1. Particle-based Model of full-size ITER-relevant Negative Ion Source
Francesco Taccogna
Pierpaolo Minelli
Nicola Ippolito
P.Las.M.I
INFN
Bari
th ICIS, 25th August 2015

2. Outline Status of present models: Weakness vs Strength
Full-size Source Model with detailed extraction: MINUS
Preliminary results
Conclusions

3. NIS Models
Simplifying 2 different kind of approaches are present
EXTRACTION ZOOMED MODEL
FULL-SIZE SOURCE MODEL y z    
Driver    

4. EXTRACTION ZOOMED MODEL
NIS Models
EXTRACTION ZOOMED MODEL
+ Fine resolution
(aperture contains tens of cells)
+ Good statistical level (number of part. er cell)
+ Realistic sheath representation
(real vacuum permittivity)
+ 3D model
+ Detailed electron deflection field
+ Surface-produced negative ions
+ Optimization of aperture size/shape
+ Meniscus shape and beam optics
- One aperture (periodic conditions)
- Strong influence of free parameters (initial conditions, re-injection system)
- Anisotropic fluxes and possible non-Maxwellian behavior
- Magnetic filter effects starts much farther from PG
NIO1 extraction type: see Poster TuePE30

5. FULL-SIZE SOURCE MODEL
NIS Models
FULL-SIZE SOURCE MODEL
+ Model of the plasma expansion and electron transport across the magnetic filter field
+ Dis-homogeneity along PG
+ Self-consistent plasma condition at the entrance of the extraction region
+ Positive ion flux at PG (ion conversion)
+ BP and PG bias effect
+ Plasma-gas coupling
+ 3D model
- Scaled model (fake vacuum permittivity)
- Cell size larger than the single aperture
- Driver not self-consistently included z    
Driver    

6. EXTRACTION ZOOMED MODEL FULL-SIZE SOURCE MODEL
NIS Models
EXTRACTION ZOOMED MODEL
+ Fine resolution
(aperture contains tens of cells)
+ Good statistical level (number of part. er cell)
+ Realistic sheath representation
(real vacuum permittivity)
+ 3D model
+ Detailed electron deflection field
+ Surface-produced negative ions
+ Optimization of aperture size/shape
+ Meniscus shape and beam optics
- One aperture (periodic conditions)
- Strong influence of free parameters (initial conditions, re-injection system)
- Anisotropic fluxes and possible non-Maxwellian behavior
- Magnetic filter effects starts much farther from PG
FULL-SIZE SOURCE MODEL
+ Model of the plasma expansion and electron transport across the magnetic filter field
+ Dis-homogeneity along PG
+ Self-consistent plasma condition at the entrance of the extraction region
+ Positive ion flux at PG (ion conversion)
+ BP and PG bias effect
+ Plasma-gas coupling
+ 3D model
- Scaled model (fake vacuum permittivity)
- Cell size larger than the single aperture
- Driver not self-consistently included

7. 2.5D PIC-MCC full source Model: minus.f
Simulation domain
Expansion + Extraction + 1st acceleration step multiaperture grid (10 apertures):
BP, PG and EG included. y
Bias plate z    PG 
58 cm
Driver  EG   
24 cm

8. 2.5D PIC-MCC full source Model: minus.f
Assumptions-Limitations
Driver/Injection
The driver is not yet simulated:
prescribed ambipolar neutral full-maxwellian flux of plasma particle
injected in a thin area located at the driver exit plane y
Bias plate z   
Te=12 eV
TH+=1 eV
TH2+=1 eV PG 
58 cm
Driver  EG   
24 cm

9. 2.5D PIC-MCC full source Model: minus.f
Assumptions-Limitations
Driver/Injection
Geometry
2.5D cartesian geometry - 2D(y,z) electrostatic [1]
Particle tracked in x (magnetic filter field direction)
but quantity considered uniform (Ex=0)
Particle-Wall interaction are considered assuming a thin sheath approximation;
A secondary electron emission coefficient ϒ=0.2 has been assumed y
Bias plate z    PG 
58 cm
Driver  EG   
24 cm
[1] Poisson equation solver:

