PURPOSE: To couple high frequency section and collector section firmly and mechanically by placing a pair of metal interval holder and insulation spacer between high frequency section and flange of collector section while fixing the insulation spacer such that they can be replaced independently.

CONSTITUTION: A sealing section 62 is formed between the bottom wall 63 of collector and an intermediate ring 63 on a tubular member 64 jointed air-tightly with first flange 34 then ceramic insulation rings 66, 67, 68 and thin metal rings 69, 70, 71, 72 such as Kovar are jointed air-tightly and the joints 73, 74 arc welded. The joint composed of these metal rings has flexibility to enable slight variation of gap (h). Metallic interval holders 81, 81,... and many insulation spacers 82, 82,... in pairs with said holders 81, 81,... and placed between the flange 61 are provided between both flanges 34, 61. Until completion of exhaustion, a metal spacer 86 having same thickness is placed in place of insulation spacer 82 and fixed.