Plasma and Fusion Research: Regular Articles

Volume 14, 3401110 (2019)

Recent Progress in High Resolution 2D Imaging Measurements ofReconnection Heating during Merging Plasma Startup in TS-3∗)
Hiroshi TANABE, Qinghong CAO1) , Haruaki TANAKA, Tara AHMADI1) , Moe AKIMITSU,
Asuka SAWADA, Michiaki INOMOTO and Yasushi ONO
Graduate School of Frontier Sciences, University of Tokyo, Tokyo 113-0032, Japan1)
Graduate School of Engineering, University of Tokyo, Tokyo 113-0032, Japan(Received 11 January 2019 / Accepted 21 April 2019)
We present recent results from high resolution 2D imaging measurement of reconnection heating during
central solenoid (CS)-free merging startup of spherical tokamak plasmas in TS-3 using an ultra-high resolution
2D ion Doppler tomography and in-situ 2D magnetic probe arrays. The new high-resolution 2D ion Doppler
tomography diagnostic has successfully resolved the formation of fine structure during magnetic reconnection
and it has been found that magnetic reconnection increases the ion temperature inside a current sheet as wellas in the downstream region of an outflow jet. The maximum ion temperature is obtained during a current
sheet ejection event and the double peak structure of ion heating becomes clearer after the end of reconnection.
The maximum ion heating increases with the reconnecting magnetic field. The high temperature region in thedownstream typically propagates vertically along a closed flux surface and MAST-like fine structure formation
has successfully been reproduced in this laboratory experiment for the first time.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: spherical tokamak, CS-free startup, magnetic reconnection, ion heating, guide field
DOI: 10.1585/pfr.14.3401110

1. Introduction
Magnetic reconnection is a fundamental process
which accelerates and heats plasmas through the rearrangement of magnetic field lines. It is known to be an eﬀective way of converting magnetic energy into plasma thermal energy in proportion to the square of the reconnecting magnetic field. Magnetic reconnection is observed inmany fusion, laboratory and astrophysical plasmas such as
geomagnetic substorms in the Earth’s magnetosphere, solar flares and sawtooth crashes in tokamaks [1, 2]. In the
1990’s, application of the self-organizing process was pioneered in TS-3 and START, with significant ion heating of
up to ∼ 200 eV and several high beta records for sphericaltokamak [3–5].
In the last three decades, magnetic reconnection wasinvestigated in a number of experiments: MRX [6–8], SSX[9,10], VTF [11], TS-4 [12], UTST [13,14], C-2U [15,16]
and MAST [17–21]. For all of them, the following common characteristics have been reported: (i) magnetic reconnection heats ions downstream and electrons aroundthe X-point where magnetic field lines reconnect [18, 22],(ii) ions are heated by the thermalization of flow energy
associated with reconnection outflow [23, 24] while electrons gain energy mostly by Ohmic dissipation of a current
sheet [22], (iii) most of the heating energy goes to ions andauthor’s e-mail: tanabe@ts.t.u-tokyo.ac.jp∗) This article is based on the invited presentation at the 27th International Toki Conference (ITC27) & the 13th Asia Pacific Plasma Theory
Conference (APPTC2018).

electron heating is small [25, 26] (ions are heated globallybut electron heating is localized around X-point); and (iv)the maximum heating rate depends on the amplitude of thereconnecting component of magnetic field: Brec (B p fortokamaks) [27]. Significant plasma heating to temperature
exceeding 100 eV was demonstrated in many merging experiments such as TS-3 [3], START [28], C-2U [16] andMAST [29].
The high field merging/reconnection experiment in
MAST resulted in ion temperatures of ∼1 keV and bulk
electron heating to temperatures upto hundreds of eVthrough ion-electron energy relaxation [29–31], successfully exceeding radiation losses due to low-Z impurities toachieve plasma duration times of over 100 ms in the central solenoid (CS)-free startup [27, 31]. As a promising
startup scenario for spherical tokamak, the high field merging experiment in MAST was also successfully combinedwith additional heating by NBI and a solenoid (hybrid
startup scenario) to establish H-mode and higher/longer
flat-top plasma current (typically hundreds of milliseconds) [27, 31, 32]. In the MAST merging experiments,
which typically operated in high guide field conditions
Bt /Brec > 3 with Bt ∼ 0.6 T and Brec ∼ 0.1 T [33], better ion energy confinement after merging helps to combine
high temperature merging plasma startup with long pulse
scenarios.
However in MAST, due to the absence of in-situ magnetic field measurements during reconnection, investiga-

