How do I enable the Fluid Acceleration / Hybrid Time stepping in Charge Plus?

Set Cycle results to coupled in the Fluid Options, set different time scales for FEM and Fluid

Plasma simulation presents a fundamental challenge. Electrons and ions operate on vastly different timescales. Electrons are light and move quickly. Ions are heavy and move slowly. Simulating both species with a single small timestep is inefficient. The total simulation time becomes prohibitively long. 

Ansys Charge Plus provides a feature to solve this problem. It uses a hybrid, multi-timescale simulation method. This approach models fast and slow physics independently. It greatly reduces the required computation time.   

The Coupled Timestepping Method 

This method separates the simulation into two loops. A fast loop resolves electron physics. A slow loop models the ion response. The solver first simulates the electrons for a short, defined period. This period is the Cycle Duration. It then averages the effects of the electrons, such as ionization and heating. 

The solver applies these averaged effects to the ions. This occurs over a single, much larger Time Step Size. This process allows the ions to advance significantly in time. The solver repeats this loop. It finds the new electron equilibrium based on the new ion positions. This process is called finding a "mini-equilibrium." 

Use Case 1: RF Simulations 

This method is well-suited for RF-driven plasmas. A common example is a Capacitively Coupled Plasma. The user sets the Cycle Duration to the period of one RF cycle. The solver then computes the cumulative effect of many cycles on the ions. This process reveals the plasma's final, stable state efficiently. 

Use Case 2: Sub-cycling for Other Problems 

The method also applies to non-RF problems. It functions as a general sub-cycling technique. Consider an electron gun ionizing a neutral gas. The Cycle Duration is set to the characteristic time for the electrons to stabilize. The simulation then models the long-term movement of the newly created ions. This allows for the analysis of problems that were previously difficult to simulate. 

Practical Setup 

You can enable this feature in the Plasma Dynamics simulation mode. 

1. Set the small Internal Time Steps (s) for fast physics. 
2. Set the large fluid Time Step Size for slow physics. 
3. Define the Cycle Duration for the fast physics loop. 
4. In the Fluid Options, set Cycle Results to Coupled. 

This technique improves simulation speed. It makes the modeling of long-timescale plasma phenomena practical. The Coupled mode provides an efficient workflow for complex plasma simulations. 

Configuring the Coupled Method 

If Fluid Cycles is set to true, this simply enables tracking quantities averaged over the cycle duration provided.  

If Fluid Cycles is set to coupled, a more specific set of controls is put in place: The FEM information on intervals (number of time steps) and sizes (time step magnitude) should correspond to exactly one cycle duration. Fluid step should be at least as large as the cycle count times the total time for one cycle.  

Fluid cycle duration will be ignored, since the cycle duration is assumed to be given by the FEM intervals and sizes.This cycle will be repeated. The simulation of cycles will continue until the number of specified fluid intervals are finished.