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Local Structures of Electron Temperature and Electrostatic Potential during ST Merging Startup

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 . Outline. Introduction

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Local Structures of Electron Temperature and Electrostatic Potential during ST Merging Startup

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

  2. Outline • Introduction • Plasma mergingtechnique 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 MergingStartup • 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. Fig: Plasma merging startup Magneticreconnection Magnetic reconnection is considered as the main factor of plasma heating.

  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 Magnetic reconnection operates in company with high guide field during in ST merging start-up • 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] • Examine the electron heating mechanism in ST merging start-up. • Investigate the electron acceleration by in-plane electric field in high guide field.

  6. Plasma merging device Basic parameter R ~ 0.45 m Bt ~ 0.10 T Br~ 0.05 T Ti ~ 20eVTe~ 5-20eV ne ~ 1x1019m-3 li-skin~ 4cm ri-larmor~ 2 mm Fig: Plasma merging device

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

  8. Measurement Method End View • Quadruple probe • Include a triple probe which acquires Te and ne • Acquire plasma floating potential tungsten plasma Glass tube 2mm 1mm P2 P3 P1 P4 5mm / 10mm Chamber One channel configuration : Vf Triple Probe Fig: quadruple probe Fig: 5-channel quadruple probe

  9. Electron Temperature Reconnection rate and magnetic flux t1 t2 t3 t4 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 ratebut the asymmetric heating at the peak of reconnection rate.

  10. Electrostatic Potential Reconnection rate t2 t1 t4 t3 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. • Increase the 2D profile resolution of and . • Find the parameter dependence among ,and

  14. Thank you for your kind attentions!

  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 drift motion : =0 Out-of-plane E component only no guide field: ( Bt = 0 ) =0 The same 1st term 2nd term under guide field: In-plane E component Out-of-plane E component

  17. Vf

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

  19. Vp [6]

  20. ne • After probe compensation:

  21. Nuclear fusion • 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 • Large amounts of super-conducting coil to generate high toroidal magnetic field • Largest portion of construction cost Low cost is essential for practical application of fusion power Fig: ITER 2020 ～

  22. High β plasma • Total Output fusion power is in proportion to value • b: ratio of plasma thermal pressure to magnetic pressure High beta plasma is expected The smaller Aspect ratio the higher beta • b value is limited in tokamak plasma • Aspect ratio A = R0/a by Troyon’s law

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

  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

  25. CS-less Plasma start-up method • Waves injection startup • Electron cyclotronwaves injection • Radio-frequency waves injection • Plasma merging startup [2] • Compact and economical • Achieve high b 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…) Magneticreconnection Fig: Plasma merging startup

  26. 2-fluid Hall Effect Fig: Two-fluid dynamics in the reconnection layer Fig: Hall reconnection simulation [9]. • 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 in experiment [9].

  27. Hall Effect in guide field Fig: Hall reconnection configuration in guide field • 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 Fig: Hall reconnection simulation [9]. Fig: Hall reconnection in experiment [9].

  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 • Triple probe • Low cost and easy alignment • Excellent in spatial resolutions • No voltage, frequency sweeps/switch • Acquire plasma parameter Te and ne … simultaneously P1 P2 P3 Probes plasma I Vd2 Vd3 A powerful diagnostic tool even for rapidly changing time-dependent plasma Fig: Triple probe

  30. Electron temperature and density Plasma electron temperature () : plasma ; P1 P2 P3 [5] Probes I Plasma electron density ( ) : Measured Vd2 Vd3 [5] (fixed) Fig: Triple probe

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

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

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

  34. Vf distributionof 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. EｘBDrift Motion of Electron drift motion : --------------------------- [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 During reconnection After reconnection Inflow (Out-plane component) (In-plane component) (total) Outflow • 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 (1st term ) • The Inflow and outflow is much similar to those in Sweet-Parker model.

  40. ExB Drift Motion (2nd 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 (1st 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 (1stt_noBt)

  44. E・B Result

  45. (E・B)/|B|^2 Result

  46. (E・B)/|B|^2 Result

  47. ExB Drift Motion

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

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

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