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c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3401110 (2019)tion of the detailed heating/transport mechanism was notpossible. The clear results from 130CH-YAG/300CHRuby Thomson scattering [34–37] and 32CH ion Doppler
tomography diagnostics [38,39] provided new insights into
magnetic reconnection, fine structure formation such as
highly peaked structure around X-point and poloidallyring-like hollow ion temperature profile [33] by the coupling of outflow heating mechanism and toroidal confinement [18], but those processes have not yet been investigated comprehensively due to the absence of proper magnetic diagnostics. Based on the feedback, a recent TS-3
experiment made a further diagnostics upgrade for MASTlike high resolution imaging measurement and has started
detailed investigation of guide field reconnection with aguide field ratio similar to that in MAST. This summary
paper on an invited talk at ITC-27 provides a brief summary of the high guide field reconnection experiment inTS-3 (section 2). We then focus on impulsive heating
events associated with current sheet ejection in section 3.
Detailed measurements of the time-evolving ion temperature profile around the X-point reveals fine structure both
inside diﬀusion region and downstream of magnetic reconnection. The characteristic hot spots formed by reconnection heating are successfully trapped by the thick layerof closed flux surface and then equilibrated/transported toform poloidally-ring-like hollow distribution after the endof merging.

center of the machine (RCS = 0.06 m: typically IT F ∼50 kA · turn and ICS = 0 kA (CS-free scenario)), a pairof EF (equilibrium field coils: REF ∼ 0.5 m and 234 turns:
typically IEF ∼ 0.1 kA in DC), PF (RPF ∼ 0.22 m and 4
turns), separation coils (R sep ∼ 0.31 m and 3 turns) anddischarge electrodes (used for preliminary discharge). The
cylindrical vacuum vessel has Rwall = 0.375 m and 1.17 m
long. It is a relatively small scale laboratory experiment
like START [4, 28, 42].
Figure 2 illustrates a typical operation scenario of CSfree merging plasma startup [14, 22] in TS-3. As shownin fast camera images (Photron: SA-Z), initial two plasmarings (hydrogen) are generated at the top and bottom of thedevice by the induction of two internal PF coils. When thepolarity of IPF is reversed, the negative current contributes
to detachment of the plasma rings from PF coils and pushes
the two plasma tori vertically toward midplane. In addition
to the rapid increment of plasma current, magnetic reconnection occurs with characteristic bright structure [43–45]
around the X-point at t ∼ 75 µs. Merging finishes roughlyaround t ∼ 80 µs and finally a spherical tokamak configuration is formed with additional shaping by separationcoil current I sep . The evolution of poloidal flux profile
was measured by 2D (60CH) internal magnetic probe arrays [3, 22] whose minimum interval is dz ∼ 10 mm and2. Merging Plasma Startup of Spherical Tokamak in TS-3
Figure 1 shows the schematic view of merging plasma
startup device TS-3 [40, 41]. TS-3 has a CS/TF coil at theFig. 1 Plasma merging device TS-3. Two internal PF coils drive
magnetic reconnection.

Fig. 2 Typical feature of merging plasma startup of sphericaltokamak in TS-3: high speed video images, operational
waveforms and poloidal flux profile.

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Fig. 4 Time evolution of ion heating during magnetic reconnection in TS-3 and reference time evolution of merging
completion ratio α [%], toroidal electric field Et (reference of reconnection speed) and reconnecting magnetic
field component Brec = Br .

