Experimental investigation of the feasibility of <i>in situ</i> plasma cleaning in normal-conducting copper cavities

High-frequency copper cavities in particle accelerators often experience dark current enhancement and performance degradation over prolonged operation. To address these issues, it is essential to remove copper oxide from the cavity walls while simultaneously suppressing field emission. A 13.56 MHz inductively coupled plasma platform with integrated coils was developed at Tsinghua University to assess the feasibility of in situ plasma treatment for restoring high-frequency performance. Experiments conducted on this platform focused on optimizing plasma discharge parameters and treatment protocols. The results demonstrate that the “argon/oxygen + argon/hydrogen” method effectively removes hydrocarbons and copper oxides from ultra-smooth, oxygen-free copper surfaces while passivating surface burrs. This dual-action treatment is helpful in reducing field emission, consequently lowering the dark current. It is also helpful to enhance the high-frequency performance and operation stability of the copper cavity. These findings validate the potential of in situ plasma cleaning as an effective technology for restoring and maintaining the performance of high-frequency copper cavities.