In a Paul trap, AC voltages are applied to four coaxial rods called a quadrupole. The voltages are applied in a checkerboard pattern, as in Fig. 1. Ions in the middle of the trap act in the same way that a ball would that is placed on a rotating saddle-shaped surface (Fig. 2). For example, a positive ion would be attracted to negative poles and repelled from positive poles. Similarly, the ball is attracted to the saddle's valleys and repelled from the saddle's hills. This situation is unstable for DC voltages and a stationary saddle. Applying AC voltages or rotating the saddle at the correct frequency allows for a stable solution.
Even though the ions are effectively trapped inside the quadrupole, they do not remain in one location-the ions are still subject to the forces from the quadrupole's electric field. Thus, the AC voltage will cause a micro-motion of the ions, and they will move in tiny circles near the axis of the quadrupole. If the frequency is too slow and the voltage is too high, these circles will be too big, and the ions will leave the trap. If the frequency is too high, and the is voltage too low, minimal confinement will occur.
Assuming that the frequency and voltage are optimized, this technique now confines the ions to the axis of the quadrupole. To trap them in the axial dimension, a positive voltage is applied to endcaps on both ends of the quadrupole, repelling positive ions inward. This now completely traps the ions in all directions.
In a Paul trap, AC voltages are applied to four coaxial rods called a quadrupole. The voltages are applied in a checkerboard pattern, as in Fig. 1. Ions in the middle of the trap act in the same way that a ball would that is placed on a rotating saddle-shaped surface (Fig. 2). For example, a positive ion would be attracted to negative poles and repelled from positive poles. Similarly, the ball is attracted to the saddle's valleys and repelled from the saddle's hills. This situation is unstable for DC voltages and a stationary saddle. Applying AC voltages or rotating the saddle at the correct frequency allows for a stable solution.
Even though the ions are effectively trapped inside the quadrupole, they do not remain in one location-the ions are still subject to the forces from the quadrupole's electric field. Thus, the AC voltage will cause a micro-motion of the ions, and they will move in tiny circles near the axis of the quadrupole. If the frequency is too slow and the voltage is too high, these circles will be too big, and the ions will leave the trap. If the frequency is too high, and the is voltage too low, minimal confinement will occur.
Assuming that the frequency and voltage are optimized, this technique now confines the ions to the axis of the quadrupole. To trap them in the axial dimension, a positive voltage is applied to endcaps on both ends of the quadrupole, repelling positive ions inward. This now completely traps the ions in all directions.
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