3. Ion Heating and Transport Processduring Tokamak Merging in TS-3
Fig. 3 Typical profile of radial component of magnetic field,
current density and toroidal electric field around X-point.

dr ∼ 40 mm around X-point. Ion heating was monitored
by following two types of new upgraded ion Doppler tomography diagnostics: a 1D (15CH) high-speed system(dr ∼ 10 mm, 10 µs/frame and dλ ∼ 0.0054 nm/pixel by
a f = 0.5 m spectrometer (g = 1800 L/mm) and an imageintensified CMOS fast camera) and a 2D (96CH) imaging system (dr ∼ 10 mm and dλ ∼ 0.0048 nm/pixel with a
f = 1.0 m spectrometer (g = 2400 L/mm) and an ICCD
camera).
Figure 3 shows typical magnetic field configuration
during tokamak merging in TS-3 [25]. The color contours illustrates radial component of magnetic field Brec ,toroidal current density jt and toroidal electric field Et .
During merging/reconnection, it typically has double magnetic axes at the top and bottom of the device and a X-pointaround the midplane. The radial component of poloidal
magnetic field Br is one of the most important componentsof the reconnecting magnetic field Brec which determines
the amplitude of reconnection heating and final temperature obtained by merging plasma startup [22, 27]. Toroidal
current density jt typically has opposite polarity around Xpoint, forming a current sheet [22, 25], and the structure
disappears after merging. The toroidal electric field is calculated from the time-derivative of poloidal flux and werefer to it as the reconnecting electric field Erec because it
characterizes the speed of the topology change during reconnection. As shown in Fig. 3, the plasma shots used inthis paper are in the regime of relatively small reconnecting magnetic field Brec = Br ∼ 0.02 T and higher guidefield ratio with limited toroidal magnetic field Bt ∼ 0.1 T(Bt /Brec ∼ 5).

Figure 4 (left) shows time evolution of ion temperature profile during merging plasma startup in TS-3 andFig. 4 (right) illustrates the reference time scale of merging completion ratio α [%], toroidal electric field Et =−(∂ψ/∂t)/2πr (rate of flux changes: reconnection speed)
and the amplitude of reconnecting (poloidal) magnetic
field component Brec = Br ∼ 0.02 T. Ion temperature increases around t = 70 ∼ 80 µs when α rapidly changes withcorresponding higher reconnecting electric field Et . Ion
temperature mainly increases in the downstream region ofreconnection by outflow heating mechanism and the amplitude of the heating is in the order of flow energy of poloidal
Alfven speed (V p,Al f ven ∼ 50 km/s with Brec ∼ 0.02 T) [25].
Figure 5 shows the detailed time evolution of 2D
toroidal current density and ion temperature profile aroundX-point during the rapid temperature rise from t = 72 µs
to t = 82 µs. During magnetic reconnection, initially antiparallel toroidal current is formed around the X-point. The
anti-parallel current structure which has opposite polarity
with plasma current starts to dissipate after t = 74 µs andforms double peak structure at t = 76 µs. Those two peaks
are ejected radially (“current sheet ejection” [46, 47]) at
t = 78 µs and the merging process completes after that.
The ion heating structure shows similar characteristics during reconnection. Before merging, the ion temperature
is just a few eV but it starts to increase at t = 74 µs initially around the current sheet. The high temperature region spreads horizontally toward outflow direction at t =76 µs and the downstream high T i region propagates vertically downstream aligned with field line direction. At t =78 µs, the ion temperature reaches its maximum value ofover 25 eV when the current sheet is completely ejectedfrom the X-point. After that, the two magnetic axes ofmerging tori approach the midplane and push the double
peak structure radially. The outboard ejection stagnates

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Fig. 7 Comparison of reconnection heating when merging ratio
reaches α ∼ 100% and reference global geometry with
2D ion temperature profile. At the end of reconnection,
the double peak structure is surrounded by thick layer ofclosed flux surface.

Fig. 5 Detailed 2D imaging measurement of current density/flux
and ion temperature profile during the double peak structure formation process by outflow heating and current
sheet ejection.

