22A03
The analysis of fast electrons by measurement hard X-ray during current driveby EC waves in QUESTQUESTにおける電子サイクロトロン波を用いた電流駆動時の硬X線計測と高速電子挙動解析
TASHIMA Saya, ZUSHI Hideki, ISOBE Mitsutaka, IDEI Hiroshi, OKAMURA Syouichi,
HANADA Kazuaki, NAKAMURA Kazuo, FUJISAWA Akihide, HASEGAWA Makoto,
NAGASHIMA Yoshihiko, ISHIGURO Masaki, KAWASAKI Syouji, NAKASHIMA Hisatoshi,
HIGASHIJIMA Aki
田島西夜, 図子秀樹, 磯部光孝, 出射浩, 岡村昇一, 花田和明, 中村一男, 藤澤彰英,長谷川真, 永島芳彦, 石黒正貴, 川崎昌二, 中島寿年,東島亜紀
IGSES, Kyushu University, Kasuga, Fukuoka, 816-8580, Japan816-8580 福岡県春日市 6-1 九州大学大学院 総合理工学府 先端エネルギー理工学科
To investigate the contribution of fast electron in the non-inductive plasma current (Ip) in electron cyclotron heated(ECH) plasma, the experiment at three magnetic configuration with different vertical field (Bz) curvatures are
performed in the QUEST. Ip and flux of the hard X-ray photon (ΓHX) are compared with different magnetic
configuration as a function of Bz. In the magnetic field of negative curvature, trapped or stagnation electron are easily
losed. In the magnetic field of moderately positive curvature, Ip and ΓHX are respectively fourth and three times lagers
than them in the magnetic field of negative curvature at week Bz (< 2 mT). In the strong curvature, Ip and ΓHX are
respectively three and ten times lagers than them in the moderately positive curvature, at strong Bz (> 3 mT).

1. Introduction
For steady state operation of tokamak devices,non-inductive current start-up and maintenance are
one of the important research fields. EspeciallyECH has been found to be effective for plasmabreakdown and production of a seed current Iseed
non-inductively. Although three mechanisms of Iseed
have been proposed in the open magnetic field
configuration, the final conclusion is still open.
1)The return current along the spiral field line thatcancels the charge separation due to the ∇B drift.
Ip = 2<P>S/RBz (1) , [1]2) The current by the electrons whose toroidal drift
is cancelled by the vertical component of v//. [2]vz=v//( Bz/Bt ) – m( v//2 + v⊥2/2 )/ eRB ≈ 0 (2)3) The toroidal precession current caused by thebanana orbits of the trapped electrons.
Here, <P>is spatially averaged plasma pressure, Sis cross-section area of plasma current, R is theplasma major radius, Bt is toroidal field. In theCDX-U device, the closed field configuration canbe obtained in ECH plasma under steady Bz, andrelationship of Iseed ∝ Bz-1 is obtained [3]. This isconsistent with first mechanism. In addition, thefact that the particle confinement time in the open
field configuration shows the maximum at theoptimized Bz has been interpreted on the balance
between the parallel loss along the spiral field lines
and radial E×B drift loss in WT-2 [4] and TORPEX[5]. In QUEST it has been also observed that Iseed
agrees with the optimized Bz[6]. In order todiscriminate the second and third mechanisms for
Iseed dependence of the decay indexes n* = -d (lnBz)/d(lnR) has been carried out in TST-2 [7]. Results
show that Iseed is independent of n*. In the present
research, to investigate the driven mechanisms of Ip
and n* dependence of fast electrons, HX
measurement is performed. Both Bz and n*aresurveyed. Furthermore how Iseed is increased withincreasing Bz and connects the driven current Ip areinvestigated.
2. Device and experimental conditions
QUEST is a medium sized spherical tokamak
device [8]. Bz can be produced by a three sets ofPoroidal field coils PFs, PF17, PF26, and PF35. n* is-0.2(PF17), 0.25(PF26) and 0.5 (PF35) at (R,Z)=(0.6m ,0 m), respectively. The O-mode waves withparallel refractive index N|| of 0.4 are injected fromthe low field side via a phased array antenna. In thepresent experiments the resonance position is fixed at
R=0.3 m. HX flux (ΓHX) is horizontally detected bythe semiconductors. The pulse height analysis for thephoton flux in the energy range of < 400 keV is doneat the dwell time of 1-5 ms.
Two cases of experiments are conducted. For thecase 1, production of Iseed is investigated at lowpower Prf of 17 kW and constant Bz in time. ThreePFs configurations are used, time averaged currentIp [kA]

