A SUPER STRONG PERMANENT MAGNET FOR THE FINAL FOCUS QUADRUPOLE IN A LINEAR COLLIDER

A super strong permanent magnet quadrupole (PMQ) was fabricated and tested. It has an integrated strength of 28.5T with overall length of 10 cm and a 7mm bore radius. Two drawbacks should be considered to this NdFeB based PMQ: the negative temperature coefficient of its field strength and its fixed strength. A thermal compensation material was added and tested to cure the first problem. The correct amount was determined to compensate the PMQ’s temperature coefficient. The required field variability can be obtained by slicing the magnet into pieces along the beamline direction and rotating these slices. But this technique may lead to movement of the magnetic center and introduction of a skew quadrupole component when the strength is varied. Materials with large temperature coefficients are widely used to compensate magnetic circuits in watt-hour meters, for instance magnetic flux is shunted by a compensation strip (high coefficient material) in parallel with the pole piece, such as shown in Figure 2. At low temperature, flux density between the pole pieces is reduced because of the shunt with high permeability. When the temperature rises, the field strength of the permanent magnet reduces, while the shunt flux is also reduced. As a result, the flux density between the pole pieces is kept constant.