Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas
An extensive (INTAS) research programme started in 2002 to investigate the correlations between on the one hand the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand ele...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2006
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| Zitieren: | Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas / G. Van Oost, V.V. Bulanin, A.J.H. Donné, E.Z. Gusakov, A. Krämer-Flecken, L.I. Krupnik, A. Melnikov, P. Peleman, K.A. Razumova, J. Stöckel,V. Vershkov, A.B. Altukov, V.F. Andreev, L.G. Askinazi, I.S. Bondarenko, A.Yu. Dnestrovskij, L.G. Eliseev, L.A. Esipov, S.A. Grashin, A.D. Gurchenko, G.M.D.Hogeweij, S. Jachmich, S.M. Khrebtov, D.V. Kouprienko, S.E. Lysenko, S.V. Perfilov, A.V. Petrov, A.Yu. Popov, D. Reiser, S. Soldatov, A.Yu. Stepanov, G. Telesca, A.O. Urazbaev, G. Verdoolaege, O. Zimmermann // Вопросы атомной науки и техники. — 2006. — № 6. — С. 14-18. — Бібліогр.: 17 назв. — англ. |
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nasplib_isofts_kiev_ua-123456789-817322025-02-09T20:06:46Z Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas Влияние турбулентности и електрических полей на установление улучшеного удержание плазмы в токамаках Вплив турбулентності і електричних полів на установлення поліпшеного утримання плазми в токамаках Van Oost, G. Bulanin, V.V. Donné, A.J.H. Gusakov, E.Z. Krämer-Flecken, A. Krupnik, L.I. Melnikov, A. Peleman, P. Razumova, K.A. Stöckel, J. Vershkov, V. Altukov, A.B. Andreev, V.F. Askinazi, L.G. Bondarenko, I.S. Dnestrovskij, A.Yu. Eliseev, L.G. Esipov, L.A. Grashin, S.A. Gurchenko, A.D. Hogeweij, G.M.D. Jachmich, S. Khrebtov, S.M. Kouprienko, D.V. Lysenko, S.E. Perfilov, S.V. Petrov, A.V. Popov, A.Yu. Reiser, D. Soldatov, S. Stepanov, A.Yu. Telesca, G. Urazbaev, A.O. Verdoolaege, G. Zimmermann, O. Magnetic confinement An extensive (INTAS) research programme started in 2002 to investigate the correlations between on the one hand the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with high spatial and temporal resolution and of various active means to externally control plasma transport . This has been done in a strongly coordinated way and exploiting the complementarity of TEXTOR and T-10 and the backup potential of the three other tokamaks, which together have all the relevant experimental tools and theoretical expertise. The authors are grateful to INTAS (International Association for the promotion of co-operation with scientists from the New Independent States of the former Soviet Union) which supported the research activities in the framework of project INTAS 2001-2056. 2006 Article Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas / G. Van Oost, V.V. Bulanin, A.J.H. Donné, E.Z. Gusakov, A. Krämer-Flecken, L.I. Krupnik, A. Melnikov, P. Peleman, K.A. Razumova, J. Stöckel,V. Vershkov, A.B. Altukov, V.F. Andreev, L.G. Askinazi, I.S. Bondarenko, A.Yu. Dnestrovskij, L.G. Eliseev, L.A. Esipov, S.A. Grashin, A.D. Gurchenko, G.M.D.Hogeweij, S. Jachmich, S.M. Khrebtov, D.V. Kouprienko, S.E. Lysenko, S.V. Perfilov, A.V. Petrov, A.Yu. Popov, D. Reiser, S. Soldatov, A.Yu. Stepanov, G. Telesca, A.O. Urazbaev, G. Verdoolaege, O. Zimmermann // Вопросы атомной науки и техники. — 2006. — № 6. — С. 14-18. — Бібліогр.: 17 назв. — англ. 1562-6016 PACS: 52.55.Fa, 52.30.-q, 52.35Ra, 52.55.-s https://nasplib.isofts.kiev.ua/handle/123456789/81732 en Вопросы атомной науки и техники application/pdf Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| topic |
Magnetic confinement Magnetic confinement |
| spellingShingle |
Magnetic confinement Magnetic confinement Van Oost, G. Bulanin, V.V. Donné, A.J.H. Gusakov, E.Z. Krämer-Flecken, A. Krupnik, L.I. Melnikov, A. Peleman, P. Razumova, K.A. Stöckel, J. Vershkov, V. Altukov, A.B. Andreev, V.F. Askinazi, L.G. Bondarenko, I.S. Dnestrovskij, A.Yu. Eliseev, L.G. Esipov, L.A. Grashin, S.A. Gurchenko, A.D. Hogeweij, G.M.D. Jachmich, S. Khrebtov, S.M. Kouprienko, D.V. Lysenko, S.E. Perfilov, S.V. Petrov, A.V. Popov, A.Yu. Reiser, D. Soldatov, S. Stepanov, A.Yu. Telesca, G. Urazbaev, A.O. Verdoolaege, G. Zimmermann, O. Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas Вопросы атомной науки и техники |
| description |
An extensive (INTAS) research programme started in 2002 to investigate the correlations between on the one hand the occurrence of
transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2,
TUMAN-3M and CASTOR, and on the other hand electric fields, modified magnetic shear and electrostatic and magnetic turbulence
using advanced diagnostics with high spatial and temporal resolution and of various active means to externally control plasma transport .
