In case anyone wonders what's the big deal about this: As someone who knows a liiittle bit about collider physics, I think a relatively clean, relatively high e

In case anyone wonders what's the big deal about this: As someone who knows a liiittle bit about collider physics, I think a relatively clean, relatively high energy electron gun has the advantage of reducing the "emittance" of the beam, i.e. creating a beam whose particles have speeds and movement directions very close together. Starting with a low emittance beam means it's easier to end up with a low emittance beam, though AFAIU starting with a very clean beam is not a hard requirement - there are "conditioning" methods, some of which lose a fraction of the particles (not ideal).