Particle-based Model of full-size ITER-relevant Negative Ion Source Francesco Taccogna Pierpaolo Minelli Nicola Ippolito P.Las.M.I lab@CNR-Nanotec INFN Bari e-mail: francesco.taccogna@cnr.it 16th ICIS, 25th August 2015

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

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

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

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    

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

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

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    ne=6x1017 @ Te=12 eV nH+=2.4x1017 @ TH+=1 eV nH2+=3.6x1017 @ TH2+=1 eV PG  58 cm Driver  EG    24 cm

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: http://www.mcs.anl.gov/petsc/petsc-as/

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

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

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).

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 days @ 4 Otta-Core Intel XeonE5/2640.v2 (2 GHz, 64 GB RAM) y Bias plate z    PG  58 cm Driver  EG    24 cm

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

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;

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

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

Electron Flux / Temperature

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

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.