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.

Result

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