Design And Test Of The E-beam Injectors For The High Energy Power System (HEPS) Fusion Power Source Experiment

In the HEPS approach to controlled thermonuclear fusion conceived by R.V. Bussard, electrons and ions are trapped within a "polywell" arrangement comprised of magnetic and electrostatic confining fields. The unique magnetic field shape used in this system is achieved from a set of special polyhedral coil elements. A major advantage of the HEPS magnetic field geometry is that it has only point cusps which minimizes particle loss. Electrons are launched into the central region of the confinement field through the cusp points. When these electrons enter the central region they are captured and form a negative potential well which helps to trap ions. This paper describes the design and performance of the electron beam injectors used in the first HEPS experiment. The basic design approach vas to use a standard Litton gun if possible to minimize cost. Numerical studies disclosed that the diode (ungridded) Pierce gun from the Litton 5868 medical accelerator klystron would be a good candidate. This gun normally runs at roughly 80 kv and 45 amps (the basic microperveance of this gun is 2). The 5868 includes a rugged dispenser cathode for resistance to poisoning from ion backbombardment. HEPS requires a gun which provides up to 50 amps at a voltage of 38. 5 kv. This is very high perveance for a Pierce gun (6.6 micropervs). Detailed computer design studies were carried out using DEHEOS (the deformable triangular mesh beam optics code of the first author). Simulations with this code disclosed that it is possible to achieve good optical quality designs from 2 micropervs all the way up to 6. 6 by modification of the anode geometry only.