# Basic Settings

The Basic Settings element contains a group of property/value pairs that define the basic setup of the simulation.

surface meshing tolerance: Determines the relative size at which small cells from a meshed geometry surface are dropped. Set to 1.0 for simulations that do not contain any geometries.

cfl number: If time step is set to zero, the time step is automatically calculated, and for EM simulations, is reduced proportionately with the cfl number. The cfl number is the ratio of time step to Courant limit.

time step: If set to a value that is non-zero, this will be used as the simulation time step. If set to zero, the time step is calculated for you based on a number of factors.

• If it is an ES simulation without particles, the time step is set to 1.0.
• If it is an ES simulation with particles, the time step is set to the minimum of ($$2/{electron plasma frequency}$$ or $$1/(DLI*{electron thermal velocity}$$) where $$DLI = \sqrt{1/{DX}^2+1/{DY}^2+1/{DZ}^2}$$ in 3 dimensions.
• If it is an EM simulation, time step is equal to the minimum of ES calculated time step or $$1/{LIGHTSPEED*DLI}$$ and multiplied by the cfl number and surface meshing tolerance.

number of steps: The number of time steps to run the simulation.

steps between dumps: The number of time steps between sequential dumps of data to hdf5 format files.

dump in groups of: If set to 1, no change. If 3, data is dumped at the period specified in steps between dumps, with an extra two dumps after each of the following 2 timesteps. For example, if steps between dumps = 20, data is written to hdf5 format files at timesteps 20,21,22,40,41,42 etc. When using this option Dump Periodicity must not be set as a Runtime Option in the Run pane.

precision: The precision of the real numbers in the simulation. For greater precision, use double.

• double
• float

length unit: The unit of length used for setting parameters.

• meter

use GPU (if found): If a GPU is found, do calculations based on faster GPU updaters.

• false

verbosity: The level of informative text to be output during the simulation run. The levels are listed below in an increasing order. So, the information level will be the least verbose, and debug level 3 is the most verbose.

• information
• emergency
• critical
• error
• warning
• notice
• debug level 1
• debug level 2
• debug level 3

dimensionality: Set to 3, 2, or 1 to indicate how many dimensions in which to run the simulation.

• 3
• 2
• 1

Note

For cylindrical coordinates, only two-dimensional electrostatic simulations are currently available in visual setup. In a 2 or 1 dimensional simulation, boundary conditions and volumes can be set in higher dimensions, but will be ignored.

grid spacing: The spacing of the grid cells.

• uniform

restore geometries: This parameter is typically set to true. Due to a bug in memory management on Windows 10 systems, it should be set to false if working with complex geometries.

MPI decomposition: Typically set to default, but can be specified to decompose along a single axis. Useful for simulations with significantly more cells in one direction than others.

• axis 0
• axis 1
• axis 2

coordinate system: The type of coordinate system to work in.

• cartesian
• cylindrical

Note

For cylindrical coordinates, only two-dimensional electrostatic simulations are currently available in visual setup.

field solver: The field solver determines which equations will be used to calculate the fields.

• electrostatic

number of guard cells: For information about guard cells, see VSim User Guide: Simulation Concepts.

• electromagnetic

background permittivity: The background permittivity of the simulation, typically 1.0

dielectric solver: The type of solver to use for any dielectrics in the simulation.

• point permittivity: Standard, sets permittivity of computational cells based on a stair step method.
• permittivity averaging: Will calculate the average permittivity in a cell that has two dielectric objects.

Cerenkov Filter: Electromagnetic problems allow for the selection of a numerical Cerenkov noise filter. These filters come in 4 varieties, or no filter.

• none: No Filter
• weak: Filters with a formula of $$(1-x) * (1+x)$$. Should be used in problems with cavities or resonant structures.
• medium: Filters with a formula of $$(1-x) * (1+2x)(1-x)$$. Should be used in problems with cavities or resonant structures.
• strong: Filters with a formula of $$(1-x)$$. Should be used in problems with high numbers of relativistic particles.
• extreme: Filters with a formula of $$(1-x) * (1-x)$$. Should be used in problems with high numbers of relativistic particles.

Strong filters will execute the fastest, while medium will execute the slowest. These are all Godfrey Filters, which use additional curl-curl operations on the electric field to update the field at the next time step, removing short wavelengths. Friedman Filters are another type of filter that remove high frequency noise rather than short wavelengths. Please contact Tech-X if you wish to implement this class of filter.

• prescribed fields: The prescribed fields solver is used with a defined electric and magnetic field that is allowed to have some time dependence. It is most commonly used for multipacting simulations where it can provide very high resolution of the electric field near PEC objects.

• no field solver: For pure particle movement simulations with no fields.

periodic directions: The directions of the simulation which should be modeled as periodic, if any. Phase shifting boundaries are used for structures which do not have a full period in the simulation space. Phase shifting boundaries cannot be used in particle simulations.

• no periodicity

• periodic x

• periodic y

• periodic z

• periodic x and y

• periodic x and z

• periodic y and z

• periodic x, y, and z

• phase shifting periodic x

phase shift x: Fields will be phase shifted in the x direction by this many radians.

• phase shifting periodic y

phase shift y: Fields will be phase shifted in the y direction by this many radians.

• phase shifting periodic z

phase shift z: Fields will be phase shifted in the z direction by this many radians.

• phase shifting periodic x and y

phase shift x: Fields will be phase shifted in the x direction by this many radians.

phase shift y: Fields will be phase shifted in the y direction by this many radians.

• phase shifting periodic x and z

phase shift x: Fields will be phase shifted in the x direction by this many radians.

phase shift z: Fields will be phase shifted in the z direction by this many radians.

• phase shifting periodic y and z

phase shift y: Fields will be phase shifted in the y direction by this many radians.

phase shift z: Fields will be phase shifted in the z direction by this many radians.

• phase shifting periodic x, y, and z

phase shift x: Fields will be phase shifted in the x direction by this many radians.

phase shift y: Fields will be phase shifted in the y direction by this many radians.

phase shift z: Fields will be phase shifted in the z direction by this many radians.

particles: Whether or not to include particles in the simulation.

• no particles

• include particles: If particles are included in the simulation, the following two properties are used to help calculate the time step.

estimated max electron density: an estimate of the maximum electron density for setting a default timestep

estimated min electron temperature (eV): an estimate of the minimum electron temperature for setting a default timestep

dump nodal fields: If True, the fields used to calculate particle pushes will be written to memory. If false, they will not. This can save hard disk space in large simulations.

collisions framework: Collisions framework to be used in the simulations.

• no collisions

• reduced: Reduced collisions framework. Also called the “Impact Collider” framework.

• monte carlo: Legacy Monte-Carlo interactions from previous VSim versions.

• reactions: Full featured particle and fluid interactions most commonly used.

collision order: Either random, constant, or rotate. A random order will perform each collision in a random order, constant in the order specified, and rotate will move down the list of collisions, performing a new one first, each time. As each particle can only be involved in one collision per timestep this can effect simulation results.

moving window: Whether or not to use a moving window, which allows the simulation window to move at the speed of light in the chosen direction. Useful for simulations such as laser pulse or particle beam moving at a velocity close to the speed of light. Can only set a moving window in an electromagnetic simulation without phase shifting boundary conditions.

• no moving window

• with moving window

shift position fraction: This determines the time at which the window will begin to move. The window will begin to move at a time equal to the shift position fraction times the size of the simulation grid divided by the speed of light.

shift speed fraction: This is the relativistic $$\beta$$ factor which determines the speed $$v$$ at which the window will move, where $$v = \beta c$$.