  A novel design of superconducting Crab Cavity was proposed and designed at
Brookhaven National Laboratory. The new cavity shape is a Double Quarter Wave
or DQWCC. After fabrication and surface treatments, the niobium
proof-of-principle cavity was cryogenically tested in a vertical cryostat. The
cavity is extremely compact yet has a low frequency of 400 MHz, an essential
property for service for the Large Hadron Collider luminosity upgrade. The
electromagnetic properties of the cavity are also well matched for this
demanding task. The demonstrated deflecting voltage of 4.6 MV is well above the
requirement for a crab cavity in the future High Luminosity LHC of 3.34 MV. In
this paper we present the design, prototyping and test results of the DQWCC.
