A plasma betatron without gas breakdown

Following Budker´s proposal^[1], experiments on plasma betatrons were carried out in several laboratories. Large conduction currents were observed as a result of gas breakdown, but the current of energetic electrons did not exceed 10 A. We adopted plasma injection to start a beam in good vacuum. The chamber consisted of outer and inner glass pipes lined with stainless steel mesh at radii of 5 cm and 2 cm, respectively. A toroidal field of 2 - 3 kG was applied along with a vertical field of up to 1.4 kG. Plasma was injected by coaxial guns, 1/4 inch in outer diameter, without increasing the neutral pressure significantly. A 20 A beam was accelerated to a peak energy of 1 MeV with little loss and then dumped to the chamber wall by a spiller field. The current was enhanced to more than 100 A by the addition of electron injection using a field emission cathode.