Polyhedral cavity structure for linac colliders

Abstract

A polyhedral superconducting cavity is being developed for possible use in linac colliders. The side view of the cavity has the ellipsoidal contour of a Tesla-type multicell string, yet the end view has the shape of a dodecahedron. Each of the twelve copper wedges has the TESLA contour cut out by EDM. The copper segments have refrigeration channels gun bored through them and the solid structure eliminates Lorentz detuning. A niobium foil is bonded to the inner surface of the wedge and then the twelve are assembled to create the superconducting cavity. There are no welds, and the seams between adjacent segments do not affect the high Q of the accelerating mode but at the same time block the azimuthal currents of deflecting modes. The power coupled into deflecting modes can be slot-coupled at the seams into waveguides integrated in the copper segments and conveyed to a warm termination. This open geometry makes each segment readily available for cleaning, polishing, inspection, and characterization. The accessibility to the surface accommodates advanced superconductors that may allow for higher gradients. The performance of these materials can be tested in a superconducting test cavity. GRADIENT AND DEFLECTING MODES: PACING ISSUES FOR LINAC COLLIDERSILC has been endorsed as the next new facility for high energy research. The TESLA technology [1] has been chosen for the project as the most cost-effective basis for the ~500 GeV linacs. The design is based upon a 1.3 GHz ellipsoidal cavity, made of pure Nb, operating at 1.8 K. Prototype ILC cavities have attained an accelerating gradient of ~30 MV/m and up to ~50 MV/m with high-power conditioning. The latter is reaching the superheating limit.The capital cost of a TESLA linac collider will be dominated by the cost of the Nb cavities, cryogenics, power couplers, and RF power systems. The operating cost will be dominated by the cost of refrigerating the accelerating structures to superfluid helium temperature. The performance of a linac collider is determined by the accelerating gradient and the beam brightness that can be sustained through the acceleration process.The TESLA cavity's compound ellipsoidal geometry is a figure of revolution, as shown in Figure 1. A 9-cell cavity string is the basic module of the linac structure. Fabrication begins by forming half-cell contours from flat niobium sheet. The half-cells are e-beam welded at the equator (Figure 1) to form single cells and then 9 cells are welded at the necks to form a module.The attainable gradient can be limited by contamination of surface chemistry and by irregularities in grain structure, both of which can be generated by the equatorial weld. These issues can affect both the surface electric and magnetic fields that can be sustained. Defects, impurities, and irregularities can lead to multipacting, field emission, and thermal breakdown, all causing quench. Once a module is complete, cleaning, polishing, inspection, and characterization of the critical surfaces are performed blindly through narrow end apertu...