PURPOSE:To give a good trigger by feeding a current between a switch valve for controlling the supply of discharge gas and the seal surface of the switch valve and producing electric discharge between the switch valve and the seal surface at the same time when the switch valve is opened. CONSTITUTION:After electrodes 1 and 2 are placed facing each other in a discharge case 3, a Laval nozzle 21 is installed in the center of the electrode 2 and the nozzle 21 is connected to a high-pressure gas bomb through a high-speed switch valve 7, thereby constituting a gas-puff-type plasma X-ray source. In addition, a trigger power supply 8' is connected between the electrode 2 and the valve 7 through an inductor 91 and a switch 9, so that a current flows through the valve 7, a seal surface 6 and the electrode 2 when the switch 9 is turned on and so that an arc develops and trigger is performed between the seal surface 6 and the valve 7 when the valve 7 is opened at high speed. As a result, it is possible to reliably perform electric discharge at the same time when a main circuit switch 5 is turned on without disturbing the gas flow in the hole 11 of the electrode 1.
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