The production of intense ion beams using a magnetized plasma

A new method of producing ion beams with large perveance is investigated. A plasma in a magnetic field is brought out from the ion source into the vacuum space. Here magnetic fields are used to impart to it the desired geometry. If the extraction of ions is carried out in a direction perpendicular to the surface of this magnetized plasma, the position of the surface changes comparatively little when the electric field is applied. This stabilization of the position of the plasma boundary enables one to obtain hydrogen ion currents of up to 3 A using quite moderate potentials (6-10 kV). The fact that the boundary conditions can be determined allows one to compute the ion trajectories, between the plasma boundary and the target, taking space charge into account. From these trajectories the current density distribution over the beam cross section and the tubes of current are determined. There is satisfactory agreement between these calculations and the experimental results. The position of the plasma boundary and its geometry, can be controlled by using a solenoid and shims. The conditions are found for forming beams having either a maximum or a minimum current density on the axis. Using a cylindrical electrode with a grid and operating under pulsed conditions, hydrogen ion beams are formed with currents of 600-800 mA and energies of 30-40 keV. An equivalent electron perveance of 5 ? 10-6 A/V3/2 is achieved for this ion source. Continuous beams of H+ and He+ ions are formed, having currents of 540 and 150 mA respectively and energies of 115 and 75 keV respectively, focused at a distance of 160 cm from the ion source.