Proceedings of ITC/ISHW2007

Impact of Dynamic Ergodic Divertor on Plasma Rotationin the Small Tokamak HYBTOK-II
Y. Hasebe, M. Okamoto, S. Kajitaa , N. Ohnob , S. Takamurac
Graduate School of Engineering, Nagoya University, Furo-cho, Nagoya 464-8603, JapanaJapan Atomic Energy Agency, 801-1, Mukoyama, Naka 311-0193, Japanbc

EcoTopia Science Institute, Nagoya University, Furo-cho, Nagoya 464-8603, Japan

Department of Electronics, Aichi Institute of Technology, Yakusa-cho, Toyota 470-0392, Japan(Received day month year / Accepted day month year should be centered 10-point type)

Plasma rotation or its shear is important on the formation of transport barrier. It is thought that the rotating
helical magnetic field generated by Dynamic Ergodic Divertor (DED) could generate a rotational torque in tokamak
plasmas and control rotation profiles as a result. In order to measure the plasma rotations and to investigate theeffect of DED on them, we developed a passive spectroscopic measurement system for the small tokamak
HYBTOK-II to measure plasma rotations. A spontaneous toroidal plasma rotation in co-current direction and thepoloidal plasma rotation in electron diamagnetic drift direction have been observed without DED. Considerable
changes of the plasma flow have been obtained with DED to show some reduction of toroidal and poloidal rotation
velocity near the resonant magnetic surface. The modification of plasma rotation velocity was found to couple tothe change of the radial electric field.
Keywords: plasma rotation, dynamic ergodic divertor, passive spectroscopy, radial electric field, HYBTOK-II

1. Introduction
To achieve high performance in magneticallyconfined plasmas, control of plasma rotations and theirprofiles play important roles on forming and sustaining
transport barrier through the modification of radial electricfield [1,2]. In burning plasma, it is not easy to control
plasma properties by external current drive or auxiliary
heating (i.e. controlling current and temperature profiles)
due to large bootstrap current and self-α heating. However,in present devices, neutral beam injection is useful tocontrol the plasma rotations as an external momentum
source.
Dynamic ergodic divertor (DED), which has beendeveloped to control energy and/or particle transport at theedge region, is believed to generate rotational torque on
plasmas. Modulation of particle and/or energy transport in
DED comes from the magnetic field structure caused bymagnetic islands induced by externally-applying Rotating
Helical magnetic perturbation Field (RHF) [3-7].
Mechanism of controlling of plasma rotation by use of
DED relates to the Lorentz torque due to interaction
between shielding current produced by the rotating
perturbation field in plasmas and perturbation field itself[7]. The torque is expected to be large when the relative
author’s e-mail: y-hasebe@ees.nagoya-u.ac.jp

angular frequency between plasma and RHF in DED islarge. However, the high frequency RHF in the plasma
becomes weak due to the shielding effect of the vacuum
vessel because the helical coils are wound outside of thevacuum vessel. On the other hand, lower frequency RHF
can penetrate much deeper inside of plasma. Therefore, inthe present paper, we focus on the lower frequency RHF (~several kHz) case to investigate the effect of stochastic
magnetic field formed inside the separatrix in this paper.
In order to study the effect of DED on plasma rotationin the small tokamak HYBTOK-II, an optical measurement
system, which may observe the inner plasma region, hasbeen developed with the working gas of helium.

2. Experimental Setup
The experiments have been performed on the small
tokamak HYBTOK-II (the major radius R0 = 40 cm, theplasma minor radius a ≈ 10 cm, the limiter radius aw = 11cm). In the machine operation, the main plasma parameters
were as follows: the plasma current Ip = 5 kA, the toroidal
magnetic field Bt = 0.28 T, and the pulse duration time is
10 ms. The HYBTOK-II is equipped with DED coils,
which consist of two sets of coils that are wound on theout-vessel locally in the toroidal direction. The RHF withProceedings of ITC/ISHW2007

(a)

(b)

Fig. 1

Arrangement of the DED coils (a) bird’s eye view (b) on poloidal cross section [5].

