I missed what did you claim. If you about pinch of combined current you are wrong.
Charged particles beams dramatically compressed passing through even the cloud of opposite charged particles.
And we will have a combination of three factors:
Compensation of space charge + magnetic attraction + low temperature.
And compressed beam will be narrower at least on order.
And if you claim "several 10's of cm" -on two orders.
The key word here is low temperature
- at least 500 times lower than in other fusion methods.
Have you ever heard about electron cooling first offered by G.I. Budker? Relativistic electrons will radiate and the temperature will not exceed a few eV-s.
Does low temperature imply proportionally low pressure of gas at the same density?
And consequently possibility of stronger compression?
So, a very effective focusing and consequently a strong self-magnetic field confining the particles. Unattainable e.g. in TOKAMKs with even mega-amperes axial (toroidal) currents.
And total current not 250 A but 560 A for large commercial scale reactor. It will be enough for focusing to up to 0.5-1 cm diameter.
Regarding initial ion source
Here is a link of Japanese constant current ions gun giving the current 3 A of particles' energy 500keV with aperture 20'000 cm2 = 2 m2.
http://www.jaea.go.jp/english/news/p100 ... ndex.shtml
So, for ion gun might to give us 250 A we need about 80 times bigger aperture = ~160 m2.
This corresponds to the diameter of about ~14.4 m in case of a single round shaped ion source (let’s call this diameter as “equivalent aperture”)
Nobody built such big ion source yet. But technically it is quite possible.