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Jet apparatus results

The initial results from the jet apparatus were comparable to those found from a conventional ion source. A spectrum of O2+  was recorded with a satisfactory signal to noise ratio, with only a small decrease in ion current of one order of magnitude (the jet source achieved 1$\times10^{-8}$ A at the first magnetic sector lens stack TIM). The distance between the source and magnetic sector is significantly larger ($\approx$ 65 cm) compared to the conventional ion source ($\approx$ 15 cm), and hence a decrease in ion current was expected due to collisional and unimolecular decomposition of the molecular ions in the beam. Extra pumping capacity in the jet source compensates for flight region losses, as higher source pressures can be used. The pressure in the source chamber can reach 1$\times10^{-4}$ torr without damage to the diffusion pump or filament. This pressure is two orders of magnitude greater than that of a conventional ion source. An initial spectrum of O2+  is shown in Figure 5.5. Linewidths in this region are approximately 500 MHz, identical to those found in a conventional EI source with an acceleration voltage of 2 kV. Both these values are higher than those reported by Cosby et al. where all transitions have widths between 275 and 450 MHz [125]. This is probably due to the low acceleration potential, in comparison to that used by Cosby et al. (3.6 kV). The signal to noise ratio is poorer in comparison to the conventional ion source spectrum, probably due to the lower beam currents from the jet source.
Comparison between intensities of 9 Q21(9.5) and 17 P21(18.5) in Figures  5.4 and 5.5 show that virtually no intensity redistribution occurs between the jet source and conventional source with a 25 $\mu $m nozzle and no carrier gas.


next up previous contents
Next: Estimation of temperature Up: Results Previous: Results   Contents
Tim Gibbon
1999-09-06