Like salt crystals, niobium crystals, or grains, can be grown in either larger or smaller sizes. The individual crystals or grains can easily be seen in large-grain niobium. (Image courtesy of JLab)

With detailed recipes for rolling, baking and rinsing, one might think that gourmet chefs are building, designing and testing the superconducting cavities for the International Linear Collider. On a quest to design the most efficient and cost-saving cavity for the ILC, the physicists who are actually developing the recipes are not a far cry from chefs. And just as chefs experiment by mixing spices to create new flavours, scientists involved in superconducting cavity technology are trying out alternative ingredients by testing such things as large grain and single crystal niobium.

Named after Niobe, the granddaughter of Zeus, niobium is a superconductor – a substance with no electrical resistance in the superconducting state. Because niobium is the elemental superconductor with the highest transition temperature, it has been the choice material for many accelerator projects using superconducting technology; it is therefore also the material of choice for ILC cavities (see symmetry magazine, September 05). A material made up of crystals or grains, niobium can be grown in larger or smaller sizes – just like salt crystals. Large-grain niobium therefore simply consists of larger niobium crystals. While the cavities in the ILC will all be made out of solid niobium (potentially a cost upwards of 300 million dollars), manufacturing cavities from large grain or single crystal niobium may cut down on expenses by eliminating some of the treatments that are necessary for fine grain cavities.

Clean and extremely smooth cavity surfaces seem to be essential for achieving the high accelerating gradient of 35 MV/m (megavolts per metre) for the ILC. In theory, the boundaries between the grains in the niobium are the energetically preferred areas in the metal to collect contaminants and might be sources of material degradation. By eliminating these boundaries (hence having fewer grains or a single crystal), the large-grain and single-crystal cavities, in principle, may perform better by providing less places for contaminants to gather.

A single cell cavity made from a single crystal of niobium at JLab. (Image courtesy of JLab)

A team at JLab, led by Peter Kneisel and Ganapati Myneni and supported by Tadeu Carneiro from CBMM, a niobium manufacturing company in Brazil, built and tested several cavities from such material in the last year and received very encouraging results -- achieving a gradient of 45 MV/m in a cavity made from a single crystal. More recently, work at Cornell University and at DESY confirmed the positive results from large grain niobium cavities. There is general agreement among the physicists, however, that there is still plenty of work to do before large-grain or single-crystal niobium can be a viable option for the ILC.

At DESY, several single-cell large-grain cavities have also been tested and gradients in the vicinity of approximately 40 MV/m were measured after electropolishing the cavities. After vertical electropolishing, scientists at Cornell University achieved a gradient of approximately 30 MV/m on a single-cell large-grain cavity. Scientists at JLab recently conducted a comparison study to test large-grain niobium from three different vendors: CBMM from Brazil, W.C.Heraeus from Germany and Ningxia from China.

"We built TESLA type single-cell cavities from each material, and all the cavities went through the same treatment," said JLab's Peter Kneisel. After comparing the three different kinds of large-grain niobium, the results indicated that there weren't any major performance differences. "Even though the materials are different, they all performed between 31.5 MV/m and 34.5 MV/m, which is a surprisingly small spread," Kneisel said. "It made us happy in the sense that our procedures seem to be reproducible, and the materials seem to be performing very uniformly. However, statistics of one is never a good thing – therefore we are building and testing five more cavities from each material."

Working with vendors to produce single crystal and large grain niobium for the ILC will be among the topics at the Single Crystal Niobium Technology Workshop this November in Araxá, Brazil, the location of the world's largest niobium mine. "We would like to get industry more involved with the procedures for producing single-crystal niobium," Kneisel said.

Manufacturing more single-crystal niobium has the potential to reduce costs for the ILC. When using fine-grain niobium, the ingot material from the electron beam melting process must first be processed into a sheet by forging and rolling steps with intermediate heat treatments to make it perfectly uniform. By producing single-crystal niobium, the material would not have to undergo these extensive treatments because the sheets are obtained by simply slicing them off an ingot, resulting in potential significant cost-savings.

The electropolishing procedure may also be unnecessary for single-crystal cavities and can be replaced by the simpler and better understood buffered chemical process step. Kneisel, who believes that this elimination may be a possibility, tested large-grain cavities after just the buffer chemical polishing process and got positive results. "If you can reduce the number of steps in the process and simplify them, you will have more cost-savings," he said.

Up to this point all of the large-grain tests have been done on single-cell cavities rather than the larger nine-cell cavities, which will be required for the ILC. JLab is now in the process of building two nine-cell cavities made with large-grain niobium, and they are scheduled for testing this fall. DESY recently received a nine-cell large-grain cavity from ACCEL Instruments and has two more under fabrication. They will also begin testing in the near future.

-- Elizabeth Clements