Fermilab scientists are using their vertical test stand, located in Industrial Building 1, to test bare 1.3 GHz 9-cell niobium cavities for the ILC. During a vertical test, physicists apply a low power CW (continuous wave) RF voltage to a superconducting cavity and measure the Q (or quality) factor – a measure of the rate of energy loss. A high Q factor means that the cavity will better retain the energy pumped into it – a desirable outcome from the vertical test stand process. In order for the ILC to smash together electrons and positrons at a high energy of 500 GeV, the superconducting cavities must produce high electric fields to accelerate the particles.
In the test stand, a cavity gets immersed in liquid helium and tested inside a vertical dewar to characterize its accelerating properties. Because the cavity essentially sits in a high tech bucket, it is more practical (rather than scientifically necessary) to test it in a vertical as opposed to horizontal orientation. Cooled down to a temperature of 2 kelvins, physicists will be able to determine how high a gradient the cavity will be able to reach – the key to accelerating particles to their highest possible energies. The goal for the ILC cavities is an impressive accelerating field of 35 million volts per meter with a Q factor of 1010. Typically, each cavity will spend a day inside the vertical test stand, including the cool down and warm up period, but a test may require more time if the cavity appears to have a problem. Once the cavity completes the vertical test stand process, it will be dressed inside a helium vessel and continue on to the next qualification test.
Construction on the vertical test stand shaft, pit installation and pouring of the concrete were all completed in July 2006 at Fermilab. For RF power, the group selected a 500-watt amplifier and an RF system based on one designed for similar test stands at Jefferson Laboratory. Throughout the design process, Fermilab worked closely with physicists and engineers from DESY, Jefferson Laboratory and Cornell University. In July 2007, the team successfully tested their first single-cell superconducting cavity, proving the capability of their system.
From October 2007 through January 2009, twenty-six cooldowns were performed at the vertical test stand. These have included vertical tests not only of 9-cell ILC-style cavities, but of single-cell ILC-style cavities and 325 MHz single-spoke resonators for the High Intensity Neutrino Source project as well. In addition, the test stand has been used to develop cavity instrumentation including a variable RF input coupler, a novel thermometry system and a cavity vacuum pumping system, and to verify cavity preparation procedures at several facilities.
A vertical test stand upgrade to increase test throughput in support of Project X is underway, and includes substantial improvements to the cryogenic system and the addition of two larger test dewars.