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Transcript of ECE586: Advanced E&M Simulation (2004) On PDX1 Program 2004. 9. 16 HyunChul Kim and J.K. Lee Plasma...
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ECE586: Advanced E&M Simulation
(2004)
On PDX1 ProgramOn PDX1 Program
2004. 9. 16
HyunChul Kim and J.K. Lee
Plasma Application Modeling, POSTECH
References:• Minicourse by Dr. J. P. Verboncoeur (PTS Group of
UC Berkeley) in IEEE International Conference on
Plasma Science (2002)• “Plasma Physics via Computer Simulation” by C.K.
Birdsall and A.B. Langdon (Adam Hilger, 1991)
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A Series of XPDX1*
r
~ LRC
Computation Space
* Developed by PTS group, UC BerkeleyAll are available at http://ptsg.eecs.berkeley.edu
XPDx1: X window (using xgrafix library),
Plasma Device, 1 Dimensional (1d3v), Bounded
(with external circuit drive), Electrostatic Code• XPDP1 (x=P) : Planar Configuration
• XPDC1 (x=C) : Cylindrical Configuration
• XPDS1 (x=S) : Spherical Configuration
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PIC Overview
• Plasma behavior of a large number of charges
particles are simulated by using a few
representative “super particles”.
• PIC codes solve fundamental equations, the
Newton-Lorentz equation of motion to move
particles in conjunction with Maxwell’s
equations (or a subset) with few
approximations.
• PIC codes are quite successful in simulating
kinetic and nonlinear plasma phenomenon like
ECR, stochastic heating, etc.
PIC Codes Overview
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Computer Simulation of Plasma
Kinetic Description
Fluid Description
Vlasov, Fokker-Planck
Codes
ParticleCodes
Hybrid Codes
FluidCodes
• The particle-particle model
• The particle-mesh model
• The particle-particleparticle-mesh model
Particle codes
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XPDx1 Flow Chart
Fig: Flow chart for an explicit PIC-MCC scheme
I II
III IV
IV
V
ix, )( v
j, )( BE
1 ,:Subcycling ktkt fastslowi
• Particles in continuum
space• Fields at discrete mesh
locations in space• Coupling between
particles and fields
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I. Particle Equations of Motion
Newton-Lorentz equations of motion
)BvE(Fv qmdt
d
vx dt
d
In finite difference form, the leapfrog method
)B2
vvE(
vv 2/2/2/2/t
ttttt
tttt
m
q
t
2/vxx ttttt
t
• Second order accurate
• Stable for 2twp
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I. Particle Equations of Motion
m
tq ttt
2
Evv 2/
• Boris algorithm
m
tq ttt
2
Evv 2/
t
m
q
tB)vv(
2
vv
v
v vv
vv
m
tq tt
2
B)
2tan(b̂t
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I. Particle Equations of Motion
t' tvvv
tt
t
'tt1
t2vvv
v
v
ttv
'v
tt
t
'tt1
t2v
Finally,
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XPDx1 Flow Chart
Fig: Flow chart for an explicit PIC-MCC scheme
I II
III IV
IV
V
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II. Particle Boundary
Secondary electron emission
• Ion impact secondary emission
• Electron impact
secondary emission
+
– se
ionelectron–
• Conductor : absorb charge, add to the global σ
Absorption
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XPDx1 Flow Chart
Fig: Flow chart for an explicit PIC-MCC scheme
I II
III IV
IV
V
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III. Electrostatic Field Model
• In electrostatics,
Maxwell’s equation in vacuum
EDDt
BE 0,,
HBBt
DJH 0,0,
EE 0
0
2
(Poisson’s equation)
Or Gauss’ law
enclosedVSQdVsdD
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III. Electrostatic Field Model
Possion’s equation
),,(),( tt xx
• Finite difference form in 1D planar geometry
,2
2
11
jjjj
x
Boundary condition : External circuit
20010
2/1
x
xE
tttt
0
0 E J
JE
A
QQdtJ
tttt
tt plasmattt
00
From Gauss’s law,
• Short circuit
0)( specified, is )(0 tt J
• Open circuit
t
tt plasmattt dtJ00
