The heat treatment of SRF cavities at medium temperature (250 °C to 350 °C), also known as “mid-T heat treatment”, is one of the R&D activities at DESY towards a high-duty-cycle (HDC) upgrade of the European XFEL. Such treated cavities exhibit an improvement in the quality factor Q0 (3E10 to 5E10) at a moderate accelerating electric field strength Eacc (10 MV/m to 20 MV/m) compared to EuXFEL cavities. In fact, cavities treated in this way do experience quenching at Eacc in the range of 20–30 MV/m, i.e. they cannot be operated at gradients above 30 MV/m. However, in this work, we have found that a heat treatment consisting of a combination of mid-T and low-T not only favorable high Q0-values were measured, but additionally high gradients of up to 40 MV/m could be achieved. This offers great potential for upgrading modern LINACs with new high usable performance. The results of 1.3 GHz TESLA-type single- and nine-cell cavities as well as the influence of the effective oxygen diffusion length l will be presented. Further insights into the surface of Nb are provided by supporting sample analyses.

A future upgrade of the European XFEL foresees High-Duty-Cycle (HDC) operation which requires a new electron beam injector. The centerpiece of such a photoinjector is a continuous wave (CW) L-band superconducting radio frequency (SRF) cavity developed at DESY. This cavity demonstrated world record peak axial electric field values of up to 50 MV/m with a copper (Cu) cathode, thread mounted to the cavity backwall. In this contribution, we report on the present status of the cavity design, describe the cold integration of the cavity including the cryostat, tuner and solenoid magnet, and show the plans for a test facility being built to verify the beam quality produced by this SRF injector.

Within the ITN (ILC Technology Network) activity led by KEK, the so-called "Medium Grain Niobium" is investigated with respect to its possible application for a large-scale SRF cavity production for the International Linear Collider ILC [1-3]. In the framework of the KEK-DESY collaboration, the niobium material for two 1.3 GHz single-cell cavities was supplied by KEK. After fabrication and initial surface treatment by electropolishing in industry, the cavities have been tested successfully at DESY with gradients above 40 MV/m. Subsequently, a heat treatment at medium temperatures around 350 °C (Mid-T heat treatment) in the DESY furnace was applied. The vertical test results of these treatments will be reported. In addition, the effect of UHV heat treatments on state-of-the-art fine grain niobium at temperatures between 800 °C and 1100 °C has been studied on samples with respect to grain growth, mechanical and thermal properties.