J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)449–452
000–000

Edge Transport Barrier in CHS
Measured Using Beam Emission Spectroscopy
OISHI Tetsutarou, KADO Shinichiro1, YOSHINUMA Mikirou2, IDA Katsumi2,
TANAKA Satoru and OKAMURA Shoichi2
Department of Quantum Engineering and Systems Science, Graduate School of Engineering,
The University of Tokyo, Bunkyo-ku 113-8656, Japan
1
High Temperature Plasma Center, The University of Tokyo, Bunkyo-ku 113-8656, Japan
2
National Institute for Fusion Science, Oroshi-cho 322-6, Toki 509-5292, Japan(Received: 12 December 2003 / Accepted: 7 May 2004)

Abstract
An edge particle transport barrier was observed as a transition phenomenon characterized by the rapid decreaseof the Hα signal and the increase of line-averaged density in the neutral beam heated plasma in the compact helicalsystem (CHS) when the heating power exceeds a certain threshold. A beam emission spectroscopy (BES) measurement
showed a rapid increase of local plasma density near the plasma edge accompanied by coherent-like density fluctuations
around 5 kHz.
Keywords:BES, density fluctuation, edge transport barrier, H-mode, CHS

1. Introduction

neutral beam atoms (denoted as “beam emission”) [5]. Since
the observable region is the intersection of beam line andsightline for each fiber channel, local values and theircorrelations are available. Provided the beam attenuation isnegligible, then the beam emission intensity can be regarded
as a function of the plasma density.
In this paper we report an initial study of a transition
phenomenon in CHS by using the BES measurement.

The formation of an edge transport barrier (ETB) at theH-mode transition [1] plays a very important role in achivinga high performance plasmas confinement. Several phenomenathat suggest a correlation between the formation of ETB andthe fluctuation of plasma parameters have been observed,including the enhancement of specific MHD instabilities suchas the edge localized mode (ELM) [2] and edge harmonic
oscillation (EHO) [3], and the suppression of turbulence near
the plasma edge with steepened gradients of temperature anddensity. Therefore, it is important to understand therelationship between the formation of ETB and fluctuationsof plasma parameters.
Recently, a transition phenomenon characterized by arapid decrease of the Hα intensity signal was observed in theneutral beam heated plasma in the compact helical system(CHS) when the heating power exceeded a certain threshold.
The increase of line-averaged density and the increase of thelocal edge density measured by means of YAG-Thomson
scattering at the transition have been reported [4], whichsuggests a formation of ETB.
We have applied beam emission spectroscopy (BES) forthe measurement of the local density variations orperturbations with high time resolution. The BES, which has
been proposed to measure long wavelength plasma density
fluctuations, detects emissions from the collisionally excited

2. Experimental setup
Figure 1 is a schematic drawing of BES systems in CHS.

Fig. 1 Schematic of the BES system on the CHS device.

©2004 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchCorresponding author’s e-mail: tetsu@flanker.q.t.u-tokyo.ac.jp1449

Oishi T. et al., Edge Transport Barrier in CHS Measured Using Beam Emission Spectroscopy

the sightline, about 1.6 cm), as seen in Fig. 2, the intensity
rises at t = 85 msec and decreases at t = 113 msec, whichcoincides with the transition and back transition observed in
the Hα signals, respectively. The normalized radius r/a isdefined by the magnetic flux surfaces calculated based on thethree-dimensional equilibrium code VMEC for β = 0 %. The
signal in the edge region can be considered to correspond to
the temporal evolution of the local plasma density because
the beam attenuation is negligible near the plasma edge. This
rapid increase in the signal is observed only in CH7 (r/a~0.91) and CH8, which indicates the formation of theparticle transport barrier in the edge region.
It should be noted that a slight decrease of the signal
inside CH6 (r/a~0.77) before the transition is caused by themoderate change of the density profile and is independent of
the occurrence of the transition. Thereafter, the signal
decreases rapidly at the transition. Near the plasma center(CH1, r/a~0.23), the signal at 100 msec (after the transition)
has decreased by approximately 35 % compared with that at80 msec (before the transition). This rapid decrease can becaused either by the enhanced beam attenuation due to theedge density building-up, by the decrease in the core density,or by both of them. Since the enhancement of the beam
attenuation estimated from the YAG-Thomson density profile
measurements is from 10 to 20 percent, there is a possibility
that the core density decreases at the transition. However, theerrors in the YAG-Thomson measurements in the edge region
are largely due to the low density and temperature. Moredetailed evaluation is required to confirm this.

