DOE OSTI2017
The goal of this project is to design and construct of a prototype of quadrupole permanent magnet for the magnet transport lattice for an electron-ion collider. The technical objectives are not only to achieve the field gradient of 34.42T/m in the region of interest, at a radius 16.0mm, relative to the quadrupole axis (z-axis), but also lower the unwanted field harmonics below one unit out of 10,000, especially for Sextupole, Octupole, and Decapole components. The innovation is 1) modular design for cost saving, and 2) adjusting mechanism of each modular magnet for field tuning. The 300-mm long, quadrupole magnet assembly consists of 12 modules of 100-mm long magnet. The magnetic field tuning was implemented by two ways: first, adjusting mechanically the pole tip position for a majority of field tuning, and second, applying the passive shims for a fine field tuning. The magnetic design and optimization of the quadrupole magnet was performed using finite element analysis. The permanent magnet choice was EEC-N44SH grade with the minimum coercivity of 987kA/m (12.4kOe), which has enhanced resistance to demagnetization due to the radiation. The 3D computer-aided design of quadrupole magnet assembly was created based on the magnetic design results. The overall dimensions are 514x500x300mm and its weight is ~470kg (1,050lbs). The weight and volume were minimized through the optimization. Electron Energy Corporation’s vertically integrated manufacturing capability and over 48-years magnet production experience enabled us to implement a “fast prototyping” process and build two quadrupole magnets. Using wire EDM machines, the pole tip profile was precisely fabricated within ±25µm tolerance. The field gradient 34.22T/m was measured using the real time data acquisition system and a 3D Hall probe mounted on the precision XYZ station with the resolution of 1µm (0.00004”). The magnetic field harmonics were measured using a harmonic coil at the Brookhaven National Laboratory, Upton, NY. Faro® laser tracker was used to track the position of each quadrupole while adjusting the pole location mechanically. The normal harmonics were measured up to 20-poles. Overall the 2 nd magnet test data is better than the 1st magnet because of a better symmetry. The adjusting pole technique was proved effective to lower the Sextupole harmonics from -3.14 to -0.71 unit. The Octupole harmonics was improved from -5.67 to -3.45 unit, still not satisfactory. It is found that the proposed approach is not practical and efficient in terms of the mass production although the modular concept can save the manufacturing cost. Another finding is in reality it is hard to adjust mechanically due to the limit of laser tracker resolution .001” and tweak magnetically the field due to the limited harmonics data. Importantly the Brookhaven National Laboratory’s harmonics data doesn’t indicate where the strong/weak field position, which requires another field map test at each measurement. To resolve these, two alternative solutions are introduced; 1) electromagnet correcting coils and 2) 8-pieces quadrupole magnet.
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