It will add to the mass of the accelerator,
but a type of vaccuum system can suck in
the particles again once it reaches the
end, hitting the surface of the electromagnetic field.
Why the outer shell is an energy field is because,
it's not propulsion if the particles are contained
within. I'm guessing the energy field, despite
being a shield itself still permits interaction
with the hollow space.
Also does not increase the mass of the overall
'ship' by much. But it depends on the mass
of the particles, it's acceleration relative
to the accelerator/ship itself.
What do I know. I'm just guessing by intuition.
In any case, I will continue to try and finish
up the chores. Sometimes my mind gets stuck
in a unnecessary loop.
I have to get out of it.