Paul Trap Ion Motion Simulator

Douglas Cameron

TrapSim Program

TrapSim.exe is a publicly available program for simulating the trajectories of charges atoms and molecules inside of a Paul trap.

A Paul trap is a metallic ring with two metallic end caps - think of a donut with small spheres placed above and below it. The end caps are electrically separated from the ring and a radio frequency voltage is put between the ring and end caps. With the proper selection of voltage waveform the interior volume can be made to hold levitating charged particles.

TrapSim.exe is a Windows executable. The executable and two support files are located in a zipped file here.

Using the program, a very brief introduction:

  • Double-click the run button for an initial run. The trajectories of 10 randomly located ions will be drawn. The two primary graphs are two separate viewing perspectives.
  • Click on "number ions drawn" and enter a new number to decrease the number of visible ions.
  • Once stopped with the 'Run' button, a new set of ions can be started with the 'Generate new ion set' button.
    • The number of ions generated is set on the 'Pulse' page. This also controls the mass (in Atomic Mass Units), initial cloud dimensions and velocities.
    • Right clicking on any graph will clear it. Stretch a rectangle in it with the left mouse button to zoom in to any region. Double-click to restore the previous setting.
  • Az and Qz can be varied by clicking on their respective gauges and entering new numbers or using the left and right arrow keys.
    • Varying Az and Qz will yield very different trajectories.
    • For examples, try:
      • Az, Qz = -0.4,1
      • Az, Qz = 0, 0.2  then turn on viscosity
      • Az, Qz = -0.6, 1.25 nearly unstable
        Try with 50 ions to see 'sloshing'.
    • See the phase diagram below for stable settings of Az, Qz.
    • 'Walls' can also be activated to realistically halt stray ions.
  • 'Viscosity' can be activated to imitate the effect of a background gas on particle behavior. This has an arbitrary coefficient. Vary as desired. (More accurate gas pressure simulation is under development.)
  • The charge interaction between ions can be activated with 'Coulomb Interaction'. Since some experimental clouds contain 10K ions and this is not easily simulated, the Coulomb interaction can be increased by a 'X' factor as if the ion charges were increased. Set to 1 for realism, larger to emulate the effect of a large cloud.
    • Activating the Coulomb interaction with a large number of ions will greatly slow the simulation.
  • A single graph is provided as a Poincare projection. When activated, points are drawn when the trajectory of one ion passes through the z-plane. Run with the fewest ions for speed. With patience, the chaotic nature of the ion trajectory is visible.

PhaseDiag.jpg (42335 bytes)
(larger image)

Note that this program is intended for experimental simulation not mass distribution. Due to its technical nature, it may require some patience to learn to use efficiently. Experimentation with the program is encouraged.

Source code is available from the author on request. It uses the AlphaSquared graphical user interface.

Development of this program was funded primarily by The Rowland Institute for Science, under the direction of Joel Parks. The program was written exclusively by Douglas Cameron.