Design and Initial Operation of a 50-Mev Microwave Linear Accelerator for Electron Beam Therapy

Abstract

A number of basic requirements for an accelerator for electron beam therapy were laid down by the Argonne Cancer Research Hospital in its contract with the High Voltage Engineering Corporation. These included: full output at 50 Mev; control of electron energy output from 10 to 50 Mev without appreciable change in output current, beam cross section, or spectral distribution; total output of 1∕10 microampere; high order of operational reliability. These were achieved with an accelerator consisting of two 8-ft. sections of waveguide of the Stanford type (fabricated by Stanford University) operated in cascade. Each section is driven by a separate 20-megawatt klystron (also fabricated by Stanford). The two klystrons are driven by a single magnetron with variable phase delay in the drive to one of the klystrons. A single knob on the control stand adjusts the phase delay, giving a smooth and continuous control of energy through control of the phase of the electrons with respect to the radio-frequency wave in the second section of the waveguide. Since the conditions of injection are not altered when the electron energy is changed, the character of the output beam is not modified appreciably. The klystrons are pulsed on at full power for a period of 1.6 microseconds repeated 60 times per second. Injection of electrons is at 100 kv for a period of 0.6 microseconds with the injector pulse delayed 0.9 microseconds from the start of the klystron pulse to allow the waveguide to be fined with radiofrequency energy. The injector uses a tantalum filament with grid control and combined electrostatic and magnetic focusing. Operational reliability is increased through the provision of a third klystron in stand-by condition that can replace one that has failed through a simple change in waveguide and coaxial line connections; through the use of D-C resonant charging of the klystron pulse lines; through the use of high-speed vacuum pumps with adequate high-vacuum valving; and through conservative design that does not place excessive load on high-voltage and high-power components. The machine operates satisfactorily to 50 Mev, and indications are that the output could be extended safely to 60 Mev.