A significant component of JLab’s SRF R&D activities is cavity testing and characterization. This is performed in the Vertical Test Area (VTA), a unique facility designed for testing and measurement of SRF cavities in superfluid helium. The VTA consists of 8 dewars, 6 of which are fitted with movable radiation shields, which permits high power testing of cavities without personnel exposure to ionizing radiation. Cavities can be tested in the VTA at frequencies from 700 MHz to over 2 GHz, and at input power levels up to 500W.
In addition to cavity testing, the VTA also supports other cryogenic tests of SRF related components such as vacuum feed-throughs, mechanical and piezo-electronic tuner mechanisms, and material electrical and thermal characterization (e.g., thermal conductivity, RRR, and T c measurements).
The VTA dewars are supplied with liquid Helium (LHe) from the Cryogenic Test Facility (CTF), which can supply the VTA with 4K LHe at the rate of about 250-300L/hr (continuously, when supported by the lab’s Central Helium Liquefier (CHL)). Dewars are pumped to sub-atmospheric pressures in order to achieve temperatures as low as 1.9K, using a vacuum pump with a capacity of about 7 g/sec. Multiple dewars can be pumped down simultaneously, in accordance with system capabilities (defined by vacuum pump mass flow limits). All Helium used in the VTA is returned to the CTF where it is purified of any contaminants (most notably nitrogen) and re-liquefied.
The VTA dewars are instrumented with thermometers, LHe level sensors, and pressure transducers, and are controlled via interlocked electro-mechanical valves, which prevents damage to or contamination of the CTF Helium supply. Some functions, such as dewar LHe filling, pumpdown, and warmup, are computer controlled, while others are performed manually. A complete test cycle for the larger (850 liter capacity) dewars can be accomplished (warm-to-warm) in 36 hours or less. Smaller dewars (100-200 liter capacity) can be cycled in 8 hours (one shift). The combination of automated and interlocked control, along with efficient cryogen and thermal management, yields a facility with an SRF cavity test throughput unequalled anywhere in the world.