3D PIC-MCC modeling of an ITER-like negative ion source

Transcription

1 3D PIC-MCC modeling of an ITER-like negative ion source G. Fubiani GREPHE/LAPLACE University of Toulouse (Paul Sabatier) Toulouse, France 1/53

2 Introduction 2/53

3 Introduction I. Numerical model II. BATMAN simulation characteristics III. Electron transport IV. Plasma asymmetry V. Positive ion transport VI. Negative ion kinetics VII. Negative ion extraction VIII. Comparison with experimental data IX. Future work 3/53

4 Numerical model 4/53

5 Numerical model 3D Particle in cell (PIC) & Monte Carlo collision (DSMC) model Runs in parallel using the OpenMP framework Poisson is solved self-consistently 3D Multi-grid (homemade) Magnetic field is prescribed Actual profile from permanent magnets Electron, positive and negative ion transport is simulated Assumptions: Neutral dynamics not calculated (flat density profiles) ε 0 is artificially increased (sheath of greater dimensions) A comprehensive physical chemistry database is used 5/53

6 Geometry Simulated device geometry: one-driver ITER-type BATMAN high power negative ion source (from IPP Garching) 24.5 cm 58 cm B f 24.5 cm 32 cm B f Permanent magnets 16 cm 40 cm 6/53

7 Magnetic filter field XY plane XZ plane B f B f Maximum on PG B 75G 7/53

8 Suppression magnets Plasma grid (PG): surface area ~570 cm holes, total extraction area= 105 cm 2 (instead of 70 cm 2 ) YZ plane XZ plane Suppression magnet bars embedded into the extraction grid (as seen from the PG surface) B s 8/53

9 Scaling 9/53

10 Scaling (cont d) 10/53

11 IPP-BATMAN ion source characteristics at 0.3 Pa 11/53

12 Simulation parameters Rectangular driver 60 kw absorbed power Filter field maximum near PG (so-called standard configuration ) Neutral species characteristics: flat density profiles with n H2 = m -3, T H2 = 0.1 ev n H = m -3 and T H = 1 ev 40 ma/cm 2 of negative ions produced on PG from neutrals e -, H 2+, H 3+, H +, and H - dynamics are simulated 3D PIC grid resolution: 128x96x192 nodes 1-2 weeks using 6 CPUs 12/53

13 Physical chemistry (61 reactions) Neutrals H 3+ ions H 2+ ions e + H 2 e + H 2, elastic and energy loss (17 reactions) e + H 2 2e + H 2+, ionization e + H 2 e + H + + H, dissociation (2 reactions) e + H 2 e + 2H, dissociation (3 reactions) e + H e + H, elastic and energy loss (5 reactions) e + H 2e + H +, ionization e + H 3+ 3H, dissociation e + H 3+ H + H 2, dissociation e + H 3+ e + 2H + H +, dissociation e + H 3+ e + H + + H 2, dissociation H 3+ + H 2 H 3+ + H 2, elastic collisions H 3+ + H H 3+ + H, elastic collisions e + H 2+ e + H 2+, coulomb collisions e + H 2+ 2H, dissociation e + H 2+ e + H + H +, dissociation (2 reactions) e + H 2+ 2e + 2H + + H 2, dissociation H 2+ + H 2 H 3+ + H, dissociation H 2+ + H 2 H 2+ + H 2, elastic collisions H 2+ + H H 2+ + H, elastic collisions 13/53

14 Physical chemistry (Cont d) Protons Negative ions H + + H H + + H, elastic and energy loss (2 reactions) H + + H 2 H + + H 2, elastic and energy loss (5 reactions) e + H 2 H - + H e + H - 2e + H, electron loss H - + H e + 2H H - + H e + H 2 H - + H H - + H, elastic H - + H 2 H - + H 2, elastic H - + H + 2H (2 reactions) H - + H + e + H 2 + H - + H 2 e + H + H 2 H - + H H + H -, charge exchange For the production of H - in volume, 1% of H 2 is assumed to be in the vibrationally excited states ν 4. 14/53

15 Particle density profiles (on axis), 60 kw absorbed power, 20V PG bias 15/53

16 Electron transport 16/53

17 Schematic description z Driver 0 x 17/53

18 Schematic description z J e x B B 0 x 18/53

19 Schematic description z Plasma response to the asymmetric current E z B 0 x 19/53

20 Schematic description z Electrons cross the filter field along the top walls B 0 x 20/53

21 Electron current density Magnetic filter field maximum 9 cm from the PG (30G) Γ e I e < I i PG XZ plane I e > I i Electron drift motion induced mostly by the diamagnetic force 21/53

22 Plasma asymmetry 22/53

23 Electron temperature profiles y z T e T e 12 ev <1 ev 0 x 0 x Electron follows the magnetic field lines Plasma asymmetry generated by the magnetic drift motion 23/53

24 Electron density n e Y=0 plane 24/53

25 Transverse potential profile vs. bias (2) > (1) (2) (1) asymmetry 25/53

26 Potential asymmetry vs. the PG bias I e > I i I e < I i A small asymmetry in the potential can significantly affect negative ion kinetics 26/53

27 Electron current vs. the PG bias Electron current collected on the PG Total electron losses on the ion source walls: 550 A Electron current on PG < 10% of total Magnetic field barrier quite efficient 27/53

28 Positive ion transport 28/53

29 Plasma potential Magnetic filter field maximum near the PG (75G) ~540 A of positive ions produced mostly inside the driver What is the amount reaching the PG? Ions are not strongly magnetized Electric field is the driving force 29/53

