23P101-P
Measurement of Energetic Electron Flow with Langmuir Probe in QUESTQUESTにおける静電プローブを用いた高速電子流計測
Masaki Ishiguro1, Kazuaki Hanada2, Haiqing Liu1, Hideki Zushi2 et al.
石黒正貴1, 花田和明2，劉海慶1，図子秀樹2、中村一男2、藤沢彰英2、出射浩2、永島芳彦2、長谷川真2、松岡啓介1、吉田直亮2、徳永知倫1、磯部光孝3、田島西夜1、高瀬雄一4、前川孝5、御手洗修6、岸本泰明5、川崎昌二2、中島寿年2、東島亜紀2
1Interdisciplinary Graduate School of Engineering Sciences Kyushu University, Kasuga 816-8580, Japan
2
Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan1
九大総理工 〒816-8580 春日市春日公園6-1
2九大応力研 〒816-8580 春日市春日公園6-1
3
核融合研 〒509-5292 土岐市下石町322-6
4
東大新領域 〒277-8561 柏市柏の葉5-1-5
5
京大エネ科 〒606-8501 京都市左京区吉田本町
6東海大 〒862-8652 熊本市渡鹿9-1-1

In spherical tokamak (ST), non-inductive current start-up with RF injection has been an important matter.
Although it was suggested that energetic electrons confined in open magnetic field configuration (OMFC)
play an important role for spontaneous formation of closed flux surface (CFS) in numerical manners,
quantitative estimation was inadequate. This time, the current carried by these energetic electrons wasdirectly measured with a newly-developed Langmuir probe and measured results were well fitted withvalues estimated numerically. The temperature of these energetic electrons was about 4~5keV and total
carried current was estimated to reach about 1.4kA, which seemed large value enough to affect theformation of CFS with pressure driven current.

1. Introduction
For realization of commercial future fusion
power plant, Spherical Tokamak (ST) is one ofthe promising candidates because of its cost
efficiency. In tokamak including ST, toroidal
loop current should be driven in plasma for
confinement of it in closed magnetic flux surface(CFS) and inductive method with central solenoid(CS) coil has been conventionally used to start-up
the plasma current. However, in ST, CS-lessstructure is required because of its tightly
restricted special margin of central part of amachine. Thus, non-inductive current start-up is
one of the most important matters for ST.
Especially, the method with radio frequency wave(RF) injection has been widely used andresearched in many ST devices such as
CDX-U[1], LATE[2], TST-2[3] and MAST[4]. In
current start-up with RF injection, energetic
electrons were focused in that current carried bythese electrons seemed to play an important rolefor spontaneous transition from open magnetic
field configuration (OMFC) to CFS [5]. Although
the presence of the current carried by energetic
electrons was suggested in numerical manners [6],no one has confirmed it experimentally andquantitative estimation was inadequate to judge

whether the current caused by these energetic
electrons has large amount enough to affect theformation of CFS. This time, the current carried
by energetic electrons was directly measured with
newly-developed Langmuir probe for the firsttime. Besides, quantitative estimation of the total
value of the current was done in numericalmanners based on the measurement results. Thistime, we report the results mentioned above anddiscuss the effect of the current for formation ofCFS.
2. Experimental Apparatus
The experiments were done in Q-shu Univ.
Experimental Steady-State Spherical Tokamak(QUEST), which is medium-sized ST in Kyushu
University [7]. RF system with 8.2GHz up to400kW was used for plasma current start-up anddriving. In Fig 1, the schematic view of probe head
used for measurements of the current caused byenergetic electrons is shown. The head has insulator
wall between two tungsten tips and one tip is set on
the wall. Half surfaces of tip A and B are coveredby insulator and tip C is full stripped. The head wasset as the wall perpendicularly crossed toroidal
direction and tip A faced to co-directional electron
flow, where co-direction is defined by electronvelocity in toroidal direction, vtoro and plasma
current Ip as follows; -vtoroIp > 0. Close pieces of
information to obtain the current and thetemperature caused by energetic electrons with thisprobe head are introduced in [8].
3. Experimental Results and Analysis
In the discharges for measurements of energetic
electrons with Langmuir probe, time-constantvertical field Bv was set at about 1.5mT withpositive and negative N-index, n~0.35 and n~-0.07
respectively at the center of the vacuum vessel.
About 30kW RF power was injected and Ip~4kA
resulting in the formation of CFS was obtained inpositive N-index configuration (PNC), while only
about 1.2kA plasma current was obtained innegative N-index configuration (NNC). It should be
noted that energetic electrons are confined only in
PNC. In PNC, the large magnitude of floating
potential more than hundred volts was detected in
tip A, while that in tip B was only several volts in
OMFC region out of last closed flux surface(LCFS). This asymmetric floating potential
suggested presence of co-directional flow ofenergetic electrons. To measure the current causedby the flow, floating potential measurement with tipA, C and ion saturation current measurement withtip B were done and the data was analyzed with themethod mentioned in [8]. As a result of themeasurements, temperature of energetic electrons,
Teh of 4~5keV and current, jehp of 1~2kA/m2 wasobtained at several positions of probe head asshown in Fig. 2. To evaluate total value of thecurrent caused by energetic electrons, thedistribution of the current was estimated with orbit
calculation. In the calculation, fundamental, 2nd and3rd ECR layers were assumed as the source ofenergetic electrons and temperature and density at
the source were decided as the calculated
temperature and current agreed with measured
value at each position of probe head as shown in
Fig. 2. In this calculation, total amount of current
estimated at 1.4kA, which reached about 35 % ofwhole plasma current. The parameters at the sourceof energetic electrons were applied to calculation in
OMFC at initial phase of discharge. The current byenergetic electrons seemed not to be enough to leadto formation of CFS only by itself. But, withpressure driven current in OMFC, which wasdetected in negative N-index configuration, CFS
was formed with current caused by energeticelectrons.
Acknowledgments
This work was supported by Grant-in-Aid for

JSPS Fellows (KAKENHI) 23/3093. This work wasperformed with the support and under the auspicesof the NIFS Collaboration Research Program(NIFS05KUTRO14, NIFS08KUTRO022).

Fig.1. A schematic view of the probe head

Fig.2 The plasma current, the temperature and current
caused by energetic electrons at each probe position areshown. Closed circles and solid line show measuredvalues and open ones show calculated values.
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