Because it is digitally switched with low voltage (30V+), according to this patent. But so is any hot-cathode source, with filament voltage as low as 3V, and possibly an extra grid or Wehnelt cup electrode.
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| Wehnelt cup (source: Lambtron at Wikipedia) |
Update March 24, 2021: A good slide from a IAEA on grid-controlled tubes use in pulsed fluoroscopy:
How fast can you switch? A 1984 paper says pulse width down to 19 microseconds (says that at the time, fluoroscopy and CT did not requires pulse widths less than 1 millisecond). The 120kVp is switched with 3.7kVp at the grid, tube current is 125mA
Update April 6, 2021: The switching grid, or focusing cup, is negatively biased toward the cathode (filament), at about 1kV (while in a conventional tube, the cup is connected to the filament)