This has been done in a strongly coordinated way and exploiting the complementarity of TEXTOR and T-10 and the backup potential of
the three other tokamaks, which together have all the relevant experimental tools and theoretical expertise. |
| format |
Article |
| author |
Van Oost, G. Bulanin, V.V. Donné, A.J.H. Gusakov, E.Z. Krämer-Flecken, A. Krupnik, L.I. Melnikov, A. Peleman, P. Razumova, K.A. Stöckel, J. Vershkov, V. Altukov, A.B. Andreev, V.F. Askinazi, L.G. Bondarenko, I.S. Dnestrovskij, A.Yu. Eliseev, L.G. Esipov, L.A. Grashin, S.A. Gurchenko, A.D. Hogeweij, G.M.D. Jachmich, S. Khrebtov, S.M. Kouprienko, D.V. Lysenko, S.E. Perfilov, S.V. Petrov, A.V. Popov, A.Yu. Reiser, D. Soldatov, S. Stepanov, A.Yu. Telesca, G. Urazbaev, A.O. Verdoolaege, G. Zimmermann, O. |
| author_facet |
Van Oost, G. Bulanin, V.V. Donné, A.J.H. Gusakov, E.Z. Krämer-Flecken, A. Krupnik, L.I. Melnikov, A. Peleman, P. Razumova, K.A. Stöckel, J. Vershkov, V. Altukov, A.B. Andreev, V.F. Askinazi, L.G. Bondarenko, I.S. Dnestrovskij, A.Yu. Eliseev, L.G. Esipov, L.A. Grashin, S.A. Gurchenko, A.D. Hogeweij, G.M.D. Jachmich, S. Khrebtov, S.M. Kouprienko, D.V. Lysenko, S.E. Perfilov, S.V. Petrov, A.V. Popov, A.Yu. Reiser, D. Soldatov, S. Stepanov, A.Yu. Telesca, G. Urazbaev, A.O. Verdoolaege, G. Zimmermann, O. |
| author_sort |
Van Oost, G. |
| title |
Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas |
| title_short |
Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas |
| title_full |
Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas |
| title_fullStr |
Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas |
| title_full_unstemmed |
Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas |
| title_sort |
role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2006 |
| topic_facet |
Magnetic confinement |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/81732 |
| citation_txt |
Role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas / G. Van Oost, V.V. Bulanin, A.J.H. Donné, E.Z. Gusakov, A. Krämer-Flecken, L.I. Krupnik, A. Melnikov, P. Peleman, K.A. Razumova, J. Stöckel,V. Vershkov, A.B. Altukov, V.F. Andreev, L.G. Askinazi, I.S. Bondarenko, A.Yu. Dnestrovskij, L.G. Eliseev, L.A. Esipov, S.A. Grashin, A.D. Gurchenko, G.M.D.Hogeweij, S. Jachmich, S.M. Khrebtov, D.V. Kouprienko, S.E. Lysenko, S.V. Perfilov, A.V. Petrov, A.Yu. Popov, D. Reiser, S. Soldatov, A.Yu. Stepanov, G. Telesca, A.O. Urazbaev, G. Verdoolaege, O. Zimmermann // Вопросы атомной науки и техники. — 2006. — № 6. — С. 14-18. — Бібліогр.: 17 назв. — англ. |
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Вопросы атомной науки и техники |
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2025-11-30T09:21:47Z |
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2025-11-30T09:21:47Z |
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| fulltext |
14 Problems of Atomic Science and Technology. 2006. 6. Series: Plasma Physics (12), p. 14-18
ROLE OF TURBULENCE AND ELECTRIC FIELDS IN THE ESTABLISHMENT
OF IMPROVED CONFINEMENT IN TOKAMAK PLASMAS
G. Van Oost1, V.V. Bulanin2, A.J.H. Donné3, E.Z. Gusakov4, A. Krämer-Flecken5, L.I. Krupnik6, A. Melnikov7,
P. Peleman1, K.A. Razumova7, J. Stöckel8,V. Vershkov7, A.B. Altukov4, V.F. Andreev7, L.G. Askinazi4,
I.S. Bondarenko6, A.Yu. Dnestrovskij7, L.G. Eliseev7, L.A. Esipov4, S.A. Grashin7, A.D. Gurchenko4, G.M.D.Hogeweij3,
S. Jachmich9, S.M. Khrebtov6, D.V. Kouprienko4, S.E. Lysenko7, S.V. Perfilov7, A.V. Petrov2, A.Yu. Popov4, D. Reiser5,
S. Soldatov5, A.Yu. Stepanov4, G. Telesca1, A.O. Urazbaev7, G. Verdoolaege1, O. Zimmermann5
1Department of Applied Physics, Ghent University, Ghent, Belgium;
2St.Petersburg State Polytechnical University, St.Petersburg, Russia;
