Fermilab’s Horizontal Test Stand is located in a shielding block cave in the Meson Detector Building. Designed to test both 3.9 GHz and 1.3 GHz 9-cell niobium cavities, the Horizontal Test Stand will play an important role in the growing ILC R&D program at Fermilab.
In order to qualify for the Horizontal Test Stand, the cavity must first get tested in the Vertical Test Stand, located in Fermilab’s Industrial Building 1. After a cavity passes the vertical test, it gets welded inside a helium vessel and dressed with a power coupler and tuner – a process that can be time consuming and reserved only for cavities that are more likely to meet performance goals. When a nine-cell bare cavity becomes a fully dressed cavity, you suddenly have something quite different that requires another test to see how it will now perform.
In the Horizontal Test Stand, approximately 300 kW (kilowatts) of RF power will get switched on for about 1.5 ms (milliseconds) at a time and repeat five times per second, the same way that the ILC will operate. As opposed to the vertical tests, which use lower power CW (continuous wave) RF, the horizontal test applies a much larger pulsed RF power to the cavity. This is the first time that the cavity will experience the pulsed RF power that will be similar to the conditions inside the ILC.
When testing, the cavity first undergoes RF processing, a procedure where physicists apply high power RF to blast away any lingering impurities. Similar to a stress test, physicists then apply as much power to the cavity as possible before it quenches, or fails, to see just how much it can tolerate. For the ILC cavities, the target is to reach an accelerating gradient of 35 MV/m (megavolts per meter) and a Q (quality) factor of 10^10. A lower Q factor will indicate that the cavity is losing power and thus not fully efficient. The tuner motor and other components also get tested while inside the Horizontal Test Stand, giving the cavity a thorough exam before it can graduate and become part of a cryomodule.
Fermilab commissioned the Horizontal Test Stand in 2007 with a 1.3 GHz ILC-style cavity. In 2008, four 3.9 GHz cavities for the FLASH free electron laser facility at DESY were tested and all achieved gradients of at least 22 MV/m, well above their required operating gradient of 14 MV/m. As operations were streamlined over the course of the year, the goal of a quick two-week turnaround time for cavities at the Horizontal Test Stand was achieved by the fourth cavity test.HTS- Update