Fig. 2

Optical arrangements for (a) toroidal and (b) poloidal flow velocity measurements.
(c) Spectrometer, detection device and synchronized system with external trigger.

the poloidal and toroidal mode numbers of m/n = 6/1
resonates with magnetic rotational transform at q = 6
magnetic surface (Fig. 1) [5]. These two sets of coils are
powered independently by insulated gated bipolartransistor inverter power supply with a phase difference of± 90°. We can control the poloidal rotation direction of
RHF by changing the phase difference between two coils.
RHF’s rotation frequency of 3 kHz was selected. Durationof the RHF was 10 msec.
The optical measurement system consists ofbifurcated 32 ch (16 ch×2) optical fiber arrays, focusinglens (103 mm focal length in a toroidal direction, 34.5 mm
in a poloidal direction), Czerny-Turner spectrometer (1800gr/mm grating, 1.0 m focal length and 0.01 nm resolution)and 1024×256 pixels CCD detector (Fig. 2). By changing

the position of the fiber arrays using by movable stages, thedistance between the line of sight and the plasma center, r,can be varied from r = 0-100 mm. The flow velocity, V, isgiven by V = c·∆λ/λ, where c is the speed of light, ∆λ theDoppler shift and λ the central wavelength of the observed
spectrum. The shifted value ∆λ was estimated by ∆λ =(λblue− λred)/2 in this system, where λblue and λred are thecentral wavelength of the spectrum observed from theupstream, and downstream of the plasma flow, respectively.
Thus, one branch of bifurcated arrays faces upstream-side
and another looks forwards the downstream-side as shownin Fig. 2. In this experiments, singly ionized helium
emission (n = 3 − 4, λ = 468.54 − 468.59 nm) was
employed for the flow measurements. The spectrum wasfitted with multi-Gaussian functions considering the fineProceedings of ITC/ISHW2007

Fig.3

Observed spectra at both upstream- (opencircle) and downstream-side (open square),with fitted curves.

Fig. 4

Toroidal rotation velocity profile without(cross) and with DED in the direction of ion
diamagnetic drift (closed circle) and in thatof electron (triangle).

structure of the transition [9].
A triple probe which is accessible up to the center ofplasma column is installed to measure the electron
temperature Te, the electron density ne and the floating
potential Vf. The Plasma potential Vp is estimated by using
the relation Vp = Vf + 3Te.

3. Results and Discussion
In the measurements, pure helium plasma wasemployed to improve S/N ratio by obtaining the strong HeII emission. Typical electron density and electron
temperature are ne ~ 6 × 1018 m-3 and Te ~ 30 eV,
respectively. At first, we briefly present the fundamental
spectral measurements obtained from the passive
spectroscopy. Figure 3 shows the spectra observed both at
the upstream-side and at the downstream-side with therelative strength of the fine structure multiplet. The
observed spectra are fitted by multi-Gaussian functions.
The ion temperature obtained from the Doppler width withconsidering an instrumental width and the fine structure isabout 5 eV, and it is found that there is no clear radial
positional dependence. The toroidal plasma rotation
velocity profiles are shown in Fig. 4 as a function adistance R between a line of sight and the center of theplasma column, which lies on 1 cm inside that of thevacuum vessel, as shown in Fig. 2(a). It is noted that thevelocity does not shows the local value because of thelimitation of the passive spectroscopy owing to line
integration. Thus, only the averaged value over the line ofsight is available at the moment, especially for the toroidal
direction. A spontaneous plasma rotation in the co-current
direction has been observed. The maximum velocity is
about 0.8 km/s. By applying RHF with 150 A of themaximum coil current, a reduction of plasma rotation isFig.5

Poloidal rotation velocity profile.
The markers of each cases are same as Fig. 4.

