## Particle Tracing Module

### New App: Red Blood Cell Separation

This app examines the separation of red blood cells and platelets in a microfluidic channel using dielectrophoresis. The red blood cell and platelet diameter are input, as well as the electromagnetic frequency and applied potential. The separation efficiency is computed and there are visual plots for the particle trajectories, electric potential, and fluid velocity.

*Red blood cells and platelets are separated by the dielectrophoretic force. The bottom-right outlet in the geometry only releases red blood cells, indicating that the sample is sufficiently pure for further analysis.*

Red blood cells and platelets are separated by the dielectrophoretic force. The bottom-right outlet in the geometry only releases red blood cells, indicating that the sample is sufficiently pure for further analysis.

### New Multiphysics Interfaces for Particle Tracing

The following new multiphysics couplings have been introduced:

*Electric Particle-Field Interaction*: Uses the positions of charged particles to generate a space charge density that can then be included in an Electrostatics interface.*Magnetic Particle-Field Interaction*: Uses the positions and velocities of charged particles to generate a current density that can be included in a Magnetic Fields interface.*Fluid-Particle Interaction*: Computes the volume force exerted on a fluid by particles.

For each new multiphysics coupling, there is a new multiphysics interface that can be used to create the necessary physics interfaces.

*The Particle Field Interaction, Non-Relativistic*interface creates an*Electrostatics*interface, a*Charged Particle Tracing*interface, and the*Electric Particle-Field Interaction*multiphysics coupling. Use this interface to model constant-current beams of charged particles at nonrelativistic speeds.*The Particle Field Interaction, Relativistic*interface creates an*Electrostatics*interface, a*Charged Particle Tracing*interface, a*Magnetic Fields*interface, and the*Electric Particle-Field Interaction*and*Magnetic Particle-Field Interaction*multiphysics couplings. Use this interface to model relativistic charged particle beams at constant current that may generate significant magnetic fields. This multiphysics coupling also requires the AC/DC Module.- The
*Fluid-Particle Interaction*interface creates a*Single-Phase Flow*interface, a*Particle Tracing in Fluids*interface, and the*Fluid-Particle Interaction*multiphysics coupling. Use this interface to model the flow of particles in a fluid when the mass flow rate is constant.

### Bidirectionally Coupled Particle Tracing Study Step

The new *Bidirectionally Coupled Particle Tracing* study step can be used to set up two-way couplings between particle trajectories and fields. It automatically creates a pair of For/End For nodes in the solver sequence, which allows time-dependent particle trajectories and stationary fields to interact with each other.

### Inelastic Collisions

The new Collisions node can be used to model several different types of interactions between charged particles and a background gas. The following subnodes, each representing a different type of interaction, can be added to the Collisions node:

- Elastic
- Attachment
- Excitation
- Ionization
- User Defined

Each of the subnodes to the Collisions node is based on a Monte Carlo scattering model in which each particle is given a probability to undergo a collision based on the collision frequency and time-step size.

The Collisions node replaces the Elastic Collision Force feature. The friction model, a deterministic force previously accessed through the Elastic Collision Force feature, can be accessed by using a dedicated Friction Force node.

### New Release Feature for Particle Beams

The new Particle Beam node can be used to release beams of charged particles by specifying the beam emittance and Twiss parameters, with either an elliptical or Gaussian distribution in phase space. In addition, new global variables allow quantities like beam emittance to be visualized easily during results postprocessing.

*Magnetic lens: Particles are released in a beam with a symmetric bi-Gaussian distribution (top left). The beam hyperemittance is plotted along the nominal trajectory (bottom left). A Poincaré map shows the particle positions at several cross sections, each indicated by a different color (right).*

Magnetic lens: Particles are released in a beam with a symmetric bi-Gaussian distribution (top left). The beam hyperemittance is plotted along the nominal trajectory (bottom left). A Poincaré map shows the particle positions at several cross sections, each indicated by a different color (right).

### Space Charge Limited Emission

A dedicated multiphysics node for space charge limited emission of particles from a surface is now available. Space charge limited emission of electrons occurs when any further increase in the current of emitted particles would generate sufficiently high space charge density to repel particles back toward the surface from which they were released. The *Space Charge Limited Emission* node and *Electric Particle Field Interaction* node can be used together to determine the space charge limited current. A new tutorial has been added to the Application Library (see screenshot) called *Child's Law Benchmark*, which demonstrates this effect.

### Improved Accumulators

Domain-level *Accumulator* features no longer require small manual time steps; in most cases, accumulated variables can now be computed accurately with the default solver settings. As a result, many models that use *Accumulator* nodes on domains are now calculated up to and above ten times faster with improved accuracy. New options are also available for determining how the accumulated variable is interpolated when particles cross over many mesh elements in a single time step.

### Release Particles from a Text File

It is now possible to initialize particle positions and velocities using data from an imported text file using the Release from Data File node.

### New Options for Sampling from Velocity Distributions

When releasing particles with a spherical, hemispherical, conical, or Maxwellian distribution, you can choose to release them with either a deterministic velocity distribution or using random sampling of this distribution.

### New Particle-Particle Interaction Force Settings

A new built-in *Particle-Particle Interaction* force option is available: Linear elastic force. Selecting the option to apply a cut-off length to any particle-particle interaction force sets the force to zero when particles are sufficiently far apart.

### Specified Combinations When Releasing Particles from a Grid

The *Release From Grid* node can now be used to release particles at either specified combinations of coordinates or all combinations of coordinates. When releasing particles, it is possible to select a Grid type: either *All combinations* or *Specified combinations*. This feature enables much finer control over the initial positions of particles, making it possible to release particles at locations other than a rectangular grid.

### New Tutorial: Relativistic Diverging Electron Beam

When modeling the propagation of charged particle beams at high currents and relativistic speeds, the space charge and beam current create significant electric and magnetic forces that tend to expand and focus the beam, respectively. The Charged Particle Tracing interface uses an iterative procedure to efficiently compute the strongly coupled particle trajectories and electric and magnetic fields for a beam operating at constant current. A mesh refinement study confirms that the solution agrees with the analytical expression for the shape of a relativistic beam envelope.

*A beam of relativistic electrons is released at the waist and begins to diverge. The electric field (red) and magnetic field (blue) of the beam are plotted along its trajectory.*

A beam of relativistic electrons is released at the waist and begins to diverge. The electric field (red) and magnetic field (blue) of the beam are plotted along its trajectory.

### New Tutorial: Child's Law Benchmark

Space charge limited emission is a phenomenon that restricts the current of charged particles that can be released from a surface. As the electron current released from the cathode increases, so does the magnitude of the space charge density in the immediate vicinity of the cathode. This distribution of charge density exerts an electric force on the emitted electrons, directed toward the cathode. The space charge limited current is the maximum current that can be released such that the emitted particles are not repelled back toward the cathode.

In this example, the space charge limited current in a plane-parallel vacuum diode is computed using the Space Charge Limited Emission node. The resulting electric potential distribution and current are compared with the analytical solution given by Child's Law. The current density is computed using a study, called Bidirectionally *Coupled Particle Tracing*, which establishes a bidirectional coupling between the particle trajectories and the electric potential.