Design and optimisation of new beamlines for iThemba laboratory for accelerator based sciences (iThemba LABS0

Abstract

The facilities at iThemba LABS are used for basic and applied research, treatment of patients with proton and neutron beams and the production of radioisotopes for hospitals and industry. The laboratory operates a 6 MV Van de Graaff accelerator and three cyclotrons. The cyclotron complex comprises of the two injector cyclotrons: the light-ion solid-pole injector cyclotron (SPC1) and the second solid-pole injector cyclotron (SPC2) which pre-accelerate and inject beams of particles into the large separated-sector cyclotron (SSC), which accelerates the particles up to a maximum energy of 200 MeV for protons. SPC1 uses an internal Penning Ionization Gauge (PIG) ion source that provides beams of light ions. SPC2 has two external ion sources; the polarized ion source and an electron cyclotron resonance (ECR) ion source, which provides beams of polarized hydrogen ions and heavy ions, respectively. In addition to the existing two external ion sources a third ion source, for acceleration of high intensity beams of protons, is being planned for SPC2. The new source, which will also be external to SPC2, has to inject a beam into one of the existing injection beamlines of SPC2. A suitable position for coupling the ion source to the injection beamlines of SPC2 has been found and in order to match the beam from the source to the existing lines a short new beamline section has been designed. A double focusing 90° bending magnet, for separation of the proton beam from unwanted molecular hydrogen ions has to be inserted between the source and the existing beamlines. Because of space limitations the bending magnet will also be used to focus the beam in both the horizontal and vertical directions. By calculation, using the computer program TRANSPORT, the best position for the ion source with respect to the magnet, and the optimum entrance and exit edge angles of 34.7° for the magnet, have been determined. Satisfactory beam envelopes could be obtained with the existing beamline elements up to the centre of SPC2. This shows that except for the bending magnet no further beamline elements are required to inject a beam from the new source into SPC2. Approximate analytical expressions were used to design and optimize the dimensions of the 90° bending magnet. An H-type magnet with a bending radius of 220 mm and a pole gap of 70 mm were decided upon. More accurate field calculations performed with the commercially available computer program TOSCA, which uses finite element analysis, verified the results obtained with analytical expressions. Saturation effects are negligible even at higher than required excitation of the coil and the field homogeneity in the beam region between the pole plates is better than 2% of the maximum field value. With the current design of the beamlines at the Van de Graaff accelerator, which dates back to 1962, the energy resolution is poor. Initially there was no need for higher energy resolution, but in recent years, with the acquisition of the multi-probe facility and the stronger emphasis on solid state physics experiments, the demand for beams with high energy-resolution increased. To obtain high energy resolution in a beamline at least one double-focusing bending magnet, with a large radius, as well as object and image slits, on which the beam should be focused, are required. The Van de Graaff beamlines were redesigned to improve their energy resolution and the quality of the beams delivered to the different users. Calculations, using the computer program TRANSPORT and emittance data obtained by fitting measured to calculated beam profiles, have shown that with two additional quadrupole magnets, an additional slit and modification of the entrance and exit edge angles of the existing 90° bending magnet in the beamline, the energy resolution can be improved from 1 % to 0.15%. This modification will not only result in better energy resolution but also in improved transmission efficiency in all the beamlines. Consequently, beams of adequate intensity and quality will become available when these modifications are implemented.