clearly observed around R = − 4 cm in both RHF rotation
directions. Figure 5 shows poloidal plasma rotation
velocities without and with 3 kHz DED. “r” in horizontal
axis means distance between the line of sight and theplasma center. Without DED, the direction of plasmarotation is the same as that of the electron diamagnetic drift
from r = − 2 to r = − 8 cm. With DED case, plasma rotationis damped around r = − 4 cm. Radial electric field Er
derived by the plasma potential profile is shown in Fig. 6
for both without and with DED cases. Er×B drift direction
matches the poloidal rotation, and reduction of Er at insider ~ 7.5 cm agrees with the reduction of the poloidalrotation. The variations of the plasma rotations by DED
were found to be coupled with radial electric field.
In the low frequency case (3 kHz DED), the changes
in rotation due to DED was observed at the same location

Proceedings of ITC/ISHW2007
changes of Er estimated from eq. (1) using the results of therotation measurements are in agreement with theexperimental results, qualitatively. On the other hand, thetoroidal rotation decrease seems to be caused by friction
increase due to variation of magnetic structure at the edge
region or friction with neutral helium which exists morenear the edge region. Thus, it could be said that changes of
the rotations originate in the magnetic island structure, andconsequently the Er changes, in these cases.

4. Conclusion
Fig. 6 Radial electric field evaluated by using thetriple probe. Negative Er means inward field.

Fig. 7

Perturbation component of radial magnetic
field The locations of rational surface of q = 6,
7, 8 are also shown.

A modification of plasma rotation properties withDED is reported. The plasma flow was measured by theoptical emission spectroscopy developed in HYBTOK-II.
Doppler shift of He II line obtained from the system islarge enough to estimate the rotation velocity and its
direction in pure helium discharge. The spontaneous
toroidal plasma rotation in the co-current direction and thepoloidal rotation in the electron diamagnetic drift direction
were observed without DED. By applying a low frequency
RHF (~ 3 kHz), both the toroidal and the poloidal rotations
decreased. It is likely that they are caused by magnetic
island formation. Modification of Radial electric field wasalso observed.
In future, it is necessary to generate plasma withhigher electron temperature in order to study an influenceof shielding current in the plasma. As a modification ofradial electric field due to enhancement of electrontransport in the stochastic region are observed on theTEXTOR [10], there is also need to consider these effects.

References
for both toroidal and poloidal rotations. Perturbation
component of radial magnetic field Br1 was measured byusing a radially movable magnetic probe and is shown inFig. 7. The Br1 in vacuum decreases with distance from thecoil placed on out-vessel. However, Br1 in the plasma wasamplified compared with that in vacuum near resonancesurface (r ≈ 7.6 cm). This can be contributed to there-distribution of plasma current due to the growth of themagnetic island structure [6,7]. It is thought that poloidal
rotation damping was caused by the enhancement ofpoloidal parallel viscosity due to the island formation. In
steady-state plasma, the radial component of equation ofmotion for ion fluid in the toroidal coordinates is asfollows:
Er = (1 eZ i ni )(∂Pi ∂r ) − Vθ Bφ + Vφ Bθ .

(1)Here, eZi, ni and Pi are the electrical charge, the number
density and pressure of the ion, respectively. V and B arethe flow velocity and the magnetic field, respectively. The

[1]
[2]
[3]

K. Ida, Phys. Fluid B 4, 2552 (1992).
Y. Sakamoto et al., Nucl. Fusion 41, 865 (2001).
S. Takamura, H. Yamada and T. Okuda, Nucl. Fusion 28,183 (1988).
[4] M. Kobayashi, H. Kojima, K. Zhai and S. Takamura, Phys.
Plasmas 7, 3288 (2000).
[5] S. Takamura, Y. Kikuchi, Y. Uesugi and M. Kobayashi,Nucl. Fusion 43, 393 (2003).
[6] M. Kobayashi et al., Nucl. Fusion 40, 181 (2000).
[7] Y. Kikuchi et al., J. Nucl. Mater. 313-316, 1272 (2003).
[8] Y. Kikuchi et al., Plasma Phys. Control. Fusion 49, A135
(2007).
[9] S. Kado and T. Shikama, J. Plasma Fusion Res. 79, 841(2001).
[10] K. H. Finken et al., Phys. Rev. Lett. 94, 015003 (2005).