Investigation of fine structure formation of guide field reconnection during merging plasma startup of spherical tokamak in TS-3U

Ion heating/transport and its fine structure formation process through magnetic reconnection have been investigated by high guide field tokamak merging experiments in TS-3 and TS-3U. In addition to the previously reported demonstration of high-temperature plasma startup without center solenoid, the detailed fine structure formation process of reconnection heating has been revealed using new 96CH/320CH ultra-high-resolution 2D ion Doppler tomography diagnostics. By identifying the double-axis field configuration with the X-point on the midplane using in situ magnetic probe diagnostics, the detailed measurement successfully revealed that the ion temperature profile forms two types of characteristic heating structure, both around the X-point and downstream. The former is affected by the Hall effect to form a tilted heating profile, while the latter is affected by the transport process which a forms a poloidal double-ring-like structure. The achieved ion heating mostly depends on the reconnecting component of the magnetic field, and the contribution of the guide field to decrease the heating efficiency tends to be saturated in the high guide field regime. Under the influence of better toroidal confinement with higher guide field, the downstream ion heating is transported vertically, mostly by parallel heat conduction, and finally forms a poloidal ring-like hollow distribution aligned with the closed flux surface at the end of merging.


Introduction
Magnetic reconnection is a fundamental process which accel erates/heats plasmas through the restructuring process of magnetic field lines. This process is known to be an effec tive 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 toka maks, geomagnetic substorms in the Earth's magnetosphere and solar flares [1,2]. In the 1990s, the application of recon nection heating was pioneered in TS3 and START, with sig nificant ion heating of up to ∼200 eV and several high beta records for the spherical tokamak [3][4][5].
In the last three decades, the energy conversion mechanism was investigated in a number of experiments: MRX [6], SSX [7],

Nuclear Fusion
Investigation of fine structure formation of guide field reconnection during merging plasma startup of spherical tokamak in TS-3U Ion heating/transport and its fine structure formation process through magnetic reconnection have been investigated by high guide field tokamak merging experiments in TS3 and TS3U. In addition to the previously reported demonstration of hightemperature plasma startup without center solenoid, the detailed fine structure formation process of reconnection heating has been revealed using new 96CH/320CH ultrahighresolution 2D ion Doppler tomography diagnostics. By identifying the doubleaxis field configuration with the Xpoint on the midplane using in situ magnetic probe diagnostics, the detailed measurement successfully revealed that the ion temperature profile forms two types of characteristic heating structure, both around the Xpoint and downstream. The former is affected by the Hall effect to form a tilted heating profile, while the latter is affected by the transport process which a forms a poloidal doubleringlike structure. The achieved ion heating mostly depends on the reconnecting component of the magnetic field, and the contribution of the guide field to decrease the heating efficiency tends to be saturated in the high guide field regime. Under the influence of better toroidal confinement with higher guide field, the downstream ion heating is transported vertically, mostly by parallel heat conduction, and finally forms a poloidal ring like hollow distribution aligned with the closed flux surface at the end of merging.
Keywords: spherical tokamak, centersolenoidfree startup, guide field reconnection, ion heating (Some figures may appear in colour only in the online journal) Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. VTF [8], TS4 [9], UTST [10], C2U [11] and MAST [12,13]. For all of the laboratory experiments, the following common characteristics have been reported: (i) magnetic reconnection heats ions downstream and electrons around the Xpoint where magnetic field lines reconnect [13,14]; (ii) ions are heated by the thermalization of flow energy of the reconnection outflow jet [15] while electrons gain energy mostly by Ohmic dissipation of the 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 near the Xpoint); and (iv) the achieved maximum reconnection heating rate depends on the amplitude of the reconnecting component of the magnetic field: B rec (B p for tokamak) [18]. Based on these characteris tics, significant plasma heating over 100 eV was demonstrated in TS3 [3], START [19], C2U [20] and MAST [21].
