Curved fringe rods

FIGURE 6.7 A 90° ion optic design nsed with curved fringe rods and an off-axis quadrupole mass analyzer (courtesy of Varian Inc.). 

The equations of motion used to describe the trajectory of an ion in a linear quadmpole (Equation [6.12], Section 6.4.2) are strictly valid only well inside the rod assembly, well removed from the entrance and exit. At each of these ends the ideal quadmpole field (Equation [6.11]) terminates abmptly, but in any real device is affected not only by the RF and DC potentials applied to the rods but also by the potentials applied to nearby ion optical elements (lenses etc.). Moreover, the field lines created by the potentials applied to the rods spill out for some distance outside the theoretical boundaries. These curved fringe fields (Section 6.4.2a) distort the ideal quadmpole field such that the ion motions in the x- and y-directions that are independent of one another in the main quadmpole field (Equation [6.11]) become coupled as a consequence of mixing radial and axial potentials, i.e. the electrical force exerted on an ion in the z-direction can be a function of the time dependent potentials applied in the X- and y-directions (but now curved in three-dimensions), and vice versa. These effects of fringe fields are important in the following discussion. 

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