Proceedings of ITC/ISHW2007

Observation of Toroidal Flow on LHD
Mikirou Yoshinuma, Katsumi Ida, Masayuki Yokoyama, Kenichi Nagaoka, Masaki Osakabe
and the LHD Experimental Group
National Institute for Fusion Science,322-6 Oroshi-cho, Toki, 502-5292, Japan
In order to investigate the formation of toroidal flow in helical systems, both NBI driven flow and spontaneous
toroidal flow are observed in the Large Helical Device (LHD). The toroidal flow driven by NBI is dominant in theplasma core while the contribution of the NBI driven toroidal flow is small near the plasma edge. The spontaneous
toroidal flow changes its direction from co to counter when the radial electric field is changed from negative topositive at the plasma edge. The direction of the spontaneous toroidal flow due to the radial electric field near theplasma edge is observed to be opposite to that in the plasma core where the helical ripple is small.
Keywords: Toroidal Flow, Spontaneous Flow, Charge Exchange Spectroscopy

1. Introduction

author’s e-mail: yoshinuma@nifs.ac.jp

65
t=1.35s3t=0.95s

iT (keV)

42
10
3.6

3.8

4

4.24.4

4.6

4.8

R (m)
10t=1.35s

0-10t

V (km/s)

The transport in the plasma is considered to besensitive to the profile of flow velocity. A moderate shearof poloidal flow can suppress the turbulence and reduce thetransport. On the other hand, it has been pointed out thatthe toroidal flow contributes the stabilization of resistive
wall mode in tokamaks [1]. Therefore the spontaneous
toroidal flow becomes important in the next fusion device
such as ITER, where the toroidal flow velocity driven byexternal momentum is expected to be not enough tostabilize the MHD mode. The mechanism of driving thespontaneous toroidal flow has a great interest in the
momentum transport physics and has been investigated intokamaks experimentally and theoretically [2-8].
Besides the toroidal flow driven by the external
momentum input of neutral beam injected tangentially,
there is the spontaneous toroidal flow driven by thecoupling of ExB force and viscosity tensor. Stress tensor
re-directs some fraction of diamagnetic and ExB flows into
parallel flows. The radial electric field can be controlled bychanging the collisionality as predicted by neoclassicaltransport in the helical system. The radial electric field at
the plasma edge changes its sign from negative to positiveby reducing the electron density in the NBI plasma, while
the radial electric field in the plasma core becomes positiveby applying the center focused ECH to NBI plasma inLHD. There are three NBI tangentially injected; one isinjected in CW direction and the other two are injected in
CCW direction. The various combinations of these three
beams give the scan of momentum input to the plasma in
the wide range. Both the toroidal and poloidal rotation
velocities are measured with the charge exchange
spectroscopy using the charge exchange line of fully
ionized carbon. In this paper, we will report theobservations of both the NBI driven toroidal flow andspontaneous toroidal flow in helical plasma.

(b)t=0.95s

-20
-30
-40
-50
-60
3.6

3.8

4

4.2

4.4

4.6

4.8

R (m)
Fig.1 Radial profiles of (a) ion temperature and (b)toroidal flow velocity before (open circle)
and after (close circle) the injection oftangential neutral beams in the plasma with
Rax=3.6m, Β=2.85Τ and γ=1.254.

Proceedings of ITC/ISHW2007

1510Electron RootCoNBI2(Co)51019(a)-30.55x10 m(with Co injection and ECH)

5r019-5-10

3.8

44.2

Ion Root-10

NBI1(Ctr)

19-3

1.0x10 m(with Ctr injection)

4.4-15
4.2

4.6

4.3

4.4

4.5R(m)R (m)Fig.2 Radial profiles of toroidal flow velocity inthe plasma with Rax=3.6m, B=1.5T and
γ=1.174. The tangential NBI is selected forco-injection(square),counter-injection(triangle), and balanced injection (circle).