The highfield merging experiment in MAST docu mented ∼1 keV of global ion heating and bulk electron heating up to hundreds of eV through ion-electron energy relaxation [21][22][23], successfully exceeding the radiation barrier of lowZ impurities to achieve a duration time of over 100 ms in the solenoidfree startup [18,23]. As a promising startup scenario for the spherical tokamak, the highfield merging experiment in MAST also achieved successful connection with additional heating by neutral beam injection and solenoid (hybrid startup scenario) to establish the Hmode and a longer flattop plasma current (typically hundreds of milliseconds) [18,23,24]. In the MAST merging experiments, which are typically operated in the high guide field condition B t /B rec > 3 with B t ∼ 0.6 T and B rec ∼ 0.1 T [25], better toroidal plasma confine ment of ion heating after merging was key to connecting the hightemper ature merging plasma startup to the longpulse scenario.
However, in MAST, due to the absence of inplane poloidal field measurements during reconnection, investigation of the detailed heating/transport mechanism was not possible. As postMAST projects, further upgrade projects have now been started in ST40 ( B t ∼ 3 T and B rec > 0.2 T: higher than MAST) by Tokamak Energy Ltd. [26] and TSU ( B rec > 0.1 T with MASTlike highresolution diagnostics) at the University of Tokyo [27]. In order to investigate the further upgraded scenario of the tokamak merging with the high guide field ( B t > 3B rec ), detailed investigations of the ion heating/trans port process have been done in TS3 and TS3U (TS6). This paper addresses the highlights of the new findings in the labo ratory merging experiments: the fine structure formation of the ion temperature profile by reconnection heating and the visualization of its global transport process, which also forms a characteristic profile using new 96CH/320CH ultrahigh resolution ion Doppler tomography diagnostics. Figure 1 shows typical features of the merging plasma startup in TS3 [14,28]. Magnetic reconnection is driven by the poloidal field (PF) coil current I PF (kA · turn) and the two plasma rings at the top and bottom of the device (t = 70 µs) merge together and form a spherical tokamak after merging (t = 90 µs) as shown in the highspeed camera images and poloidal flux profile. The toroidal current density j t (MA m −2 ) has the opposite polarity around the Xpoint (current sheet) during magnetic reconnection (t = 76 µs) and the fast camera detects a toroidal ringlike structure where the current sheet exists [29]. The ion temperature starts to increase around this phase and forms a double peak structure at r ∼ 0.15 m and r ∼ 0.25 m where the reconnection outflow jets dissipate. Figure 2 shows the 2D ion temperature profile measured by new 96CH 2D ion Doppler tomography, which was upgraded from the previous 35CH system [30,31] as a TSU project (upgrade project for UTokyo spherical tokamak). As illus trated in figure 2(a), it spans 16CH radially and 6CH axially (∆r ∼ 10 mm and ∆z ∼ 20 mm) to resolve the detailed struc ture around both the Xpoint and global profile downstream. In previous TS3 experiments using the 7 × 5 or 8 × 4 chord system (∆r ∼ 35 mm and ∆z ∼ 20 mm), its spatial resolu tion and the measurement range were not sufficient and the old tomography system could not resolve such a detailed distribution for more than a doubly peaked profile [14,30]; however, the upgraded system can successfully resolve the micro/macroscale fine structure formation process. During the characteristic three time frames within 10 µs at t = 70, 75 and 80 µs (before, during and after merging), the ion temper ature starts to increase and forms the characteristic heating profile shown in figure 2(b). During reconnection at t = 75 µ s, the ion temperature increases around the Xpoint as well as downstream from the outflow jet, while after merging at t = 80 µs, the highT i region downstream propagates ver tically, aligned with the closed flux surface of the tokamak configuration. Figure 3 highlights these two characteristic time frames during reconnection (acceleration phase) and after merging (transport/confinement phase). During merging (phase 1), the ion temperature profile is affected by the accelerating effect of guide field reconnection (the coupling of the Hall effect with B t [32]) as the T i profile around the Xpoint changes. For the hydrogen merging experiment (ion gyro radius ρ i ∼ 5 mm and ion skin depth of c/ω pi > 20 mm), the T i profile forms a horizontally straight structure on the midplane, while for helium merging (ρ i ∼ 10 mm and c/ω pi > 40 mm), the ion temperature profile around the Xpoint forms a poloidally tilted structure in the anticlockwise direction (highlighted by red arrows) because of the enhancement of the contribution of the Hall effect by the larger scale length with higher mass ratio [33]). After merging (transport/confinement phase), the recon nection heating profile forms another structure downstream. For the experimental condition of the tokamak merging with the high guide field ratio of B t /B rec ∼ 5 ( B rec ∼ 0.02 T and B t ∼ 0.1 T), the ratio of ion thermal diffusivity χ i /χ i ⊥ ∼ 2(ω ci τ ii ) 2 > 10 and the parallel heat transport term dominates the heat conduction, the high T i region propagates vertically and forms a poloidal ringlike hollow distribution.

