afterb_sm_li.inp

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afterB_SM_Li.inp
{

High-energy electron bunch enters a neutral lithium gas in cylindrical geometry --
Modeling the SLAC "afterburner" concept of Tom Katsouleas with self-ionization.

This input file includes the effects of tunneling ionization.
Collisional effects are ignored for both beam and plasma electrons.

This simulation models a beam-plasma wake-field accelerator:
a)  The background gas is NOT pre-ionized.
b)  Electron/ion pairs are created by tunneling ionization.
c)  Beam density exceeds generated electron plasma density, so the beam
    "blows out" plasma electrons near the symmetry axis.
d)  The electron beam is Gaussian in z and r.
e)  The electron beam is overfocussed by these fields and so executes
    betatron oscillations;  however, the focussing force varies axially.

Moving window:
a)  Once the electron beam has entered the grid and is close to the far
    edge of the simulation region, a moving window algorithm is invoked
    so that the beam can be modeled for long times.

Boundary conditions:
a)  The simulation region must be bounded by either conductors or
    insulators, in order to capture lost particles.
b)  Conductors were chosen, to avoid any charge build up.
c)  The choice of conducting boundary conditions means that electric
    fields parallel to the boundaries are forced to zero;  however,
    fields near the boundaries of the simulation must be small anyway
    to accurately model a semi-infinite plasma, so this is OK.
}

// Define variables that can be used throughout this input file.

Variables
{
// First, define some useful constants.
  pi = 3.14159
  speedOfLight = 2.998e+08
  unitCharge = 1.602e-19
  electronCharge = -1 * unitCharge
  electronMass = 9.1095e-31
  electronMassEV = electronMass * speedOfLight * speedOfLight / unitCharge
  ionCharge = unitCharge
  unitMassMKS = 1.6606e-27
  lithiumMassNum = 6.942
  lithiumMass = unitMassMKS * lithiumMassNum

// Next, define the parameters of the high-energy electron beam.
  beamEnergyEV = 50.0e+09
  beamTempEV = 0.0
  thermalBeamSpeedEV = 0.5 * beamTempEV
  totalNumBeam = 2.0e+10
  totalBeamCharge = totalNumBeam * electronCharge
  rmsBeamRadius = 1.e-05
  rmsBeamLength = 3.e-05
  rmsBeamTime = rmsBeamLength / speedOfLight
  radialCutoffFac = 3
  axialCutoffFac = 3
  totalBeamRadius = radialCutoffFac * rmsBeamRadius
  totalBeamLength = 2 * axialCutoffFac * rmsBeamLength
  beamAspectRatio = totalBeamLength / totalBeamRadius
  totalBeamArea = pi * totalBeamRadius * totalBeamRadius
  rmsBeamVolume = pi * rmsBeamRadius * rmsBeamRadius * rmsBeamLength
//	rmsBeamEmittanceNormPiMRad = 1.25e-04

// Define the number of grids in R and Z
  lengthOverRadiusAspectRatio = 3
  simRadiusOverBeamRadius = 8
  numRgridsAcrossBeam = 8
  numZgridsAcrossBeam = numRgridsAcrossBeam * beamAspectRatio
  numRgrids = numRgridsAcrossBeam * simRadiusOverBeamRadius
  numZgrids = numRgrids * lengthOverRadiusAspectRatio
  numCells = numRgrids * numZgrids

// Number of beam particles
  numBeamPtclsPerCell = 100
  numBeamCells = numRgridsAcrossBeam * numZgridsAcrossBeam
  numBeamPtcls = numBeamPtclsPerCell * numBeamCells
  beamNumRatio = totalNumBeam / numBeamPtcls

// Intermediate calculations for modeling Gaussian shape of the beam.
  invSigRsq = 1.0 / ( rmsBeamRadius * rmsBeamRadius )
  invSigZsq = 0.5 / ( rmsBeamLength * rmsBeamLength )
  invSigTsq = invSigZsq * speedOfLight * speedOfLight

// Calculate the size of the simulation region, grid spacings, time step.
// We are assuming the same grid size in both z and r	
  maxRadiusMKS = simRadiusOverBeamRadius * totalBeamRadius
  rGridSize = maxRadiusMKS / numRgrids
  zGridSize = rGridSize
  maxLengthMKS = numZgrids * zGridSize
  timeStep = 0.5 * rGridSize / speedOfLight

// This is the desired delay time before the moving window algorithm activates.
  movingWindowDelay = 0.97 * maxLengthMKS / speedOfLight

// Calculate peak currents for defining emission of the high-energy beam.
  peakCurrentDensity=totalBeamCharge*speedOfLight/rmsBeamVolume/sqrt(2.*pi)
  peakCurrent = peakCurrentDensity * totalBeamArea
  pulseLengthSec = totalBeamLength / speedOfLight
  oneHalfPulse = pulseLengthSec/2.
  oneEighthPulse = pulseLengthSec/8.
  threeEighthsPulse = 3.*oneEighthPulse
  sevenEighthsPulse = 7.*oneEighthPulse

