We investigated the pressure dependence of the gas flow and the field ion intensity of a coaxial ion source operating at room temperature over a wide pressure range, testing various gases and ionization voltages. Flow conductance measurements taking into account the different gases’ viscosity and molecular mass consistently exhibit a generic pattern. Three different flow regimes appear with increasing upstream pressure. Since the coaxial ion source supplies the gas locally, very near the apex of the tip where ionization occurs, large ionization currents can be obtained without degrading the propagation conditions of the beam. Compared with field ionization in a partial pressure chamber, using the coaxial ion source increases the ion current a hundredfold for the same residual low pressure. We also show that the gas flow regime does not impact the ionization yield. Although a fuller characterization remains to be performed, brightness reaches 3×1011 A/m2/sr at 12 kV extracting voltage.

The development of bright sources is allowing technological break- throughs, especially in the field of microscopy. This requires a very ad- vanced control and understanding of the emission mechanisms. For bright electron sources, a projection microscope with a field emission tip provides an interference image that corresponds to a holographic recording. Image reconstruction can be performed digitally to form a "real" image of the object. However, interference images can only be obtained with a bright source that is small: often, an ultra-thin tip of tungsten whose radius of curvature is of the order of 10nm. The contrast and ultimate resolution of this image-projecting microscope depend only on the size of the apparent source. Thus, a projection microscope can be used to characterize source brightness: for example, analyzing the interference contrast enables the size of the source to be estimated. Ultra-thin W tips are not the only way to obtain bright sources: field emission can also be achieved by ap- plying voltages leading to a weak macroscopic electric field (< 1V/μm) to insulating micron crystals deposited on conductors with a large ra- dius of curvature (> 10μm). Moreover, analyzing the holograms reveals the source size, and the brightness of these new emitters equals that of traditional field emission sources.