Contributed Paper
Instability in the Frequency Range of Alfvén Eigenmodes Driven
by Negative-Ion-Based Neutral Beams in JT-60U
SHINOHARA Kouji1), ISHIKAWA Masao1), TAKECHI Manabu1), KUSAMA Yoshinori1), TODO Yasushi2),
GORELENKOV Nikolai3), CHENG Chio3), FUKUYAMA Atsushi4), KRAMER Gerrit3), NAZIKIAN Raffi3),
MATSUNAGA Go1) and OZEKI Takahisa1)
1)Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka, Ibaraki, 311-0193, Japan
2)
National Institute for Fusion Science, Oroshi-cho, Toki, Gifu, 509-5292, Japan3)
Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543, USA
4)
Department of Nuclear Engineering, Kyoto University, 606-8501, Japan(Received 25 November 2004 / Accepted 9 May 2005)

Alfvén eignemode experiments have been carried out by using the negative-ion-based neutral beam in JT-60U.
For bursting instabilities in the frequency range of Alfvén Eigenmodes, two types of mode activity have been observed.
In one type called Abrupt Large-amplitude Event (ALE), enhanced transport of the energetic ions was detected. The
enhanced transport showed the clear energy dependence based on measurements performed using a natural diamonddetector. In another type of bursting event called the fast Frequency Sweeping (fast FS) mode, frequency up-down
chirping is observed on the time-scale of a few ms, much faster than equilibrium changes in the background plasma.
This frequency chirping was well reproduced by the non-linear simulation code, MEGA, for magnetohydrodynamics(MHD) with energetic particles. The slow up-frequency sweeping of the Reversed-Shear-induced Alfvén Eigenmode(RSAEs), due to the slow evolution of qmin was also observed. These experiments were analyzed by using the NOVA-K
code. It is shown that the most unstable modes are localized around the minimum in the magnetic safety factor (qmin)in the analysis.
keywords:
Alfvén eigenmode, negative-ion-based neutral beam, tokamak, JT-60U, MEGA, non-linear activity, Reversed-Shear-induced Alfvén
Eigenmode, NOVA-K

1. Introduction

quency ranges of Alfvén eigenmodes have been observed [5].
Some of them have had a “steady” temporal behavior andhave been identified; e.g. Global Alfvén eigenmode (GAE)[6,7], Toroidicity-induced Alfvén eigenmode (TAE) [8,9], Ellipticity-induced Alfvén eigenmode (EAE) [10], Noncirculartriangularity-induced Alfvén eigenmode (NAE) [10], etc.
in contrast to these steady frequency modes, several instabilities are observed with rapid frequency sweepings. Oneof frequency sweepings has been recently explained by the
temporal evolution of qmin, for Reverse Shear Alfvén Eigenmodes (RSAEs). (RSAE) [11-13] or Alfvén Cascade [14,15].
However, some frequency sweepings has a very short timescale, which cannot be explained by the temporal behaviorof the bulk plasma and equilibrium, and are sometime called“frequency chirping modes” [16-18]. The instabilities of thistype appear as bursts, namely each mode has a short duration,
typically less than a few milli-second. The short-time-scalefrequency sweeps are expected to couple with the temporal
evolution of the distribution of energetic ions, though thedetailed physics has not been fully understood yet.

In a reactor-relevant plasma, energetic ions, namely α
particles, as a product of D-T fusion reactions may interact
with the bulk plasma and may induce instabilities in the frequency ranges of Alfvén eigenmodes. Alfvén eigenmodes
can enhance the transport of the energetic ions and may leadto the deterioration of the plasma performance, or lead todamage of plasma facing components [1]. Meanwhile, someauthors suggest possibilities of usefulness for these instabilities, which are called “α-channeling”; one application is forplasma current profile control by controlling the transportof the energetic ions [2] while another is a bulk ion heating
through Alfvén eigenmodes [3,4]. In order to more fully understand the potential of these instabilities to improve plasma
performance, or to avoid deleterious effects from the excitation of these modes, we need to understand the characteristics
and the mechanism of the wave-particle interactions.
So far, in experiments with energetic ions produced byhigh energy neutral beams (NB) or Ion-Cyclotron Range ofFrequency (ICRF) heating, several instabilities in the freauthor’s e-mail: shinohak@fusion-naka.jaeri.go.jp
This article is based on the invited talk at the 21st JSPF Annual Meeting.

