next up previous contents
Next: Flight Region Up: Apparatus Previous: Overview   Contents

Ion source

Gas enters the source chamber through either one or two gas lines regulated by a leak valve and passes into a confined region (source block) through two Teflon feedthroughs where it is bombarded with ca. 70 eV electrons from a thoriated tungsten filament. The electrons are detected at a trap, which allows regulation of the current of electrons using a feedback loop. Trap currents of 500 $\mu $A for a filament current of 4 A are typical. Two small permanent magnets cause the electrons to spiral, increasing their flight path length and subsequently increasing the ionisation yield. The source block is kept at a high voltage (up to +5 kV) with respect to an earthed extraction slit outside the source. Positive ions are accelerated from the source region, whereupon ion optics align and focus the beam. The experimental apparatus is shown in Figure 2.4.

Figure 2.4: Ion beam apparatus (not to scale).
\resizebox{5in}{8in}{
\includegraphics{figures/ionbeam.eps}}



Tim Gibbon
1999-09-06