Monte Carlo Interactions Introduction

Interaction:

Any of the interactions handled by the MonteCarloInteractions block. These interactions can be separated into two distinct phases. First, each interaction is assumed to have a random probability for occurrence, which depends on the initial state of the interacting objects. Second, each interact is assumed to have a randomly determined final state, which also can depend on the initial state of the interacting objects. MonteCarloInteractions interactions are grouped into three main categories depending on the kind of initial state modeled by the interaction. The three main categories of interactions are:

Null Interaction:

any interaction that does NOT depend upon the full state (position and/or velocity) of any particle in the initial state. The interaction is forced to occur at every time step (i.e., have a probability of 1). For some null interactions, the initial state of the interaction does not depend upon kinetically modeled particles, such as field ionization of a gas modeled as a fluid. Alternately, other null interactions are random processes that always occur, such as the integrated effect of multiple scattering and energy loss of a particle traveling through a moderating gas.

Unary Interaction:

any interaction that depends upon the full state (position and velocity) of only one particle in the initial state. The interaction is randomly occurring, and whose probability and final state depend on only one kinetically modeled particle in the initial state. A common kind of unary interaction is one that involves the ionization of a gas, modeled as a fluid, by an incident particle.

Binary Interaction:

any interaction that depends upon the full state (position and velocity) of two distinct particles in the initial state. The interaction is randomly occurring, and whose probability and final state depend on two kinetically modeled particles in the initial state. The most common kind of binary interaction involves the collision of two kinetically modeled particles.

Some interactions model the collision of one fundamental particle with another (though not necessarily kinetically modeled). For these interactions, the probability for occurrence depends upon the cross section for the associated scattering event. These cross-section-dependent interactions include:

  • unary chargeExchange

  • all of the unary impact collisions

  • all of the binary interactions

Impact collisions

Impact collisions:

Models the interaction of incident ions or electrons with a background fluid neutral gas.

It also models the interaction of incident ions or electrons with kinetic neutral particles. In this setup, the kinetic neutral particles within each computational cell are assembled together to calculate the neutral particle density results which will be used in handling the collision algorithm. This model is completely different from the binary interactions model. In binary interactions, each pair of particles is checked for the possibility of collision and then the collision is handled. This approach is computationally expensive when there are many particles per cell in a simulation.

All impact collisions are cross section based. The user can either choose to use the builtIn cross section, the eedl cross section, in which case an EEDL data file name must be specified, or the functionDefined cross section, in which case the cross section is defined via an OAFunc function (examples below). The EEDL (Electron Evaluated Data Library [PCS91] contains electron collision cross section information for monatomic elements Z = 1 to 100 defined by the International Atomic Energy Agency (IAEA)’s Nuclear Data Services. Vorpal has interfaces, via the TxPhysics library, that can parse this EEDL data library and obtain the necessary collision cross section data for the collision handling.

Each impact collision interaction can be represented by an Interaction block, inside the

MonteCarloInteractions block, in the Vorpal input file. There are different kinds of impact collisions:

  • impactElastic

  • impactExcitation

  • impactIonCollisions

  • impactIonization

  • negativeIonDetachment

  • threeBodyRecombination

Electrons/Ions interactions with background fluid neutral gas: In this setup, all impact collisions model interaction between a Species (sortSpecies or cellSpecies) and a Fluid representing the neutral gas. These two blocks must be defined in the input file and impactSpecies and neutralGas parameter values must refer to the name of these two blocks respectively.

Electrons/Ions interactions with kinetic neutral gas: In this set up, all impact collisions model interaction between a incident electron/ion Species and a kinetic neutral particle species. The inNeutrals parameter value must refer to the name of the Species block for kinetic neutral particle.

Note

These interactions replace the ImpactCollider/ImpactCollision blocks previously present in Vorpal.