Globals

Global variables can be declared outside of any particular block, and they control various general aspects of the simulation. For example, the global variable dimension defines the number of dimensions for your simulation. When running your simulation from the command line, you can override values set in your .pre file for required global variables by using the variable’s command line parameter to define a new value. Not all global variables have command line parameters, in which case they must be defined in your .pre file.

Required global variables are as follows:

floattype (string, required)

Variable that defines the precision of real numbers in the simulation. For greater precision, use the value double, otherwise use the option float. You must define floattype in your input file.

Example use of floattype:

floattype = double
dimension (integer, required)

Variable that defines the dimensionality (1D, 2D, or 3D) of the simulation. Its command line parameter is -dim.

Example use of dimension:

dimension = 2
dt (real, required)

Time step size for your simulation. When choosing the step size, you also must consider stability requirements. For example, you must satisfy the Courant condition when selecting a step size in electromagnetic simulations. Its command line parameter is -dt.

Example use of dt:

dt = 1.49731212265e-16
nsteps (integer, required)

Number of steps to take. (In the case of a restart, nsteps is the number of additional steps.) Its command line parameter is -n.

Example use of nsteps:

nsteps = 22277

Optional global variables include the following, and are discussed in VSim Reference: Global Variables.

  • dumpPeriodicity
  • dumpSteps
  • maxcellxing
  • sortPtcls
  • dnSortMin
  • dnSortMax
  • useGridBndryRestore
  • copyHistoryAtEachDump
  • stepPrintPeriodicity
  • timingAnalysisPeriodicity

Global Variables Specific to Moving Windows

The moving window feature allows the simulation window to move at the speed of light in the chosen direction. This feature is used to reduce the size of the simulation box while following the physics phenomenon of interest, such as a laser pulse or a particle beam that is propagating at a velocity close to the speed of light.

This feature and its parameters are as follows:

  • moving window
  • downShiftDir
  • downShiftPos
  • OAFuncshiftSpeed

These, as well as example code, are given in detail in VSim Reference: Global Variables.

Global Variables from Inside a Block

It is possible to declare your own global variable in VSim. This is done by first defining the variable, then declaring it global. For example:

<Block>
  $ X = 4
  $ global X
</Block

This will cause the variable X to equal 4 outside of the block. The variable must be defined and declared global on separate lines. For example,

$ global X = 4

will not define X as a global variable with value 4.

Defining Basic Simulation Parameters

Your simulation will include at least the following global parameters:

  • dimension
  • floattype
  • dt
  • nsteps
  • dumpPeriodicity

You can either set these equal to actual values, or you can set them equal to variables already defined in your .pre file.