3FOM-Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM, Nieuwegein, The Netherlands;
4Ioffe Institute, St Petersburg, Russia;
5Institute of Plasma Physics, Forschungszentrum Jülich GmbH, EURATOM Association, Jülich, Germany;
6 Institute of Plasma Physics, NSC “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine;
7RRC Kurchatov Institute, Moscow, Russian Federation;
8Institute of Plasma Physics, Association EURATOM/IPP.CR, Prague, Czech Republic;
9Laboratory for Plasma Physics, ERM/KMS, Association EURATOM-Belgian State, Brussels, Belgium
An extensive (INTAS) research programme started in 2002 to investigate the correlations between on the one hand the occurrence of
transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2,
TUMAN-3M and CASTOR, and on the other hand electric fields, modified magnetic shear and electrostatic and magnetic turbulence
using advanced diagnostics with high spatial and temporal resolution and of various active means to externally control plasma transport .
This has been done in a strongly coordinated way and exploiting the complementarity of TEXTOR and T-10 and the backup potential of
the three other tokamaks, which together have all the relevant experimental tools and theoretical expertise.
PACS: 52.55.Fa, 52.30.-q, 52.35Ra, 52.55.-s
1. INTRODUCTION
About a decade ago local zones (called internal transport
barriers, ITBs) with reduced transport were discovered in
tokamaks. These ITBs can act on the electron and/or ion
fluid.The understanding and reduction of turbulent transport
in magnetic confinement devices is not only an academic
task, but also a matter of practical interest, since high
confinement is chosen as the regime for ITER and possible
future reactors because it reduces size and cost.
Generally speaking, turbulence comes in two classes:
electrostatic and magnetic turbulence. Over the last decade,
step by step new regimes of plasma operation have been
identified, whereby turbulence can be externally controlled,
which led to better and better confinement.The physical
picture that is generally given is that by spinning up the
plasma, it is possible to create flow velocity shear large
enough to tear turbulent eddies apart before they can grow,
thus reducing electrostatic turbulence. This turbulence
stabilization concept has the universality, needed to explain
ion transport barriers at different radii seen in limiter-and
divertor tokamaks, stellarators, reversed field pinches, mirror
machines and linear devices with a variety of discharge- and
heating conditions and edge biasing schemes. The electron
heat conduction, however, which normally is one to two
orders above the collisional lower limit, remained strongly
anomalous also in the regime with suppressed electrostatic
turbulence. In that case it became the dominant heat loss
channel. From this, it is conjectured that magnetic turbulence
drives the anomalous electron heat conduction..
Although turbulence measurements have been performed
on many magnetic confinement devices during the last
decades, the additional insight gained from these experiments
is relatively limited. This can be attributed to a number of
reasons: Firstly, only a very coarse spatial resolution was
achieved in many measurements of electric fields and
turbulence. Secondly, simultaneous measurements of different
fluctuating quantities (temperature, density, electric potential
and magnetic field) at the same location, needed for a
quantitative estimation of the energy and particle transport due
to turbulence were only performed in a very limited number of
cases. Thirdly, theoretical models were often only predicting
the global level of turbulence as well as the scaling of this level
with varying plasma parameters.