43PF17PF2621812x100.00.2

0.4

0.6

0.8

1.0

Γ HX [CPS]

x10

1.2

1.4

1.6

1.8

20.0400.0 0.4 0.8 1.2 1.62.0x10-3-3

For PF 35 case ΓHX 1.8 times greater than in PF26
case at Bz = 3 mT, and trend of ΓHX is same as Ip.
4. Discussion and Summary
In order to investigate the electron confinementin these magnetic configurations, the electron orbits,
which are launched at the vessel center (R = 0.6 m
and z = 0 m), are numerically investigated for theenergy range from 1 keV to 30 keV, at various pitchangles. In the PF17 case, all electrons are lost at thewall, because no trapped particles can be allowed.
Therefore the Bz dependence of <Tseed> and orbit
calculations suggest that the first mechanism may playa role for current generation.
The equilibrium Bz is estimated by eq. (3)µ0 I p
8 R li 3( In+ − + βp)
4πRa 2 2(3)Here, βp = 8π2a2<P>/µ0Ip2, <P> = neTe, Te is theelectron temperature (~100 eV), ne is the electron
density (~1×1017 m-3). For the PF 26 case, the relation
indicates a conventional low βp equilibrium. However,for PF35 case, the larger Bz is required for higher βp.
The role of the energetic electrons on βp is essential.
120Bz [T]Fig. 1 Time averaged Ip and ΓHX function of Bz.
♦; PF26coil configuration○; PF17coil configuration
Figure 2 shows the comparison of Bz
dependence of <Ip> for the PF35 case, with those of
the PF26 case. Although both the maximum of <Ip>
are ~ 5-6 kA, the relation with Bz is quite difference.
For the PF35 case the maximum value of <Ip> isobtained at Bz of 8 mT, while no current isgenerated for the PF26 case for Bz>1.6 mT. Inorder to increase Ip for the PF 26 case the Bz
ramp-up scenario (the case 2) must be used, as
denoted by open diamonds in the Figure 2. For thePF35 case, when Prf > 60 kW, more than 10 kA canbe easily accessed without ramping–up Bz.
Therefore, the confinement of the energetic
electrons and generation mechanisms of Iseed seemto be strongly dependent on n* from 0.25 to 0.5.

5.0x10|Bz| [T]-382.5Fig. 2 Time averaged Ip function of Bz. ♦ ; PF26coil
configuration with constantly Bz. ▲; PF35coil
configuration with constantly Bz ◊ ; Bz ramp-up

Bz =2.06403

PF35PF26Bz Ramp-up

10|Ip| [kA]

<Iseed> is used. Iseed and ΓHX are studied as a functionof Bz and n*. For the case 2 ramp-up of Iseed and ΓHXis studied with temporally increasing Bz and then arelation between Ip and Bz is compared with theconventional tokamak equilibrium. In order toramp-up Ip Prf is raised up to 40 kW.
3. Experimental results
Figure 1 shows the Bz dependence of <Iseed>andΓHX for the PF17 and PF26 configurations. For theformer, no current is generated for Bz < 0.8 mT,<Iseed> is ~ 1 kA at Bz ~ 1.0 mT and decays for Bz >1 mT. For the latter, <Iseed> increases linearly up to
4 kA with Bz of 1.6 mT , but <Iseed> cannot becreated avove 1.6 mT. Thus, the clear n* dependence
on <Iseed >is confirmed. ΓHX is not observed for Bz <0.8 mT, but appears for Bz> 0.8 mT for both cases.
For the PF17 case ΓHX peaks at Bz of 1.2 mT, but
ΓHX for the PF26 case peaks at slightly higher Bz of
1.5 mT. At the higher Bz ΓHX is reduced, but a finiteintensity is observed though <Iseed> is < 1 kA.

135150165

105 90302520
15105PF35 3 mT
7512060

13545150

3015165

105 9030252015105PF26 3 mT
756045300 180

180
0 5 10 15 20 25 30

1500 5 10 15 20 25 30Fig. 3 The confined areas in the velocity space○;Lost ●;confined
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[8] K. Hanada et al., 22nd IAEA FEC FT/P3-25,2008
Res. Letters 3, 026 (2008)[9]T. Yoshinaga, et al., PRL 96, 125005 (2006)