Tanabe DOI:10.1088/1741-4326/ab1cdf EX/P3-22
Investigation of Fine Structure Formation of Guide Field Reconnection During Merging Plasma Startup of Spherical Tokamak in TS-3U H. Tanabe1, H. Hatano1, T. Hayashi1, Q. Cao1, A. Sawada1, M. Akimitsu1, M. Inomoto1, and Y. Ono1 1Graduate School of Frontier Science, University of Tokyo, Tokyo, Japan Corresponding Author: H. Tanabe, [email protected] We present the latest results of high-resolution 2D imaging measurement of merging/recon- nection heating during the central solenoid (CS)-free plasma startup of spherical tokamak using a new 96CH 2D ion Doppler tomography diagnostics. In the last decade, magnetic reconnection research made a major progress such as a) achievement of „ 1 keV plasma heating in MAST 2 both for ions and electrons; b) demonstration of Brec scaling of ion heating ranging 0.01 keVă Ti ă 1.2 keV with 0.01 Tă Brec ă 0.15 T in many plasma merging experiments based on outflow heating mechanism; and c) elucidation of fundamental heating characteristics: localized electron heating around X-point mostly by current sheet dissipation and global ion heating downstream where kinetic energy of outflow jet dissipates. Namely in the last three years, it was found that reconnection heating forms fine structure under high guide field condition of Bt ą 3Brec. From 2017, the formation process of the fine structure has been investigated in TS-3U (Bt „ 5Brec) with direct measurement of magnetic field profile and high-resolution 2D imaging measurement of ion temperature profile using a new 96CH ion Doppler tomography. As a new finding, it was found that ion temperature increases inside the current sheet as well as downstream. The high temperature region around the X-point is affected by Hall current jHall from the decoupling of ions and electrons, the characteristic heating profile rotates poloidally toward jHall ˆ Bt direction. This characteristic is clearer in high field side (Bt depends on major radius in tokamak configuration) and with higher mass ratio (enhancement of jHall ˆBt due to the larger scale length than current sheet width). While at the end of merging, ion heating downstream is surrounded by closed flux surface formed by reconnected field lines and forms another fine structure. The high temperature profile downstream propagates vertically and finally forms poloidally double-ring-like structure under the influence of better toroidal confinement with higher guide field which strongly suppresses perpendicular heat k K 2 transport (χ {χ „ 2, ωciτiiq " 10). This work was supported by JSPS KAKENHI Grant Numbers 15H05750 and 17H04863, and NIFS Collaboration Research Program NIFS16KLER048.
Published as a journal article in Nuclear Fusion http://iopscience.iop.org/article/10.1088/1741-4326/ab1cdf H. Tanabe, H. Hatano et al.
INVESTIGATION OF FINE STRUCTURE FORMATION OF GUIDE FIELD RECONNECTION DURING MERGING PLASMA STARTUP OF SPHERICAL TOKAMAK IN TS-3U
H. Tanabe, H. Hatano, T. Hayashi, Q. Cao, A. Sawada, M. Akimitsu, M. Inomoto and Y. Ono Graduate School of Frontier Sciences, University of Tokyo, Tokyo 113-0032, Japan Email: [email protected]
Abstract
We present the latest results of high resolution 2D imaging measurement of ion heating during central solenoid (CS)- free plasma startup of high-beta spherical tokamak. A new ultra-high resolution 96CH/320CH ion Doppler tomography was installed on TS-3U and following two fine structures of high guide field reconnection have successfully been resolved for the first time such as (a) tilted ion heating profile caused by the Hall effect around the X-point during the acceleration phase of magnetic reconnection (b) global ion heating and parallel heat transport process at downstream region where reconnected flux forms thick layer of closed flux surfaces, which confine the high temperature region and forms poloidally double-ring- like structure. Under the condition of high guide field limit (Bt > 3Brec), the contribution of higher guide field to suppress ion heating tends to be saturated and the promising upgrade scenario based on tokamak merging with higher guide field ratio is successfully demonstrated.
1. INTRODUCTION
Magnetic reconnection is a fundamental process which accelerates/heats plasmas through the restructuring process of magnetic field lines. This process is known as an effective way of converting magnetic energy into plasma energy in proportion to the square of the reconnecting magnetic field. Magnetic reconnection is observed in many fusion, laboratory and astrophysical plasmas such as sawtooth crashes in tokamaks [1], geomagnetic substorms in Earth’s magnetosphere and solar flares [2]. In the 1990’s, the application of reconnection heating was pioneered in TS-3 and START, with significant ion heating of up to ~ 200eV and several high beta records for spherical tokamak [3-5].
