Variable circular beam apertures

TL;DR: In this paper, the authors describe two types of variable beam apertures used in the converted MP-Tandem at Munich, one of which uses eight sectors, four of which are insulated allowing beam position measurements.

Abstract: 2014 Two types of variable beam apertures used in the converted MP-Tandem at Munich are described. One of them uses eight sectors, four of which are insulated allowing beam position measurements. Due to its relatively small power dissipation this aperture is used only in the low energy region. The other type is an iris aperture designed for high power dissipation to be used in the terminal. The total beam current of this aperture can be measured. Both apertures can be completely closed and can therefore be used as Faraday cups. They are designed in UHV-techniques and can be baked at 200 °C. REVUE DE PHYSIQUE APPLIQUÉE TOME 12, OCTOBRE 1977, PAGE Two types of variable and nearly circular apertures matched to the circular beam shape of the converted MP-Tandem were developed. The first type (Fig. 1) is called four sector aperture although it actually consists of eight sectors which are arranged in four planes allowing complete closure of the aperture. FIG. 1. Variable four sector aperture with magnetic feedthrough and motor (schematic). Four sectors are electrically insulated for individual beam current measurements. The four remaining sectors, completing the octagon, are grounded. The mechanical gear mechanism resembles the clamping device in a lathe. The use in ultrahigh vacuum requires special care with regard to outgassing rates and friction. Materials used are stainless steel, titanium, aluminium and polyimide. Ball bearings are lubricated with MoS 2. A prototype was cycled 80 000 times at 5 x 10-9 torr and temperatures up to 250 °C without showing any wear. Without motor the assembly may be baked at 400 °C. The maximum permissible power is estimated to be 10 W. This restricts the use of this type of aperture to the low energy region of the accelerator. Four of these apertures have been installed between injector and low energy tube entrance. Three of them located after lens 1 [ 1 ], at the buncher waist and before lens 2 are used to define precisely the emittance of the injected beam [2]. The apertures furthermore greatly facilitate and accelerate the handling of the beam, they are also used to vary beam intensities without touching the ion source controls. The current signals can be used to automatize the setting of corresponding steerers. No defects have occurred in half a year of routine operation. The second type, designed to withstand high power loads, is an iris aperture (Fig. 2), similar to the shutter in a photographic camera. Eight segments move between fixed axles and a rotable cage. The segments are made from titanium, which retains its shape even at very high local temperatures. The cage rotates on ceramic balls to provide electrical insulation. Other materials used are stainless steel and polyimide. The cage is moved by two radially magnetized permanent magnets, whose fields interact through the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:0197700120100157900