19 are fixed to the forks 14 of the inner tube 2 each coupling plate 19 and corresponding fork 14 are coupled together by a pin 20 the radiated light output frame 3 has a disk like shape and a rectangular or arc shaped window matching a cross section of a radiated light flux is formed in the frame 3 at its central area the window is hermetically sealed with a beryllium thin film 21 which is welded or soldered to the output frame 3 the output frame 3 is coupled to the flange 5 of the large diameter outer tube unit 1 via the vacuum bellows 4 and flanges 3 39 and 5 39 as shown in fig3 and 4 the cross section of the inner tube 2 has a shape covering the radiated light flux as shown in fig2 one end of the inner tube 2 is mounted on a flange 3 34 of the radiated light output frame 3 and the other end thereof protrudes from the large diameter outer tube unit into the upstream vacuum duct 5 the drivers 6 1 and 6 2 drive the inner tube 2 in the vertical direction and receive a force in the horizontal direction generated by a pressure difference between the atmospheric pressure applied to the radiated light output frame 3.