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J. Plasma Fusion Res. Vol.81, No.7 (2005) 547 - 552

Journal of Plasma and Fusion Research

In this article, recent research on instabilities in the frequency ranges of Alfvén eigenmodes in JT-60U is reported
relating to the above issues. In the recent experiments, weused Negative-ion-based Neutral Beam (NNB) injection.
NNB in JT-60U can be used to inject energetic neutral beamions with energy of 300 – 400 keV tangentially in the same
direction of the plasma current. Thus, we can investigate thecharacteristics of the modes and the transport of energetic
ions induced in the ITER relevant regimes. This use of the
NNB injection is one of unique characteristics of Alfvén wave
research on JT-60U [19].
We describe some topics of bursting instabilities in thefrequency range of Alfvén eigenmodes in Sec. 2. In the bursting instabilities, we observed two types of modes phenomenologically. In one type, the mode duration was less than one
milli-second and the mode amplitude was so large that wewere able to detect enhanced transport of the energetic ions.
The enhanced transport showed clear energy dependence. In
another type, the mode amplitude is weaker compared with thefirst type of bursting mode. The frequency up-down chirpingis clearly observed on the time-scale of a few milli-seconds.
This frequency chirping was reproduced by the simulation
code, MEGA [20].
In Sec. 3., the observation of RSAEs is reported. RSAE
comes from the analogy of GAE, which were mainly observedin cylindrical or helical systems. To demonstrate this ideal
MHD model of the RSAE, we have performed experimentsin the condition where we can measure the q profile with ahigh signal-to-noise ratio. We observed the slow up-frequency
sweeping of the RSAE due to the evolution of qmin, where qmin
is the minimum of the value of safety factor profile. Recently,
we also analyzed this experiment by using the NOVA-K code[8].
Finally, we summarize our findings in Sec. 4.

Vol.81, No.7

July 2005

Fig. 1 Temporal evolutions of (a) frequency spectrum ofmagnetic fluctuations measured by Mirnov coil, (b)amplitude of magnetic fluctuations with frequency of
30 – 70 kHz, (c) neutron emission rate.

amplitude of magnetic fluctuations measured using Mirnov
coils reaches B̃pol /Bpol ~ 10–4 at the wall. On the occurrence of
the ALE, we observed drops in the neutron emission rate forthe larger amplitude modes.
The enhanced transport of energetic ions has been observed during ALEs. An energy dependence of the enhanced
flux of charge exchange neutral particles is measured using a
E//B type neutral particle analyzer [22]. It is thought that theobserved enhanced flux is produced by energetic ions transported to peripheral region of plasma where the neutral density is relatively large. The energy dependence suggests thatenhanced transport of energetic ions is induced by a resonant
wave-particle interaction [21]. Recently, we have installeda new charge exchange neutral particle flux measurement
system by using natural diamond detectors (NDD) [23]. A
clear bump in velocity space in the flux was observed, whichenabled a clear identification of the resonant interaction [24].
We also estimated the change in the energetic ion population
spatial profile by using a neutron emission profile monitor[25]. The estimated change of the energetic ion spatial profile
induced by a large ALE is shown in Fig. 2. [24].

