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Kinematic Compression

The ion beam is not perfectly mono-energetic due to the different positions of the molecules inside the source block upon ionisation, their different motions and the geometry of the source region. This results in ions leaving the source with a finite energy spread $\Delta$E. However, improved resolution is obtained by accelerating the molecules from the ionisation region using high voltages, resulting in velocity bunching or kinematic compression (see Figure 2.3.) Ions which are accelerated from the ion source with high velocities (high voltages) have a reduced velocity spread (and hence Doppler width) for a given energy spread in the ion source.

Figure 2.3: Figure showing kinematic compression in a fast ion beam. Note the narrowing of the velocity spread at high acceleration potentials.
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Tim Gibbon
1999-09-06