Use of tapered glass capillaries for producing microbeams

The Pacific Northwest Consortium CAT is developing tapered glass capillaries using fiber optics techniques for fabrication. Previous calculations showed that, under ideal conditions and using Undulator A as a source, glass capillaries can produce higher intensity gains than those of mirrors for an output diameter of 5 micrometer or less. Recently, we fabricated several capillaries with approximately linear tapers and tested three of them with outlet diameters of 1.3, 1.4, and 12.4 micrometers and lengths of about 35–90 cm. We obtained intensity gains over a pin hole of about 270, 240, and 30, respectively. They are about half of the calculated values expected from a perfectly linear profile. Also the angular spread of the output beam was much larger than the calculated value. We measured the profile of one capillary in detail and calculated the intensity for the capillary with the measured profile and still found significant differences. We also made calculations assuming several types of misalignment and found that they are important factors in understanding the larger angular divergence and reduced intensity. These results suggest improvements in the fabrication of the capillaries and in ways to support them during experiments.