/we need an "accelerating field" that is capable of speeding projectiles up to high enough velocities/
Obviously, you would use pulsed laser air ionization, using the plasma paths created as the rails for your own ad hoc one time railgun.
There is no mechanism I can think of for a homogenous field to accelerate a projectile. Instead, your vehicle will have many fields, each the width of a laser beam. Any one of these lasers will be able to increase intensity to the point that air traversed is ionized.
Ionized air can conduct electricity: it is how the lightning does it. Parallel paths can serve as railgun rails for you to shoot your own projectile.
I envision all of these lasers to be turned on all the time, pointing in all directions. Like a colander placed over a lightbulb. As needed one or more can be turned up to produce the plasma path. Or perhaps they all are that strong all the time.
It would be very easy to see this vehicle coming. Given that the element of surprise is gone, I propose that the vehicle also broadcast sounds which are an appropriate match for the belasered appearance.