In the last three decades, the energy conversion mechanism was investigated in a number of experiments: MRX [6], SSX [7], VTF [8], TS-4 [9], UTST [10], C-2U [11] and MAST [12, 13]. For all of the laboratory experiments, following common characteristics have been reported: (i) magnetic reconnection heats ions downstream and electrons around the X-point where magnetic field lines reconnect [13, 14], (ii) ions are heated by the thermalization of flow energy of reconnection outflow jet [15] while electrons gain energy mostly by Ohmic dissipation of current sheet [14], (iii) most of the heating energy goes to ions and electron heating is small [16, 17] (ions are heated globally but electron heating is localized area near X-point) and (iv) achieved maximum reconnection heating rate depends on the amplitude of reconnecting component of magnetic field: Brec (Bp for tokamak) [18]. Based on the characteristics, significant plasma heating over 100eV was demonstrated in TS-3 [3], START [19], C-2U [20] and MAST [21]. The high field merging experiment in MAST documented ~ 1keV of global ion heating and bulk electron heating upto hundreds of eV through ion- electron energy relaxation [21-23], successfully exceeding the radiation barrier of low-Z impurities to achieve the duration time over 100ms in the solenoid free startup [18, 23]. As a promising startup scenario for spherical tokamak, the high field merging experiment in MAST also achieved successful connection with other additional heating by NBI and solenoid (hybrid startup scenario) to establish H-mode and longer flat-top plasma current (typically hundreds of milliseconds) [18, 23, 24]. In the MAST merging experiments, which typically operated in high guide field condition Bt/Brec > 3 with higher toroidal magnetic field of Bt ~ 0.6T and Brec ~ 0.1T [25], better toroidal plasma confinement of ion heating after merging is a key to connect the high temperature merging plasma startup to long pulse scenario [13, 18].
However in MAST, due to the absence of in-plane poloidal field measurements during reconnection, investigation of the detailed heating/transport mechanism were not possible. As a post MAST projects, now further upgrade projects were started in ST40 (Bt ~ 3T and Brec > 0.2T: higher than MAST) by Tokamak Energy Ltd. [26] and TS-U (Brec > 0.1T with MAST-like high resolution diagnostics) by univ. Tokyo [27]. In order to investigate further upgraded scenario of tokamak merging with high guide field (Bt > 3Brec), detailed investigations of ion heating/transport process have been done in TS-3 and TS-3U (TS-6). This paper addresses the highlights of the recent results from our laboratory experiments: the new findings of fine structure formation by reconnection heating and full-2D imaging measurement of in-plane heat transport processes.
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2. MERGING PLASMA STARTUP OF SPHERICAL TOKAMAK IN TS-3
Figure 1 shows typical features of merging plasma startup in TS-3 [14, 28]. Magnetic reconnection is driven by
PF coil current IPF [kA∙turn] and the two plasma rings at the top and bottom of the device (t = 70s) merge together and forms a spherical tokamak after merging (t = 90s) as shown in the high speed camera images and 2 poloidal flux profile. Toroidal current density jt [MA/m ] has opposite polarity around the X-point (current sheet) during magnetic reconnection (t = 76s) and the fast camera detects toroidally ring-like structure where current sheet exists [29]. Ion temperature starts to increase around those phase and forms double peak structure at r ~ 0.15m and r ~ 0.25m where reconnection outflow jets dissipate [14, 16, 28].
FIG. 1. Typical features of merging plasma startup of spherical tokamak in TS-3. Magnetic reconnection is driven by two merging driving coil current (IPF) and it forms toroidally ring-like bright struture around the midplane where the X-point exists. Ion temperature increases during the merging phase and typically forms double peak structure after merging.
3. FINE STRUCTURE FORMATION OF RECONNECTION HEATING
Figure 2 shows 2D ion temperature profile measured by a new 96CH 2D ion Doppler tomography which was upgraded from the previous 35CH system as a TS-U project [30, 31]. As illustrated in Fig.1 (a), it spans radially 16CH and axially 6CH (r-z: 16 × 6) to resolve both the detailed structure around the X-point and global profile downstream. During the characteristic three time frames within 10s at t = 70, 75 and 80s (before, during and after merging), ion temperature starts to increase and forms characteristics heating profile. During reconnection at t = 75s, ion temperature increases around the X-point as well as downstream of outflow jet; while after merging at t = 80s, high Ti region downstream aligned with closed flux surface of tokamak configuration.
Figure 3 highlights those two characteristic time frames during reconnection (acceleration phase) and after merging (transport/confinement phase). During merging (phase 1), ion temperature profile is affected by the accelerating effect of guide field reconnection as Ti profile around the X-point changes. For Hydrogen merging experiment (ion gyro radius i ~ 5mm and ion skin depth of c/pi > 20mm), Ti profile forms horizontally straight structure on midplane; while for Helium merging (i ~ 10mm and c/pi > 40mm), ion temperature profile around the X-point forms poloidally tilted structure in anti-clockwise direction because of the enhancement of the contribution of Hall effect by larger scale length with higher mass ratio). After merging (transport/confinement), reconnection heating profile forms another structure downstream. For the experimental condition of tokamak merging with the high guide ratio of Bt/Brec ~ 5 (Brec ~ 20mT and Bt ~ 0.1T), the ratio of ∥ ⊥ 2 ion thermal diffusivity ~ 2(ciii) >> 10 and the parallel heat transport term dominates the heat conduction, the high Ti region propagates vertically and forms poloidally double-ring-like structure.
