Modeling the fringe field of permanent magnet multipoles using numerical simulations

Abstract: Permanent Magnet multipoles (PMM) are widely used in accelerators to either focus particle beams or confine plasma in ion sources. The real magnetic field created by PMM is calculated by magnetic field simulation software and then used in particle tracking codes by means of 3 dimensional magnetic field map. A common alternative is to use the so-called 'hard edge' model, which gives an approximation of the magnetic field inside the PMM assuming a null fringe field. This work proposes an investigation of the PMM fringe field properties. An analytical model of PMM magnetic field is developed using the Fourier multipole expansion. A general axial potential function with a unique parameter ${\lambda}$, able to reproduce the actual PMM magnetic field (including its two fringe fields) with an explicit dependence on the PMM length is proposed. An analytical first order model including the axial fringe field is derived. This simple model complies with the Maxwell equations (curl(B)=0 and div(B)=0) and can replace advantageously the 'hard edge' model when fast analytical calculation are required. Higher order analytical multipole expansion model quality is assessed by means of ${\chi^2}$ estimators. The general dependence of the potential function parameter ${\lambda}$ is given as a function of the PMM geometry for quadrupole, hexapole and multipole, allowing to use the developed model in simulation programs where the multipole geometry is an input parameter.