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Skimmer and ion optics

A skimmer (Beam Dynamics nickel type 1, orifice diameter 0.4-0.8 mm) is mounted on a partition wall of the source chamber. The mounting design is such that the skimmer can be mounted and removed quickly and easily, without damage to the thin nickel walls: it is placed into a recess (2.3 cm with a 1 cm hole through the center) of a stainless plate, over which a mounting ring is then placed and held securely by three stainless steel screws. A Teflon coated copper wire is screwed onto the plate, passing through the feedthroughs, and is connected to the high voltage supply (see Figure  5.3). The plate is placed inside the vacuum chamber and screwed into place using six mounting bolts. Teflon isolates the plate from the chamber partition, to allow the application of voltages to the skimmer. Typically voltages for the skimmer can vary up to $\pm 50$ V from the source voltage, controlled and read by a VG M71 source control unit.

Ion optics are mounted on the partition wall protruding into the flight region chamber, positioned axially to the nozzle and skimmer. These consist of several earthed plates and two plates in the Y plane and two in the Z plane. As the plates are earthed individually, a TIM signal can be measured after skimming the ion beam. The lens stack design is similar to the stack at the first magnetic sector; however, it is possible to vary the voltages over a greater range than for the conventional lens stacks using a separate ion optics control box (in-house device). Typical ranges for these lenses are 0-10 % of the acceleration voltage, applied to the ion optic stack via a feedthrough port mounted vertically in the flight chamber. Teflon coated copper wires connect the lens stack plates to this feedthrough plate.

Figure 5.3: Illustration of the nickel skimmer mounting and the lens stack construction inside the flight region chamber.
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next up previous contents
Next: Alignment of the nozzle Up: Experimental Previous: Ion source   Contents
Tim Gibbon
1999-09-06