2. Bursting Instability in the Alfvén Eigenmode Frequency Range
In most Alfvén eigenmode experiments using NNB injection, bursting instabilities are observed in the frequency rangeof Alfvén eigenmodes [17,18,21]. One example is shown in
Fig. 1. Phenomenologically, we classified the bursting instabilities into two groups. The first one has a time-scale of afew milliseconds and its frequency sweeps upward and/or
downward by 10–20 kHz in a few milliseconds. The starting
frequency of this frequency sweeping lies in the TAE gap
frequency. We named this group of modes fast Frequency
Sweeping (fast FS) modes. On the occurrence of the fast FS
mode, a small drop in the neutron emission rate, which suggests enhanced transport of energetic ions, was observed inseveral cases with relatively large mode amplitude measured
by Mirnov coils [18]. The second bursting mode is named the
Abrupt Large-amplitude Event, ALE. This mode has a timescale of less than 1 millisecond. It is difficult to say whether
the ALE always has a frequency sweeping, basically due to its
short-time-scale behavior. However the ALE sometimes follows a frequency sweeping. The mode amplitude is large. The
548

Contributed Paper

Instability in the Frequency Range of Alfvén Eigenmodes Driven by Negative-Ion-Based Neutral Beams in JT-60U

K. Shinohara et al.

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Fig. 2

Fig. 3 Temporal evolution of frequency spectrum of radial
magnetic field fluctuations in simulation by MEGA
code.

Fast ion density profiles before and after ALE. These
profiles are derived from the data obtained by a neutron profile monitor.

The fast FS mode in a JT-60U plasma has been studied
using the nonlinear MEGA code for magnetohydrodynamics (MHD) and energetic particle simulation. The plasma is
divided into two parts; energetic ions and bulk plasma in thiscode. The bulk plasma and the electromagnetic field are described by the MHD equations and a drift-kinetic description
is employed for the energetic ions. To complete the equation
system in a self-consistent way, the energetic ions affect thebulk plasma through the MHD momentum equation via theenergetic ion current. The δ f particle simulation method wasapplied to the energetic ions. The frequency chirping of fast
FS mode was reproduced by the simulation code [20]. The
results are shown in Fig. 3 [26]. The mechanism of frequency
chirping is considered to be due to “spontaneous hole-clump
pair creation”. In Ref. [27], the spontaneous hole-clump paircreation appear when γ d /γ L > 0.4, where γ d is the linear damping rate and γ L is the linear growth rate. In our simulation, theratio γ d /γ L is 0.68. From the analysis using the MEGA code,
the fast FS mode is considered not to be a TAEs but to be
an Energetic Particle Mode (EPM) which is located near thecenter of the plasma where the energetic ion density is highest[28]. The mode structure measurement is an issue for future
consideration. The EPM is a mode, which is strongly affectedby the energetic ion population. It is expected that the behavior of Fast FS mode could be changed when the energetic ion
density profile is changed in the experiments. The frequency
behavior of fast FS was compared with the neutron emission
profile measurement. Figure 4(a) shows a temporal evolutionof magnetic fluctuations. During the time period from 4.2 to
4.3 s, upward frequency sweeping FS are dominant. On theother hand, up and downward FS are observed during thetime period from 4.4 to 4.5 s, though the start frequency of
Fast FS is almost same as that in the earlier time period. The
time separation between ALEs is also longer in the later time

Fig. 4(a) Temporal evolution of frequency spectrum of magnetic fluctuations. (b) Time averaged neutron emission
profiles measured by neutron profile monitor for twotime intervals of t = 4.2s to 4.3s and t = 4.4s to 4.5s.

period. In the period from 4.2 to 4.5 s, the equilibrium plasma
parameters and profiles do not change significantly, except for
the toroidal rotation. The toroidal rotation changes from ~20km/s to ~40 km/s, which corresponds to the change in themode frequency of the n = 1 mode from ~1 to ~2 kHz. The
549

