Study of Plasma Meniscus and Beam Halo in Negative Ion Sources Using 3D3VPIC model
Shu Nishioka
Faculty of Science and Technology, Keio Univ.
1st year Master’s degree
Simulation model
B
136, 152, 152x y zN N N
・ Initial number of superparticles electron: Ne = 9×106 H+: NH+ = 1×107 H-: NH- = 1×106
・ Number of meshes:
0.625 mmDex y z ・ mesh size:
・ real size of simulation domain: 17mm, 19mm, 19mmx y zL L L
・ Scale factor: 23.72×10
*PG magnetic filter is taken into account and it is parallel to the y-axis. It was based on JT-60U negative ion source.
Geometry of simulation Model
Simulation model
PIC calculation cycle
Integration of the motions of the charged particles
dm q
dt
vE v B
Solve the Poisson equation
HeH nnnq
0
2
jjj E22, ttttttt vvxx
First-order particle weighting
jj x
( )( )
( )( )
( )
( )
A
B
C
D
x l y m z nq
x y z x y z
q l y m z n
x y z x y z
x l m z nq
x y z x y z
q lm z n
x y z x y z
charged particle
G
H
x l mnq
x y z x y z
q lmn
x y z x y z
( )
( )
E
F
x l y m nq
x y z x y z
q l y m n
x y z x y z
Simulation model
y~ z
0
0
180
・ The periodic boundary conditions are used in the y and z directions.
・ The surface produced H- taken to be a parameter and launched at the PG surface by a given number of particles per time step.
Boundary Condition
Simulation model
4.19×105 m/sElectron thermal velocity
5.64×1010 rad/sElectron plasma frequency
7.43×10-6 mElectron Debye length
1018m-3Electron density
0.25 eVHydrogen ion temperature
1 eVElectron temperature
ValueSymbolPhysical parameters
eT
HH TT ,
en
De
pe
thv
Current density
density
Magnetic flux density
Electrostatic potential
Velocity
First 2D space coordinate
Time
NormalizationSymbolPhysical quantities
tpet~
x~ y~
v~Dex Dey
thvv
~ ekTe
B~
J~
n~
emB pee
thevenj
e x y znN N N N
z Dez
Computatinal resouceCPU : Intel - Core i7-3820 @3.60GHz (4core 2thread): 8 MPI process.GPU : GTX titan : GPU is used only to solve Poisson eq.RAM : 16GB13s per PIC cycle.
Fig.1 2D density profile of the 2D3V PIC model
Fig. 2 2D density profile of the 3D3V PIC model
Comparison between 2D3VPIC and 3D3VPIC ~
-Hn
H- density
H- density
Emittance diagram (at x=17mm)
Plasma meniscus
Plasma meniscus
θ
Electron density
Magnetic filter field
Magnetic filter field
Electron densityEmittance diagram (at x=17mm, z=0mm)
・ Plasma Meniscus in the 2D model penetrates more deeply into the plasma source region and curvature is larger.・ The beam halo fraction to the total beam current is estimated to be 51.5% in the 2D model while around 6.3% in the 3D model.
Why is Penetration of the Meniscus small in the 3D modelPotential Profile during vacuum condition
Schematic view of 2Dmodel geometry
3Dmodel geometry
2D model
3D model
・ The figure shows potential profile each 2D, 3D model during vacuum condition. In this 2D model, because extraction hole is modeled as a slit, the equipotential surface penetrate more deeply into the plasma source region. Therefore plasma meniscus penetrates deeply than 3D model.
Fig. 2 2D density profile of the 3D3V PIC model (Z=0)
Result ~perpendicular and parallel to PG filter field plane~-H
nH- density
Plasma meniscus
Magnetic filter field
Electron densityEmittance diagram (at x=17mm, z=0mm)
Magnetic filter field
Plasma meniscus
Emittance diagram (at x=17mm, z=0mm)H- densityElectron density
Fig. 2 2D density profile of the 3D3V PIC model(Y=0)
・ Asymmetry of the electron density profile due to the E×B drift is observed.
・ Asymmetry of the plasma meniscus is observed. It induce asymmetry of negative ion current density profile.
1. The H- beam halo ratio to extraction beam current is dependent on the penetration of plasma meniscus.
2. The ratio of beam halo is about 6% in the 3D3V-PIC model. This value reasonably agree with the experimental result1,2.
3. Asymmetricity of the electrons and negative ions due to the E×B drift is observed with 3D3V-PIC model.
Conclusion
1. K. Miyamoto, Y. Fujiwara, T. Inoue, N. Miyamoto, A. Nagase, Y. Ohara, Y. Okumura, and K. Watanabe, AIP conference proceedings, 380, 360 (1996)
2. H.P.L. de Esch, L. Svensson, Fusion Engineering and Design 86, 363 (2011).
Benchmark problem
19mm×17mm×17mm
Include One PG aperture
PG: hole Radius =14mm, thickness=2mm
Peak value of the PG filter filed: 50Gauss
Geometry
Physical parameter
Electron thermal velocity
Electron plasma frequency
Electron Debye length
Electron density
Hydrogen ion temperature
Electron temperature
ValueSymbolPhysical parameters
eT
,H HT T
en
De
pe
thv
15kV, 0EG PGV V
Effect of E×B drift
×E B
Direction and magnitude of the E×B drift
・ The electron transport depends on along the direction of E×B drift close to PG.
Magnetic filter field
Electron density and direction of E×B
Temporal Evolution of Extracted current
t~
J~
ElectronNegative Ion
SP start
J
t~
(Am-2)1750
1500
1250
1000
750
500
250
0
Definition of Plasma Meniscus
0
The contour surface with defines Plasma Meniscus. This is almost same as the contour along which grad φ=0.
The H+ ions cannot go forward beyond this contour and they return back towards the left hand side into the source region. On the other hand, H- ions is accelerated when beyond this contour, then they are extracted.
0H
n
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