5(b)Ion Root19-3

1.0x10 m(with Ctr injection)

Co-15-30.45x10 m(with Co injection)pV (km/s)

Ctrt

V (km/s)

-5-20
3.6

Negative E Positive Er

NBI1&2(Balance)
0-5t

V (km/s)

Ctr

0

2. Lines of Sight of CXS on LHD
The charge exchange spectroscopy (CXS) has been
widely used to measure the profiles of ion temperature,
toroidal flow velocity, and impurity in neutral beam
injected plasmas. In LHD, charge exchange line of thefully ionized carbon is used for the CXS measurement.
The extreme hollow profile of carbon impurity isobserved associated with the peaking of the ion
temperature profile in LHD. Because of the finite beamwidth comparable to the half of the plasma minor radius,
the integration effect along the line of sight can be aserious problem in the poloidal view when the carbon
density profile become hollow. Therefore the CXS
measurements with the toroidal view are applied tomeasure the ion temperature more accurately in theplasma with peaked ion temperature where the carbon
density profile tends to be hollow. Figure 1 shows theexample of the CXS measurement with toroidal line ofsight in the plasma with the magnetic axis Rax of 3.6m,magnetic field strength B of 2.75T, pitch parameter γ of
1.254 and counter dominant neutral beam injection. The
central ion temperature achieved to 4.5keV after theinjected of three tangential neutral beams. The increase of
the ion temperature gradient and strong toroidal flow near
the magnetic axis in the direction parallel to themomentum input of NBI are observed. The spontaneous
toroidal flow in the opposite direction to NBI is observedat the mid minor radius (R=4.2m) where the ion
temperature gradient becomes steep. Thus, the-10

Electron Root
19-3

19 -3
0.55x10 m
0.45x10 m(with Co injection and ECH) (with Co injection)

-15
4.2

4.3

4.4

4.5R(m)Fig.3

Radial profiles of (a) poloidal rotation
velocity and (b) toroidal rotation velocity
near the plasma edge in the case of electronroot (triangle and square) and ion root
(circle), in the plasma with Rax=3.6m,
B=1.5T and γ=1.174.

combination of NBI driven flow near the plasma core andthe spontaneous flow at mid minor radius produces theinteresting radial profile of toroidal flow.

3. NBI driven Toroidal Flow
Figure 2 shows the radial profiles of the toroidal flow
velocity in the plasma with co-injected (parallel to theequivalent toroidal plasma current), counter-injected(anti-parallel to the equivalent toroidal plasma current),
and balance-injected of tangential NBI. The toroidal flows
in the counter-direction are observed whole the major
radius in the plasma with balance-injected NBI shows theexistence of spontaneous component. The toroidal flow

Proceedings of ITC/ISHW2007

55
R=4.4m t=1.5s
NBI 1 and 24ECH ON
1.3s - 1.8s

1.5s (with ECH)etT (keV)V (km/s)

0-532
1-10

051015020

1.2s(without ECH)
2.8

3.2E (kV/m)

4.4

Fig.5 Radial profile of electron temperature with(square) and without (circle) ECH in theplasma with Rax=3.6m, B=1.5T and γ=1.174.

1.5
ECH ON 1.3s - 1.8s
1.25s (without ECH)
1iT (keV)near the magnetic axis depends on the direction of NBI,
while no significant change of toroidal flow is observed
near the plasma edge. It is suggested that the NBI driven
toroidal flow is small at the plasma edge, because of thestrong helical ripple and small deposition of the NBI power.
Therefore the effect of the radial electric filed on thespontaneous flow becomes more visible at the edge.

4R (m)

r

Fig.4 Dependence of toroidal rotation velocity onradial electric field at R=4.4m in the plasma
with Rax=3.6 m , Β=1.5Τ and γ=1.174.

3.6

4. Spontaneous Toroidal Flow at the Edge
Figure 3 shows profiles of poloidal flow and toroidal
flow near the plasma edge where the effect of the radial
electric filed on the spontaneous flow is large. The poloidal
flow, which has a dominant contribution to the radial
electric field, is changed from negative value to positive
value by decreasing the plasma density and by increasing
the heating power. The magnitude of the toroidal flow
velocity in the counter direction increases when the sign ofthe poloidal flow velocity changes from negative value (Er< 0 : the ion root) to positive value (Er > 0 : the electronroot). Figure 4 shows the relation between the toroidal flow
velocity and on the radial electric field at the plasma edge
(R=4.4m). The counter flow (negative) increases when theradial electric field becomes more positive. Theses results
are consistent with the experiment in CHS, where thespontaneous toroidal flow in the counter direction appears
associated with the transition from the ion root with small
negative Er to the electron root with large positive Er [9]. It
should be noted that the direction of the spontaneous
toroidal flow is anti-parallel to the direction of <ErxBθ>drift. The spontaneous toroidal rotation flows in thedirection reducing the radial electric field rather. This is

