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SRF Cavities

The common approach to superconducting radio frequency (SRF) cavities is using bulk niobium, a proven technology. Niobium, a type II superconductor, is the material of choice for its high critical temperature (Tc = 9.2 K) and its high superheating critical magnetic field, which is of the order of the thermodynamic critical field (Hc = 2000 Oe). Well prepared niobium SRF cavities can have electric fields exceeding 20 MV/m with a quality factor Q of the order 1010. Cavities capable of higher field strength and with higher quality factor have been made, although Q generally decreases as the field is pushed to the limits of breakdown and quenching. 

The cost of niobium is high, especially high purity material qualified to have a high residual resistivity ratio (RRR, of order 300). Making the cavities of bulk 
niobium may be prohibitively expensive for high energy linacs that make use of 650 MHz or lower, implying large cavities. A possible approach is to move to niobium coatings, reducing the need for niobium to a tiny fraction

We explore the use of high-power impulse magnetron sputtering (HiPIMS) for the fabrication of thin films and multilayers for SRF applications.

Deposition chamber dedicated to the coating of  
1.3 GHz cavities with niobium using the HIPIMS process. 
The cylindrical magnetrons are mounted on opposing 
arms, indicated by the arrows, which allows them to 
slowly move in a synchronized manner driven by 
computer-controlled stepper motors.

Dual HIPIMS magnetron operating at the 
relatively low average power of 200 W, with 520 V
applied to the target: the light emission is mainly from 
excited argon.