The effect of guide field and reconnecting field on startup performance
For the application of reconnection heating, it has long been discussed that the toroidal field contributes to the confine ment after merging, while the reconnection heating rate also decreases because of the suppression of the ion viscosity coeffi cient and acceleration efficiency in the high guide field regime. However, for the tradeoff problem, recent laboratory experi ments and particleincell simulation demonstrated that suf ficient acceleration efficiency could be arranged by triggering fast reconnection by the driving inflow even in high guide field conditions [34,35]). Regarding the dissipation of flow energy by viscosity heating: (reduced form of Braginskii's viscosity heating term [36] in ω ci τ ii 1 [37]), figure 4(a) shows the guide field dependency of viscosity coefficients η D and η R . The coef ficient η R = 0.3nT i /(ω 2 ci τ ii ) is strongly suppressed by the guide field as it decreases with the square of ion gyro fre quency. On the other side, another viscosity coefficient has a DC term 0.3nT i τ ii which is not affected in the highguide field regime. Figure 4(b) shows the experimental results of reconnection heating as a function of the guide field ratio. Ion heating is suppressed in the high guide field case com pared with the low guide field case but such features are satur ated in the higher guide field regime B t /B rec ∼ 3: toroidal field B t ∼ 0.1 T (a higher reconnecting field B rec typically increases the amplitude of heating by increasing the inflow speed [38] (∝ B rec ) and the saturation point is not necessarily B t = 0.1 T). A similar feature is also demonstrated in MAST with a higher guide field ratio ( B t /B rec ∼ 5 and B t /B rec ∼ 10 achieved the same bulk ion heating downstream [13,18] and successfully achieved ∼1.2 keV at maximum [21]). Figure 4(c) shows the scaling of ion heating as a function of reconnecting field B rec (three different guide field conditions are plotted: counterhelicity spheromak merging [3] (no guide field: B rec includes reconnecting component of B p and B t ), co helicity spheromak merging [39] (low guide field: B rec ∼ B p ) and tokamak merging (high guide field: B rec ∼ B p ) [16,25]. The heating efficiency is slightly higher in the low guide field condition but its contribution is mostly negligible for the prac tical performance of plasma startup.

Progress of the upgrade project TS-U: construction of TS-3U (TS-6)
As a postTS3/MAST project, the University of Tokyo started the upgrade project of the merging/reconnection startup experiment [27]. At the end of 2017, TS3 finished its opera tion and its vacuum vessel was replaced by a new chamber TS3U (φ0.750 m ×1.44 m). The new device radially keeps the same major radius and axially extends 1.2 times longer than TS3. In addition to the improvement of ellipticity, the vertical position of merging driving coils is more separated (from 0.4 m (TS3) to 0.6 m (TS3U) in the present opera tion) in the new experiment to improve the detachment of the two plasma rings from internal PF coils during reconnec tion, which causes significant heat loss during/after merging.  Furthermore, the flexibility of diagnostic access is significantly improved in the new experiment and it is possible to perform both in situ probe measurement and optical diag nostics with −0.3 m < z < 0.3 m (in TS3, optical access was limited to −0.075 m < r < 0.025 m). Figure 5 shows the schematic view of the new experiment and the visible images of the first plasma produced in the spring 2018 plasma commissioning: both PF coils (φ0.44 m/4turns and φ0.62 m/3turns) and capacitor banks of TS3 (40kV/18.75 µF) are reused in the initial campaign). As recorded in the fast camera images, two plasma rings are formed at the top and bottom of the device, which merge together around the mid plane with a similar time scale as in TS3 (τ PF swing ∼ 100 µs). For the first campaign in 2018, several diagnostics were still under construction and the upgraded high field merging operation was not yet available. However, the installation of 150CH 2D magnetic probe arrays (r − z : 6 × 25CH) and 2D ion Doppler tomography (96CH and 320CH) was com pleted and these were used for the measurement. The flexible diag nostic access in TS3U enabled full 2D visualization of reconnection heating ranging from −0.25 m < z < 0.25 m, which covered the full volume of the two merging flux tubes for the first time in the 30year history of experimental recon nection studies. Figure 