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A quadrupole with equally charged electrodes could be used to focus as well as to accelerate ion beams. The equality of electrode charges causes the driving electric field of this device to be qualitatively different from that of other devices with a similar structure composed of four rod-like electrodes. For the same reason, the area of strong field influence on the beam is shifted towards the entrance and exit of the device. Consequently, it is expected that the device's performance depends on the shapes of its entrance and exit regions. The device parameters whose influence is studied are aperture radius, gap size, and rod length. Performance of the device is quantified with accelerating and focusing potentials, which are obtained as integrals of axial and radial electric fields. These variables are further used to choose optimal values of the device parameters for desired operation of the device.