The investigation of the correlations between on the one
hand the occurrence of transport barriers and improved
confinement in magnetically confined plasmas, and on the
other hand electric fields, modified magnetic shear and
electrostatic and magnetic turbulent fluctuations necessitates
the use of various active means to externally control plasma
transport. It also requires to characterize fluctuations of various
important plasma parameters inside and outside transport
barriers and pedestal regions with high spatial and temporal
resolution using advanced diagnostics, and to elucidate the role
of turbulence driving and damping mechanisms, including the
role of the plasma edge properties. The experimental findings
have to be compared with advanced theoretical models and
numerical simulations.
The Consortium of the INTAS project 2001-2056 disposes of
5 tokamaks (the medium-size and similar tokamaks TEXTOR
and T-10, and the smaller-scale tokamaks FT-2, TUMAN-3M
and CASTOR), equipped with advanced diagnostics with high
spatial and temporal resolution. Research activities are strongly
coordinated and exploit the complementarity of TEXTOR
(mainly ion heating, Dynamic Ergodic Divertor) and T-10
(electron heating, Heavy Ion Beam Probe, HIBP) as well as the
backup potential of the three other tokamaks, which together have
all the relevant experimental tools and theoretical expertise. A
15
substantial effort was made for the improvement and
development of diagnostics which were necessary for the
successful execution of the project: HIBP[1],correlation
reflectometry (CR) [2,3], Doppler reflectometry [4], correlative
enhanced scattering [5], fluctuation reflectometry theory [6,7],
advanced Gundestrup probe [8].
The most important results obtained in the
investigations of the physical mechanisms underlying
different types of transport barriers are presented in
Section 2. The results of studies on turbulence
characteristics are discussed in Section 3.
2. TRANSPORT BARRIERS: PHYSICAL
MECHAMISMS
2.1. ELECTRON INTERNAL TRANSPORT
BARRIERS
Recent research in the T-10 and TEXTOR devices has
concentrated on understanding the physical mechanisms
that are responsible for the generation of electron internal
transport barriers (e-ITBs) and also on finding out in
which way they are related to the concept of profile
consistency, in which the plasma pressure and
temperature profiles have a tendency to organize
themselves [9] into an ‘universal’ profile shape, in
agreement with the plasma minimum free energy
principle. If ∇p exceeds a certain critical value,
instabilities connected with the pressure gradient will
counteract the formation of an even steeper gradient. The
radial distribution of transport coefficients is determined
by the necessity to maintain the self-consistent pressure
profile under different external impacts.
Previous work [10] has shown that e-ITBs are formed
when dq/dr is low in the vicinity of rational magnetic
surface with low m and n values. The investigation of
effects bound with ITB formation was continued in T-10
experiments in 2005. For this purpose experiments with a
rapid plasma current ramp up were performed. In this
case, due to (βp + li/2) ∼1/Ip
2 a rapid change of the
magnetic surface densities in the central part of plasma
takes place, while current penetration in this region occurs
only after t>50ms. So confinement changes observed in
the plasma core are the result of a magnetic surface
density change only. The results are under analysis [11].
Experiments with Internal Transport Barrier (ITB)
formation and the maintenance of self-consistent plasma
profiles under the action of Electron Cyclotron Resonance
Heating and Current Drive ECRH/ECCD were performed at
T-10. The results are still being analyzed. A joint analysis of
T-10 and TEXTOR experimental results enabled to analyze
effects bound with plasma self-organization. It was shown
that the plasma pressure profiles obtained in different
operational regimes and even in various tokamaks may be
represented by a single typical curve, called the self-
consistent pressure or canonical profile, also often referred to
as profile resilience or profile stiffness.
The investigation of self-consistent profile effects was
carried out under different experimental conditions, such as
regimes with plasma density near the Greenwald limit and
regimes with deuterium pellet injection. It can be concluded
that the effect takes place in a wide region of plasma density
up to that, which leads to disruption. The conditions
described by this self-consistent profile are realized in a very
short time, less than the experimental time resolution
∆t ≥ 2…4 ms. During ECRH it is realized by a plasma
density redistribution: ne decreases in the plasma heating
zone. This implies that the famous “density pump out” is the
result of plasma self-consistent organization. Experimentally
this means that, when one tries to distort the self-consistent
pressure profile, the heat (cold) pulse spreads much more
quickly than can be expected from transport coefficients,
calculated from a radial power balance. However, in ITB
regions ∇p can largely exceed that from the self-consistent
pressure profile.