0.5
1.45s
(with ECH)
03.6

3.8

4

4.2

4.4

4.6R(m)Fig.6

Radial profile of ion temperature with(square) and without (circle) ECH in theplasma with Rax=3.6m, B=1.5T and γ=1.174.

because the viscosity tensor re-directs some fraction of
ExB flows into the direction of minimum gradient ofmagnetic field strength. The direction of the parallel flows
re-directed by the viscosity in helical plasma is opposite tothat in tokamaks because the pitch angle of minimum
gradient B is larger than the pitch angle of magnetic fieldaveraged in magnetic flux surface in helical plasma, while
the pitch angle of minimum gradient B is zero in tokamaks.

Proceedings of ITC/ISHW2007
In tokamak, the spontaneous toroidal rotation in thecounter direction is observed in the plasma with negative
Er as reported in JFT-2M [3]. The spontaneous toroidal
rotation becomes most significant in the ITB region where
the strong negative Er appears [14]. The direction of thespontaneous toroidal rotation in tokamaks is parallel to thedirection of <ErxBθ> drift.

10
ECH ON 1.3s - 1.8s
51.45s (with ECH)

-5tV (km/s)

06. Summary-10-15-20
3.6

1.25s
(without ECH)
3.8

4

4.2

4.4

4.6R(m)Fig.7

Radial profile of toroidal rotation velocity
with (square) and without (circle) ECH in theplasma with Rax=3.6m, B=1.5T and γ=1.174.

5. Spontaneous Toroidal Flow in the Core
The improvement of electron heat transport near themagnetic axis (electron ITB) is observed in various helicalsystems by applying the center focused ECH to the lowdensity plasma [10-13]. When the transition to the electron
ITB takes place, the positive radial electric field is formedin the plasma core. The spontaneous toroidal flow driven
by the ExB flow associated with the electron ITB isobserved in the plasma core. The tangential NBI isbalanced to make the NBI driven toroidal flow to beminimized small in the plasma core. The profiles of theelectron temperature, the ion temperature and the toroidal
flow velocity in the plasma with and without the ECH are
shown in Fig 5, Fig. 6 and Fig.7, respectively. The electron
ITB profile is observed in the electron temperature profile
during the ECH pulse (t=1.3–1.8 sec) while no significant
change in the ion temperature profile is observed.
Associated with the transition to electron ITB, which ischaracterized by the peaked electron temperature, a large
positive radial electric field appears in the plasma core [10]as predicted by neoclassical theory. The toroidal flow in theco-direction during the ECH is clearly observed. This
result shows that the positive electric field drives thetoroidal flow in the co-direction in the plasma core wherethe modulation of the magnetic field due to helical ripple issmaller than that due to toroidal effect (εh < εt). The
direction of the spontaneous toroidal flow near themagnetic axis is parallel to the direction of <ErxBθ> drift,
which is in contrast to the spontaneous toroidal flow
anti-parallel to the direction of <ErxBθ> drift near theplasma edge as shown in Fig.3 and Fig.4.

Radial profiles of the plasma flow velocity both in thetoroidal direction and poroidal direction are measured withthe charge exchange spectroscopy using the charge
exchange line of fully ionized carbon. Toroidal flow
parallel to the momentum input of NBI is observed to belocalized in the core region of the plasma. The relations
between the spontaneous toroidal flow and radial electricfield are investigated and the positive radial electric field
drives spontaneous rotation in the counter direction near
the plasma edge and in the co direction near the magneticaxis. This observation shows the spontaneous rotation near
the magnetic axis is opposite to that near the plasma edge.
The difference in the direction of spontaneous flow
between core and edge is considered to be due to thedifference in the ratio of the toroidal effect to the helicalripple. In LHD, the spontaneous toroidal flow re-directed
ExB flow by the viscosity tensor in the core region isparallel to that observed in tokamaks, while thespontaneous toroidal flow near the plasma edge isanti-parallel to that in tokamak.
We would like to thank technical staff for their effort
to support the experiment in LHD. This work is partly
supported by a Grant-in-aid for Scientific research(18206094) of MEXT Japan. This work is also partly
supported by NIFS05LUBB510.

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