30 Plasma potential vs. the PG bias 30/53

31 Particle wall losses (20V bias) Total current generated in the source: ~540A 247 A of H A of H A of H + Particle currents (kinetic energy) on PG Electrons: 2 A H 2+ : 5.5 A (15 ev) H 3+ : 3.8 A (12 ev) H + : 7 A (10 ev) A small amount of particles reach the PG 98% of electrons impact the driver and back of the expansion chamber 77% of H % of H 2+ lost on the lateral walls 31/53

32 Particle impact properties on PG vs. bias 60 ma/cm 2 of H - produced by neutrals give on PG: ~30 A The negative ion current produced by positive ions is low for the typical BATMAN ion source working conditions PG bias such that I PG >0 Production yield on PG from Seidl et al. JAP 79 (1996) 32/53

33 Average positive ion kinetic energy in expansion chamber (20V PG bias) T H+ = 1 ev T H3+ = 1.6 ev T H2+ = 3.2 ev Middle of the driver Potential drop Δφ= 25 V 2 cm from the PG surface 33/53

34 Positive ion mean-free-paths in the expansion chamber H 2 + H 3 + H + Ion velocity= Positive ions are collisional Destruction mean-free-paths larger that the ion source dimensions Except for H 2+ near driver exit (converted into H 3+ and protons) 34/53

35 Negative ion kinetics 35/53

36 Ion-ion plasma near the PG (20V PG bias) Transverse density profiles 4 cm from the PG (Y=0 plane) Magnetic filter field maximum on PG (75G), standard configuration 36/53

37 Average negative ion kinetic energy in the expansion chamber (20V PG bias) T H- = 0.7 ev Middle of the driver Potential drop Δφ= 25 V 2 cm from the PG surface 37/53

38 Negative ion mean-free-paths in the expansion chamber Ion velocity= 38/53

39 Negative ion density profiles vs. the PG bias Magnetic filter field maximum 9 cm from the PG (30G) n H- Y=0 plane 35V PG bias I PG = +90 A I H- toward volume= 8.5 A I H- back on PG= 27 A 15V PG bias I PG = -58 A I H- toward volume= 22 A I H- back on PG= 23 A Negative ion motion driven by the asymmetric electric field 39/53

40 Negative ion extraction 40/53

41 Context Where does the extracted negative ions originate from? Charge exchange? Direct extraction? What is the current fraction resulting from the dissociation of H 2? Is the current profile on the PG from volume produced ions asymmetric? Model characteristics: Magnetic filter field maximum 9 cm from PG (30G on axis) 60 ma/cm 2 of negative ions produced on the PG surface by neutral atoms 1% of H 2 assumed to be in the vibrationally excited states ν 4. Flat neutral profiles: n H m -3, T H2 = 0.1 ev n H m -3 and T H = 1 ev 41/53

42 Negative ion flux profiles (35V bias) Γ H- Y=0 plane H 2 dissociation CEX Extracted ions 5 A produced from H 2 7% reach the PG (0.33 A) Area <1 cm from the PG was excluded 30 A of surface produced ions undergo a CEX collision 0.8 A back on the PG Negative ion current profiles on PG are highly asymmetric Ion motion driven by the asymmetric plasma potential 42/53

43 Transverse potential asymmetry (I PG = +90A) Top potential profile remains always above the bias voltage even for I PG >>1 43/53

44 Asymmetric negative ion current profiles on PG *1 cm from the PG toward the grid Bottom Top PG area 44/53

45 What is the current fraction actually extracted? E x ~1 cm PG V PG = 35 V V EG = 60 V <0 >0 35% of volume produced negative ions reaching the PG are extracted Used as a reference (generated far for the PG) 35% extraction translates into: 1.1 ma/cm 2 from H 2 dissociation 2.5 ma/cm 2 due to CEX (produced >1cm from the PG, <1 cm unknown) A calculation with the real plasma density (un-scaled) is required near the PG 45/53

46 Benchmark 46/53

47 Simulation characteristics Rectangular driver 15 kw absorbed power (40 kw HF in the experiment) Filter field peaks on axis at 9 cm from PG (75G max) Neutral species characteristics: flat density profiles with n H2 = m -3 (0.6 Pa), T H2 = 0.1 ev n H = m -3 and T H = 1 ev 60 ma/cm 2 of negative ions produced on PG from neutrals 18.5V of PG bias e -, H 2+, H 3+, H +, and H - dynamics are simulated 3D PIC grid resolution: 128x96x192 nodes 47/53

48 Plasma characteristics n e n e XY plane XZ plane j e T e 7.5 ev 0.4 ev φ φ 29V 18.5V 48/53

49 Comparison with experimental measurements Lines points= experimental data Ref: Schiesko et al., Plasma Phys. Control Fusion 54 (2012) /53

50 Comparison with experimental measurements (cont d) 50/53

51 Comparison with experimental measurements (cont d) 51/53

52 Perspectives 52/53

53 Perspectives/collaborations Model neutral dynamics RF plasma coupling in the driver Provide input data to other numerical models Ones dedicated to ion extraction for instance Simulate other types of negative ion sources We could work on common projects What about founding? We could share students as a link between labs. 53/53

54 Extras 54/53

55 Numerical error in electron orbits with large ω ce Δt? Reference: C. E. Parker and C. K. Birdsall, J. Comp Phys 97, p. 91 (1991) Boris finite differenced scheme is stable for large time steps ω ce Δt>>1 Rotation angle varies by ~π per one time step Error on the Larmor radius is bounded, max: This is small compared to the field scale length. Example: similar profiles are observed between ω ce Δt 1 (max) and ω ce Δt 5. 11% error on Larmor radius estimate, 7% on the gyro-phase for ω ce Δt 1 55/53