6 shows the time evolution of the full 2D highreso lution imaging measurement of the ion temperature and magn etic flux profile in the new merging experiment TS3U (TS6). As in the TS3 experiment, ion heating by magnetic reconnec tion initially makes hot spots in the downstream region where the outflow jet dissipates [14]. The ion temper ature continues to increase during the reconnection process, while the high T i region starts to propagate vertically under the influence of toroidal effect and forms a doubleringlike structure aligned with the closed flux surface of the two merging tokamaks as in the twofluid modeling of the reconnection experiment in MAST [40]. Figure 7 shows the ion heat flux vector profile at the charac teristic time frame of t = 85µs based on the full 2D T i profile measurement: (a) ion temperature gradient vector −∇T i with T i , (b) the ratio of parallel and perpendicular heat diffusivity χ i /χ i ⊥ and (c) heat flux vector q [36] with T i . Ion temperature gradient vector −∇T i itself has the strongest perpendicular components at the highT i region around r ∼ 0.1 m at the mid plane ( z ∼ 0 m) where reconnection heating forms a hot spot. The parallel heat transport coefficient has higher value at the inboard region (highfield side: B t ∝ r −1 ) and perpendicular heat transport around the hot spot (r ∼ 0.1 m) is strongly sup pressed. Therefore, the heat flux vector q is strongly aligned with the closed flux surface and forms a poloidal doublering like structure.
In comparison with the no guide field operation (χ i /χ i ⊥ ∼ 1 in the nullhelicity reconnection experiment in MRX), crossfield thermal transport is strongly suppressed in tokamak merging: χ i /χ i ⊥ > 100 in the highfield side and χ i /χ i ⊥ ∼ 10 in the lowfield side (in MAST, χ i /χ i ⊥ > 100 is satisfied both in the inboard and outboard region: such condition could be satisfied in TS3U with a 3 times higher toroidal magnetic field when the highfield operation started in 2019). As a related postMAST project, the ST40 experi ment in tokamak energy proposes to have a further high guide field regime with toroidal magnetic field B t ∼ 3.0 T at max imum [26,41]. If the merging/reconnecting plasma startup scenario in such a high guide field condition successfully produces hightemperature plasma as suggested in figure 4, the improved confinement should lead to a further highper formance scenario and it is expected to exceed the records in MAST [21] in the near future [41].

Conclusion
In this study, ion heating profiles and their transport pro cess during guide field reconnection have been investigated in TS3 and TS3U (TS6). New 96CH/320CH ion Doppler tomography was installed for the upgrade project of TSU and the ultrahigh resolution 2D imaging diagnostics successfully resolved the fine structure formation process of guide field reconnection. The conclusions of the paper are summarized as follows: (i) Magnetic reconnection heats ions globally down stream of the outflow jet and forms a hollow temperature pro file; (ii) the ion temperature increases around the Xpoint as well as downstream; (iii) ion heating around the Xpoint forms a poloidally tilted structure as the contribution of the Hall term is enhanced with higher mass ratio; (iv) global downstream ion heating is transported aligned with the closed flux surface of the tokamak configuration; (v) a higher guide field strongly suppresses crossfield thermal transport and the ion temper ature profile finally forms a poloidal ringlike hollow distribu tion; and (vi) the achieved reconnection heating depends on the amplitude of the reconnecting component of the magnetic field B rec while the guide field (B t ) dependency is negligibly small for the highfield regime.
In addition to the detailed highresolution measurement which supports the previously reported heating characteristics ((i) and (vi)), the new experiment successfully led to new find ings such as microscopic heating around the Xpoint ((ii) and (iii)) and the global structure formation process ((iv) and (v)). Although the confinement/transport processes (iv) and (v) were proposed in many previous reports, it should be noted as an important milestone that such dynamic processes have successfully been demonstrated/resolved in a real tokamak merging experiment for the first time. 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 suppresses perpendicular heat conduction and the highT i region forms a characteristic doubleringlike structure by the fieldaligned heat transport process.