Journal of Plasma and Fusion Researchtime-averaged neutron emission profiles are depicted in Fig.
4 (b). The time average was carried out from 4.2 to 4.3s andfrom 4.4 to 4.5 s. A difference was barely noticeable in theneutron emission profile.
As discussion, we present the following speculations.
First a small change in the toroidal rotation might change
the behavior of the frequency sweeping through the change
in the damping or driving of the mode because the balance
between damping rate and growth rate is considered to beimportant in some fast frequency change activities [27]. There
are reports in which TAEs are suppressed, namely the damping rate of the TAE was increased, by changing the toroidal
rotation [29,13]. A similar mechanism might be working inour observations and these changes in damping and drive may
affect the nonlinear behavior with more sensitive manner inthis nonlinear activity. Alternatively, structures in the energetic
ion density profile in time and/or space can play an importantrole in the driving and damping mechanism. Fine structures ofthe energetic ion density profile in time and/or space might beimportant, e.g. a local steepening in the gradient that could not
be measured at present, might affect the driving mechanism ofthese modes. A third suggestion is that a small change in thethermalized plasma might change the damping of the mode
as some part of the damping can come from the thermalized
plasma, such as ion Landau damping.

Vol.81, No.7

Fig. 5

July 2005

Schematic drawing of Alfvén continuum spectra forn = 1 in the RS plasma with qmin = 2.8. Here, thebroken curves stand for Alfvén continua in a cylindrical plasma.

3. Reversed-Shear-Induced Alfvén Eigenmode
Alfvén eigenmodes in reversed shear plasmas were investigated by using the kinetic full-wave TASK/WM codeand the model of Reversed-Shear-induced Alfvén Eigenmode(RSAE) was proposed to explain these observations [11,13].
In reversed shear plasmas, a spectral gap is induced aroundthe position of the minimum of the magnetic safety factor,
ρqmin. The RSAE is excited in this gap, and that the mode consists of a single poloidal mode number similar to the Global
Alfvén Eigenmode (GAE). The frequency of the RSAE lies
just below the upper continuum of the gap and just above thelower continuum as shown in Fig. 5. Thus the frequency is approximately expressed as ~ (n – m/qmin)vA /2π R for the upper
mode and ~(m/qmin – n)vA /2π R for the lower frequency mode,
where n is the toroidal mode number, m is the poloidal mode
number, vA is the Alfvén velocity and R is the major radius.
From this expression, we notice that the mode frequency
is sensitive to the value of qmin. It is expected that this qmin
dependence lead to the frequency sweeping of the mode whenthe qmin is changing in the reversed shear discharge.
RSAE experiments using NNB injection have been carried out on JT-60U [12]. Figure 6 shows magnetic fluctuation
spectra from experiment. An n = 1 mode was observed at the
expected RSAE frequency. The mode was identified as an
RSAE from the comparison among the observed temporal
evolution of the mode frequency, the mode amplitude, andthe temporal evolution of the gap structure by the simplified
model calculation.
Here, we show the stability analysis of the observed

Fig. 6 (a) Temporal evolution of qmin, (b) line averaged electron density, (c) a time trace of frequency spectrum of
the n = 1 instability. Note: three horizontal vague lines
around 48 kHz and 36 kHz in the figure come fromnoise in Mirnov coil system.

mode using NOVA-K code. The NOVA-K code evaluates thegrowth rate of Alfvén eigenmodes due to kinetic effects using
perturbative analysis. In addition to the driving terms, theNOVA-K code calculates the damping rates, such as thermal
electron and ion Landau damping, and the trapped electron
collisional damping with finite orbit width (FOW) and finite
Larmor radius (FLR) effects [8,30,31]. Furthermore, the treatment of the anisotropic beam ion produced by neutral beam
injection was improved recently [32]. In this analysis, we use
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Contributed Paper

Instability in the Frequency Range of Alfvén Eigenmodes Driven by Negative-Ion-Based Neutral Beams in JT-60U

K. Shinohara et al.

Fig. 8 Stability curve (circles) in the domain of central energetic ion beta, βh0, and vb /vA. Open markers are forclassical estimation, by OFMC code, of βh0 /〈βh〉~10,
and closed markers are for assumed parameter of
βh0 /〈βh〉~2. Square symbols mean assumed parametersof beam ions for the experiment; open square for classical estimation and closed square for the profile of
the reduced peaking factor of βh0 /〈βh〉~2.

