SPARK3D User Manual
Setting a Multipactor configuration

Adding a new Multipactor configuration

It is possible to create as many Multipactor configurations as needed in the Multipactor Configuration Group. This way, you can analyze Multipactor discharge in the same device with different Multipactor parameters. For example, you can change the SEY properties of the material.

In order to create a new Multipactor configuration, right-click on the Configuration Group tree item and select Add Multipactor Configuration option as is shown below


A new Multipactor configuration item will appear in the tree in the framework of the Multipactor Configuration Group.

Setting Multipactor configuration parameters

Multipactor configuration parameters are set from its corresponding window, which can be opened from the Multipactor configuration tree item by double clicking on it or using Open Multipactor Configuration right-click option.

Configuration window

The configuration window allows setting the multipactor simulation parameters.




Through this option the user can choose the specific regions of the structure where the analysis will be carried out by simply enabling their check-boxes. If a region, that has been previously simulated, is disabled, its results will be preserved and shown in the Multipactor window. This way, the user can incorporate new regions of analysis keeping the results of the already defined ones. 

It is also possible to access the Analysis Regions window from the Edit Regions button. It is important to point out that all modifications made on the regions from that window will apply to all configurations. So, if a region, which is used in several configurations both of Corona and Multipactor, is changed or deleted all existing results corresponding to it will be erased.


Allows choosing materials with well studied Secondary Electron Yield (SEY) properties. It also allows creating new materials and save them for future simulations.

Material name Six materials are included with their SEY properties. User defined materials are saved and loaded with the Solution.            
Maximum secondary emission coefficient Maximum SEY of the material. Typical values are between 1.5 and 3.
Secondary emission coefficient below lower crossover SEY of elastically reflected electrons at low impact energies. By default, 0.5.
Lower crossover electron energy (eV) The lowest electron impact energy at which the SEY crosses the value of 1. This is a typical value between 15 and 100 eV.
Electron energy at maximum SEY (eV) The electron impact energy at which the SEY is maximum. Typical values are between 150 and 300 eV.

It is also possible to use a custom SEY by importing it from an input file. The file must be in CSV (comma-separated-value) format, which is text file with .csv extension that consists on tabulated data. The SEY file should have 2 (or three) columns: the first one contains the electron impact energy in eV and the second one corresponds to the SEY of the material at normal incidence. The third column is optional and contains the elastically reflected electrons. If not present, these are assumed to be zero at all energies.  SPARK3D will automatically add the angle dependence for each electron impact. For energies outside the range defined in the input file, the SEY will be set to 0.

Press the button with the icon   to open a new window with a plot of the selected SEY curve.

The selection and definition of the SEY curve has an important effect on the multipactor simulation. See some practical considerations when selecting the material properties.

DC fields

By selecting the check boxes inside Uniform Fields, uniform DC fields are added to the simulation. Units are Tesla and V/m respectively.

If External DC Fields have been added to the model, these will be listed in the table. They can be independently activated or deactivated for the simulation. A Scale Factor can be applied to the fields magnitude.

Simulation preferences


Automatic power loop

If selected, the multipactor module will search automatically for the multipactor threshold, starting from the initial power and stopping when the desired precision is reached. Bisection method is employed, and the multipactor criterion (to determine whether there has been a discharge or not) is set by the Multipactor criterion menu below. The parameters are:

  • Precision (dB): This parameter sets the precision in power level desired for the multipactor breakdown onset. The default is 0.1 dB.
  • Initial power (W): This will be the initial input power used to search the multipactor breakdown onset. This can be changed to an input power level close to the final breakdown onset if some information is known a priori.
  • Maximum power (W): Sets the maximum allowed power for multipactor breakdown search. If the simulation reaches this power and no multipactor is observed the element is considered multipactor free. The default is 1 MW.
Custom power loop

If selected, the input power steps are selected by the user by pressing the edit button, see the figure below. A multipactor simulation will be done for each step. The criterion for stopping the simulation can be chosen from:

  • Stop based on multipactor criterion: The simulation will stop if a discharge (or not discharge) is detected, using the selected criterion in the menu below.
  • Stop on fixed time: The simulation time is fixed, no matter whether there is a discharge or not, unless the number of electrons decreases to 0, or reaches the maximum allowed number of electrons (1e15 for numerical stability reasons).
Initial number of electrons This defines the initial number of electrons launched in a particular component element. This number can vary in order to obtain reliable results. The default value of 100 electrons should be quite accurate in single-carrier mode and in waveguide elements where the parallel plate approximation holds. However, if the length of the waveguide element is of the order of its height more electrons could be necessary. For a complete simulation, the best idea is to start with a low number of electrons in order to get a fast idea of the approximated breakdown power level. After that, more electrons can be launched using an input power level close to the one obtained in the simulation with few electrons.
Multipactor criterion

Multipactor criterion is the mechanism that automatically decides whether there is a discharge or not at a certain input power. There are three different criteria, all of them based on the electron population:

  • Charge (automatic): This is the default mode. At each RF half-cycle, the ratio between the current number of electrons and the initial ones are checked. This criterion establishes a factor depending on the current number of simulated half-cycles. If the number of electrons is above such a factor, multipactor is detected. Basically, it sets higher factors for lower number of half-cycles (beginning of the simulation) and more relaxed ones for larger number of half-cycles (longer simulations). This is done in order to avoid false detection during the initial stages of the simulation. Additionally, if after a certain number of cycles, the ratio is below a certain number, the simulation is stopped and no multipactor is detected. This is done in order to avoid excessively large simulations in which there is not a clear electron growth.
  • Charge (fixed factor): It is similar the automatic one, but the factor is not automatic but set by the user. Gives more control on the simulation but needs of more knowledge on the specific problem from the user side. It does not have any check for low number of electrons. Only populations decreasing to zero are considered no discharges. Therefore there is a risk of long simulations.
  • Charge trend: It fits the electron evolution to a exponential curve and checks whether there is positive or negative growth. It detects both discharges and no discharges. In general, this method detects multipactor much faster than the others. However, it may suffer from higher variability between consecutive simulations. In such cases, it is advisable to use a high number of initial electrons.
Write 3D stats

It writes advanced statistics in Paraview mesh format that can be visualized from the results tab (see output section):

  • Average SEY: It shows the average SEY of the impacting electrons in each surface mesh triangle.
  • Average Impact Energy: It shows the average impact energy of the impacting electrons in each surface mesh triangle.
  • Impact Density: It shows the electron impact density (impacts/m2) for each surface mesh triangle.
  • Emission Density: It shows the electron emission density (emitted electrons/m2) for each surface mesh triangle. It can be positive (more electrons were emitted than absorbed) or negative (more electrons were absorbed than emitted).

 If Custom power loop is selected, the user can choose the input power steps by pressing the edit button. The following window appears.

Advanced Parameters Dialog

The Advanced Parameters Dialog allows for setting extra simulation parameters which are not usually needed for typical simulations but that provides extra control for advanced users.

The Advanced Parameters Dialog allows for setting the following parameters:

Any modification in the above parameters can be confirmed with the OK button and will lead to deleting all existing results. Otherwise, the user can also cancel this action through Cancel button.

There is only one exception to this behavior: the selection of regions. If one region which was already simulated is disabled for analysis, its results are kept and shown in the results window.