Fermilab is positioning itself to be a key player in the superconducting radio-frequency cavity R&D Program in the US. Together with the Americas Regional Team for the ILC, Fermilab is building the necessary infrastructure to fabricate, process, treat and test R&D cavities for the ILC.
The ILC will accelerate electrons and their opposites, positrons, close to the speed of light. In order to provide the necessary acceleration to make particles collide at 500 billion electron volts, the ILC will use superconducting radiofrequency cavities made of pure niobium that are chilled to 1.8 degrees above absolute zero. As many as 16,000 cavities, each roughly a meter long, and placed end-to-end in vessels called cryomodules, will drive the electrons and positrons forward with an accelerating gradient of more than 30 million volts per meter (MV/m). The higher the gradient, the shorter, and hence cheaper, the ILC can be made.
How do superconducting cavities work? A voltage generator fills each hollow structure with an electric field. The voltage of the field changes from plus to minus with a certain frequency: a radio frequency, or RF. Charged particles feel the force of the electric field, and if the cavity is arranged correctly, accelerate them. Build the cavity out of superconductor, such as niobium, and chill it to near absolute zero and you have a “superconducting RF cavity.” SCRF cavities conduct electric current with extremely small loss of energy, which means that almost all the electrical energy goes into accelerating the beam, rather than into heating up the accelerating structures themselves. The Quality factor (or Q) of a cavity is a measure of how much energy the cavity stores divided by how much it loses on each oscillation of the RF electric field. The ILC goal is a Q of 1010. To get a feeling for just how amazing this is, imagine you could strike a church bell that had a Q of 1010 and rang at 2000 cycles per second. The bell would continue to ring for several months after being struck.
Designing and building a cavity such impressive performance is not simple. The cavities are made from smooth cells of Niobium, carefully electron beam welded together to form 9 cells. The cavities are electrochemically polished to provide a mirror-like surface . The surface must be free of fine particles and surface blemishes. Defects at the level of a few microns can cause cavities to lose their superconductivity and quench so that they cannot sustain the electric field needed to accelerate particles. For comparison, a human hair is 25 microns in diameter, so we are talking about really clean cavities.
For the past several decades physicists, engineers and technicians from around the world have been working together to develop improved recipes for producing the optimal cavity shape and surface. Such techniques as buffer chemical polishing, electropolishing, and high pressure water rinses are already proven to be effective treatments for achieving high accelerating gradients in superconducting cavities. Further R&D will determine precisely which cavity shape and processing recipe is the best one for the ILC.
Although cavity development occurs at laboratories, there is a strong effort to move the fabrication and eventually the processing of cavities to industry. In FY2005-06, the Americas Regional Team ordered 20 fine-grain niobium cavities for ILC R&D. A total of ten 9-cell cavities have been ordered from ACCEL Instruments, a medium-sized company near Cologne, Germany. A total of ten, 9-cell and six 1-cell cavities have been ordered from Advanced Energy Systems (AES), a small company in Medford, New York. Fermilab is also working with two new US vendors, Roark in Indiana and NioWave in Michigan, for technology transfer to fabricate these ILC cavities.
ACCEL and AES each delivered four 9-cell cavities that have undergone electropolishing processing and vertical testing at Jefferson Laboratory. One ACCEL cavity achieved a gradient of 41.5 MV/m. The maximum gradient achieved by an AES cavity was approximately 29 MV/m. The Newman Laboratory Superconducting Radiofrequency Department at Cornell University has also electropolished and tested these cavities and achieved a gradient of 31.5 MV/m. In collaboration with Argonne National Laboratory, Fermilab is developing local electropolishing, processing and testing capabilities. Fermilab is also working to develop US industrial capabilities for electropolishing cavities.
Cavity hydroforming workshop was held at Fermilab on the 1st of September 2010. Details available here.