Download PDP-11 Formation of Closed Flux Surfaces in Reconnection Current

PDP-11
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Formation of Closed Flux Surfaces in Reconnection Current Layer by
Accelerated Electrons during Merging Start-up of Spherical Tokamak
M. Inomoto1, K. Yamasaki2, T. Ushiki1, X. Guo1, S. Kamio3, R. Yanai2, T. Sugawara1,
Y. Fukai1, H. Yamanaka1, R. Tamura1
1
Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561, Japan
2
Graduate School of Engineering, The University of Tokyo, Bunkyo 113-8656, Japan
3
National Institute for Fusion Science, Toki 509-5292, Japan
E-mail contact of main author: [email protected]
Abstract. Electron parallel acceleration in merging start-up phase of a spherical tokamak plasma was
experimentally investigated. Since the merging start-up involves magnetic reconnection process in the presence
of strong toroidal magnetic field perpendicular to the reconnecting radial magnetic field, strongly magnetized
electrons are accelerated along the toroidal magnetic field by the reconnection electric field to have much higher
energy than the bulk electrons. Those accelerated electrons form a localized current channel inside the current
layer where the poloidal magnetic field is negligibly small, resulting in generating closed flux surfaces in which
electrons are well-confined. This mechanism will extend the electron travel distance inside the current layer and
increase the energy conversion ratio to electron kinetic energy.
1. Introduction
Spherical tokamak (ST) concept suggests a potential for high-beta plasma confinement,
however, downsizing or removal of center solenoid (CS) coil is strongly required because of
restricted room in the ST central region. Many kinds of CS-free start-up methods have been
studied in various experimental devices [1]. As one of candidates for CS-free start-up
methods, combination of torus discharge by using external poloidal field (PF) coils and axial
merging technique has been investigated in the UTST experiment to establish dynamic
formation scheme of an ST target plasma for neutral beam injection (NBI) heating/current
drive.
The merging start-up method [2,3] provides high-power plasma heating via magnetic
reconnection process to form a high-beta ST plasma because magnetic reconnection converts
magnetic energy to plasma kinetic/thermal energies within a short period. In the simple
picture of anti-parallel reconnection, plasma is accelerated by the magnetic tension force in
the reconnection outflow region. Thus Ions attain a larger part of the released energy than
electrons [4]. On the other hand, electron heating is essentially required to apply NBI heating
on the merged ST plasma, but its mechanism in collisionless reconnection process has not
been well understood yet.
In the ST merging start-up scheme, magnetic reconnection in the presence of strong toroidal
magnetic field (TF) Bt > 10Bp, which is perpendicular to the reconnecting magnetic field Bp,
takes place and this perpendicular field is considered to modify the reconnection mechanism
drastically [5]. In this research, electron heating mechanism has been investigated in the
merging start-up scheme of ST on the UTST device [3].
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FIG. 1. Cross-sectional view of the UTST device together with flux surfaces before
merging (a), during merging (b), and after merging (c) measured by internal pickup
coil arrays.
2. Experimental Setup
2.1.UTST Merging Experiment
The UTST device was constructed to investigate CS-free start-up scheme of STs by using
external PF coils and plasma merging technique. FIG. 1 (a-c) show flux surface evolution
during merging formation of an ST. Three pairs of external PF coils are utilized to form two
null points inside the vessel and to apply toroidal electric field to initiate torus discharge at the
two null points. Two initial STs with Te ~ 10 eV and total current Ip < 100 kA are successfully
formed at the upper/lower regions of the UTST device. The formed two STs are merged on
the midplane of the device and finally form a single-axis ST (FIG. 1 (c)) through reconnection
of poloidal fields (FIG. 1(b)).
2.2.Diagnostics
Plasma merging process is mainly monitored by a set of internal magnetic probe arrays.
About two-thirds of a poloidal cross-section is covered by two-dimensional magnetic probe
array with 145 measurement points, each of which has pickup coils detecting both axial (Bz)
and toroidal (Bt) components of magnetic field with 6-8 cm resolution. High-frequency
pickup coils are also inserted on the midplane (Z = 0) to detect electromagnetic fluctuations
during plasma merging process.
Temperature and density of the bulk electrons are measured by a YAG Thomson scattering
system which evaluates the electron temperature perpendicular to the toroidal magnetic field.
Energetic electrons are detected by surface barrier detectors (SBDs) with absorption filters.
Fast camera and photodiodes are employed to observe light emission from the X-point region
through interference filters (for CII and CIII lines).
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FIG. 2. Typical waveforms of (a) total plasma current and (b) soft X-ray emission
from an ST plasma.
3. Experimental Results
3.1.Localized Emission from Current Layer
Typical waveforms of total plasma current and soft X-ray emission from merging/merged ST
plasma are shown in FIG. 2 (a) and (b). A sharp spike of soft X-ray emission was observed
during the merging phase, indicating that energetic electrons are generated during
reconnection with TF.
Since the TF strongly magnetizes electrons even at the X-point and reduces electron
gyroradius e ~ 0.03 mm much shorter than electron skin depth c/pe ~ 1 mm or typical
FIG. 3. Snapshots of (a-c) poloidal magnetic flux surfaces (black lines) with out-ofplane current density (color), and (d-f) visible light emission images.
