was found by systemic measurements of the ion transmission to a Faraday cup as a function of the applied source voltage. Absolute ion numbers were calculated from measurements using an electrostatic ion analyser and a Faraday cup ion collector. From these measurements it was possible to deduce the transmission losses through the Low Energy Beam transport line between the LIS and an RFQ; they were found to be as high as 80%. In summary, the measurements detailed here allow the prediction of the source parameters required for extraction of a higher current, higher charge state beam required for a final LIS implementation capable of supplying the ion beam for the Large Hadron Collider (LHC). In addition, simulations of the beam extraction using a time dependent macro-particle and static ray-tracing software package are described which provide a reasonably good modelling of the beam extraction.