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A conceptual design is presented for the 50 Tesla superconducting solenoids that are required for an optimized fast cooling ring in current designs for multi-TeV muon colliders. The solenoid utilizes high-performance multifilament Bi-2212/Ag round strand.The conductor is a cable-in-conduit consisting of six such strands cabled around a thin-wall spring tube then drawn within an outer sheath. The spring tube and the sheath are made from high-strength superalloy Inconel. The solenoid coil comprises 5 concentric shells supported independently in the conventional manner. Each shell consists of a winding of the structured cable, impregnated in the voids between cables but empty inside so that the spring tubes decouple stress so that it cannot straindegrade the fragile strands, and a high-E stress shell.An expansion bladder is located between the winding and the stress shell. It is pressurized and then frozen to provide hydraulic compressive preload to each shell. These provisions makes it possible to accommodate ~10 T field contribution from each shell without degradation, and to distribute refrigeration so that heat is removed throughout the volume of the windings.

The block coil geometry utilized in recent high-field dipole development has significant benefit for applications requiring rapid cycling, since it intrinsically suppresses coupling currents between strands. A conceptual design for a 6 Tesla dipole has been studied for such applications, in which the intra-strand losses are minimized by using bronze-process Nb 3 Sn superconducting wire developed for ITER. That conductor provides isolated fine filaments and optimum matrix resistance between filaments. The block-coil geometry further accommodates placement of He cooling channels inside the coil, so that heat from radiation and from AC losses can be removed with minimum temperature rise in the coil. The design could be operated with supercritical helium cooling, and should make it possible to operate with a continuous ramp rate of 5-10 T/s.

Virtual impactor fractionation has been used to remove all particles over a selectable micron-sized threshold in samples of precursor powders for MgB 2 and Nb 3 Sn superconductors. In a virtual impactor the powder is dispersed in an aerosol stream and passed through a vane geometry in which particles less than a critical size follow the gas streamlines which turn abruptly into a collection chamber, while particles larger than the critical size pass undeflected into a reject chamber. The aerosol dispersion was made in an inert gas flow in order to prevent degradation of the powder by exposure to oxygen or moisture.

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...