The Electron Gun for the Stanford Two - Mile Accelerator

The design of the electron gun for an electron linear accelerator is of prime importance in achieving a small emittance volume and good reliability. This paper discusses phase space concepts useful in gun design, the design of the SLAC electron gun, and tests of its performance. A convenient definition for the effective area in phase space of a finite set of calculated points is proposed and a discussion of the increase in transverse phase space within a linear accelerator is presented. The s U C gun is a Pierce spherical triode with a 50-ohm coaxial input to the grid. The gun is designed to operate with 80 kV dc between the cathode and the anode. The current in the beam can be varied from 2 A peak to less than A peak by varying the grid to cathode voltage over a range of about 1000 volts. The design permits use of either an oxide cathode radiantly heated or a tho-riated tungsten cathode heated by electron bombardment. The ease with which a high energy particle beam can be transported over long distances, the precision with which the beam energy can be measured, and the precision with which scattering angles of particles can be determined in a physics experiment all depend on the transverse phase space area (or emittance) of the beam. The emittance of an electron linear accelerator is largely determined by the emittance of the electron gun. Thus, the quality of the gun influences beam transport systems and the resolution of physics experiments carried out using the beam. tance L define a beam radius r1 and a radial momentum interval Two apertures of radius r I a d r 2 separated a dis-where p is the momentum of the particles. The a r e a of the inscribed ellipse is r r r p 1 2 L 1 (1) A = which is commonly defined as the admittance of two apertures. If the particles a r e accelerated from an energy of y1 at the first aperture to 7 2 at the second, the admittance is