openEMS for magnetostatics

[Icefirejm]

Hello everyone,

I have an interest in magnetic levitation, and for a while I have been studying and becoming familiar with openEMS to try and see if it is possible to simulate the magnetic field created by electromagnets (starting off with wires) driven by a constant current source. openEMS is generally not intended to be used for such applications, but it seems like a good option when compared to the often pricy proprietary programs used in such simulations. Something similar was discussed in #231.

I've been trying to emulate a current source by using a 50 ohm lumped port with a step-excitation in the middle of a rectangular strip with the same conductivity as copper. This strip is connected to a ground plane though the use of tiny conductors at each end of the strip.

When simulated and visualized with paraview it shows there is no current density while there is an electric field throughout the structure. This surprises me because current density is calculated as the product of the electric field (which exists) and the conductivity of the material (which it has). Maybe someone can tell me what I am doing wrong?

Perhaps the idea I have for the creation of a current source is wrong. Is a voltage applied by use of an electric field over a resistance the same as a voltage source in series with a resistor? The electric field in the excitation box should be exerting a force on the electrons causing them to move. But if we look al classical circuit theory it looks like there is a shorted voltage source. I'll put the code I used below.

---beginning code---

close all
clear
clc

%% setup the simulation %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
physical_constants;
unit = 1e-3; % specify everything in mm

t_length = 3000;
t_width = 100;
t_height = 2;

m_length = t_length+500;
m_width = t_width+500;
m_height = t_height+500;

%% setup FDTD parameters & excitation function %%%%%%%%%%%%%%%%%%%%%%%%%%%%
FDTD = InitFDTD('NrTS', 5000);

F0 = 10e8;
lambda = c0/F0;

res_global = 10;
res_local = 30;

%resolution around 3mm
resolution_global = lambda/res_global/unit;
resolution_local = lambda/res_local/unit;

FDTD = SetStepExcite(FDTD);

BC = {'MUR' 'PEC' 'MUR' 'MUR' 'MUR' 'MUR'};
FDTD = SetBoundaryCond( FDTD, BC );

%% setup CSXCAD geometry & mesh %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
CSX = InitCSX();

mesh.x = SmoothMeshLines([-t_length/2 t_length/2], resolution_local, 1.3);
mesh.x = SmoothMeshLines([-m_length/2 -mesh.x mesh.x (m_length/2)], resolution_global, 1.3);

mesh.y = SmoothMeshLines([-t_width/2 t_width/2], resolution_local, 1.3);
mesh.y = SmoothMeshLines([-m_width/2 mesh.y m_width/2], resolution_global, 1.3);

mesh.z = SmoothMeshLines([-t_height/2 t_height/2], resolution_local, 1.3);
mesh.z = SmoothMeshLines([-m_height/2 mesh.z m_height/2], resolution_global, 1.3);

CSX = DefineRectGrid( CSX, unit, mesh );

%% Define port

start = [-10 -50 -5];
stop = [10 50 5];

[CSX,port{1}] = AddLumpedPort( CSX, 250, 1, 50, start, stop, [1 0 0], true);

%% Model ground plane, and connectors

copper_c = 56e6;

%ground plane
CSX = AddMaterial(CSX,'copper', 'kappa',copper_c);
CSX = AddBox(CSX,'copper',10,[-1000 -50 -5],[1000 50 5]);

%ground plane
CSX = AddBox(CSX,'copper',10,[-1500 -200 -11],[1500 200 -6]);

%connector 1
CSX = AddBox(CSX,'copper',10,[-999 -10 -11],[-989 10 1]);

%connector 2
CSX = AddBox(CSX,'copper',10,[999 -10 -11],[989 10 1]);

%% Dump box horizontal

boxStart = [mesh.x(1), mesh.y(1), 0];
boxStop = [mesh.x(end), mesh.y(end), 0];
CSX = AddDump(CSX,'At','DumpType',2,'FileType',0,'SubSampling','1,1,1');
CSX = AddBox(CSX,'At',0 , boxStart,boxStop);

%% write/show/run the openEMS compatible xml-file
Sim_Path = 'wire_port_test';
Sim_CSX = 'msl.xml';

[status, message, messageid] = rmdir( Sim_Path, 's' ); % clear previous directory
[status, message, messageid] = mkdir( Sim_Path ); % create empty simulation folder

WriteOpenEMS( [Sim_Path '/' Sim_CSX], FDTD, CSX );
CSXGeomPlot( [Sim_Path '/' Sim_CSX] );
RunOpenEMS( Sim_Path, Sim_CSX );