HIGH VOLTAGE CATHODE RAY AND X-RAY TUBES AND THEIR OPERATION
Authors: Coolidge W. D. , Dempster L. E. , Tanis H. E. 
Published: January 2005

The following recommendations are made as bearing upon tube design. The glass used should be free from such elongated gas bubbles as are to be found in drawn tubing—only mold-blown tubing should be employed. The larger the bulb the better and, for use in air, thickness of bulb wall is conducive to good operation. The anode arm should be so dimensioned with respect to the anode as to provide a rather long and relatively narrow space between the two, as this helps to keep reflected electrons from getting too far into the anode arm and so building up a destructive potential gradient in the glass. While under ideal conditions a cathode arm could be dispensed with, it is, in general, better to have one and to give it essentially the design of the anode arm. For currents of as much as several milliamperes, a hollow anode is desirable, as it reduces the number of reflected electrons which get into the anode arm. It also lends itself to the easy attainment of adequate x-ray protection. With a target consisting of a wrought tungsten face attached to a water-cooled mass of copper, the tungsten face must be thick enough to avoid local melting of the copper at any point behind it. If the tube is intended for operation on intermittent current, the temperature gradient between the acting face of the target and the cooling medium must not be too high, as otherwise the forces of heat expansion and contraction will in time wreck the target. To avoid troublesome field currents, the electrodes should have their cooperating ends facing one another rather than being placed one within the other. They should consist of metal which has had a thorough degassing treatment. All sharp edges and corners should be avoided in both cathode and anode structures. The canal systems caused by disruptive discharges in the glass, resulting from field currents, resemble those produced in various colloidal materials under cathode-ray bombardment in air. During assembly great care must be exercised to keep all parts scrupulously clean, and during exhaust equal care must be taken to avoid sputtering, as otherwise a troublesome conducting layer will be formed on the inside of the bulb. Cascading may be resorted to for voltages higher than can be sustained by a single section, and can apparently be made to take care of any voltage which can be generated. With a hollow anode, magnetic focusing may be required, to reduce the size of the cathode ray beam, and with a cascade tube system, it is better to apply it at the anode end rather than at an intermediate electrode. Reference is made to several forms of high voltage generator, including a transformer without rectifier, a full-wave transformer-kenotron outfit, a constant-potential direct-current outfit and an induction coil. One embodiment of the last named is described in detail. A complete induction coil x-ray outfit is described for 900,000 volts and 5 milliamperes, which has just been installed for experimental therapy at the Memorial Hospital in New York City. In the industrial field, it seems almost certain that such high voltage x-ray equipment will be used for the radiographic examination of thick sections of metal. A tube can be much smaller, for a given voltage, when operating in oil instead of air, and it is predicted that, in future, x-ray or cathode ray tubes for very high voltages will be operated immersed in the same oil with the high voltage source and with no portion of the high voltage circuit exposed to the air. This will make for a great reduction in space occupied and for reduced electrical hazard.