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FIG. 2. Detailed 2D ion temperature profile measurement using the new 96CH ion Doppler tomography on TS-3. Ion temperature increases locally around the X-point and globally downstream.
FIG. 3. Two types of fine strucuture formation by reconnection heating during acceleration phase (when localized heating structure is clearer around the X-point) and transport/confinement phase (propagation of heating profile after merging). Under the influence of toroidal guide field., the polarity of downstream heating profile forms poloidally tilted structure by Hall effect, while the characteristic high Ti region propagate vertically after merging and forms poloidally ring-like structure.
4. THE EFFECT OF GUIDE FIELD AND RECONNECTING FIELD ON STARTUP PERFORMANCE
For the application of reconnection heating, it has long been discussed that toroidal field contributes the confinement after merging, while reconnection heating rate also decreases because of ion viscosity coefficient suppression in a high guide field regime (acceleration efficiency is also decreased but recent laboratory experiments and PIC simulation demonstrated that fast reconnection could be arranged by driving inflow even 2 in the high guide field regime [32, 33]). About dissipation of flow energy by viscosity heating: P = ∫[D (div ui) 2 + R (rot ui) ]dV (reduced form of Braginskii’s viscosity heating term in ciii >> 1 [34]), Fig.4 (a) shows guide 2 field dependency of viscosity coefficients D and R. The coefficient R = 0.3nTi/(ci ii) is strongly suppressed by guide field as it decreases with the square of ion gyro frequency. On the other side, another viscosity 2 2 coefficient D = (1+(ciii) )R = 0.3nTi/(ci ii)+0.3nTiii has a DC term 0.3nTiii which is not affected in the high guide field regime. Figure 4 (b) is the experimental results of reconnection heating as a function of guide field ratio. Figure 4 (a) shows that ion heating is suppressed in high guide field case compared with low guide
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field condition but such feature is saturated in the higher guide field regime [Bt/Brec ~ 3 (toroidal field Bt ~ 0.1T)]. Similar feature is also demonstrated in MAST with higher guide field ratio (Bt/Brec ~ 5 and Bt/Brec ~ 10 achieves same bulk ion heating downstream [18] and successfully achieved ~1.2keV [21]). Figure 4 (c) shows the scaling of ion heating as a function of reconnecting field Brec (three different guide field conditions are plotted: counter-helicity spheromak merging (no guide field: Brec includes reconnecting component of Bp and Bt), co-helicity spheromak merging (low guide field: Brec = Bp) and tokamak merging (high guide field: Brec = Bp)). The heating efficiency is slightly higher in low guide field condition but its contribution is mostly negligible for practical performance of plasma startup.
FIG. 4. Guide field dependency for reconnection heating. Ion viscosity decreases in high guide field regime but it saturates in high guide field limit because of the DC term of viscosity coefficient and experimental results also shows similar characteristic to saturate in high guide field regime. The amplitude of achieved reconnection heating mostly depends on reconnecting component of magnetic field Brec.
5. PROGRESS OF THE UPGRADE PROJECT TS-U: CONSTRUCTION OF TS-3U (TS-6)
As a post-TS-3/MAST project, univ. Tokyo started the upgrade project of merging/reconnection startup experiment [27]. At the end of 2017, TS-3 finished its operation and its vacuum vessel was replaced to a new chamber TS-3U (750mm × 1440mm). The new device radially keeps the same major radius and axially extends 1.2 times longer than TS-3. In addition to the improvement of ellipticity, the vertical position of merging driving coils are separated more (from 400mm (TS-3) to 600mm (TS-3U) in the present operation) in the new experiment to improve the detachment of the two plasma rings with internal PF coils during reconnection which cause significant heat loss during/after merging. Figure 5 shows the schematic view of the
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new experiment and the visible images of first plasma produced in 2018 spring (low energy operation during plasma commissioning). As recorded in the fast camera images, two plasma rings are formed at the top and bottom of the device, merge together around the midplane with the similar time scale in TS-3. In the new device, diagnostics access was improved from previous TS-3 experiment, full vertical scan is available both for probe measurement and optical diagnostics. For the first campaign with limited diagnostics and low field merging experiment, 150CH 2D magnetic probe arrays (r-z: 6 × 25CH) and 2D ion Doppler tomography (96CH and 320CH) were installed and starts the measurement in 2018 summer.
