SRF SCIENCE AND TECHNOLOGY


Superconducting rf is one of Jefferson Lab’s core competencies and will be a key component of an increasing number of future accelerator projects. The excellent performance of today’s superconducting accelerator is the result of continuing R&D that has taken place for more than 30 years. New applications, however, will be even more demanding on the technology, and will require further advances, some incremental, others more radical.

R&D in srf science and technology will occur in two main directions:


Surface resistance of Nb at high rf fields

There is, at present, no complete theory of the surface resistance of superconductors at high rf fields. Some experiments indicate a nonlinear behavior resulting in an increase of the surface resistance. This phenomena and its origin are still poorly understood. Whether this effect is a result of preparation techniques or more fundamental might put a limit on the highest fields at which superconducting cavities can operate.


Rf critical fields of superconductors

Superconductors can have different critical fields that are well understood in dc fields. At rf frequencies, however, where the time scale is short, it is not well understood which one of these critical fields will impose a limit. The answer will have a direct impact on the usefulness of the more exotic materials for high-power rf applications.


Influence of the surface oxide structure and composition on the surface resistance of Nb

Superconducting rf is essentially a surface phenomenon where currents flow in a surface layer approximately 50 nm thick. Such a thin layer will be quite different from the bulk material since Nb can develop a number of complex oxides. Understanding the influence of both the chemical composition and structure of these oxides, as well as control them, is important in order of achieve the full potential of the Nb technology.


Effect of free-hydrogen concentration on the mechanical and electromagnetic properties of Nb

In order to achieve a low surface resistance, about 150 mm need to be removed from the current-carrying surface of superconducting Nb cavities. This removal takes place through wet chemistry, either chemical or electropolishing. These processes can generate large amount of hydrogen that can be absorbed in the niobium; hydrogen moves relatively freely in niobium but can also segregate at certain sites. The presence of this hydrogen can adversely affect the mechanical and electromagnetic properties of niobium. A better understanding of the processes involved in the motion and segregation of hydrogen, as well as means to prevent its absorption or help its removal would contribute to improving the performance of superconducting Nb cavities.


Plasma etching of Nb for SRF applications

A radically new way of processing superconducting surfaces would involve plasma etching. Such a process has been identified before as a possibility but little work has been done so far. Successful development of this dry process would eliminate the hydrogen dissolution problem mentioned above; it might also allow processing of the superconducting surfaces in situ, thus eliminating any subsequent handling and possible contamination.


Nb thin films on high thermal conductivity substrates

As superconducting Nb cavities operate at higher and higher gradients, the power dissipation increases and, unless the heat can be removed efficiently to he liquid helium, thermal instabilities can result. One approach to alleviate this problem is to use cavities made of thin Nb films on high-thermal conductivity substrates. Sputtered Nb films on Cu substrates have been used in the past, but their performance have been to date inferior to bulk Nb. New deposition techniques or new substrates may allow a further increase in the operating gradient of superconducting cavities.


Electromagnetic properties of Nb alloys

Further improvements that can be made in the niobium technology will be incremental and the ultimate limit of its performance is within reach. The next breakthrough in SRF will require new materials. The likeliest candidates are niobium alloys; however, their electromagnetic properties and their preparation techniques are still poorly understood and it still is an open question as to whether they will find application in the next generation superconducting accelerators.


Accelerating Cavities Based on Novel Superconductors

New superconducting materials that are potential candidates for rf accelerating cavities applications, whether metallic alloys or ceramics, have mechanical properties that are fundamentally different from that of niobium. They do not behave like conventional metals and are extremely brittle. Conventional fabrication techniques will not be applicable and, if these new materials are to find a use, new fabrication and processing techniques will have to be developed.