Fig. 6 Time evolution of the half width of current sheet thickness δ at the X-point and eﬀective resistivity η∗ during
merging. When the current sheet separates to form a double peak structure, its half width at the X-point becomescomparable to the ion Larmor radius ρi , and a significant
increase of eﬀective resistivity is observed at this phase.

around r ∼ 0.28 m and a clear double peak structure is
obtained after merging.
Figure 6 illustrates the time evolution of current sheet
thickness δ evaluated at the X-point and eﬀective resistivityη∗ = Et / jt . During guide field reconnection, the ion gyroradius ρi tends to be smaller than the current sheet thickness and meandering motion is initially suppressed. However, when current sheet ejection is triggered, the thickness of the diﬀusion region around the X-point becomescomparable to the ion gyro radius. The apparent reconnection speed Erec is not maximum at this time but the eﬀective resistivity increases to a much higher value than theclassical resistivity ηSpitzer ∼ 10−2 [mΩm]. Although theradial spatial resolution of the magnetic probes is not sufficient to detect the radial motion of the split current sheet,
Akimitsu [48] reported associated island structure formation and ejection-like features, detected by developing new
PCB-type radially high-resolution magnetic sensors. Ion
temperature rapidly increases during the ejection event andits profile changes to a double peak structure at the end of
merging.
Figure 7 shows the dependence of ion heating on reconnecting field Brec . At the end of merging (α ∼ 100%),
reconnection outflow and current sheet ejection initiallyforms a double peak structure of ion temperature profile
downstream. In comparison with counter-helicity merging
of spheromaks [41], which involves the collision of two
spheromaks with opposite toroidal magnetic field polarity
and resulted in maximum ion temperature up to ∼ 250 eVin the 1990’s [3], the obtained maximum reconnection
heating is smaller for tokamak merging. One of the major diﬀerence is the available magnetic energy which is released during magnetic reconnection. In co-helicity typereconnection [49, 50] (spheromak-spheromak or tokamaktokamak merging with same polarity of toroidal magnetic
field), the reconnecting magnetic field component is thepoloidal magnetic field and the toroidal field does not dissipate during reconnection. In counter helicity merging,on the other hand, the toroidal field also contributes tothe reconnection process and it drives outflow acceleration in the toroidal direction, so called the sling-shot effect [3,49]. The sheared toroidal rotation dissipates aroundthe magnetic axis after merging, and the final temperature tends to form a parabolic profile with the maximum
value around the null-point; in contrast, co-helicity merging typically forms a hollow distribution as shown in Fig. 7
and [50–52]. Although the amplitude of ion heating issmaller for co-helicity reconnection, tokamak merging has
the advantage of better plasma confinement via the toroidal
magnetic field. Without a guide field, the ratio of parallel/perpendicular ion heat diﬀusivity typically has a small

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Fig. 8 Global 2D ion temperature profile measurement before
and after merging in TS-3U. Upstream plasma initiallyhas singly peaked distribution but magnetic reconnectionchanges the profile to form a high temperature region
downstream. The new experiment successfully demonstrated/visualized that the impulsively formed high temperature region downstream is successfully confined inside the closed flux surface and equilibrated to form
poloidally-ring-like hollow distribution mostly by parallel heat transport process.