FIG. 5. New merging plasma startup experiment: TS-3U (TS-6: fast plasma in 2018 spring). The new vacuum chamber keeps the same major radius with TS-3 but is verticall extended (750mm×1440mm)
Figure 6 shows the time evolution of the full-2D high resolution imaging measurement of ion temperature and magnetic flux profile in the new merging experiment TS-3U. As in TS-3 experiment, ion heating by magnetic reconnection initially makes hot spots in the downstream region where outflow jet dissipates. Ion temperature continues to increase during the reconnection process; while high Ti region starts to propagate vertically and finally forms double-ring-like structure aligned with closed flux surface of two merging tokamaks as in the two fluid modeling of the reconnection experiment in MAST [35]
Figure 7 shows ion heat flux vector profile at the characteristic time frame of t = 85s based on the full 2D Ti profile measurement. Each plot shows ion temperature gradient vector −∇Ti, parallel heat transport component ∥ ⊥ ∥ ⊥ − ∇Ti, perpendicular one − ∇T, the ratio of parallel and perpendicular heat diffusivity and the summation of parallel and perpendicular heat conduction vector. Ion temperature gradient vector −∇Ti itself has the strongest perpendicular components at the high Ti region around r ~ 0.1m at the midplane (z ~ 0m). The parallel heat transport coefficient has higher value at the inboard region and the parallel heat transport is faster in the high field side. While, perpendicular heat transport is suppressed in the high field side while became stronger in the low field side. Such characteristics is caused by weight of the transport coefficient ∥ and ⊥, to ∥ ⊥ 2 have the higher ratio of ~ 2(ciii) . The ratio is higher in high field side and smaller (~10) in low field ∥ ⊥ side. Therefore, the summation of parallel and perpendicular heat conduction vector − ∇Ti − ∇T strongly aligned with closed flux surface and finally forms poloidally double ring structure. In comparison with no-guide field operation (∥⊥~ 1 in null-helicity reconnection experiment in MRX), cross field thermal transport is strongly suppressed in tokamak merging. While in MAST, ∥⊥>>100 is satisfied both inboard and outboard region. It suggests that the absolute value of toroidal guide field in TS-3U needs to be improved by upgrading toroidal magnetic field 3 times higher than previous scenario. As a post MAST project, ST40 experiment in Tokamak energy proposes to have further high guide field regime with toroidal magnetic field Bt ~ 3.0T at maximum. If merging/reconnection plasma startup scenario in such a high guide field condition successfully produces high temperature plasma as suggested in Fig.4, the improved confinement should lead to further high performance scenario and it is expected to exceed the records in MAST [21] in the near future [36].
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FIG. 6. Full-2D imaging measurement of ion temperature profile in TS-3U (TS-6) with direct magnetic probe measurement in the core region of magnetic reconnection. Merging/reconnection heating forms hollow profile based on outflow heating mechanism and the high Ti region propagates vertically downstream, forming poloidally double-ring-like structure.
FIG. 7. 2D ion heat flux profile based on the experimental results of 2D ion temperature profile. Ion temperature gradient formed by reconnection heating has large radial component but the improvement of transport coefficient by higher guide field strongly supresses perpendicular heat conduction and high Ti region forms characteristic double-ring-like structure by the field aligned heat transport process.
6. CONCLUSION
In this study, ion heating profiles and its transport process during guide field reconnection has been investigated in TS-3 and TS-3U. A new 96CH/320CH ion Doppler tomography has been installed as the upgrade project of
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TS-U and the ultra-high resolution 2D imaging diagnostics successfully resolved the fine structure formation process of guide field reconnection.
The conclusion of the paper is summarized as follows: (i) magnetic reconnection heats ions globally downstream of outflow jet and forms hollow temperature profile, (ii) ion temperature increases around the X- point as well as downstream, (iii) ion heating around the X-point forms poloidally tilted structure as the contribution of Hall term is enhanced with higher mass ratio, (iv) global downstream ion heating is transported aligned with the closed flux surface of tokamak configuration (v) higher guide field strongly suppresses cross- field thermal transport and ion temperature profile finally forms poloidally double ring structure, and (vi) the achieved reconnection heating depends on the amplitude of reconnecting component of magnetic field Brec while guide field (Bt) dependency is negligibly small for high field regime. Based on those characteristics, it is concluded that the application of high guide field reconnection with sufficient reconnecting field to exceed the radiation barrier over 100eV (Bt/Brec > 3 and Brec > 0.1T) is essential to connect the high temperature plasma formation by merging plasma startup to a long pulse operation scenario.
ACKNOWLEDGEMENTS
This work was supported by Grant-in-Aid for Scientific Research 15H05750, 17H04863 and 18K18747, and NIFS Collaboration Research Programs (NIFS16KLER048, NIFS17KNSS091 and NIFS17KKGR006).
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