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Ion source

The ion source is mounted in a stainless steel chamber above the Edwards E06K diffusion pump. The jet nozzle is mounted at the end of a stainless steel tube, which in turn is mounted on bellows to allow axial translation of the jet assembly. Horizontal and vertical translation of the nozzle can be obtained by the adjustment of six translational screws. The gas line passes through an O-ring seal and an electrically isolated Teflon Swagelok seal and enters the nozzle region. A pinhole (Ealing [diameter 12.5-75 $\mu $m]) is mounted inside a threaded holder, screwed to end of the gas line. An O-ring seal prevents gas from escaping at the gas line/nozzle holder interface.

A Micromass MM6 filament and trap are mounted on either side of the nozzle, the normal orientation being with the filament mounted vertically above the nozzle, the trap diametrically opposite. A copper cage surrounds the ion source to prevent penetration of stray fields into this region. The ion source can be translated with respect to the nozzle so that the distance between filament and nozzle can be adjusted. Typical filament/nozzle horizontal separations are between 2 mm and 10 mm. The filament is regulated in Electron Impact (EI) mode i.e. the electron current arriving at the trap was held constant by varying the current through the filament. It was found that the jet source could maintain emission currents of 1000 $\mu $A for several weeks, in comparison to the conventional ion source which can maintain currents of this order for a few days only. This is due to the open design of the nozzle: the flowing gas is ionised in an open space, as opposed to the normal EI source (which confines gas in a source block). The electron beam passes unhindered through to the trap in the jet source, whereas in a standard EI source it has to pass through an entrance hole and an exit slit in a source block. Filaments are usually destroyed when too large a filament current (typically above 4 amps) is needed to keep a constant emission current. In the jet source, the filament is under less strain due to the lower current (typically 2 Amps) needed to obtain the same emission current. This open design of ion source allows filament changes and source cleaning to be carried out quickly and simply due to the ease of access. Magnets mounted above the filament and below the trap cause the electrons to spiral, increasing their flight path as occurs in a conventional ion source. The source and copper cage, filament and trap are connected to a high voltage programmed power supply (VG M18A) using Teflon coated copper wires connected via electrical feedthroughs. Trap and filament currents are read/regulated using an emission control and source control VG M93 and M71 respectively. The emission control unit has a small modification which allows the trap current (normally held at +50 V), to be varied between 24 and 55 V.
A high vacuum viewport (68 mm diameter) is mounted at the side of the chamber, centred on the nozzle/skimmer region, allowing visual nozzle adjustments to be made without danger of touching the skimmer.

Figure 5.2: Nozzle design
\resizebox{5in}{!}{\includegraphics{figures/jetsource.eps}}


next up previous contents
Next: Skimmer and ion optics Up: Experimental Previous: Vacuum Apparatus   Contents
Tim Gibbon
1999-09-06