1. Local Structures of Electron Temperature and Electrostatic Potential during ST Merging Startup *Boxin Gao, Akihiro Kuwahata Inomoto Lab The University of Tokyo School of Engineering Department of Electrical Engineering and Information Systems

2. Outline Introduction – Plasma merging technique for economical fusion reactor – Magnetic reconnection – Research purpose Experimental device and measurement – Plasma merging device – Measurement methods – Multi-channel Langmuir probes array Experiment result – 2D Electron Temperature – 2D Electrostatic Potential Summary and Future work

3. Plasma Merging Startup Spherical Tokomak (ST) is one of the promising concepts for magnetically confined fusion reactor because of its high beta and economic efficiency. To establish center-solenoid-free startup, various schemes such as RF, helicity injection and plasma merging, have been proposed. Magnetic reconnection is considered as the main factor of plasma heating. Magnetic reconnection Fig: Plasma merging startup

4. Magnetic Reconnection Reconfigure magnetic field to a lower- energy state Release magnetic energy to surroundings – Heat plasma – Produce plasma flows Magnetic field lines of opposite polarity are reconnected each other. Fig: Magnetic reconnection in space Fig: Magnetic reconnection

5. Research Purpose Examine the electron heating mechanism in ST merging start-up. Investigate the electron acceleration by in- plane electric field in high guide field. Experiment observation on ion heating at reconnection outflow through fast shock[1] Simulation on electron acceleration by parallel electric field at X point in high guide field[2] Magnetic reconnection operates in company with high guide field during in ST merging start-up

6. Plasma merging device Basic parameter R ~ 0.45 m B t ~ 0.10 T B r ~ 0.05 T Ti ~ 20eV Te ~ 5-20eV ne ~ 1x10 19 m -3 l i-skin ~ 4cm r i-larmor ~ 2 mm Fig: Plasma merging device

7. Plasma merging process Fig: TS-4 device Fig: Plasma merging process Create 2 torus plasma Reverse PF current – make them emerge with each other Reconnection point

8. Measurement Method P1P1 P2P2 P3P3 Triple Probe plasma Fig: quadruple probe Quadruple probe – Include a triple probe which acquires T e and n e – Acquire plasma floating potential P4P4 VfVf Chamber Fig: 5-channel quadruple probe 5mm / 10mm Glass tube tungsten 2mm 1mm End View One channel configuration :

9. Electron Temperature t1 t2 t3 t4 Reconnection rate and magnetic flux Electron temperature ： Electron heating both in current sheet and in outflow. Maximum electron temperature at peak of reconnection rate. Symmetric outflow electron heating at low reconnection rate but the asymmetric heating at the peak of reconnection rate.

10. Electrostatic Potential t1 t2 t3 t4 Reconnection rate Floating potential and magnetic flux ： Quadruple distribution is observed at the peak of reconnection rate. Great gradient of floating potential at the peak of reconnection rate.

11. Ep Distribution High in-plane electrostatic filed occurred during reconnection.

12. ExB Drift Motion Strong in-plane electric field is induced to keep the ExB drift motion. Particles’ motion will be strongly affected by this in-plane electric field.

13. Summary & Future work 2D profile of temperature was measured during magnetic reconnection with high guide field. 2D profile of the in-plane electric field was measured during magnetic reconnection with high guide field.

14. Thank you for your kind attentions! 14

15. Reference [1] Y. Ono and H. Tanabe: “Ion and Electron Heating Characteristics of Magnetic Reconnection in Tokamak Plasma Merging Experiments”, Plasma Physics and Controlled Fusion, Vol.54, No.12, 124039 (2012) [2] G. Lapenta and S. Markidis: “Scales of Guide Field Reconnection at the Hydrogen Mass Ratio”, Physics of Plasmas, Vol.17, No.8, 082106 (2010) [3] P. L. Pritchett, Collisionless magnetic reconnection in a three-dimensional open system, Journal of geophysical research, Vol.106, Nov 1, 2001 [4] J. F. Drake, M. A. Shay, The Hall fields and fast magnetic reconnection, Physics of plasmas, 2008 [5] S. Chen, T. Sekiguchi, Instantaneous direct-display system of plasma parameters by means of triple probe, Journal of applied physics, Vol.36, No.8, Aug, 1965 [6] J. Yoo, M. Yamada, Observation of ion acceleration and heating during collisionless magnetic reconnection in a laboratory plasma, Vol. 110, 215007, 2013 [7] J. Egedal, W. Fox, Laboratory observations of spontaneous magnetic reconnection, Physical review letters, Vol. 98, 015003, 2007 [8] J. P. Eastwood, M. A.Shay, Asymmetry of the Ion Diffusion Region Hall Electric and Magnetic Fields during Guide Field Reconnection: Observations and Comparison with Simulations, Physical review letters, Vol. 104, May 21, 2010 [9] T. D. Tharp, M. Yamada, Study of the effects of guide field on Hall reconnection, Physics of Plasmas, Vol. 20, 055705, 2013

16. Reconnection in guide field Bt (out-of-plane) appears as guide field no guide field: ( Bt = 0 ) no guide field: ( Bt = 0 ) =0 under guide field: 1 st term 2 nd term Out-of-plane E component In-plane E component The same Out-of-plane E component only

17. Vf

18. ne G. Lapenta and S. Markidis, Physics of Plasmas, Vol.17, No.8, 082106 (2010)

19. Vp

20. ne After probe compensation:

21. Nuclear fusion Fig: ITER 2020 ～ Low cost is essential for practical application of fusion power Large amounts of super-conducting coil to generate high toroidal magnetic field –Largest portion of construction cost Next generation of electric power plant – Less pollution – Low risk and radiation – Powerful and stable supply The international tokamak facility called ITER is now under construction 21

22. High β plasma High beta plasma is expected  value is limited in tokamak plasma – Aspect ratio A = R 0 /a by Troyon’s law The smaller Aspect ratio the higher beta 22

23. Spherical tokamak(ST) A promising candidate for fusion reactor core plasma –High  is achievable (up to 50%) [1] –Better confinement property –Compact, low cost in construction and operation 23

24. Problem in ST Problem in ST : Little space for Centre Solenoid (CS) coil Plasma start-up method without CS coil is investigated Fig: CS coils used in tokamak 24

25. CS-less Plasma start-up method Waves injection startup –Electron cyclotron waves injection –Radio-frequency waves injection Plasma merging startup [2] –Compact and economical –Achieve high  plasma –Heat plasma through magnetic reconnection process –Form a stable ST configuration efficiently Unnecessary of instability prevention process Less usage of external heating instrument(such as NBI,RF…) Fig: Plasma merging startup Magnetic reconnection 25

26. 2-fluid Hall Effect Fig: Two-fluid dynamics in the reconnection layer Difference movement between ion fluid and electron fluid – Ion : big mass; less magnetized; big Larmor radius – electron : few mass; strongly magnetized; small Larmor radius Magnetic energy => kinetic energy and thermal energy – Ion and electron outflow are observed [3] Symmetry quad-pole distribution Fig: Hall reconnection simulation [9]. Fig: Hall reconnection in experiment [9].

27. Hall Effect in guide field Fig: Hall reconnection configuration in guide field Fig: Hall reconnection simulation [9]. Fig: Hall reconnection in experiment [9]. Guide field always existed in ST Asymmetry quad-pole distribution Recent observation of hall effect in guide field – Magnetic field distribution [9] – Magnetic fluctuation [10] – Ion temperature distribution [11] Undefined – Electron temperature distribution – Electrostatics potential distribution – Electrostatics fluctuation

28. Research purpose Invest the mechanism of energy transformation in collisionless magnetic reconnection with guide field – Find how electron is heated in reconnection region Measure electron temperature distribution – Find whether electrostatic potential contribute to ion energy Acquire electrostatic potential distribution – Find whether electrostatic waves influence on plasma heating Obtain electrostatic fluctuation

29. Measurement method P1P1 P2P2 P3P3 I V d2 V d3 Probes plasma Fig: Triple probe Triple probe – Low cost and easy alignment – Excellent in spatial resolutions – No voltage, frequency sweeps/switch – Acquire plasma parameter T e and n e … simultaneously A powerful diagnostic tool even for rapidly changing time-dependent plasma

30. Electron temperature and density (fixed) P1P1 P2P2 P3P3 I V d2 V d3 Probes plasma Fig: Triple probe ; Measured Plasma electron density ( ) : [5]

31. Quadruple probe array Fig: 5-channel quadruple probe 2 Fig: 5-channel quadruple probe 1 5mm / 10mm Glass tube tungsten 2mm 1mm End View Probe End View 20mm One channel configuration : Probe array configuration: Fig: End view of 5-channel probe 1

32. 3D Fluctuation probe Fig: 3D fluctuation probe End View : 4mm 0.5mm 1mm 2mm 0.5mm 1mm 2.5mm 2mmSide View : Probe configuration : Tungsten Ceramic 32

33. Alignment Fig: Plasma merging device 5 channel probe 1 5 channel probe 2

34. V f distribution of Hydrogen Quad-pole distribution of floating potential was observed – A typical evidence of hall effect in magnetic reconnection – About 10[eV] ion kinetic energy transformed from electrostatic energy are confirmed

35. Reconnection Rate

36. E ｘ B Drift Motion of Electron --------------------------- [ プラズマ物理入門 P20] Vf は t 方向に一様（軸対象性）

37. ExB Drift Motion Inflow Outflow (Out-plane component) (In-plane component) (total) During reconnection After reconnection ExB Drift motion component by in-plane electric field is dominate over that by out-plane electric field.

38. Et Distribution

39. ExB Drift Motion (1 st term ) The Inflow and outflow is much similar to those in Sweet-Parker model.

40. ExB Drift Motion (2 nd term) ExB drift motion caused by electrostatic field comes significant during reconnection period Electrostatic field greatly increases the speed of inflow and outflow in magnetic reconnection area

41. ExB Drift Motion ExB drift motion of electron in guide field is dominant by electrostatic field

42. ExB Drift Motion (1 st term) Vp (Et only) --> (0.0000 1.0427) km/s Vp (Ep only) --> (0.0191 35.6529) km/s Vp (Et with Ep) --> (0.0331 35.4518) km/s Et (Et only) --> (-137.7242 56.7613) V/m Ez (Ep only) --> (-1129.2 1755.9) V/m Er (Et with Ep) --> (-1051.5 1146.9) V/m

43. ExB Drift Motion (1 st t_noBt)

44. E ・ B Result

45. (E ・ B)/|B|^2 Result

47. ExB Drift Motion

48. Electron Temperature Electron heating at outflow region is observed During reconnection After reconnection Te in mid-plane (z=0) :

49. Electron Density Electron density is greatly affected by plasma confinement in stead of magnetic reconnection Electron density is not obviously changed between outflow region During reconnection After reconnection ne in mid-plane (z=0) : Before reconnection