A schematic engineering drawing of the vertical test stand that is now under construction at Fermilab.

A view of where the cryogenic piping and other instrumentation will be connected to the vertical test stand and the covered 7 metre deep shaft in the floor of the Industry Building at Fermilab.

Fermilab has a new hole in the ground – a 7 metre deep hole to be precise. While a very large rabbit on site might find that it makes for a cosy home, this hole located in Fermilab's Industrial Building is the start of construction for the lab's first Vertical Test Stand for qualifying superconducting cavities for the International Linear Collider.

Starting in mid-2007, Fermilab will use this vertical test stand to test bare 1.3 Ghz 9-cell niobium cavities. During a vertical test, physicists apply an 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. A good Q factor means they will do this as efficiently as possible, without large energy losses to the cryogenics system. The vertical test stand thus becomes part of the qualification process for determining that the cavities meet all of the extraordinary needs of the ILC. Vertical tests also serve to verify whether the cavity preparation procedures are sufficient.

In the test stand, a cavity gets immersed in liquid helium and tested inside a vertical dewar to characterise 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 metre 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.

The ILC will require 16,000 superconducting cavities and each one needs to spend at least a day in a vertical test stand. How do physicists plan to test these cavities? Right now there are only a handful of vertical test stands around the world, and every cavity is tested. This is likely to be true at least for the next few years. In addition to measuring maximum cavity gradients, much of the initial period of test stand operation will be dedicated to understanding the reproducibility of cavity fabrication and surface preparation, as prescribed by the GDE R&D task force on high-gradient cavities. "Eventually, we hope vertical tests will be necessary for only a fraction of identically produced cavities, assuming we can first verify that cavity characteristics are reproducible when the manufacturing process is stable," said physicist Camille Ginsburg, task leader for the vertical test stand at Fermilab. "The key is to develop a process that is good enough to consistently make high quality cavities." At the moment, however, this goal may be far off, and in the end, it may turn out that all the cavities must be tested. In that case, there may eventually be a lot more rabbit holes in the world.

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. Cryogenic piping, RF instrumentation and readout sensors are starting to arrive. Ginsburg expects the cryostat, which is currently being built by a vendor, to arrive soon, and commissioning will commence this spring to be ready for testing in mid-2007. In the meantime, physicists and engineers are designing the radiation shielding and support structures for the test stand. "Building a test stand with all of this instrumentation is like a puzzle," Ginsburg said. "Fitting everything into a small space is difficult."

Throughout the design process, Fermilab's group has consulted with physicists and engineers at DESY, Jefferson Laboratory and Cornell University for information on how to build a vertical test stand for the ILC. "We really appreciate the collaboration," Ginsburg said. "I anticipate that we will have a lot more of it in the future."

-- Elizabeth Clements