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current layer thickness  >1 cm in the experimental condition, electrons near the X-point
travel long distance along the toroidal field with effective acceleration by the reconnection
electric field Ereco [5] up to much higher velocity than the bulk electron velocity. Maximal
electron parallel velocity ve// achieved during acceleration near the X-point is estimated as
B
1
m e v e2 // ~ L t eE reco ,
2
Bp
(1)
where L is the characteristic length of current layer. In ST merging in the UTST experiment,
toroidal magnetic field is 10-20 times larger than the typical reconnection magnetic field Br
measured at the edge of the current layer, but it is much thicker than the electron gyroradius at
the X-point. Hence, the poloidal magnetic field in the region near the X-point where the
magnetized electrons stay could be much smaller that the reconnection magnetic field Br,
leading to longer acceleration length.
FIG. 3 (a-c) show evolution of poloidal magnetic flux surfaces (black lines) with out-of-plane
current density jt (color) during ST merging and (d-f) show corresponding visible light
emission images. Thin current layer was formed during merging phase (FIG. 3 (b)) between
the approaching two STs. At the same moment, localized emission from the reconnection
current layer, which mainly consists of line emissions from higher-order impurity carbon ions
ionized/excited by electron impacts, was observed (FIG. 3 (e)). Since the soft X–ray emission
intensity from the current layer shows linear dependence with (Bt/Bp)Ereco [7], it is supposed
that the electrons are accelerated inside the reconnection current layer and achieve high
parallel energy. The acceleration time estimated from the reconnection electric field is about 2
s and is much shorter than the merging duration of 20-30 s in the UTST device.
Time evolutions of (a) soft X-ray (>70eV) and (b) CIII (~30eV) emissions from the current
layer are shown in FIG. 4. Soft X-ray signal showed steeper rise than CIII emission,
indicating that the electrons can achieve higher parallel energy in initial low density
condition. Then CIII emission in current layer (b) rose about 10 s before the peak of the CIII
FIG. 4. Time evolutions of (a) soft X-ray and (b) CIII emissions from current layer. CIII
emission from outflow region is shown in (c). Definition of the current layer/outflow
regions are presented in (d).
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emission from the outflow region (c), whereas the collisionless reconnection theory predicts
that the electrons are quickly flow out from the reconnection region within 1 s. These
experimental results suggest that the energetic electrons with large parallel (toroidal) velocity
are trapped inside the current layer and thus achieve larger portion of released magnetic
energy.
3.2.Magnetic Structure in Current Layer
In high Lundquist number plasmas, elongated reconnection current layer shows tearing mode
instability to have multiple X-points and forms magnetic islands with closed flux surfaces, or
“plasmoids”, between the two X-points. These plasmoids are supposed to change the
reconnection rate and energy release mechanism [8,9]. The current layer observed in the
UTST merging was, however, not so elongated because the downstream region is limited by
the device boundary. Nevertheless, emergence of small scale magnetic structures was found
inside the current layer.
FIG. 5 (a) shows the time evolution of radial profile of reconnected field Bz at Z = 0
measured by an internal probe array located inside the current layer. The location of nullpoint (Bz = 0) is shown by white lines. The initial Bz inside the current layer was almost zero
and flat with small gradient in radial direction. Then positive and negative Bz regions arose
alternatively and slowly grew for ~ 5 s. Multiple null-points which consist of X- and Opoints, i.e. closed flux surfaces, were observed from t = 0.696 ms to 0.701 ms and traveled
FIG. 5. Time evolutions of radial profiles of (a) reconnected magnetic field Bz and (b)
out-of-plane current density jt inside the current layer (Z = 0).
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toward outboard direction because a plasmoid inside the current layer might suffer from
position instability along radial direction.
The alternating Bz profile was formed by enhanced current density channels inside the current
layer. FIG. 5 (b) shows the evolution of radial profile of toroidal current density jt in which
high current density regions are enclosed by red lines. Inside the current layer, locally
enhanced current channels were generated and reversed the nearby magnetic field to form Opoints, as shown in FIG. 5 (a) in which high current density region is superposed by red lines.
The locations of the high current density agree with the locations of O-points, in which
electrons with high parallel velocity are well-confined.
To begin with, the current channel was considered to be driven by the accelerated electrons
inside the current layer. Since the poloidal magnetic field inside the current layer was quite
weak, the localized current channel easily forms closed flux surfaces which further extend the
confinement time of the energetic electrons. Thus the electrons inside the closed flux surfaces
will be accelerated by reconnection electric field for longer-term to achieve higher parallel
energy during reconnection.
The deduced mechanism of closed-flux-surface generation inside the current layer derives
from plasma merging in the presence of strong TF, in other words, magnetic reconnection in
the presence of strong guide field perpendicular to the reconnecting field. This phenomenon is
expected to enhance the conversion ratio to electron kinetic energy and lead to more efficient
electron heating by ST merging formation.
4. Conclusion
Electron dynamics was investigated by using emission diagnostics and local magnetic
measurements during ST merging formation in the UTST experiment. The current layer
thickness was much wider than the electron gyroradius due to the strong TF and thus the
electrons inside the current layer were significantly accelerated by the reconnection electric
field to form localized current channels. Locally enhanced current then formed closed flux
surfaces inside the current layer and improve electron confinement. The electrons are likely to
stay inside the reconnection layer where large toroidal electric field exists for longer term and
attain larger parallel energy.
This phenomenon would lead to more effective electron heating by magnetic reconnection
which improves the plasma performance as a target ST for NBI heating and current drive.
Acknowledgement
The authors appreciate many helps and inputs from TS group members. This work was
supported by JSPS A3 Foresight Program "Innovative Tokamak Plasma Startup and Current
Drive in Spherical Torus", Grant-in-Aid for Scientific Research (KAKENHI) 16K14525,
15H05750, 15K14279, 26287143, 25820434, and NIFS collaboration research
NIFS15KBAR012.
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[3]
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