// Define plasma temperature and resulting flux of electrons into the simulation region.
  gasTempEV = 0.00001
  gasDensityMKS = 1.8e22
  gasPressureTorr = 1.20e-21 * gasDensityMKS * gasTempEV

  numFlatCells = numZgrids/2
  numRampCells = 4
  numZeroCells = numZgrids - numFlatCells - numRampCells
  zeroEnd = (numZeroCells + .5) * zGridSize
  rampEnd = (numZeroCells + numRampCells + .5) * zGridSize

  numPtclsPerCell = 20
}

// This simulation has only one "region", which contains grid, all particles, etc.
Region
{

// Define the grid for this region.
Grid
{

// Define number of grids along Z-axis and physical coordinates.
  J = numZgrids
  x1s = 0.0
  x1f = maxLengthMKS
  n1 = 1.0

// Define number of grids along R-axis and physical coordinates.
  K = numRgrids
  x2s = 0.0
  x2f = maxRadiusMKS
  n2 = 1.0
}

// Specify "control" parameters for this region
Control
{
// Specify the time step.
  dt = timeStep

// Turn on the moving window algorithm.
  movingWindow = 1
  shiftDelayTime = movingWindowDelay

// Turn on damping for the high-frequency EM fields
  emdamping = 0.49

// Turn off the initial Poisson solve
  initPoissonSolve = 0

// Use bilinear current weighting
  CurrentWeighting=1
}

// Define the plasma ions.
Species
{
  name = lithium
  m = lithiumMass
  q = ionCharge
// advance the ions only once every 100 steps, because they
//   are essentially stationary
  subcycle = 100
// prevent out-of-control growth in # of ptcls
  particleLimit = 8.e+05
}

// Define the plasma electrons.
Species
{
  name = electrons
  m = electronMass
  q = electronCharge
// prevent out-of-control growth in # of ptcls
  particleLimit = 8.e+05
}

// Define the beam electrons.
Species
{
  name = beam_electrons
  m = electronMass
  q = electronCharge
//  collisionModel = 1
}


// Define the beam emitter, which introduces the high-energy beam into the
// simulation.
VarWeightBeamEmitter
{
  speciesName = beam_electrons
  I = peakCurrent

// Define the 2-D function F(x,t) that specifies beam emission profile.
  xtFlag = 3
  nIntervals = 32
  F=exp(-invSigRsq*x*x)*exp(-invSigTsq*(t-oneHalfPulse)*(t-oneHalfPulse))*step(pulseLengthSec-t)

// Macroparticles are emitted from the left boundary, close to the axis of symmetry.
  j1 = 0
  j2 = 0
  k1 = 0
  k2 = numRgridsAcrossBeam
  normal = 1
  np2c = beamNumRatio

// Emit particles, directed along the Z-axis,  with specified energy and temperature.
  units = EV
  v1drift = beamEnergyEV
}


// Specify the tunneling ionization parameters for background Li gas
MCC
{
  gas = Li
  pressure = gasPressureTorr
  temperature = gasTempEV
  analyticF = gasDensityMKS * step(x1-zeroEnd) * ( ramp( (x1-zeroEnd)/(rampEnd-zeroEnd) ) * step(rampEnd-x1) + step(x1-rampEnd) )

  eSpecies = electrons
  iSpecies = lithium

// turn off collision ionization
  relativisticMCC = 1
  collisionFlag = 0

// turn on tunneling ionization 
  tunnelingIonizationFlag     = 1 
  ETIPolarizationFlag = 1

// fix the number of macro particles to be created in each cell
  TI_numMacroParticlesPerCell = numPtclsPerCell
}


// Specify a perfect conductor along the left boundary.  This serves as a particle
//   boundary condition (catches particles that leave the simulation) and as a
//   field boundary condition (E_r is forced to vanish).
Conductor
{
  j1 = 0
  j2 = 0
  k1 = 0
  k2 = numRgrids
  normal = 1
}

// Specify a perfect conductor along the radial boundary.  This serves as a
//   particle boundary condition (catches particles that leave the simulation)
//   and as a field boundary condition (E_z is forced to vanish).
Conductor
{
  j1 = 0
  j2 = numZgrids
  k1 = numRgrids
  k2 = numRgrids
  normal = -1
}

// Specify a perfect conductor along the right boundary.  This serves as a
//   particle boundary condition (catches particles that leave the simulation)
//   and as a field boundary condition (E_r is forced to vanish).
Conductor
{
  j1 = numZgrids
  j2 = numZgrids
  k1 = numRgrids
  k2 = 0
  normal = -1
}

// Define the cylindrical symmetry axis.
CylindricalAxis
{
  j1 = 0
  j2 = numZgrids
  k1 = 0
  k2 = 0
  normal = 1
}

}