Permanent Magnet Quadrupole for Final Focus Lens in a Linear Collider

采用扩展Halbach配置的超强永磁体技术可达到高达5.5T的强度，软磁体材料部分取代了永磁体。虽然超导磁体可以产生比超强永磁体更强的磁场，但当极数超过4个时，由于线圈线轴和低温恒温器产生的死区，超导磁体并不优越。为此，我们研究了超强永磁体四极杆(PMQ)作为直线对撞机最终聚焦二重线的可行性。第一个用扩展Halbach结构制作的PMQ原型在第一个FY上实现了世界纪录的强磁场梯度，证明了扩展超强永磁体技术在四极磁体上的原理。采用双环结构，外环旋转，内环固定，最大限度地减少了磁铁中心的偏移。旋转角度位置仅限于0度和90度，以消除倾斜效应。这个变量PMQ是在第三个财年在SLAC测量的。结果表明：1)强度较高，2)变化率的动态范围大于5倍，3)磁心位置重复性好于3μm，减小内环内径可进一步提高磁场梯度。这样的内环是在去年制造的。设计了一种带有旋转线圈的新型磁体测量系统，并在过去一年内制作了部分部件。

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.