The invention relates to a system for controlling the power of a high-voltage electron beam generator having a cathode and a control electrode associated with the cathode, having a regulator at ground potential and a pulse generator for the production of square-wave voltage pulses symmetrical with the null line, for whose transfer to the part of the system that is at high-voltage potential a high-voltage isolating transformer is present, having a primary winding at substantially ground potential and a secondary winding at high-voltage potential and connected to a full-wave rectifier which is connected to the cathode and control electrode by means of a circuit having passive elements and acting as a filter.
Control systems of the kind described above are used in many fields of electron beam technology, but principally for the melting, welding, processing and vaporizing of materials of all kinds by means of electron beams, preferably under vacuum. To vary the intensity of an electron beam, the heat output of the incandescent cathode, the bias voltage of the control electrode, and/or the applied acceleration voltage can be varied.
In high-voltage electron beam generators which are generally operated with an anode voltage in excess of 20 kilovolts and therefore have a relatively low beam current at a given output, a cylindrical control electrode is disposed in the vicinity of the cathode, which is often referred to as a Wehnelt cylinder. With this control electrode it is possible to vary the beam current between 0 and 100%. For this purpose a negative voltage of variable amplitude with respect to the cathode is applied to the control electrode, and suppresses the emergence of the electrons in the direction of the anode, doing so to a greater or lesser extent according to the magnitude of this control voltage. The voltage level required for the sure blocking of the electron beam generator is one to two kilovolts, depending on the way the control electrode is made. The maximum beam current is reached when the cathode and control electrode are at virtually the same potential.
Through German Auslegeschrift No. 1,131,760 a system of the kind described above is known in which the control electrode is periodically fed with pulses which are superimposed on a direct-current voltage. This is intended to bring it about that the electron beam will be turned on intermittently. What is involved, therefore, is a constant turning on and off of an electron beam of always constant power, while nevertheless the power output is variable. The power can also be influenced in this case by the relative on-time of the electron beam. In this known system, however, the DC component of the control voltage is not applied to the control electrode, so that continuous power control of the electron beam generator is not possible. A continuous power control, however, is necessary for a number of purposes, such as for example in welding for raising the power at the beginning and at the end of a seam, in the welding of workpieces of different thickness, when vaporizing, etc.
A continuously varying or constant DC voltage can also be achieved by the homopolar concatenation of square and/or trapezoidal pulses, in which the envelope curve of the pulses is then the determining factor.
Through German Offenlegungsschrift No. 2,514,805 a system is known for the power control of high-voltage electron beam generators in accordance with the initial description in the background the present specification, which makes use of the principle of the concatenation of square-wave pulses on the secondary side of the high-voltage transformer.
In this case the transformer is equipped on the low-voltage side with a third winding, and the circuit that is on the high-voltage side and acts as a filter is in the form of an RC circuit. The third winding has the disadvantage that it is suitable only for frequencies up to a maximum of about 1 KHz. The RC circuit has an excessively long time characteristic, which is to be attributed to the fact that two requirements are diametrically opposed: on the one hand a low ripple is required in the high voltage, and this calls for a large capacity in the condenser. On the other hand, a rapid response is required in the controlling action, and this requirement can be satisfied only with a condenser of small capacity. The filter in any case determines the ripple in the high DC voltage. Hence, the known system is not suitable for the required high regulating speeds or frequencies to be handled.