Fabrication and performance of "Muffin-tin" microwave cavities for accelerator use

Several 1-cell, 2-cell and 6-cell s-band "muffin-tin" type cavities have been fabricated out of 1.6mm thick Nb sheet metal. Without any heat treatment, the 6-cell cavities achieved Q
o
's between 3-6 × 10
⁹
and effective accelerating field gradients, E
eff
(for electrons) up to 2.7 MeV/m. After firing at 1900°C, Q
o
values between 6-14 × 10
⁹
and E
eff
up to 6.9 MeV/m were reached. Comparable Q
o
values and significantly higher field values were obtained in the 1-cell and 2-cell cases. A summary of the fabrication technique together with test results are presented. We also discuss results of other tests related to cavity performance such as the visual observation of the interior of cavities during operation and a study of heat pulses detected outside the cavity during and prior to breakdown. Tests have also been made of single cell cavities on which a surface layer of Nb was deposited by sputtering. A Q
o
of 3 × 10
⁹
and E
eff
of 4.3 MeV/m was achieved without any surface treatment after sputtering. We believe these results show that the sputtering technique is capable of producing high quality RF surfaces.