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III. Electrostatic Field Model
• Voltage driven series RLC circuit
From Kirchhoff’s voltage law,
)()()(
)()()(
0
2
2
tttVC
tQ
dt
tdQR
dt
tQdL
J
― One second order difference equation
where
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XPDx1 Flow Chart
Fig: Flow chart for an explicit PIC-MCC scheme
I II
III IV
IV
V
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IV. Coupling Fields to Particles
Particle and force weighting
: connection between grid and particle quantities
• Weighting of charge to grid • Weighting of fields to particles
a point charge
grid point
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IV. Coupling Fields to Particles
• Nearest grid point (NGP) weighting
fast, simple bc, noisy
• Linear weighting
: particle-in-cell (PIC) or cloud-in-cell (CIC)
relatively fast, simple bc, less noisy
• Higher order weighting schemes
slow, complicated bc, low noisy
NGP
Linear spline
Quadratic spline
1.0
0.5
0.0
Cubic spline
Fig: Density distribution function of a particle atfor various weightings in 1D
xxi xxi 2ixxxi xxi 2
Position (x)
ix
)( ixxSx
i
ijij xXSqX )()( j
ijjii xXSExqF )(
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Weighting in 1D
• For linear weighting in cylindrical coordinates,
( 0 < j < N )
IV. Coupling Fields to Particles
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XPDx1 Flow Chart
Fig: Flow chart for an explicit PIC-MCC scheme
I II
III IV
IV
V
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Collisions
Electron-neutral collisions
• Elastic scattering (e + A → e + A)
• Excitation (e + A → e + A*)
• Ionization (e + A → e + A+ + e)
Ion-neutral collisions
• Elastic scattering (A+ + A → A+ + A)
• Charge exchange (A+ + A → A + A+)
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V. Monte-Carlo Collision Model
• The MCC model statistically describes the collision processes, using cross sections for each reaction of interest.
• Probability of a collision event
])(exp[1 tnP iiTgi
j ijiT )()( where
• For a pure Monte Carlo method, the timestep is chosen as the time interval between collisions.
iiTgi n
Rt
)(
)1ln(
However, this method can only be applied when space charge and self-field effects can be neglected.
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V. Monte-Carlo Collision Model
• There is a finite probability that the i-th particle will undergo more than one collision in the timestep.
Since XPDx1 deals with only one collision in the timestep, the total number of missed collisions
.1
2
i
i
k
ki P
PPr
Hence, XPDx1 is constrained byfor accuracy.
1max tv
))((max where max Tgnv
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V. Monte-Carlo Collision Model
• Computing the collision probability for each particle each timestep is computationally expensive.
→ Null collision method
].exp[1 max tPT
1. The fraction of particles undergoing a collision each time step is given by
3. The type of collisions for each particle is determined by choosing a random number, .0 maxR
2. The particles undergoing collisions are chosen at random from the particle list.
Fig: Summed collision frequencies for the null collision method.
Null collision
Collision type 3
Collision type 1
Collision type 2
Tc PNN
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Numerical Parameters
Choose Δx and Δt to resolve the smallest important
physical feature
Require Δx < Debye length, sheath length, wave
length, Larmor radius, boundary feature, etc.
Require for all species (“particle
Courant”) for accurate sampling of fields.
Require for accuracy of explicit leap
frog mover or for accuracy when space charge forces
are important.
Require when collisions are important.
max/xt
pwt /2.0
/1t
Require # of superparticles per cell > 10. It should
be larger in simulations where particles remain
trapped for long times.
