Collisionless Plasma Flow in External Magnetic Field
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University of Nevada, Reno
Collisionless Plasma Flow in External Magnetic Field
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Plasmafully ionized hydrogenplasma length : 4 mmcollision frequency : νce = 2x 108 1/sec
flow velocity usw=4x107 cm/s
External Magnetic Fieldprofile : electron gyrofrequency : ωce = 1.76 x 1011 rad/sec @ B =
10 kGelectron gyroradius : re = 16 μm @ B = 10 kG
ion gyroradius : ri = usw/ωci = 0.42cm @ B = 10 kG
Bex0 /(1cm x);7kG@x 0 65kG@x 1cm
Bex0 /(1cm x);7kG@x 0 65kG@x 1cm
Computational Parametermesh size : Δx = 1.426 μmtime step : Δt = 4.65 fstotal simulation time : 20 ns
1D-PIC SIMULATION PARAMETER
x D re ri
x D re ri
t 1/ pe 1/ ce 1/ ci ~1ns < 20ns
t 1/ pe 1/ ce 1/ ci ~1ns < 20ns
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TIME EVOLUTION OF ION DENSITY
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MAGNETIC FIELD IS PARTIALLY NEUTRALIZED BY SELF-INDUCED FIELD
Negative B-field is excited by electrons circular motion at the plasma expansion front.
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NEGATIVE ELECTROSTATIC FIELD IS EXCITED AT PLASMA FRONT
Ion Expansion Front
Since electrons are magnetized, ions go ahead and make a negative sheath field at the plasma
front.
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Figure : Electron phase plot X-px (left) & X-py (right) at t=9.3ns
ELECTRONS ARE ACCELERATED BY NEGATIVE SHEATH FIELD AND DO CIRCULAR MOTION
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Figure : Ion phase plot X-Px at 4.6 ns (left) and 9.3 ns (right).
IONS ARE DECELERATED BY NEGATIVE SHEATH FIELD
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Figure : Energy spectrum at t=9.3ns.
Te~1.2keV (Initial Te ~ 400eV)
ENERGY SPECTRA SHOW IONS LOSE ENERGY AND ELECTRONS ARE HEATED
~ Flow velocity
Low energy ions increases.
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Ex u ec
Bnet
Ex u ec
Bnet
i ~ eExs
i ~ eExs
Ions are stopped when they transfer half of their own energy to the electrons
i eu ec
Bnet re
eu ec
Bnet p ec
eBnet
u e p e ~ 2e
i eu ec
Bnet re
eu ec
Bnet p ec
eBnet
u e p e ~ 2e
SUMMARY OF WHAT’S GOING ON
(Assumed the effective collision frequency is the electron cyclotron frequency )
(Assumed the ion energy equals to the sheath potential )
Y. Sentoku, K. Mima, P. Kaw, and K. Nishikawa, Phys. Rev. Lett. 90 (2003) 155001
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IONS START FLOWING BACKWARD AFTER 10 NSEC
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ne=2x1016 ccm
Te=5keV
usw=8x108 (0.0267c)
Plasma = 1.8mm x 0.72mm
External B-field (Z-direction)100kG@x=0 - 500kG@x=5.4mmωce=1.8x1012 rad/sec @B=100kG
re=20μm@B=100kG
Δx=3.6μmLx=7.1mm, Ly=1.8mm
2D-PIC PARAMETERS
Density is normalized by n0=2x1016 ccm
200kG
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t=120 ps
t=360 ps
t=240 ps
TIME EVOLUTION OF ION DENSITY
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TIME EVOLUTION OF ELECTRON DENSITY
t=120 ps
t=360 ps
t=240 ps
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Ex @ t=60ps
Ey @ t=60ps
Bz @ t=60ps
FIELD STRUCTURE AT T=60ps
Bz = 0.0002~20kG
Ex = 0.0002~5.8 V/m
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FIELD STRUCTURE AT T=175ps
Ex @ t=175ps
Ey @ t=175ps
Bz @ t=175ps