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In this paper, a permanent-magnet-quadrupole doublet lens with strong-focusing is designed and manufactured. Such an instrument could solve the problem of irregular beam spots produced by the weak focusing of conventional solenoid-focusing systems in low-energy electron irradiation accelerators. It could provide a better and more appropriate initial beam spot before the beam enters the subsequent magnetic spreading system or magnetic scanning system, thus, improving the irradiation uniformity. The association of the parameters of the rectangular permanent magnet with the quadrupole magnetic field is investigated. Focusing results, obtained using irregular beam spots with different profiles and different energies, and magnetic field measurement results, both from the instrument, verify the engineering feasibility of this permanent-magnet-quadrupole doublet lens.

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