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Example of XPDP1 Input FileRF DISCHARGE(IN MKS UNITS) Voltage-driven with electron-neutral collisions (Argon atom)
-nsp---nc---nc2p---dt[s]---length[m]--area[m^2]--epsilonr---B[Tesla]---PSI[D]-- 2 400 1.8e6 8e-12 0.03 0.01 1.0 0 .0 0.0-rhoback[C/m^3]---backj[Amp/m^2]---dde--extR[Ohm]--extL[H]---extC[F]---q0[C]- 0.0 0.0 0.0 0.0 0.0 1.0 0.0-dcramped--source--dc[V|Amp]--ramp[(V|Amp)/s]---ac[V|Amp]---f0[Hz]--theta0[D]- 0 v 0.0 0.0 100 13.56e6 0--secondary--e_collisional---i_collisional---reflux---nfft--n_ave--nsmoothing--ntimestep-- 1 1 2 0 0 276549 6 0--seec(electrons)---seec(ions)---ion_species----Gpressure[Torr]---GTemp[eV]---imp-- 0.0 0.2 2 100e-3 0.026 0---GAS----psource--nstrt-- 1 0 0
SPECIES 1----q[C]-------m[Kg]---j0L[Amp/m^2]---j0R[Amp/m^2]----initn[m^-3]----k-- -1.602e-19 9.11e-31 0.0 0.0 5e14 1--vx0L[m/s]---vxtL[m/s]--vxcL[m/s]---vxLloader(0=RNDM,1=QS)-- 0.0 4.19e5 0.0 1 --vx0R[m/s]---vxtR[m/s]--vxcR[m/s]---vxRloader 0.0 4.19e5 0.0 1--v0y[m/s]---vty[m/s]---vyloader---v0z[m/s]---vtz[m/s]--vzloader-- 0.0 4.19e5 1 0.0 4.19e5 1--nbin----Emin[eV]----Emax[ev]---maxnp— 200 0.0 20.0 300000-For-Mid-Diagnostic---nbin----Emin[eV]---Emax[eV]----XStart--XFinish— 300 0.0 20.0 0.0 0.03
SPECIES 2----q[C] ------m[Kg]---j0L[Amp/m^2]---j0R[Amp/m^2]----initn[m^-3]----k- 1.602e-19 6.68e-26 0.0 0.0 5e14 1-vx0L[m/s]---vxtL[m/s]--vxcL[m/s]---vxLloader(0=RNDM,1=QS)-- 0.0 97.8 0.0 1 --vx0R[m/s]---vxtR[m/s]--vxcR[m/s]---vxRloader 0.0 97.8 0.0 1--v0y[m/s]---vty[m/s]---vyloader---v0z[m/s]---vtz[m/s]--vzloader-- 0.0 97.8 0 0.0 97.8 1--nbin----Emin[eV]----Emax[ev]---maxnp-- 100 0.0 100.0 300000-For-Mid-Diagnostic---nbin----Emin[eV]---Emax[eV]----XStart--XFinish-- 200 0.0 0.3 0.0 0.03
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Some Input Parameters
nsp : Number of species.
nc: The number of spatial cells. Δx=length/nc
nc2p: Superparticle to actual particle weight. The initial
number of superparticles is N=initn·area·length/nc2p.
dt: Timestep for simulation in seconds.
length: The length of the system (distance between
electrodes) in meters.
B: Applied homogeneous magnetic field in Tesla
PSI: Angle of the B-field in degrees
extC: The external circuit capacitance in Farads. Used in
conjuction with extL, extR and the driving source.
source: Either a voltage (v) or current (i) source
f0: AC frequency of the source.
GAS: The type of gas, important when collisions are
turned on. Helium = 1, Argon = 2, Neon = 3, Oxygen = 4.
Gpressure : Background gas pressure in Torr.
q: Charge of the particle in Coulombs.
m: Mass of the particle in Kgs.
initn: Initial particle number density
For details, refer the source code itself or the manual inside the package of source file.
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Example of Result (driven by RF)
Vx vs. x for electrons
Density vs. x
Vx vs. x for ions
Potential vs. x
Ion flux vs. Ion Energy Electron Temperature vs. x