J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)345–348
000–000

On MHD Oscillations in High Beta H-Mode Tokamak Plasma in
JFT-2M
HOSHINO Katsumichi, NAGASHIMA Yoshihiko1, IDO Takeshi2, TSUZUKI Kazuhiro,
KAWASHIMA Hisato, OGAWA Hiroaki, BAKHTIARI Mohammad, SHINOHARA Kouji,
UEHARA Kazuya, OASA Kazumi, KASAI Satoshi and KUSAMA Yoshinori
Japan Atomic Energy Research Institute, Ibaraki 319-1195, Japan
Faculty of Science, The University of Tokyo, Tokyo 113-0033, Japan2National Institute for Fusion Science, Toki 509-5292, Japan1(Received: 19 December 2003 / Accepted: 6 July 2004)

Abstract
In the recent high beta H-mode experiments in JFT-2M tokamak, total beta value above ~2.5% is obtained. The
high performance plasma is obtained in high density region (normalized density ~1) with high triangularity ~0.7 inthe limiter configuration. Suppression of the low frequency (< 10kHz) MHD (magnetohydrodynamic) oscillations isvery important to obtain such high beta plasma. We find a stabilization of low mode MHD oscillation by current
profile control by ramp-up of plasma current, counter neutral beam injection and inward shift of the plasma column.
In the study of MHD oscillations in the edge transport barrier, we observe precursor MHD oscillations in density
before the intermittent ELMs (edge localized modes), or two kinds (low frequency and high frequency) of characteristic
quasi-coherent MHD oscillations (~ 70kHz to ~ 310kHz) in density, or in magnetic field, or in potential. These quasicoherent modes are not found in the ELM-free H-mode phase but appear in the slightly degraded H-mode phase (H’mode) or in enhanced Dα phase. A comparison of these quasi-coherent MHD with background turbulent fluctuationlevel is presented.
Keywords:
MHD oscillation, high beta, H-mode, EDA mode, ITB, ETB, JFT-2M, Tokamak, quasi-coherent mode, turbulence

1. Introductiontoroidal direction) for the additional heating in theseexperiments. The beam energy is 32keV and the total power
is up to ~1.5MW.

To achieve the high beta plasma is indispensable for therealization of the thermonuclear fusion reactor. In such a highbeta plasma, various MHD oscillations appear according to
the evolution of plasma current density profile and pressure
profile during the course to achieve the high beta plasma. We
present the recent observation of such MHD oscillations,whose frequency ranges from a few kHz to a few hundred
kHz, observed in the course to obtain high beta H-mode in
the JFT-2M tokamak in section 2.1, and in the study of theedge transport barrier (ETB) in section 2.2.
JFT-2M tokamak is a medium size tokamak (R = 1.31m)
with non-circular cross section (a × b = 0.35m × 0.53m). Themaximum toroidal field and plasma current are 2.2T, 0.5MA,respectively. The vacuum chamber is made of SUS with innerwall made of thin (~10mm) ferritic steel (F82H). The limiteris made of graphite. The wall conditioning is done by baking,Taylor discharge cleaning, boronization (Tri-methyl-boron)and daily He glow discharge cleaning. The two co- andcounter-neutral beams are injected obliquely (~38° to the2. Experimental results
2.1 MHD in high triangularity limiter H-mode
High elongation κ and high triangularity δ is preferredfor obtaining high beta, because in these configuration, one
can raise the total plasma current with the same toroidal field,
namely the operation in the high normalized current Ip /a/
Bto~0.8 is possible. Edge stability is improved in high δ. We
attained κ~1.7, δ~0.7 (Fig. 1(a)) by increasing the push of
the plasma by increasing the OH (ohmic heating) coil current.
By the combination of the current ramp-up and neutral beam
heating (NBH), successive counter-injection first and coinjection next, we could get weak shear configuration inwhich 1<q(r)<2 in the plasma core region (r/a<0.65) (Fig.
1(b)). Therefore the plasma is sawteeth-free and we could getnormalized beta βN~3.2 (βt~2.5%) with H factor ~1.5, βN H~
4.8, the Greenwald density ~ 0.95 (〈ne〉~0.7×1020m-3). The

Corresponding author’s e-mail: hoshino@axjft3.tokai.jaeri.go.jp1345©2004 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchHoshino K. et al., On MHD Oscillations in High Beta H-Mode Tokamak Plasma in JFT-2M(a)(d)Fig. 1(a) High elongation κ, high triangularity δ limiter configuration in the high β experiment. Ip = 0.16MA, Bt0 = 0.77T, κ = 1.7, δ =0.66, qs = 5.7, βp = 1.8, li = 0.80, βt = 2.5%, βN = 3.2 by equilibrium fit code (EQFIT). (b) Safety factor profile q(r). (c) Timeevolutions of βN and Greenwald normalized density, Ip , li . The q(r) at the time indicated by arrows are shown in (b). (d) Soft Xray contour. Tearing mode appears (from 530ms) at R~1.02m and R~1.55m (R0 = 1.29m) during the soft βN degradation.

