23F07
Onset of Fast Magnetic Reconnection in Plasma Merging Experiments
プラズマ合体実験における高速磁気リコネクションの発現
Michiaki Inomoto, Yoshinori Hayashi, Hiroshi Tanabe, Akihiro Kuwahata, Boxin Gao and Yasushi Ono
井 通暁，林 由記，田辺博士，桑波田晃弘，高博シン，小野 靖
Graduate School of Frontier Sciences, The University of Tokyo,
5-1-5,Kashiwanoha,Kashiwa,Chiba 277-8561,Japan東京大学大学院新領域創成科学研究科 〒277-8561 千葉県柏市柏の葉 5-1-5

Anomalous dissipation in magnetic reconnection is usually recognized as a result of current
sheet instabilities such as lower hybrid drift instability followed by a long-wavelength
electro-magnetic mode called drift kink instability. In plasma merging experiments, excitation of
low frequency wave was observed in the vicinity of the X-point associated with the increase of
the magnetic reconnection rate under a guide field. Plasmoid development was also observed
to enhance the reconnection rate in the guide field reconnection.

1. Introduction
Magnetic reconnection plays important roles in
rapid eruption and structure formation events inmagnetized plasmas. Fast magnetic reconnection isprovided by large magnetic dissipation, or theanomalous resistivity in the diffusion region, whichis induced by microscopic instabilities in the current
sheet. One of the primary candidates of the micro
instabilities is the lower hybrid drift instability(LHDI), which is often observed in space andlaboratory
experiments[1,2].
Recentthree-dimensional particle-in-cell simulation studies
have pointed out that the drift kink instability (DKI)
is triggered after the nonlinear saturation of theLHDI mode when the half-width of the current
sheet decreases below the ion gyroradius [3],resulting in the reconnection enhancement andanomalous ion heating [4]. Local current sheet
behaviors have been experimentally investigated byusing toroidal plasma merging devices, in which
self-organized magnetic reconnection events
develops with small constraint from boundary
conditions.
2. Experimental setup
Measurement of low frequency magnetic
fluctuation in the vicinity of a current sheet wascarried out in the TS-3 plasma merging device,
shown in Fig. 1. Two plasma toroids are produced
and merge through magnetic reconnection. Current
sheet behaviors were observed by a 2D magnetic
probe array and four arrays of fast pickup coils.
Typicalplasma
parameters
ofmagnetic
reconnection with a guide field are shown in table I.

Fig. 1. Schematic view of TS-3 device

Table I. Typical plasma parameters
Reconnecting field
Guide fieldElec. density
Elec. / ion temperature
Current sheet width
Current sheet length
Ion gyroradius
Ion skin depth
Ion cyclotron freq.
Lower hybrid freq.

Br ~ 50 mT
Bt ~ 40 mT
ne ~ 1x1020 m-3
Te ~ Ti ~ 10 eV
2 ~ 4 cm
2L ~ 10 cm
i ~ 1 cm
c/pi ~ 2 cm
fci ~ 1 MHz
fLH ~ 40 MHz

3. Experimental results
The existence of the guide field brings drasticchanges in the appearance of the magnetic
fluctuation. Large magnitude of fluctuation with ion
cyclotron range frequency (~2ci) and parallel
wavelength in the order of ion gyroradius wasobserved during the reconnection with a guide field.

Fig. 2 (a) shows the magnetic fluctuation signals
observed inside the current sheet. Large-amplitude
magnetic fluctuation with frequency of about 2MHz(see fig. 2(b)) was detected during the magneticreconnection. Enhancement of the effective
resistivity in the current sheet was also observed
around the same time when the maximal wave
amplitude up to 10 % of the reconnecting field wasdetected. These experimental results suggest thatthe drift-instability like current sheet modulation
develops and enhances the reconnection rate.

current sheet is then suddenly released to developtransient fast magnetic reconnection, which issometimes accompanied by current sheet / plasmoid
ejection events.
4. Discussion
Current sheet behaviors have been investigated inthe plasma merging experiment with various
conditions of magnetic reconnection. The guidefield primarily slows down the reconnection ratesince it prevents the current sheet compression toincrease the stability for microscopic mode whichdevelops in ion or electron scales. However, once
the inflow flux exceeds the outflow rate, the stored
flux / energy around the current sheet brings somekinds of instabilities to enhance the reconnection
rate.

(a)

(b)

Fig.2. Magnetic fluctuation signals (a) and their
Fourier spectrums (b) observed during magnetic
reconnection with guide field.
The existence of the guide field provides
propagation condition for kinetic Alfvén waves
with wave number and frequency observed in theexperiment. Thus, the long-wavelength current
sheet instability in the guide field reconnection
might couple with the kinetic Alfvén mode todevelop to a large-amplitude and coherent waves
inside the current sheet.
On the other hand, magnetic reconnection
without guide field is accompanied by magnetic
fluctuation near the end of and after thereconnection. The observed fluctuation frequencyof 200-400kHz is close to that estimated from themotion of the reconnected field lines with m=1 and2 deformation. This low frequency fluctuation may
account for the significant ion heating in thereconnection without guide field observed in detailby a two-dimensional Doppler spectroscopy.
Plasmoid development was also observed in theguide field reconnection, resulting in a non-steadyfast magnetic reconnection. Externally driven large
inflow provokes tentative imbalance of the inflow
and outflow fluxes, resulting in a density pile-up
near the X point. The accumulated plasma in theAcknowledgments
This work was supported by JSPS KAKENHI(22246119 and 22686085) and the Global COE
Program “Secure-Life Electronics”, MEXT, Japan.
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