Transverse emittance growth along the Alvarez drift tube linac (DTL) section is a major concern with respect to the preservation of beam quality of high current beams at the GSI UNILAC. In order to define measures to reduce this growth, appropriate tools to simulate the beam dynamics are indispensable. This paper is about the benchmarking of three beam dynamics simulation codes, i.e. DYNAMION, PARMILA, and PARTRAN against systematic measurements of beam emittances for different transverse phase advances along the DTL. Special emphasis is put on the modeling of the initial distribution for the simulations. The concept of rms equivalence is expanded from full intensity to fractions of less than 100% of the beam. The experimental setup, data reduction, preparation of the simulations, and the evaluation of the simulations are described. In the experiments and in the simulations, a minimum of the rms-emittance growth was observed at zero current phase advances of about 60. In general, good agreement was found between simulations and experiment for the mean values of horizontal and vertical emittances at the DTL exit.
The therapeutic advantages of scanned electron beams from medical linear accelerators are accompanied by difficulties in electron dosimetry. Owing to short-time dose inhomogeneities in the treatment field, implicit in the system, time-consuming point-by-point measurements of dose distributions are normally required. However, by triggering the measuring time synchronously with the slow scan frequency, the time necessary to record reliable dose distributions can be shortened considerably. The instrumentation for the computer-controlled dosimetry system developed for this task is described. The results of our measurements show that reproducibility in dose within +/- 1% is achievable in less than 6.3 s per point for scanned electron beams.
The small multicusp ion source developed at Lawrence Berkeley Laboratory (LBL) has been equipped with a low voltage ratio, single aperture extraction system. The influence of the potential of the plasma electrode and of a dipole filter field on the beam emittance are measured. A simple method to reduce hash is suggested. The aim of these investigations is to produce nitrogen ion beams with a high atomic ion fraction and a low emittance as required for a RFQ-accelerator, which will be built for ion implantation.
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