Spatial channels of BES consist of 8 optical fibers with object
lenses which focus on the position along minor radius ofCHS. The observation regions for channels 1 and 8 are close
to the center and edge, respectively. One system for thehydrogen neutral beam injection (NBI) heating with theacceleration voltage of about 28 keV is used for the probe
beam for BES. The Hα wavelength in the beam emission isDoppler-shifted to 651.9 to 652.4 nm for the observation
angle of 23° to 34° with respect to the beam line, so that it
can easily be separated from Hα emission from bulk plasmas(656.28 nm). Narrow band interference filters, whosetransmission wavelength can be controlled by the filter holder
temperature, are used to select the Doppler-shifted emissions,which are detected using avalanche photo-diode detectors(APD) combined with a 100-kHz low-pass filter.

3. Experimental results
3.1 Temporal evolutions of the intensity ofBES signals
A typical temporal evolution of the beam emission
intensity for the discharge with transition is shown in Fig. 2
together with that of Hα intensity. Plasma was initiated byelectron cyclotron heating (ECH). It was further heated by an
NBI (#2) with the port-through power of 620 kW from 43 to143 msec, and then another neutral beam (#1) with the portthrough power of 670 kW was superposed from 83 to 113
msec. The standard magnetic field configuration in CHS wasadopted in which the vacuum magnetic axis position Rax was
92.1 cm from the torus center and the toroidal magnetic fieldstrength at the magnetic axis Bax was 0.95 T. The BES signal
is observed during the period in which the probe beam, thefirst NBI (#2), applies.
For the near-edge region (CH8, R = 110.3 cm,normalized radius r/a~1.02 with a spatial resolution, width of

3.2 Density fluctuations during barrier
formation
The left-hand side of Fig. 3 shows the contour plot of
the fluctuation power spectrum in the beam emission signals

Fig. 3Fig. 2

Temporal evolutions of the intensity of the beamemission.

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Fourier power spectrogram of the density fluctuationnear the plasma edge (r/a~1.02).
Left: the contour plot against frequency and time.
Right: the average of the contour (a) before thetransition (60–80 msec), (b) during barrier formation(90–110 msec), and (c) after the back transition (120–140msec).

Oishi T. et al., Edge Transport Barrier in CHS Measured Using Beam Emission Spectroscopy

at r/a~1.02 as a function of time and frequency obtained
through a fast Fourier transform (FFT) analysis over every 1msec. The Fourier spectra averaged (a) from 60 to 80 msec
(before the transition), (b) from 90 to 110 msec (during barrierformation), and (c) from 120 to 140 msec (after the backtransition) are shown on the right-hand side. The spectralenergy before transition is concentrated in the low-frequency
region and monotonically decreases with frequency. However,coherent-like fluctuations around 5 kHz were observed
intermittently only during the barrier-formation phase.
Figure 4 shows the temporal evolution of the fluctuationlevel of the BES intensity averaged over 4–7 kHz for (a) CH1(r/a~0.23) and (b) CH8 as the amplitude values obtained froman FFT analysis over every 250 µsec starting from the time
indicated in the context. The signal at 91.25 msec for CH1(indicated by a filled arrow) can be considered to correspondto the density fluctuation of interest, which exists in theobservable region of CH1. On the other hand, in both CH1and CH8 the signals are detected at 98.25 msec and 101.25
msec (indicated by open arrows). The signals in CH8
correspond to the density fluctuation near the plasma edge.
However, the signals in CH1 that appears with those in CH8
can be caused by the density fluctuation in the region of CH1
and the beam density fluctuation. In order to distinguish thesephenomena, we investigated the phase and the level offluctuations.
Phase shift between measurement channels: When theperturbation originates in the edge region, it may affect thebeam density in the downstream of the beam line. In this case,
it is expected that the phase of fluctuations in this frequencychanges the sign between the outer channel and the inner one.
The reason is that the increase in the edge density leads tothe increase in the beam attenuation, which results in thedecrease in the inner beam density. The raw signals around(a) from 98.25 to 98.5 msec and (b) from 101.25 to 101.5
msec are expanded as shown in Fig. 5. The phase of thisperturbation changes the sign both between CH6 and CH7 asshown in Fig. 5(a) and between CH7 and CH8 as shown inFig. 5(b). One might conclude that the signal in the inner
region consists of the information in the edge density
fluctuation.
Estimation of the beam density fluctuation level forthe inner region: We estimated the perturbations in thedownstream signal, which arises in response to that in theupstream events, based on the following model.
(1) The density profile measured by the YAG-Thomson
scattering is used, although there are ambiguities in
the low-density edge region as described previously.
(2) The density change near the edge is from 2 to 4
percent, which corresponds to the amplitude ofcoherent fluctuation of 5 kHz near the edgemeasured by BES, as shown in Fig. 4(b).
(3) The region where density is fluctuating is r/a > 0.9,
which corresponds to the width where the outer two
BES channels are observing.
The intensity fluctuation component expected from theFig. 4

