In the drift space you have a field going from 1 MV at the edge to 0 V in the center (at least near the collector). If the particle is at the 0V gradient when it enters the drift space it is not going to gain any energy. If you are not cutting across the equipotential lines you don't gain energy. It is not possible. Unless the sky is green in your universe.
I don't see this.
What I see, in terms of fields, is very strong E-fields between the end of the tube and the emitter, no E-field within the main body of the tube (at least, NOT near the collector) and very strong E-fields between the body of the tube and the collector plate. I envision the E-Field lines terminating perpendicular to the surface of the collector and to the inside surface of the tube, since E-Fields are perpendicular to conductive surfaces.
My schooling tells me that the equipotential surfaces are perpendicular to the E-Field, so I see a very strong gradient (tight spacing of equipotential surfaces) near the collector, no gradient within the main body (drift space) of the tube, and another strong gradient between the tube and the emitter.
When you fire the 2MeV alpha particle from the emitter to the collector, I see it crossing lots of equipotential surfaces as it heads towards the tube to have near-0 kinetic energy as it enters the drift space. I see no electrostatic forces acting on it while in the drift space (and my intuition feels the currents aren't large enough to have significant magnetic effects). But when it gets close to the collector, it is going to be accelerated by the potential gradient, crossing lots of equipotential surfaces, near the collector, and slam into the collector with lots of energy. If we assume the emitter and collector have the same voltage, it'll hit the collector with 2MeV of energy.
I'll freely admit I'm not an expert. I've done lots of book-reading of physics, and passed with flying colors a college-level calculus-based EM course in high school -- 20 years ago. I'm old enough that I find FETs easier to understand that bi-polar transistors because FETs work like tubes, but that doesn't mean I understand tubes.
You seem to claim my analysis is wrong. I welcome an opportunity to sharpen my intuition and knowledge. Where is my mental picture failing me?