# Strong Landau Damping

If we only use the subsystems $H_p$ and $H_E$, we are solving Vlasov–Poisson system. In this test, initial condition is as follows:

$$$f_0(x,v) = \frac{1}{\sqrt{2\pi} \sigma}e^{-\frac{v^2}{2\sigma^2}}(1+\alpha \cos(kx)), \quad E_{10} (x) = \frac{\alpha}{k}\sin(kx), \ x \in [0,2\pi/k ), \ v \in \mathbb{R}^2.$$$
using GEMPIC

The physical parameters are chosen as $\sigma = 1$, $k = 0.5$, $\alpha = 0.5$, and the numerical parameters as $\Delta t = 0.05$, $n_x = 32$ and $2 \times 10^5$ particles. We use second order Strang splitting method in time.

σ, μ = 1.0, 0.0
kx, α = 0.5, 0.5
xmin, xmax = 0, 2π/kx
∆t = 0.05
nx = 32
n_particles = 100000
mesh = OneDGrid( xmin, xmax, nx)
spline_degree = 3

df = CosSumGaussian{1,2}([[kx]], [α], [[σ,σ]], [[μ,μ]] )

mass, charge = 1.0, 1.0
particle_group = ParticleGroup{1,2}(n_particles)
sampler = ParticleSampler{1,2}( :sobol, true, n_particles)

sample!(particle_group, sampler, df, mesh)

xp = view(particle_group.array, 1, :)
vp = view(particle_group.array, 2:3, :)
wp = view(particle_group.array, 4, :);

p = plot(layout=(3,1))
histogram!(p[1,1], xp, weights=wp, normalize=true, bins = 100, lab = "")
plot!(p[1,1], x-> (1+α*cos(kx*x))/(2π/kx), 0., 2π/kx, lab="")
histogram!(p[2,1], vp[1,:], weights=wp, normalize=true, bins = 100, lab = "")
plot!(p[2,1], v-> exp( - v^2 / 2) * 4 / π^2 , -6, 6, lab="")
histogram!(p[3,1], vp[2,:], weights=wp, normalize=true, bins = 100, lab = "")
plot!(p[3,1], v-> exp( - v^2 / 2) * 4 / π^2 , -6, 6, lab="")

Initialize the arrays for the spline coefficients of the fields

kernel_smoother1 = ParticleMeshCoupling1D( mesh, n_particles, spline_degree-1, :galerkin)
kernel_smoother0 = ParticleMeshCoupling1D( mesh, n_particles, spline_degree, :galerkin)
ParticleMeshCoupling1D(1, 0.39269908169872414, 32, 32, 100000, 3, 4, 1.0, 2, [0.0, 0.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0], [-0.5773502691896258, 0.5773502691896258], [1.0, 1.0], GEMPIC.SplinePP{4}(3, [-0.16666666666666666 0.5 -0.5 0.16666666666666666; 0.5 -1.0 0.5 0.0; -0.5 0.0 0.5 0.0; 0.16666666666666666 0.6666666666666666 0.16666666666666666 0.0], [-0.041666666666666664 0.125 -0.125 0.041666666666666664; 0.16666666666666666 -0.3333333333333333 0.16666666666666666 0.0; -0.25 0.0 0.25 0.0; 0.16666666666666666 0.6666666666666666 0.16666666666666666 0.0], 32), 0.0, 12.566370614359172)

## Initialize field solver

p = plot(layout=(1,2))
rho = zeros(Float64, nx)
efield_poisson = zeros(Float64, nx)
maxwell_solver = Maxwell1DFEM(mesh, spline_degree)
solve_poisson!( efield_poisson, particle_group, kernel_smoother0, maxwell_solver, rho )
xg = LinRange(xmin, xmax, nx)
sval = eval_uniform_periodic_spline_curve(spline_degree-1, rho)
plot!(p[1,1], xg, sval, title=:rho, label=nothing)
sval = eval_uniform_periodic_spline_curve(spline_degree-1, efield_poisson)
plot!(p[1,2], xg, sval, title=:ex, label=nothing )

## Simulation function

You will get better performance if your simulation is inside a function:

function run( steps)

σ, μ = 1.0, 0.0
kx, α = 0.5, 0.5
xmin, xmax = 0, 2π/kx
dt = 0.05
nx = 32
n_particles = 100000
mesh = OneDGrid( xmin, xmax, nx)
spline_degree = 3

df = CosSumGaussian{1,2}([[kx]], [α], [[σ,σ]], [[μ,μ]] )

mass, charge = 1.0, 1.0
particle_group = ParticleGroup{1,2}(n_particles)
sampler = ParticleSampler{1,2}( :sobol, true, n_particles)

sample!(particle_group, sampler, df, mesh)

kernel_smoother1 = ParticleMeshCoupling1D( mesh, n_particles, spline_degree-1, :galerkin)
kernel_smoother0 = ParticleMeshCoupling1D( mesh, n_particles, spline_degree, :galerkin)

rho = zeros(Float64, nx)
efield_poisson = zeros(Float64, nx)

maxwell_solver = Maxwell1DFEM(mesh, spline_degree)

solve_poisson!( efield_poisson, particle_group, kernel_smoother0, maxwell_solver, rho )

efield_dofs = [efield_poisson, zeros(Float64, nx)]
bfield_dofs = zeros(Float64, nx)

propagator = HamiltonianSplitting{1,2}( maxwell_solver,
kernel_smoother0,
kernel_smoother1,
particle_group,
efield_dofs,
bfield_dofs)

efield_dofs_n = propagator.e_dofs

thdiag = TimeHistoryDiagnostics( particle_group, maxwell_solver,
kernel_smoother0, kernel_smoother1 )

for j = 1:steps # loop over time

strang_splitting!(propagator, dt, 1)

solve_poisson!( efield_poisson, particle_group,
kernel_smoother0, maxwell_solver, rho)

write_step!(thdiag, j * dt, spline_degree,
efield_dofs,  bfield_dofs,
efield_dofs_n, efield_poisson)
end

return thdiag.data

end
run (generic function with 1 method)
@time results = run(1000);
200.876794 seconds (1.11 M allocations: 65.866 MiB, 0.03% gc time, 0.09% compilation time)

## Time evolution of electric energy

plot(results[!,:Time], log.(results[!,:PotentialEnergyE1]))