22F09
Formation of spherical tokamak at 10 times the plasma cutoff density on LATELATEでの遮断密度の10倍を超える電子密度領域での球状トカマク形成
Masaki Uchida, Fumitake Watanabe, Hitoshi Tanaka, Yuto Noguchi, Jun Katsuma,Toshiyuki Kanemitsu, Ryota Hayashi, Tadahiko Fukunaga, Hikaru Mizogami, Shota Omi,
Kengoh Kuroda and Takashi Maekawa
打田正樹, 渡辺文武，田中仁，野口悠人，勝間淳，金光俊幸，林良太，福永忠彦，溝上晃，逢見翔太，黒田賢剛，前川孝
Graduate School of Energy Science, Kyoto University
Kita Shirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
京都大学大学院エネルギー科学研究科 〒606-8502 京都市左京区北白川追分町
This paper reports on start-up and formation of a spherical tokamak (ST) plasma with ~ 10 times theplasma cutoff density solely maintained by electron Bernstein (EB) waves. The plasma current reaches up
to Ip = 10 kA under a 2.45 GHz microwave power of 60 kW, 250 ms pulse. The dependence on theelectron cyclotron resonance (ECR) location for start-up discharges has been investigated. It is found thatthe bulk electron density significantly increases when the ECR layer locates slightly inboard side of thevacuum vessel center. In this configuration, better coupling to the EB waves at the first propagation band
(between the fundamental and 2nd ECR) and effective heating of bulk electrons at the plasma core areindicated.

1. Introduction
Tokamaks need the toroidal plasma current,
which is usually initiated and maintained byinduction from a center solenoid (CS). If CS canbe eliminated from the device, the structure oftokamak reactors will be greatly simplified,
which may enable economical fusion reactors [1].
For this purpose, start-up and formation of ST byelectron cyclotron heating and current drive(ECH/ECCD) have been studied in the Low
Aspect ratio Torus Experiment (LATE) device.
ECH/ECCD method is eminently favorable forreactors since the wave beam can be injected via
a small launcher remote from the plasma.
This paper reports a startup and formation of
an ST plasma with ~ 10 times the plasma cutoff
density solely maintained by electron Bernstein(EB) waves, where the plasma current is ramped
up to Ip = 10 kA by 2.45 GHz microwave powerof 60 kW, 250 ms pulse.
2. Experimental Setup
The experiments are performed in the LATE
device, whose vacuum vessel is a cylinder with an
inner diameter of 1m and a height of 1m. The center
post with an outer diameter of 11.4cm encloses 60
turns of conductors for the toroidal field. There is
no CS for the inductive current drive.
Three 20 kW and one 5 kW magnetrons at
2.45 GHz are used for EC heating. Each power islaunched obliquely to the toroidal field from theFig.1. Typical discharge

low field side on the midplane, via a cylindricallauncher of open waveguide type. The polarization
is linearly polarized one where the electric field lieson the midplane, and is mainly in the O-mode.

3. Experimental Results
Figure 1 shows a typical discharge. A steadytoroidal field of Bt = 720 G and a vertical field ofBv = 20 G (both at R = 25 cm) are applied before
the microwave injection. A plasma current isinitiated and closed flux surfaces are formed via acurrent jump under a steady vertical field [2]. Afterthe closed flux surfaces are formed, the plasma
current ramps up with an increase of the microwave
power and the equilibrium Bv and reaches
Ip = 10 kA at Bv = 120 G (Figs. 1(a) and (b)). The
three chords interferometer measurement showsthat the electron density inside the last closed flux
surface significantly increases as Ip increases(Figs. 1(c)-(e)). At the final stage, the line-averaged
density along R = 27 cm reaches ~ 10 times theplasma cutoff density, suggesting that the plasma is
maintained solely by EB waves.

temperature also increases by EB heating.
On the other hand, when we set the ECR layer atR = 19.2 cm, the electron density significantlydecreased, while the plasma current is ramped up to
the same value of Ip ~ 10 kA under the same
injection power. In this case, the current profile
expands to the outside of the 2nd harmonic ECRlayer and the UHR layer is estimated to be close toor outside the 2nd harmonic ECR layer. Magnetic
analysis [3] shows a large perpendicular pressure
region exists near the 2nd harmonic ECR layer.
These suggest that a larger portion of the injected
power is coupled to the high energy trappedelectrons by the 2nd harmonic heating of EB waves
in this configuration.

Fig. 3 Dependence on ECR location

Fig.2 Poloidal flux contours and current profile

Figure 2 shows the poloidal flux contours and thecurrent profile estimated from the magnetic analysis
at Ip = 10 kA (t = 0.25 s). In this discharge, theupper hybrid resonance (UHR) layer location isestimated to be between the fundamental EC
resonance layer (R = 20.6 cm) and the second
harmonic one. Incident electromagnetic waves are
expected to be mode-converted to EB waves in thisfirst propagation band and then effectively heat thebulk electrons at fundamental ECR located near theplasma core. Soft X-ray and extreme ultraviolet
emission profiles show significant increases justoutside of the ECR layer as Ip increases. In addition,
impurity line radiations at higher excitationenergies such as CV (304 eV) and OV (72 eV)strongly increase compared with the density
increment. These suggest that the bulk electron

Figure 3 shows the dependence on the ECR
location for similar discharges where plasma
currents are ramped up to Ip ~ 4kA. It is clearlyshown that the electron density significantly
increases when the fundamental ECR is located at
R = 20-22 cm (Bt ~ 0.07 – 0.08 T), slightly inboard
side of the vessel center, and the 2nd harmoniclayer is located at close to the vessel wall of
R = 50 cm (Figs.3 (b)-(d)). In this configuration, theUHR layer is located between the fundamental and2nd harmonic ECR layer, resulting in a better
coupling to EB waves at the first propagation bandand the effective heating of bulk electrons at thefundamental ECR.
References[1] S. Nishio, et al.: Proc. 20th Int. Conf. on Fusion
Energy (Vilamoura, 2004) FT/P7-35.
[2] T. Yoshinaga, et al.:Phys. Rev. Lett.96(2006) 125005[3] M. Uchida, et al.:Phys. Rev. Lett. 104 (2010) 065001