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:
- Acquire a better understanding of the limitations of the
current materials and technology. Differentiate between the
engineering limitations and the fundamental ones. Develop methods
to overcome the engineering limits in order to operate close
to the fundamental physics limits.
- Acquire a physics understanding of the properties of novel
materials and develop techniques to make them suitable to accelerator
applications.
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.