PURPOSE:To obtain an electric-discharge emission with a high luminance by positioning insulating projections, which are in the axial direction, on parts of the inner wall of the second electrode which is positioned inside the first electrode. CONSTITUTION:A cylindrical airtight case 1 is prepared from pyrex glass, and both end surfaces 8 and 8 of the case 1 are provided with transparent window parts. The first and the second electrodes 2 and 3 made of stainless steel plates. A slot 5 which is parallel to the cylinder axis, is provided in the center of the side wall of the second electrode 3. A glass pipe, which is provided with slots 41, 42, 43 and 44 parallel to the axis, is positioned so that the center lines of the slot 5 of the second electrode 3 and the slot 2 correspond to one another, and inserted in such a manner that it becomes in close contact with the inner wall of the electrode 3. Parts of the pipe 4 which remain between the slots 41, 42, 43 and 44 form four projections which closely touch the inner wall of the electrode 3. The airtight case 1 is charged with helium gas at a pressure of 18 Torr.
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