Valve Devices

VALVE DEVICES, used in radiology to prevent the make-phase of high-voltage current from passing through the X-ray tube. This relates primarily to ionic tubes, since electronic tubes do not pass reverse current up to a certain voltage. The passage of reverse current through an ionic tube can quickly cause its damage, because the anticathode, having become the cathode, will begin to emit electrons (cathode rays) which, striking the glass of the tube, can melt it due to the thermal effect they produce. In addition, the passage of reverse current through an ionic tube causes rapid sputtering of the anticathode, since it is made of a substance with a high atomic number (platinum, tungsten). This leads to the absorption of the residual gas in the tube by metal particles, making it so hard that it becomes unfit for work. All valve devices can be divided into 3 groups: 1) stationary spark valve gap, 2) rotating spark valve gap, and 3) valve tubes. The stationary spark valve gap (see Fig. 1) is a glass cylinder with metal bases; an opening is made in one base, into which a metal rod is inserted on an insulated handle, ending in a point and having a screw thread at the place of passage through the cylinder base, allowing the end of the rod to be brought closer to or removed from the opposite base of the cylinder. It turns out that the current easily passes from the rod to the base, but not vice versa, due to the fact that the current density at the point is greater than at the plate, and the current easily overcomes the air resistance in this direction. The current is switched into the high-voltage circuit so that the rod is connected by a conductor to the cathode of the tube, and the plate to the cathode of the high-voltage generator. This device has the disadvantage of causing noise and the formation of a large amount of nitrogen oxides. The rotating spark valve gap (see Figure 2) consists of two metal plates (a) located along the circumference, between which a metal wire rotates. This wire is mounted on the extension of the axis of the interrupter motor (D), but is isolated from it and rotates during the operation of the interrupter. As can be seen from Fig. 2, this device is connected in series into the high-voltage circuit, and the wire is set so that at the moment of breaking the current, the current passes through the valve gap, and thereby

through the tube; at the moment of making, however, the current does not pass through the tube. The length of the plates is chosen such that when the breaking ceases, the wire does not touch the plates, and therefore only the breaking phase passes through the tube.

The disadvantage of this device is that the appearance of a spark at the moments of making and breaking the current causes the formation of high-frequency current oscillations, which can damage the tube.

Valve tubes according to the principle of their operation are divided into two large groups: 1) ionic valve tubes and 2) electronic valve tubes, or kenotrons. The former are constructed like ionic X-ray tubes, but the vacuum in them is lower; one of the types of regenerators for ionic X-ray tubes is used for their regeneration. A feature

Figure 3.

of their design is that their anode is placed in the depths of a narrow glass process of the tube and does not protrude into the cavity of the tube, while the cathode is placed in the wide part of the tube. Current easily passes from the anode to the cathode, but not vice versa, because with reverse current, the anode, having become a cathode, will begin to emit electrons, which will charge the glass of the process with negative electricity due to its proximity, while the remaining electrons will begin to repel from it, which will create unfavorable conditions for the passage of reverse current. Ionic valve tubes differ from each other only in the arrangement of their electrodes; in some the anode has the shape of a point and the cathode of a plate, in others the anode has the shape of a rod and the cathode of a spiral, in still others the anode has the shape of a plate and the cathode of a tube (see Figure 3). In order to be able to pass a stronger current, several (2-3) valve tubes are connected in parallel.

Electronic valve tubes, or kenotrons, are built on the type of electronic X-ray tubes (Coolidge). They are airless and have an incandescent cathode in the form of a tungsten spiral and an anode in the form of a tantalum plate. They pass current in only one direction (along the path of the electrons). When using kenotrons, it is also necessary to have a source of electricity (accumulator, transformer) to heat the cathode of the tube in order to obtain electron emission. Currently, there are kenotrons that do not pass reverse current up to voltages of 227 kW (see Figure 4). Recently, kenotrons have become very widespread in X-ray engineering, finding application also in the construction of high-voltage current rectifiers. Ionic valve tubes and kenotrons are connected into the high-voltage circuit so that their anode is connected to the cathode of the X-ray tube, and the cathode to the cathode of the high-voltage generator.