value (χi /χi⊥ ∼ 1 in MRX [53]) and the strong heat flux
driven by −∇T i propagates directly in the radial direction.
However, under the influence of higher toroidal guide field
in tokamak merging, χi /χi⊥ ∼ 2(ωci τii )2 > 10 is typicallysatisfied and most of the heat flux driven by ion temperature gradient propagates vertically on the thick layer ofclosed flux surface because perpendicular heat conduction
is strongly suppressed in the presence of higher toroidal
magnetic field. In addition, the amplitude of downstream
heating could be increased by having a higher poloidal
magnetic field in the upstream plasma. Tokamak merging leads to high confinement as a trade oﬀ against strong
heating.
Nevertheless, previous TS-3 experiments have not investigated detailed heat transport processes after merging
because the possible diagnostics access for ion temperature measurement was limited to −75 mm < z < 25 mm
due to the finite size of the midplane window. In 2018,the access was improved by upgrading the vacuum vessel to TS-3U (TS-6). It reuses the same PF coils as TS3 (τPF ∼ 100 µs as in Fig. 2) for the initial campaign andits merging time scale is nearly the same as that of TS3. Figure 8 shows the global time evolution of the 2D ion
temperature profile before and after merging measured in
the new device with 2D (150CH) internal magnetic probearrays. In TS-3, it was assumed that reconnection heating profile is quickly equilibrated in a few microseconds
on the closed flux surface because the thermal speed of hydrogen plasma is high: vi,th ∼ 50 km/s = 50 mm/µs. However the actual thermal transport process is not 2D and it
is aﬀected by the toroidal configuration (actual propagation path of parallel heat transport is rotated toroidally),
finite delay of equilibration was resolved with the detailed
measurement in TS-3U. The poloidally localized high temperature region propagates vertically with finite delay timeand forms poloidally-ring-like distribution via heat transport processes [51,52]. Under the influence of higher guidefields (Bt /Brec ∼ 5 and Bt ∼ 0.1 T around the X-point (r ∼0.2 m)), parallel heat transport dominates the equilibration process and perpendicular heat conduction is strongly
suppressed by the toroidal magnetic field. The ratio ofparallel/perpendicular heat diﬀusivity χi /χi⊥ ∼ 2(ωci τii )2strongly depends on the guide field and it exceeds 100 on
the high field side (Bt ∼ 0.2 T at r ∼ 0.1 m). In comparison with a no guide field experiment (χi /χi⊥ ∼ 1 [53]),
tokamak merging typically satisfies χi /χi⊥ > 10 even on
the low field side and has a characteristic hollow distribution. Although the initial heating profile is poloidally
localized and is not necessary symmetric (typically hightemperatures occur in high field side because of a smaller
volume in the toroidal configuration and radially asymmetric acceleration [54–56]), its equilibration process toform poloidally-ring-like hollow distribution has successfully been visualized/demonstrated as shown in Fig. 8.

4. Summary and Conclusion
Recent clear 2D imaging measurements of ion heating
during merging/reconnection plasma startup of sphericaltokamak has been briefly reviewed in the paper. Recent
new findings and achievement from TS-3 experiment aresummarized as follows:• Merging plasma startup successfully demonstrates
CS-free plasma startup for spherical tokamak by utilizing reconnection heating• Guide field reconnection forms fine structures botharound the X-point and in the downstream region• In tokamak merging, the radial component of thepoloidal magnetic field contributes to the heating as
a reconnecting magnetic field• Ions are heated during a fast reconnection phase andthe ion temperature typically forms a double peakstructure after current sheet ejection• During current sheet ejection, the thickness of the current sheet at the X-point becomes comparable to theion gyro radius and a significant increase of the eﬀective resistivity occurs• After merging, the double peak high temperature region propagates vertically on the closed flux surface of the spherical tokamak and finally forms a
poloidally-ring-like characteristic hollow distribution
The recent advances of MAST-like high-resolution plasma
diagnostics in laboratory experiments successfully lead tothe new finding of fine structure formation during merging plasma startup of spherical tokamak. In addition to themagnetic characteristics of current sheet ejection, a clear
double peak ion heating structure formation has successfully been detected synchronized with the magnetic features. The measured ion temperature profile successfully
reproduces the fine structure obtained in MAST and it has
been established that the poloidally-ring-like temperature
structure is aligned with closed flux surfaces by direct measurement of the magnetic field. Although the equilibra-

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Volume 14, 3401110 (2019)tion process itself is not necessary surprising, it should be
noted as an important milestone that the new experimenthas clearly demonstrated/visualized the successful confinement of high power reconnection heating on the closed flux
surface of merging startup spherical tokamak.

Acknowledgement
This work was supported by Grant-in-Aid for
Scientific Research 15H05750, 17H04863, 18K18747
and 19H01866, and NIFS Collaboration Research
Programs (NIFS16KLER048, NIFS17KNSS091 andNIFS17KKGR006).
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