The European XFEL continuous wave upgrade requires cavities with reduced surface resistance (high Q-values) for high duty cycle while maintaining high accelerating gradient for short-pulse operation. A possible way to meet the requirements is the so-called nitrogen infusion procedure. However, a fundamental understanding and a theoretical model of this method are still missing. The approach shown here is based on sample RD, with the goal to identify key parameters of the process and establish a stable, reproducible recipe. To understand the underlying processes of the surface evolution , which gives improved cavity performance, advanced surface analysis techniques (e.g. SEM/EDX, TEM, XPS, TOF-SIMS) are utilized. Additionally, a small furnace just for samples was set up to change and explore the parameter space of the infusion recipe. Results of these analyses, their implications for the cavity RD and next steps are presented. In this paper we focus on one parameter: The "line-of-sight"(LOS) protection that is used in the current nitrogen infusion recipe [1]. The recipe consists of a niobium surface heat treatment at 800°C for 3 h and ramping down to 120°C for 48 h while applying a partial pressure (25 mTorr) of nitrogen at 120°C. It has been discovered in [2] that niobium foils wrapped around the cavity flanges during annealing at and above 800°C act as a LOS protection and can help to avoid further post chemical treatment of a cavity without losing the performance benefits that come along. Contamination with hydrocarbons and titanium particles from the NbTi-flanges of the cavities inside the furnace was suspected to be the cause of cavity performance deterioration after heat treatments without any LOS protection. In order to do a successful nitrogen infusion of a cavity, the avoidance of subsequent chemical treatment is a crucial factor. Cavity performance before and after the first N-infusion process w/o applying nitrogen, i.e heating at 800°C for 3 h under vacuum conditions followed by a ramping down to 120°C, and without subsequent chemical surface removal at DESY is shown in Fig. 1. The cavity preparation and testing is explained with further detail in [3]. Although niobium foils were used as a LOS protection for the treatments at * christopher.bate@desy.de Figure 1: Cavity performance from the first attempts of nitrogen infusion and heat treatment without post chemical surface removal at DESY. For 1DE18 no nitrogen supply was given due controller failure. For 1DE16 and 1DE17 no Nitrogen was used on purpose to test our furnace vacuum environment for heat treatment and if Q degradation happens afterwards. DESY, the cavity performance degraded as shown in Fig. 1 and look similar to ones observed in [2] when no LOS protection was used. During the heat treatment runs of Figure 2: Coverage of cavity witness samples beneath a HOM coupler to mimic LOS protection of the caps. the cavities labeled as 1DE16, 1DE17 and 1DE18, witness samples were placed under a niobium HOM coupler housing to mimic the LOS particle protection of the caps as shown in Fig. 2. Samples of all of these runs showed carbide formation on the surface. Although SEM images of star-shaped structures as in Fig. 3 look very similar to nitrides that occur under nitrogen doping [4], TEM analysis of fibbed lamellas and EDX mapping proved those to be carbides as shown in Fig. 4. A carbon-rich atmosphere beneath the HOM
The European XFEL continuous wave upgrade requires cavities with reduced surface resistance (high Q-values) for high duty cycle while maintaining high accelerating gradient for short-pulse operation. A possible way to meet the requirements is the so-called nitrogen infusion procedure. However, a fundamental understanding and a theoretical model of this method are still missing. The approach shown here is based on sample R&D, with the goal to identify key parameters of the process and establish a stable, reproducible recipe. To understand the underlying processes of the surface evolution, which gives improved cavity performance, advanced surface analysis techniques (e.g. SEM/EDX, TEM, XPS, TOF-SIMS) are utilized. Additionally, a small furnace just for samples was set up to change and explore the parameter space of the infusion recipe. Results of these analyses, their implications for the cavity R&D and next steps are presented.