You can change the radius or distance between two coil and set up a magnetic mirror field. The field can trapped charged particle between two coils.

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Enjoy the fun of Physics! You can access this simulation online from
Helmholtz coil / particle trapped in magnetic mirror field
Designed by Fu-Kwun Hwang (http://www.phy.ntnu.edu.tw/ntnujava/)
Deptartment of Physics, National Taiwan Normal University, Taiwan