Superconducting prototype cavities for the Spallation Neutron Source (SNS) project

The Spallation Neutron Source project includes a superconducting linac section in the energy range from 186 MeV to 1000 MeV. For this energy range two types of cavities are needed with geometrical /spl beta/ values of /spl beta/=0.61 and /spl beta/=0.81. An aggressive cavity prototyping program is being pursued at Jefferson Lab, which calls for fabricating and testing four /spl beta/=0.61 cavities and two /spl beta/=0.81 cavities. Both types consist of six cells made from high purity niobium and feature one HOM coupler of the TESLA type on each beam pipe and a port for a high power coaxial input coupler. Three of the four /spl beta/=0.61 cavities will be used for a cryomodule test at the end of 2001. Two cavities of each type have been fabricated and the first tests on both cavities exceeded the design values for gradient and Q value: E/sub acc/=10.1 MV/m and Q=5/spl times/10/sup 9/ at 2.1 K for the /spl beta/=0.61 and E/sub acc/=12.5 MV/m and Q=5/spl times/10/sup 9/ at 2.1 K for the /spl beta/=0.81.

1 ELECTROMAGNETIC DESIGN

The designs for the two types of SNS cavities were dominated by the desire to keep the electric peak surface field at or below 27.5 MV/m and the magnetic peak surface field below 60 mT. At the same time the Lorentz force detuning coefficient should not exceed a value of . By choosing a cell-to-cell coupling of 1.5% for these six-cell cavities a reasonably small ratio of peak surface fields and accelerating fields could be realized [1], if four different half cell shapes were used. The fundamental power coupler (FPC) requirement for a value of about implied larger iris and equator diameters for the end half cells at the FPC side (see figure 1) compared to the center cells. Simulation calculations done with the center cell geometries and the full cavities showed that multipacting was unlikely. The specifications for the Lorentz coefficient require welded stiffening rings between the cell irises at a radius of 80 mm. The SNS cavities have Nb55Ti flanges throughout (DESY-type), sealed with AlMg₃ gaskets: one at each tapered beam pipe end, two ports (one for each end) for the HOM couplers, rotated 115 degrees with respect to each other, a port for the field probe and one for the FPC. The coaxial FPC is the KEK design scaled to 805 MHz [2]. Table 1: SNS cavities' electromagnetic parameters

Cavity	0.61	0.81
Frequency [MHz]	805.000	805.000
	2.71	2.19
[mT/(MV/m)]	5.72	4.72
	279	483
	179	260
Cell-to-cell [%]	1.53	1.52
)	−2.07	−0.43