Proton and electron in a static EM field

This example demonstrates tracing a single proton and electron motion under a uniform B field in real physical parameters. The E field is assumed to be zero such that there is no particle acceleration. Due to the fact that $m_p / m_e \doteq 1836$, the proton gyro-radius is 1800 times larger than the electron, if they start with the same velocity as in this case. In more common cases we would compare electrons and protons with the same energy, and their gyro-radii differ by a factor of $\sqrt{m_p/m_e} \sim 40$.

using TestParticle
using OrdinaryDiffEq
using CairoMakie

### Initialize grid and field
x = range(-10, 10, length=15)
y = range(-10, 10, length=20)
z = range(-10, 10, length=25)

B = fill(0.0, 3, length(x), length(y), length(z)) # [T]
E = fill(0.0, 3, length(x), length(y), length(z)) # [V/m]
B[3,:,:,:] .= 1e-11
E[3,:,:,:] .= 5e-13

### Initialize particles
stateinit = let
   x0 = [0.0, 0.0, 0.0] # initial position, [m]
   u0 = [1.0, 0.0, 0.0] # initial velocity, [m/s]
   [x0..., u0...]
end
param_electron = prepare(x, y, z, E, B, species=Electron)
tspan_electron = (0.0, 15.0)

param_proton = prepare(x, y, z, E, B, species=Proton)
tspan_proton = (0.0, 10.0)

### Solve for the trajectories
prob_e = ODEProblem(trace!, stateinit, tspan_electron, param_electron)
prob_p = ODEProblem(trace!, stateinit, tspan_proton, param_proton)

sol_e = solve(prob_e, Vern9())
sol_p = solve(prob_p, Vern9())

### Visualization
f = Figure(fontsize=18)
ax = Axis3(f[1, 1],
   title = "Electron and Ion Trajectories",
   xlabel = "x [m]",
   ylabel = "y [m]",
   zlabel = "z [m]",
   aspect = :data,
)
plot!(sol_e, idxs=(1, 2, 3), color=:tomato, label="electron")
plot!(sol_p, idxs=(1, 2, 3), color=:deepskyblue3, label="proton")

##TODO: wait for https://github.com/MakieOrg/Makie.jl/issues/3623 to be fixed!
ax.scene.plots[9+2*1-1].color = :tomato
ax.scene.plots[9+2*2-1].color = :deepskyblue3

axislegend()

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