state was maintained throughout the NBH [1] for ~0.1s (~7τE)(Fig. 1(c)). In such weak shear cases with current ramp-up,
we found the growth of the internal transport barrier (ITB)[2] which was characterized by rapid linear increase in corepressure accompanied by the slight decrease in the outerpressure by PIN diode array measurement.
There is a soft degradation in βN from 0.55s. We observe
low frequency tearing mode ( f~6kHz) grows at the outer partr~0.7a near q = 2 surface (Fig. 1(d)). It seems that by thedecrease in βN , the mode activity decreases at ~0.57s.
The tearing mode seems to affect the quality of the ITBand ETB as shown in the next case with sawteeth oscillation(co-NBH first then ctr-NBH case) with q = 1 rational surface
in the plasma as shown in Fig. 2. With the sawteeth, usually
we cannot obtain high β plasma. In this case, by the inner
shift of the plasma column, a sudden increase of the stored
energy (~0.61s) by the formation of the ITB occurs even withthe sawteeth. By the inward shift, edge pressure (R > ~1.50m)
begins to decrease by the increase of plasma-wall interaction
at the high field side, but core pressure increases linearly
indicating ITB formation. Decrease in radiation power,decrease in Dα level, fast increase in the stored energy occur.
As the βN increases, the low frequency tearing mode isdestabilized (~0.63s at R = 1.50m). The effect of the mode
extends from the ETB region (~0.9a) to the ITB region(~0.3a) at 0.64s. The outward shift (~0.65s) terminates theITB at the timing of the occurrence of the sawtooth. In thiscase, high βN state was not maintained long in contrast to theprevious low shear sawteeth-free case. It seems that m = 1mode (sawteeth) and m = 2 mode (tearing mode) both exert
effects to cause the destruction of the ITB.

2.2 Study of MHD at the edge transportbarrier(ETB)
2.2.1 Radial electric field and quasi-coherent
MHD oscillation measured by HIBPIn the course of obtaining high beta, the MHD stabilityof the ETB,which has steep gradient in pressure, is important.
The structure of the negative electric field in the ETB was
studied with CXRS (charge exchange recombinationspectroscopy) diagnostics [3] and with HIBP (heavy ion beamprobe) diagnostics [4] in JFT-2M.
We were aware that there are several grades in ETB
strength, weak to strong, which was measured by the edge
gradients and deuterium line radiation (Dα ) level [5]. ELMfree phase has the most strong ETB. We alredy found a2346

Hoshino K. et al., On MHD Oscillations in High Beta H-Mode Tokamak Plasma in JFT-2MFig. 3Plasma potential profile and beam intensity profile ofthe HIBP measurement. ELM free H-mode (t = 618ms)
and H’-mode (t = 622ms). ds = 0 is the position of theseparatrix. Minus represents the inside of the plasmaand plus represents in the SOL. Density fluctuation at
the ETB affects the IHIBP .

2.2.2 Turbulent fluctuation and the quasicoherent MHD oscillations in the ETB
during the H’-mode and EDA-like mode

Fig. 2We observe quasi-coherent MHD oscillation in the ETB
when the edge Dα intensity is comparatively large [8] asshown in Fig. 4(a). During the injection of NBH, we observe,
successively, L-mode, ELM-free H-mode, H’-mode and EDA[9]-like mode (Fig. 4). Fig. 4(a) shows the density fluctuation
spectrum at ne~1.8×1019m-3 (in the ETB) measured by the38GHz microwave reflectometer [10]. The turbulent density
fluctuation spectrum extends to ~100kHz during L-mode, but
decreases to ~20kHz (with ~200kHz component) during theELM-free H-mode. The quasi-coherent density fluctuation
appears during the H’-mode. The frequency downshift occurs
from ~110kHz to ~70kHz during this phase with its width∆ f~20kHz. During this H’-mode, another quasi-coherent
fluctuation ~300kHz [11] appears in the magnetic probe
signal. As the density fluctuation in the ETB has not this300kHz high frequency component (as in Fig. 4(a)) andreciprocating electrostatic probe measurement [8] detects the300kHz oscillation in the SOL (scrape-off layer) region, themagnetic fluctuation seems to originate around the separatrix
region to the SOL plasma region and not deep in the pedestal.
In the EDA-like mode, the background turbulent density
fluctuation spectrum widely extends to ~120kHz as in the Lmode, but low frequency part of the spectrum still muchsmaller than the L-mode. The ~300kHz quasi-coherent
magnetic fluctuation has ∆ f~60kHz and wider than the quasicoherent mode with ~70kHz in the H’ phase.
Figure 4(b) shows the power spectra which shows thequasi-coherent mode and turbulent density fluctuation in theL-mode phase, ELM-free H-mode phase, H’ phase and EDAlike phase. The L-mode phase has turbulent density(a) Time evolutions with sawteeth (NBH Co. 776kW
than Ctr. 710kW).
(b) Time evolutions of βN and inductance li .
(c) Time evolution of the Soft X-ray by PIN. R = 1.32m(center), R = 1.57m (edge).

