TU Darmstadt, Darmstadt, Germany
IKP, Mainz, Germany
Current state-of-the-art superconducting rf (srf) accelerators are mostly using cavities made of high RRR bulk niobium (Nb). The maximum field gradients and quality factors (Q0) of these cavities are basically reached now. To further increase the srf cavity properties for future accelerator facilities, research of new materials for srf cavity applications is necessary. The current research at the S-DALINAC* is focused on the development of bake-out procedures of Nb samples and cavities in nitrogen (N) atmosphere of up to 100 mbar to nucleate the delta-phase (d-phase) of the Nb-N binary system. The d-phase has superconducting properties which exceed the properties of bulk Nb. This makes the d-phase attractive for srf applications. The vertical test cryostat (vt) at the S-DALINAC has been upgraded and recommissioned to allow investigations of the quality factor and accelerating field gradients of cavities before and after bake-out. The vt upgrade includes a newly developed variable input coupling to allow matching of the external q-factor (Qex) to Q0. The results of the ongoing research of the nitrogen atmosphere bake-out procedures and the upgrade of the vt will be presented.
*N. Pietralla, Nuclear Physics News, Vol. 28, No. 2, 4 (2018)
TU Darmstadt, Darmstadt, Germany
Niobium is the standard material for superconducting RF (SRF) cavities for particle acceleration. Superconducting materials with higher critical temperature or higher critical magnetic field allow cavities to work at higher operating temperatures or higher accelerating fields, respectively. Enhancing the surface properties of the superconducting material in the range of the penetration depth is also beneficial. One direction of search for new materials with better properties is the modification of bulk niobium by nitrogen doping. In the Nb-N phase diagram, the cubic delta-phase of NbN has the highest critical temperature. Niobium samples were annealed and N-doped in the high-temperature furnace at TU Darmstadt and investigated at its Materials Research Department with respect to structural modifications. Secondary ion mass spectrometry showed at which conditions N-diffusion takes place. X-ray diffraction (XRD) confirmed the appearance of NbN and Nb2N phases for the optimized doping process. XRD pole figures also showed grain growth during sample annealing.
TU Darmstadt, Darmstadt, Germany
TEMF, TU Darmstadt, Darmstadt, Germany
The superconducting injector section of the S-DALINAC (superconducting Darmstadt linear electron accelerator) [1] constists of two cryomodules with three 3-GHz SRF cavities in total. The first cavity of this pre-accelerator is currently a 5-cell structure designed for relativistic particle velocities. Since the gun delivers a 250 keV beam (β=0.74), this cavity is not suited for an efficient capture of the low-energy electron bunches provided by the normal-conducting section of the injector. Beam dynamics simulations and operational experience have shown a large low-energy tail in the phase-space distribution of the bunch downstream of the injector, which arises from the large phase-slippage during the capture in the 5-cell. It is therefore intended to replace the cavity with a beta-adapted 6-cell, re-using most of the cryostat parts. This contribution presents the status of the injector upgrade and the layout and manufacturing status of the new cavity.
*N. Pietralla, Nuclear Physics News, Vol. 28, No. 2, 4 (2018)