IMP is currently constructing three major SRF linacs: the High Intensity Heavy Ion Accelerator (HIAF), the China Initiative Accelerator Driven System (CiADS), and the Isotope Platform based on a high current superconducting linac (IP-SAFE), and operating one superconducting linac for Super Heavy Elements (CAFE2). This talk will report recent progress of these projects with emphasis on SRF equipment, operation stability, and discuss lessons learned. R&D activities to meet high demands on mass production and testing will also be presented, which includes robot-assisted clean assembly, RF testing, cryomodule design and testing, performance analysis, as well as new methods and innovative structures.

The European XFEL is in operation since 2017 with a maximum energy of 17.5 GeV in short-pulse (SP) mode, consisting of 0.65 ms-long bunch trains at 10 Hz repetition rate. The accelerator can deliver up to 2700 electron bunches every 100 ms, with a spacing between bunches of 220 ns. After eight years of successful operation the accelerator team, with strong support from the XFEL strategy process, is working to define an accelerator upgrade scenario for possible implementation in the next decade. The main goal of the upgrade is to facilitate more bunches per second with larger bunch spacing while maintaining the high energy of the beam, a world record amongst FEL machines. Possible scenarios include continuous-wave (CW) and long-pulse operating modes, collectively referred to as high duty cycle (HDC). This paper describes the different operating modes under investigation and the R&D activities ongoing at DESY to support the upgrade. The main focus of the paper is on the cryomodule and cavity design modifications, while also giving a brief introduction of the other challenging aspects connected to the upgrade.

The China Spallation Neutron Source (CSNS) is the fourth pulsed accelerator-driven neutron source in the world. Meanwhile, it is one of the core large-scale scientific facilities of the Guangdong-Hong Kong-Macao Greater Bay Area Comprehensive National Science Center. The planned China Spallation Neutron Source Phase II (CSNS-II) started construction in 2024 and is scheduled to be completed in July 2029. To achieve a beam power of 500 kW for target station, the beam energy of the linear accelerator needs to be increased to 300 MeV. Therefore, a superconducting linear accelerator composed of two types of superconducting cavities, namely 324 MHz double-spoke cavities with β0 is 0.5 and 648 MHz 6-cell elliptical cavities with βg is 0.62, will be added after the Drift Tube Linac (DTL). We have completed the R&D of a prototype double-spoke cavity cryomodule and two prototype elliptical cavities. The test results showed that the maximum gradients of the two double spoke cavities at a pulse width of 4 ms and a repetition frequency of 25 Hz was 15.2 MV/m during horizontal test, while the maximum gradient of the elliptical cavity reaches 25.7 MV/m during vertical test. Both type cavities test results indicate that the design and post processing are very reliable，the mass production of superconducting cavities, couplers, tuners, and cryostats has been initiated, with plans to complete the manufacturing of all cryomodules by early 2027.

A specialized setup was designed to carry on a mid-T baking of SRF niobium cavities. It utilizes resistive heaters installed on the outer cavity walls, with a cryostat serving as a vacuum vessel. Based on our material studies with the real-time in-situ synchrotron XPS, a single-cell 1.3 GHz cavity was thermally treated in the regime providing contamination-free oxygen doping of niobium. RF tests of the cavity showed a significant reduction in surface resistance, primarily due to a decrease in residual resistance, with no field emission or degradation of the maximum accelerating field. This developed procedure can be potentially applied to bake “dressed” cavities prior to cryomodule assembly without breaking the cavity vacuum, thereby preventing surface re-oxidation and allowing the full benefits of mid-T baking to be realized in a real accelerator environment.

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.

Theories predict that Superconducting-Insulating-Superconducting (SIS) multilayers delay vortex penetration allowing for operation gradients more than twice of bulk Nb cavities and significantly higher Q-values [1]. The University of Hamburg focuses on Atomic Layer Deposition (ALD) as the most promising technique to coat SIS multilayers. A proof-of-principle experiment to coat cavities with an insulator has been successfully carried out, and the complex coating process was numerically modelled, which resulted in a further process time reduction while maintaining the high film quality [2,3]. For SIS multilayer deposition, plasma-enhanced ALD (PEALD) is used to deposit AlN and NbTiN as dielectric and superconducting material, respectively. The deposition process and post-deposition treatments have been optimized by studying the superconducting properties of the NbTiN thin film [4]. Moreover, properties such as flux-trapping behaviour and thermal transmittance of SIS multilayers have been measured. Furthermore, various material characterization techniques were applied to investigate the contribution of vacancy densities, recrystallization eVects due to the annealing past the deposition and the impact of the insulating layer on the properties of SIS multilayers. This talk will show the aggregated results of all those measurements and present the status of the PEALD single-cell cavity coating device at the University of Hamburg.