The Development of High Voltage X-ray Tubes at the California Institute of Technology

Abstract

THE difficulties encountered in constructing x-ray tubes for operation at voltages above two or three hundred kilovolts are well known. Stray charges may accumulate on the envelope in such a manner that a dangerously high potential is applied through the wall, with the result that it is punctured. Punctures may also result from tiny pieces of metal torn out of the electrodes by the intense electric fields. Gas may be liberated by stray bombardment caused by the field and resulting in destructive discharges. These difficulties may be at least partly eliminated by several means. We have attempted to decrease the stray discharges by using electrodes with large curvature and large spacing, so as to decrease as far as possible the field strength at the surfaces. In addition, the glass or other envelope is protected against stray discharges and excessive potentials by proper subdivision and shielding, and any gas which may be liberated during operation is quickly removed by fast pumping. The first tube to be used for experimental x-ray work at potentials in excess of 300 kv. was constructed in 1928. This tube was operated intermittently at 750 kv. peak, and cold emission was depended on for the source of electrons. Figure 1 shows a section through this tube which is mounted in a temporary wooden structure. The potential is applied between the long inner electrode and the bottom plate, which is at ground potential. It will be noted that the glass wall is nowhere exposed to the full potential and that it is thoroughly protected against stray bombardment. It was a simple matter to equip this tube with suitable filament and to provide adequate protection for operation and observation at close range. Continuous operation was obtained at 600 kv. peak and from 4 to 5 ma. electron emission. Such a tube is shown in Figure 2. Since no equipment of comparable voltage was at the time available elsewhere, it seemed highly desirable that an attempt be made to learn if x-rays produced at such high potentials are in any respect superior to other forms of radiation for the treatment of malignant disease. After much preliminary work with animals and many consultations with men of high standing in the medical profession, it was decided to treat selected cases of advanced deepseated malignant tumors. Treatment was started in October, 1930, and has been continued up to the present time. In 1931, Mr. W. K. Kellogg became interested in the work and generously provided funds for erecting and equipping a new laboratory to be devoted to clinical research in radiation. The laboratory and a new tube, designed to operate at 1,000 kv., were completed the same year, but actual operation of the plant was delayed for nearly a year due to difficulties in design and construction of the transformers, and regular treatment did not begin until September, 1932. Since that time this equipment has been in daily use for the treatment of malignant disease.