Temporal evolution of the fluctuation level of the BES
intensity averaged over 4–7 kHz for (a) CH1 (r/a~0.23)
and (b) CH8 (r/a~1.02).

Fig. 5

Enlarged view of BES signals around (a) from 98.25 to98.5 and (b) from101.25 to 101.5 msec. Solid bold lines
are fitting curves (5-kHz sine).

downstream beam density fluctuation is approximately from
0.1 to 0.2 percent of the central density, which is considerably
lower than the detector root mean square noise level of 1 %.
Nevertheless, if the fluctuations are coherent in the low
frequency range and their mean values vary, they are reflected
in a trend of the signals. Indeed, the fluctuation level observedin the central channel is about 1 to 1.5 percent (Fig. 4(a)), so
it is difficult to distinguish the density fluctuation from beam
fluctuation in the inner region.

4. Discussion
As described in Section 3.2, coherent-like fluctuationsof BES signals around 5 kHz were observed intermittently
while the barrier was formed. They had a radially inversed
phase due to the downstream beam density fluctuation causedby the edge density fluctuation and/or due to the density
3451

Oishi T. et al., Edge Transport Barrier in CHS Measured Using Beam Emission Spectroscopy

fluctuation that had a radially inversed phase. The sawtoothoscillation can be regarded as an example of the phenomenon
with the radial phase inversion of plasma parameter
fluctuation because it is characterized by the phase inversionof temperature oscillation [6]. In CHS, the density fluctuation
around 5 kHz associated with the sawtooth oscillation has
been observed near the edge with electric probes [7]. To
clarify the mechanisms of phase inversion of density
fluctuations, the correlations between these phenomena andour observations should be examined.
We suspect that this fluctuation is related to the transition
phenomenon; presumably it is enhanced by the increase in
the local density or pressure gradient near the last closed flux
surface (LCFS) accompanied by the transition. The coherent
oscillations related to the density or pressure gradient havebeen reported in both Tokamaks [3,8,9] and Helicals [10,11],although the detailed mechanisms and identifications are still
under investigation. We are also analyzing the characteristicsof this oscillation.

(2) A coherent-like density fluctuation around 5 kHz isenhanced intermittently while the barrier is formed.
The characteristics of this fluctuation, such as therelationship between the increase in the local density orpressure gradient near the LCFS and the enhancement of thefluctuation, should be investigated.

Acknowledgement
One of the authors (T.O.) was partly supported thoroughthe 21st Century COE Program, “Mechanical SystemsInnovation,” by the Ministry of Education, Culture, Sports,
Science and Technology.

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(2003).
[4] S. Okamura et al., J. Plas. Fus. Res. 79, 977 (2003).
[5] R. Fonck et al., Rev. Sci. Instrum. 61, 3487 (1990).
[6] S. von Goeler et al., Phys. Rev. Lett. 33, 1201 (1974).
[7] K. Ohkuni et al., Proc. 28th EPS on Plasma Phys. andControl. Fusion (Maderia, 2001) P2.077.
[8] K.H. Burrell et al., Plasma Phys. Control. Fusion 44,A253 (2002).
[9] K.H. Burrell et al., Phys. Plasmas 8, 2153 (2001).
[10] S. Kado et al., J. Phys. Soc. Jpn 65, 3434 (1996).
[11] M.G. Shats et al., Phys. Plasmas 2, 398 (1995).

5. Summary
We applied a BES measurement in CHS and studied thetransition phenomenon characterized by the rapid decrease of
the Hα signal and the increase of line-averaged density
observed in the neutral beam heated plasma. We observed thefollowing:(1) The local plasma density near the plasma edge
increases rapidly at the transition, which suggests theformation of ETB.

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