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New design for superconducting cavities for linear colliders
:: 31 August, 2007
At Texas A&M University, a team led by Peter McIntyre has developed a new design for superconducting cavities for linear colliders, perhaps even the International Linear Collider. Their polyhedral cavity design, which is only in its beginning stages and requires a significant amount of R&D, could offer such benefits as increasing the accelerating gradient and making the cavities more cost-efficient.
In the new design, the traditional smooth shape of the cavity is replaced by a polyhedral shape, made up of 12 contoured strips. Unlike traditional cavities, each one of McIntyre's cavities would be built as strings, not individual cells. To build each nine-cell cavity string, scientists would start with a continuous flat strip of niobium with copper on the underside and bend it into the contoured nine-cell shape. They would then bond the superconducting foil to a copper bar and cut it into a wedge. “What you've done is made a pie slice of a string of cavity cells,” McIntyre says. “Now you take the 12 slices and stack them around in a symmetric way to form a cavity.”
Cavities are traditionally made of purely niobium, but the copper wedge on the polyhedral cavity is used to create a simple solution to complex problems. The idea of a cavity consisting of both niobium and copper originated at KEK in Japan. Stress produced by the fields can cause the cavity to deform, a problem known as Lorentz detuning. The copper backing provides a rigid support for the cavity that prevents such deformation. The cooling system in the new design also raises the potential for cutting costs. The helium cooling for the polyhedral cavity is provided by a closed-circuit flow in cooling channels through the copper wedge, so there is no need for totally immersing the cavities in superfluid helium--the currently practiced method.
McIntyre explains that the polyhedral design would increase the current capacity of the cavity because it does not require welding. Current cavities are joined together by surface welds, which alter the grain structure of the cavity and lower the critical current. In the new design, however, metallic bands will hold the 12 pieces together, so no welds are needed. Also, if one of the 12 pieces gets damaged, it can be easily replaced. With the polyhedral design, there would be small gaps in the cavity where the slices fit together, but they do not interfere with the accelerating gradient and are very beneficial. In fact, McIntyre's team argues that these gaps help create a more focused beam and reduce the heat load.
Scientists could also more easily clean and polish the inside of the cavity because the new design allows for open access to the interior surface. The surface could be inspected directly before assembly, which cannot be done with current cavity designs. The open access also provides an easy way for scientists to apply advanced superconducting layers to the inside surface of the cavity that would make linear colliders double or triple in energy for the same power being put in.
McIntyre and his team have submitted funding requests to further develop and evaluate model polyhedral cavities. They are also seeking increased collaboration for the project. Currently, the only results the team has are from simulations and theoretical models, but they expect to have experimental data in a few months. While the new design may hold promise for future linear colliders, it will require further development and testing before it can be considered for the ILC.
News inside News:
Superconducting cavity
The superconducting cavity is of the Fabry Perot type, compatible with the static electric field required for the circular states stability. At variance with closed cavities, the surface quality of the mirrors is of the utmost importance. We have developed specific polishing techniques allowing us to get photon lifetimes in the 100 to 300 µs range. Recent improvements led to a 1ms lifetime.
Cavity design
Mirrors preparation
Photograph of polished mirrors
Cavity design:
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The two mirrors have a 50 mm diameter and a 40 mm radius of curvature. The distance between them is about 27 mm. They sustain two orthogonally polarized TEM900 modes with gaussian distributions (waist 6 mm). The degeneracy of the two modes is slightly lifted (about 100 kHz) by the mirrors imperfections. One of the mirrors is on an elastic mount, actuated by a micrometer screw and an elastic blade. This provides a gross tuning with a few tens of MHz range. Fine tuning is achieved by a piezo stack holding the other mirrors. A small dc electric field (0.3 V/cm) is maintained between the mirrors. It can be used to tune the atomic transition in and out of resonance with the cavity by Stark effect.
Mirrors preparation
The surface quality of the mirrors is of utter importance to prevent diffusive losses to the outside. Mirror roughness should be kept below 10 nm r.m.s. to get a 1 000 000 000 Q value. Mirrors preparation involves quite many steps:
Machining from pure niobium on a numerical lathe
Removal of a 100 µm crystal damage layer in a 1:1:1 mixture of HNO3;H3PO4,HF
Diamond paste polishing: roughness below 1µm
Electropolishing (see below): roughness of 20 nm rms.
Annealing at 800C for outgassing and recrystallization
Electropolishing (few µm) for oxide layers removal
0.2mm coupling holes piercing by electro-erosion
Electropolishing (few µm) for holes cleaning)
Photograph of polished mirrors
Two mirrors at a late preparation stage (before coupling hole). The optical quality of the mirrors is rather good (as shown by M. Brune's image)
Superconducting cavities for accelerators -
Abstract. Superconducting cavities have been in operation in accelerators for 25 years. In the last decade many installations in storage rings and linacs have been completed. Meanwhile, nearly 1 km of active cavity length is in operation in accelerators. Large-scale applications of superconducting radiofrequency systems are planned for future linear colliders and proton linacs.
Superconducting cavities have been proved to operate at higher gradient, lower AC power demand and more favourable beam dynamics conditions than comparable normal conducting resonators. The performance of the best single-cell cavities comes close to the intrinsic limitation of the superconducting material. Complete multicell structures with all auxiliaries (couplers, tuner, etc) lag behind in performance because of their complexity.
In this paper, an overview of accelerators with superconducting cavities is given. Limitations of superconducting performance are described and research and development efforts towards understanding and curing these effects are discussed in detail. Fundamentals of superconductivity and radiofrequency cavity design are briefly explained.
Print publication: Issue 5 (May 1998)
Received 11 November 1997
(Ref.-http://www.iop.org/EJ/abstract/0034-4885/61/5/001)
In The Images-
1.View of an assembled polyhedral cavity from the outside.
2.An inside view of how an assembled polyhedral cavity looks
3.Assembly process for a 9-cell polyhedral cavity.
4.cavity by Stark effect.
5.Mirrors preparation
6.Photograph of polished mirrors
Release link: http://www.linearcollider.org
Tags: superconducting cavities , linear colliders , polyhedral cavity design , cavities cost-efficient , . To build each nine-cell cavity string , symmetric cavity , metallic bands ,
