| 摘要 |
<p>An electron gun (40) has a cathode (41) and three sequentially-spaced plate-like electrodes (42, 43, 44). Each electrode has an aperture (45, 56, 48) aligned with a line (x-x) extending normally away from a point of the emitting surface (58) of the cathode. A power supply (60) is arranged to supply respective voltages (Vc, V1, V2, V3) to the cathode and the three electrodes. Electrons issue from the emitting surface as a substantially-laminor flow, and are focused on a cross-over (59) beyond the third electrode after the electrons have been accelerated to substantially their maximum velocity. The velocity of the electrons at the cross-over is substantially greater than in prior art guns. The cross-over is of greater resolution due to a diminished space charge effect. The cross-over remains at a substantially fixed location from the emitting surface at all beam current levels. The invention also provides an improved method of operating an electron gun, which method includes dynamic switching of the various voltages applied to the cathode and the three electrodes, so as to maintain the resolution of a reformed image point on the screen within a predetermined bandwidth throughout the entire range of possible beam currents.</p> |
