Permanent Magnet Quadrupole for Final Focus Lens in a Linear Collider

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Super strong permanent magnet technology reaches as strong as up to 5.5T with the extended Halbach configuration, which partially replaces permanent magnet by soft magnet material. Although superconducting magnets can generate stronger field than even super strong permanent magnets, they are not superior when the number of poles is more than four, because of their dead space caused by coil bobbin and cryostat. Thus we have investigated the feasibility of the Super strong Permanent Magnet Quadrupole(PMQ) as a final focus doublet for a linear collider.The first prototype PMQ fabricated with the extended Halbach configuration achieved the strong field gradient of a world record as a permanent magnet on the first FY, which proved the principle of the extended super strong permanent magnet technology on a quadrupole magnet.The variable PMQ, whose feature is required as a component of an optics, was designed and fabricated in second FY. The double ring structure is adopted, where the outer ring is rotated while the inner ring is fixed to minimize the shift of magnet center. The rotation angle positions are restricted only at 0 and 90 degrees to eliminate the skewing effect. This variable PMQ was measured in the third FY at SLAC. It is found that1)the strength is fairly high, 2)the dynamic range of the variability is more that 5 times, 3)positional reproducibility of the magnetic center is better than 3 μm.The field gradient can be further raised by reducing the bore size of the inner ring. Such an inner ring was fabricated in the last year. A new magnet measurement system with a rotation coil was designed and some of the parts were fabricated within the last year.