Electrode configurations for fast separation of trapped ions

We study the problem of designing electrode structures that allow pairs of ions to be brought together and separated rapidly in an array of linear Paul traps. We show that it is desirable for the electrode structure to produce a d.c. octupole moment with an a.c. radial quadrupole. For the case where electrical breakdown limits the voltages that can be applied, we show that the octupole is more demanding than the quadrupole when the characteristic distance scale of the structure is larger than 1 to 10 microns (for typical materials). We present a variety of approaches and optimizations of structures consisting of one to three layers of electrodes. The three-layer structures allow the fastest operation at given distance ρ from the trap centres to the nearest electrode surface, but when the total thickness w of the structure is constrained, leading to w < ρ, then twolayer structures may be preferable.