Fig. 7The open square symbol in Fig. 8 represents the beam ion
parameter by classical estimation using the OFMC code forthe experimental parameters. The critical βh0 is much smaller
than the value of βh0, open square in Fig. 8, of the OFMC
calculation at vb// /vA~0.53 where experiments are carried out.
However, the mode activity was moderate in the experiments
and the experiment parameter seems to be marginal. Thereseems a large difference between the stability curve and the
OFMC estimation, which might come from an excessively
peaked fast ion profile in the OFMC result or from the lackof continuum damping in the calculations. It is expected
the fast ion profile should be modified due to the enhanced
transport induced by the mode but no direct confirmation ispossible so far. We carried out the stability calculation witha reduced fast ion peaking factor (by about 5), keeping 〈βh〉the same. The stability boundary in this case (closed circle)
increased and came closer to the reduced βh0. The energetic
ion profile, estimated using the neutron profile monitor andprofile information of the thermalized plasma, might give us
some more information on the fast ion beta, however we do
not have such data so far. Thus this is an interesting futureissue to be clarified. The inclusion of continuum damping is
also an important issue.

(a) q profile, (b) continuum spectrum of n = 1, (c)mode structure with similar mode frequency to theexperiment in Fig. 6.

this latest version of NOVA-K code.
The energetic ion parameters using in NOVA-K are
based on the OFMC code calculation, in which the enhanced
transport induced by MHD is not taken into account. Namely,
the value taken is the so-called classical value [33].
Figure 7(a), (b) and (c) show the q profile, the continuum
spectrum for n = 1 and the calculated mode structure for themost unstable mode at t~ 6.5 s in Fig. 6, respectively. Here,ωA0 = B/qs R OJ .J and ni0 is the central ion density and
Mi is the ion mass. The value (ω /ωA0)2 of the most unstablemode is 0.3112, which is similar to the experimental value of
0.33–0.36. As expected, we can see that the mode is localizedaround ρqmin for qmin~2.8, and that the mode with the single
poloidal mode number of m = 3 is dominant.
The stability curve (open circles), which is the boundary
where the damping rate overcomes the growth rate in thedomain of central fast ion beta, βh0, and vb// /vA is depicted inFig. 8. An important note for this analysis is that continuum
damping is not included. Thus, the damping rate and criticalenergetic ion beta are underestimated.

4. Summary
Bursting modes in the frequency range of TAEs wereobserved during NNB injection in JT-60U. To investigate
the transport of energetic ions under the presence of ALEs,
neutron emission profile measurements were performed. The
neutron measurements showed the redistribution of ener551

Journal of Plasma and Fusion Research

getic ions by ALEs. Energetic ions in the center region of theplasma were expelled significantly to the outer region. We
also observed the clear energy dependence of the loss fromthe core by using a new diagnostics; the natural diamonddetector. In another type of bursting mode called fast FS mode,the frequency up-down chirping is observed on the time-scaleof a few ms. This frequency chirping was reproduced by the
newly developed simulation code, MEGA.
To understand the physics of Reversed-Shear-induced
Alfvén Eigenmodes, RSAE, we performed experiments inconditions where the q profile could be accurately measured
with a good signal-to-noise. We observed the slow up-frequency sweeping of the RSAE due to the downward evolutionof qmin. This experiment was analyzed using the NOVA-K
code. We observed that the most unstable mode was localizednear qmin with a single dominant poloidal mode number.

Vol.81, No.7

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Acknowledgment
The authors would like to appreciate the JT-60 team fortheir supports on experiments. This work was partly supported by JSPS, Grant-in-Aid for Scientific Research (B)No.14208056 and is also partly implemented under the International Energy Agency’s cooperation agreement on Large
Tokamak Facilities established between JAERI, DOE, and
EURATOM.

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