28pA05
合体球状トカマクの電子加熱
Electron Heating in Merging Spherical Tokamaks
山田琢磨1, 神尾修治1, 渡辺G岳典1, 田辺博士1, GRYAZNEVICH Mikhail2,山﨑広太郎1, 曹慶紅3, 板垣宏知1, 竹村剛一良1, 井通暁1, 小野靖1
Takuma YAMADA1, Shuji KAMIO1, Takenori G WATANABE1, Hiroshi TANABE1,Mikhail GRYAZNEVICH2, Kotaro YAMASAKI1, Qinghong CAO1, Hirotomo ITAGAKI1,Koichiro TAKEMURA1, Michiaki INOMOTO1, Yasushi ONO1
東大新領域1, カラム研2, 東大工3
GSFS, Univ. Tokyo1, CCFE2, School of Eng., Univ. Tokyo3
When two plasmas merge together to form a single
plasma, magnetic field lines reconnect, and theirenergies are converted to the plasma kinetic orthermal energies during a very short period. It has
been reported that electrons are heated inside thecurrent sheet during magnetic reconnection, whileions are heated at around the two downstream areas[1]. However, compared to the ion heating, theelectron heating has not been clarified in detail, e.g.,when and where in the current sheet electrons areheated. This work determined that the electrons are
locally heated at the merging X-point in the Mega
Ampere Spherical Tokamak (MAST) device, whichis the world’s largest plasma merging device.
MAST has the highest magnetic field (~0.6 T)
and Lundquist number (106–108) among themerging laboratory plasma devices. The plasma
merging start-up method in MAST is called
merging-compression, which creates two initialplasmas around a pair of in-vessel poloidal fieldcoils by its current ramping down and merges themat the mid-plane. In MAST, initial currents up to
500 kA and initial electron heating up to 1.2 keVwere measured during the merging-compression
plasma formation. Figure 1 shows the twodimensional electron temperature profile at 10 ms,
which is about 5 ms after the merging. Several
shots, which were vertically controlled by thein-vessel coils, were used to reconstruct thetwo-dimensional electron temperature profile. The
narrow central electron temperature peak isapproximately circular, having almost the same
width in the radial and vertical directions. In
addition, there is a structure surrounding the central
peak like an outer rim. Data from a different seriesof shots with larger outer rim structure, in which thefilling gas pressure was lower than those in theshots of Fig. 1, made it clear that the outer rim
structure also had a circular shape. The origins ofthese structures are considered to be as follows. Theelectrons are heated inside the current sheet by thetoroidal electric field produced by reconnection via
Joule heating. This expectation is supported by thefact that the electron heating was observed bothwith the CS and without the CS. However, most of
the heated electrons flow to the downstream andonly a narrow peak at the X-point region remains.
The relaxation from the hot ions, which were heatedby reconnection in the downstream areas, makes
another electron temperature peaks in the plasmaedge. After the plasma merges and forms closed
surfaces, the temperature peaks at the edge create
an outer rim structure surrounding the central peak.
[1] Y. Ono et al., Phys. Rev. Lett. 107, 185001 (2011).

Fig. 1. Two-dimensional electron temperature profile at 10 ms measured with ruby Thomson scattering.
Several vertically controlled shots were used. Almost circular narrow peak at the X-point and a structure likean outer rim surrounding the central peak were observed.