10. 2.5D PIC-MCC full source Model: minus.f
Assumptions-Limitations
Driver/Injection
Geometry
Scaled model: ε’0=25ε0  Δt’=5Δt;
Δz’=5Δz
It allows keeping the detailed mesh of one-aperture model
The aperture (D=10 mm, flat) contains 25 cells y
Bias plate z    PG 
58 cm
Driver  EG   
24 cm

11. 2.5D PIC-MCC full source Model: minus.f
Assumptions-Limitations
Driver/Injection
Geometry
Input data
- Filter field: bell shaped z-profile with zmax=3 cm from PG and Bx,max=7 mT
- Electron deflection field: prescribed 2D(y,z) map with alternating y-direction for adiacent apertures
- Fixed H and H2 density with vibrational Boltzmann distribution y
Bias plate z    PG 
58 cm
Driver  EG   
24 cm

12. 2.5D PIC-MCC full source Model: minus.f
Assumptions-Limitations
Driver/Injection
Geometry
Input data
- Most relevant collisions included
- Self-consistent production of volume H- and surface by ion conversion [1]
- Fixed current density J=660 A/m2 of surface-produced H- by neutral conversion; uniformly launched along y at PG BP
JH-,0=660 A/m2 PG EG
[1] M. Seidl, H.L. Cui, J.D. Isenberg, H.J. Know, B.S. Lee, S.T. Melnychuk, J. Appl. Phys. 79, 2896 (1996).

13. 2.5D PIC-MCC full source Model: minus.f
Simulation parameters-Performances
t = 2.5x10-11 s
x(=4x10-4 m) ~ D -> Ng=Ny x Nz=1450x589
Npart = 2x108 (w=2x108)
OpenMP/MPI hybrid paradigm
Ttot = 0.5 s in 7 4 Otta-Core Intel XeonE5/2640.v2 (2 GHz, 64 GB RAM) y
Bias plate z    PG 
58 cm
Driver  EG   
24 cm

14. Results: Temporal Evolution of Extracted Currents
- Steady state after probably 30 s; at the moment the simulation has reached 13 s;
The surface production is activated after 10.5 s and the extracted H- ions jumps from 40 A/m2 (volume contribution) to 200 A/m2;
H- ions from ion conversion on PG not yet produced;
- Co-extracted electron current evolution has an intermittent behavior due to a turbulent transport across the filter typical of low pressure discharges (Magnetron Hall-effect thruster, etc.);
H- surface production starts

15. Results: Electric Potential
- Bottom-top y-asimmetry starting from the filter region;
Hall and diamagnetic drifts contributes to create a space charge unbalanced along y and an electric field self-consistently develops to restore the plasma neutrality
- Meniscus penetrates too much because plasma is not yet arrived to screen EG field;
- Bulk value  10 V increasing;

16. Results: Electric Potential
- Bottom-top y-asimmetry starting from the filter region;
Hall and diamagnetic drifts contributes to create a space charge unbalanced along y and an electric field self-consistently develops to restore the plasma neutrality
- Meniscus penetrates too much because plasma is not yet arrived to screen EG field;
- Bulk value  10 V increasing;
- Electric potential modulation along y at the filter region
-> anomalous cross-field electron trasnport

17. Results: Electric Potential - Zoomed view of extraction region
- Plasma from the expansion region not yet able to screen the EG field
- Nevertheless plasma potential asymmetry is yet evident along the different apertures
- Meniscus shape is already forming
- The potential well attached to PG has a depth of 5 V

18. Electron Flux / Temperature

19. Extracted current aperture by aperture
Bias plate
Je (A/m2)
JH- (A/m2)
0.5
178
0.6
263
0.7
111
146
0.8
156
1.4
153
1.3
145
140 9
255
244
194 PG EG

20. Conclusions
First attempt to develop a 2.5D realistic PIC-MCC model of the full-size source with detailed extraction region
Keep the fine mesh used for the extraction region (cell size even smaller)
The work is going in 2 directions:
- inclusion of the IC driver module
- speed-up the execution of the code by programming optimization
- application to NIO1
Preliminary results (not yet steady state reached) show:
- the presence of y-modulation of electric potential responsible for the electron cross-field
anomalous transport in the filter field region
- y-dishomogeneity along the PG inducing an electron extraction current
- the electrostatic nature of the negative ion extraction is confirmed
- about 30% of the surfaced-produced negative ions is extracted
- the extracted negative ion current rise up 5 times when the surface production is activated.