The X ray (bremsstrahlung) suppression is due to ultra-high magnetic field that Eric theorized occurs during the pinch. Because with a stronger magnetic field, electrons as charged particles will have to closely orbit the magnetic field lines and wont be able to hit atoms nucleus (I think) so much, and reducing collisions will reduce the amount of X-rays. This was widely theorized, but not sure sure how much it was tested due to difficulty of achieving these level of magnetic fields:
http://aip.scitation.org/doi/abs/10.1063/1.862333
Also beryllium electrode is X-ray transparent so it will reduce the corrosion due to X-ray heating and thermal evaporation.
