| 摘要 |
<p>In a CT system, the electron beam (12) is focused by controlling the distribution of beam-generated ions electrostatically. The relative lengths of the upstream (self-expanding, de-focusing) beam region and downstream (converging, self-focusing) beam region are controlled such that beam de-focusing in the upstream region compensates for beam self-focusing in the downstream region. Thus, essentially zero external focusing strength is required. Located downstream from the electron gun (32), a positive ion electrode ('PIE') (48) determines the relative length of each region. The PIE is mounted coaxially to the beam optic axis and coupled to a large positive potential. Varying the PIE potential varies the boundary position, and thus the magnitudes of the beam de-focusing and self-focusing effects. Positive ions are removed by a periodic ion clearing electrode ('PICE') (52) whose rate of change of axial potential creates the alternating axial fields that rapidly sweep away the ions.</p> |
