We know that the beams are accelerated to about 2 Mev–we can measure their velocities. We can’t directly measure the distance it takes to accelerate them and thus the field. The maximum distance is the length of the plasmoid, 1.5 mm, so that would be about 1GV/m. Our theory indicates that the real distance is only about 5 microns so that would be a field of 400GV/m. The calculation from Maxwell’s laws is a bit more complicated than the questioner thinks and i will go over it on this thread when I get a minute.
