The Charged Particle Tracing (cpt) interface (), found under the AC/DC>Particle Tracing branch () when adding a physics interface, is used to model charged particle orbits under the influence of electromagnetic forces. In addition, it can also model bidirectional coupling between the particles and fields. Some typical applications are particle accelerators, vacuum tubes and ion implanters. The physics interface supports time-domain modeling only in 2D and 3D.

When this physics interface is added, these default nodes are also added to the Model Builder: Wall and Particle Properties. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and release features. You can also right-click Charged Particle Tracing to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is cpt.

The Formulation, Relativistic Correction, Store extra time steps for wall interactions, and Maximum number of secondary particles settings are the same as for The Mathematical Particle Tracing Interface, except that only the Newtonian, Newtonian, first order, and Massless formulations are available.

Select a Particle release specification: Specify release times (the default) or Specify current. This setting affects the way particle-field interactions are modeled, and also affects the values of certain built-in accumulated variables.

If Specify release times is selected, then each model particle is treated as the instantaneous position of one or more charged particles for the purpose of modeling particle-field interactions. This means, for example, that if the Space Charge Density Calculation node is used, the space charge density is only nonzero in mesh elements that are currently occupied by particles.

If Specify current is selected, then for the purpose of modeling particle-field interactions, each model particle traces a path that is followed by a number of charged particles per unit time. This means that the charge density computed by the Space Charge Density Calculation feature is nonzero in all mesh elements that the particle trajectories pass through, not just at the instantaneous positions of the particles. In other words, the particles leave behind a trail of space charge as they propagate.

The Specify current option is primarily used to model charged particle beams in which the charge and current density do not change over time. Changing the Particle release specification affects some inputs in the settings windows for release features such as the Release and Inlet nodes. In addition, the Surface Charge Density node is only available with the Specify release times option, while the Current Density, Heat Source, and Etch nodes are only available with the Specify current option.

Except for the option described below, these settings are the same as for The Mathematical Particle Tracing Interface, except as follows.

The Maximum number of consecutive null collisions (dimensionless) is a positive integer. The default is 100. This value is used to cap the number of collisions that a single particle can undergo within a single time step taken by the solver when performing a Monte Carlo collision simulation. This value is only used in the model if the Collisions node is present and Null collision method, cold gas approximation is selected from the Collision detection list.

For more information on the mathematics involved, see The Null Collision Method in Theory for the Charged Particle Tracing Interface.

These settings are the same as for The Mathematical Particle Tracing Interface.

The dependent variables (field variables) are the Particle position, Particle position components, Particle velocity, and Particle velocity components. The degrees of freedom for particle velocity are only used if Newtonian, first order is selected from the Formulation list. The name can be changed but the names of fields and dependent variables must be unique within a model.