ANL, Argonne, USA
TU, Philadelphia, USA
Studies on the application of magnesium diboride(MgB2)high-Tc superconducting films have shown promise for use with rf cavities. Studies are directed towards applying the films to niobium cavities with the goal to increase accelerating gradients to greater than 50 MeV/m. However, studies also have shown that MgB2 films, with a critical temperature over four times higher than Nb, have surface resistances equal, or nearly equal, at 8-12 K, to what is achieved with niobium at 4 K. It might be possible to design and operate cavity systems in the 8-12K temperature range with cryocoolers that are currently available. The current cryocoolers can remove as much as 20 watts per unit in the range of 8-12K. This suggests that helium-free superconducting RF systems are possible for future light sources and possible industrial and medical linear accelerators. Our current research is directed towards depositing MgB2 films onto copper, or other high thermal conductivity metal, substrates which would allow future cavities to be fabricated as film coated copper structures. We have started atomic layer deposition and Hybrid chemical vapor deposition studies of MgB2 on 2-inch copper coupons.
TUP089
TU, Philadelphia, USA
MgB2 thin films grown by hybrid physical-chemical vapor deposition (HPCVD) have been investigated for SRF cavity applications. Clean MgB2 thin films have a low residual resistivity (<0.1 μΩcm) and a high Tc of 40 K, promising a low BCS surface resistance. Its thermodynamic critical field Hc is higher than Nb, potentially leading to a higher maximum accelerating filed. The lower critical field Hc1, which marks the vortex penetration into the superconductor and the vortex motion related dissipation, is lower for MgB2 than Nb, but it can be enhanced by decreasing the film thickness below the penetration depth. I will present results of research in two directions: enhancement of Hc1 in thin MgB2 films and multilayers, and the coating of RF cavities by MgB2. By reducing the thickness of the MgB2 film from 300 nm to 100 nm, Hc1(0) increases systematically from 38 mT to about 200 mT. Both the in-situ and two-step processes have been used for the coating of a 6 GHz cavity. Samples from various locations of the cavity show good superconducting properties. The RF characterization of the MgB2-coated cavities will be presented.
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
JLab, Newport News, Virginia, USA
TU, Philadelphia, USA
Due to the very shallow penetration depth of the RF fields, SRF properties are inherently a surface phenomenon involving a material thickness of less than 1 micron thus opening up the possibility of using thin film coatings to achieve a desired performance. The challenge has been to understand the dependence of the SRF properties on the detailed characteristics of real surfaces and then to employ appropriate techniques to tailor these surface properties for greatest benefit. Our aim is to achieve gradients >100 MV/m and no simple material is known to be capable of sustaining this performance. A theoretical framework has been proposed which could yield such behavior [1] and it requires creation of thin film layered structures. I will present our systematic studies on such proof-of-principle samples. Our overarching goal has been to build a basic understanding of key nano-scale film growth parameters for materials that show promise for SRF cavity multilayer coatings and to demonstrate the ability to elevate the barrier for vortex entry in such layered structures above the bulk value of Hc1 for type-II superconductors and thus to sustain higher accelerating fields.
[1]. A. Gurevich, Appl. Phys. Lett. 88, 012511 (2006).