slightly weaker ETB, called as H’ phase, which ischaracterized by a characteristic quasi-coherent density
fluctuation with the frequency of ~100kHz measured by amicrowave reflectometer with increased Dα level [6].
Figure 3 shows the profiles of the plasma potential (φ)
measured by the HIBP and the intensity of the secondary
beam of the HIBP which has the information on density. We
find that in the ELM-free phase, negative electric field at theETB (–15kV/m) is stronger than that in the H’ phase (–8kV/m) and that, during the H’ phase, density fluctuation offrequency ~130kHz localizes in the ETB region.
We are aware that such frequency is close to thefrequency of the density fluctuation of the ELM precursor(~160kHz to ~100kHz) observed in ELMy H-mode [7].
Though we can not identify the MHD, it seems that the MHD
affects the radial electric field, ETB strength or triggering ofthe ELM events at ETB.

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Hoshino K. et al., On MHD Oscillations in High Beta H-Mode Tokamak Plasma in JFT-2Mfluctuation. In the EDA-like-phase, the enhanced transportcompared to the ELMfree H-mode may due to the enhanced
background turbulence level (>10kHz). But as the turbulence
below ~40kHz is still suppressed compared to the L-mode,
EDA-like phase still has improved confinement compared tothe L-mode which indicates that low frequency part(<~50kHz) plays the dominant role for the transport in theseJFT-2M conditions.
The role of the ~300kHz magnetic fluctuation around theSOL is not clear yet. We only point a possibility that a threewave coupling (geodesic electron acoustic mode, Alfven
wave, low frequency quasi-coherent mode ~100kHz)
enhances the transport [13].

3. Discussion on the relation to the“quasi-coherent mode” observed
during EDA mode in the Alcator C-Modtokamak(b)Fig. 4The “quasi-coherent mode” observed in the Alcator CMod tokamak has frequency of f = ~100kHz and ∆ f/f~0.05
to 0.2 [14]. In our “coherent mode” has f = ~110– ~70kHzand ∆ f/f = ~0.2. In this respect, these two modes are quitesimilar. These two modes exhibit the frequency downshiftpresumably by decrease in edge plasma rotation. The radial
electric field measured by HIBP is –8kV/m in our case is 1/3of their BOUT calculation for C-Mod. The “quasi-coherentmode” is considered as a form of resistive ballooning mode
known as the resistive X-point mode. We observe the highfrequency mode (~300kHz) in magnetic field and low
frequency mode (~100kHz) in density fluctuation at the sametime. It may be the next task to measure the MHD over 1MHz
to detect Alfven wave etc.

Quasi-coherent mode observed in the edge transportbarrier.
(a) density fluctuation spectrum measured by 38GHz
reflectometer, spectrum of magnetic probe and Dαintensity during the H-mode.
(b) power spectrum of the density fluctuations at Lphase, H-phase, H’-phase and EDA-like phase.

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[4] T. Ido et al., Phys. Rev. Lett. 88, 055006-1 (2002).
[5] K. Hoshino et al., J. Phys. Soc. Jpn 56, 1750 (1987) andJ. Phys. Soc. Jpn 58, 1248 (1989).
[6] K. Shinohara et al., J. Plasma Fusion Res. 74, 607(1998).
[7] K. Hoshino et al., presented in H-mode Workshop(2001).
[8] Y. Nagashima et al., presented in the H-mode workshop
2003. Submitted to Plasma Phys. Control. Fusion.
[9] M. Greenwald et al., Phys. Plasmas 6, 1943 (1999).
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[13] W.M. Nevins, private communication.
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fluctuation with half width ~70kHz. In the ELM-free H-mode
phase, turbulent density fluctuation below 120kHz is
suppressed to ~1/10. The half width of the backgroundturbulence is 10~20kHz. In the H’ phase, turbulence below
70kHz is suppressed and the quasi-coherent mode has δ f/f =20kHz/70kHz. In the EDA-like phase, turbulence below~40kHz is suppressed, though turbulence above 100kHz isenhanced. We point out that the fluctuation induced particle
flux measured by the HIBP is also dominated by ~40kHz
component during the L-mode [12].
Therefore it is plausible that the suppression of theturbulence below ~40kHz brings the significant confinement
improvement during the EDA-like mode.
From these observation, following speculation may bederived. If we assume that the measured density fluctuation
at ne~1.8×1019m-3 contributes mainly to the global particle/
energy transport, the improvement of the transport during theELM-free H-mode compared to the L-mode is due to thedecrease in the background turbulence level (<100kHz).
During the H’-phase, slightly enhanced transport than that in
ELMfree case is due to the quasi-coherent ~100kHz
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