Nitrogen doping with dual-vacuum furnace at IHEP

Abstract

A dual-vacuum furnace with the heater in the outside vacuum has been developed at the Institute of High Energy Physics (IHEP) to perform nitrogen doping (N-doping) heat treatment of 1.3 GHz 1-cell niobium cavity. The inner chamber is made of stainless steel, where the cavity is inside. This furnace equipped with cryopump can reach less than 1×10−4 Pa and 1×10−6 Pa at 800 °C and room-temperature, respectively, which is favorable for N-doping. During the doping process, nitrogen is injected by a mass flow controller with feedback of a film gauge to stabilize the doping pressure. The performance of this furnace during the doping process is introduced in details. Two 1.3 GHz 1-cell cavities made of fine grain niobium have received N-doping at this furnace, and the vertical test results are promising. The best intrinsic quality factor (Q0) of these cavities have exceeded 3×1010 at 24 MV/m, which is the target of vertical test for Circular Electron Positron Collider (CEPC). Besides, the phenomenon of anti-Q-slope is also obvious. It seems that the stainless steel is harmless to the cavity performance.