The work at TEXTOR has made it also possible to
give an answer to a long-standing question why the
electron temperature profiles during off-axis Electron
Cyclotron Resonance Heating (ECRH) in the late
Rijnhuizen Tokamak Project are hollow. These
experiments have been repeated in TEXTOR with a much
more advanced set of diagnostics and the conclusion is
that during the first 100 ms of off-axis ECRH application,
the ohmic input power in the plasma core drops below the
power lost by the electrons to the ions via collisions.
Rational surfaces thus play a key role in the
establishment of e-ITBs, as has been observed in
stellarators, too. However, this does not exclude a
possible supporting role of ExB shear in ITB formation
near rational surfaces. Recent work on DIII-D and
gyrokinetic simulations [12] hint at possible synergy
between ExB shear and effects of rational surfaces. Large
profile corrugations in electron temperature gradients at
lowest-order singular surfaces lead to the buildup of a
huge zonal flow ExB shear layer which provides a trigger
for the low power ITB observed in DIII-D.
2.2. TRANSPORT BARRIERS INDUCED BY DED
IN TEXTOR
The influence of a magnetic perturbation field,
generated by the Dynamic Ergodic Divertor (DED), on
the turbulence and transport properties is studied and
compared to plasmas without such a field perturbation.
The external magnetic field breaks up the magnetic field lines
structure and causes an ergodization of the plasma edge [13].
The strength and radial range of the perturbation field can be
widely varied. Together with tangential neutral beam injection
in co–and counter–current directions, the turbulent transport
has been investigated.
One main effect of the DED is the modification of the
radial electric field. The ergodization of the magnetic field
lines leads to an increased electron loss rate which charges
the plasma edge more positively. The application of the DED
increases the rotation in the scrape-off-layer, where the
original rotation is in the ion diamagnetic drift direction.
Since the rotation at radii smaller than the limiter radius is in
the electron diamagnetic drift direction, the DED slows
down the rotation. The inversion point of the radial electric
field (as well as the poloidal rotation velocity) is shifted
further inside. This effect does not depend on the DED
configuration (m/n= 3/1 or 12/4), but on the field strength of
the perturbation field. Note that this conclusion concerns
only DC DED operation; the AC DED scenarios are the
subject of future work.
The data obtained in a single discharge with by the fast
scanning Gundestrup probe (Fig. 1) clearly demonstrate the
effects of DED on the plasma edge parameters.
16
The combination of counter-current neutral beam injection
and the DED can lead to the formation of a transport barrier at the
plasma edge [14]. The turbulence rotation is decreased at the
barrier, which again demonstrates the braking effect of the DED.
The acceleration of rotation by counter neutral beam injection and
braking by the DED yields an increase in the velocity shear at
r/a = 0.9. At the barrier, the level of density fluctuations is
constant, the turbulence decorrelation time is increased and the
turbulence wavelength is decreased. The evaluation of turbulent
diffusion using a random walk model yields the reduction of
transport by about 50 % within the barrier.
Fig. 1. Radial profiles (#99777) of the (a) toroidal flow vφ ,
(b) poloidal flow v , (c) electron temperature eT , (d) electron
density en , (e) floating potential fφ and, (f) radial electric
field rE before (thin line) and during (thick line) DED in
TEXTOR. The vertical dashed line marks the position of the
Last Closed Flux Surface(LCFS). The dashed-dotted line
indicates the end of the reliability of the Gundestrup probe data
2.3.TRANSPORT BARRIERS DURING OH
DISCHARGES IN TUMAN-3M
The influence of low frequency magnetohydrodynamic
(MHD) activity bursts during ohmic H-mode in the TUMAN-3M
tokamak [4] has been studied focusing on the measurements of
plasma fluctuation poloidal velocity performed by microwave
Doppler reflectometry. During the MHD burst a transient
deterioration of improved confinement was observed. As shown
in Fig. 2 the plasma fluctuation poloidal rotation observed before
the MHD burst in the vicinity of the edge transport barrier was in
the direction of plasma drift in the negative radial electric field.
During the MHD activity the measured poloidal velocity was
drastically decreased and even changed its sign. Radial profiles
of the poloidal velocity measured in a series of reproducible
tokamak shots exhibited the plasma fluctuation rotation in the
ion diamagnetic drift direction at the location of the peripheral
transport barrier.
The positive Er perturbation at the plasma edge obviously
leads to a transient deterioration of the H-mode transport
barrier. On the other hand, the inward propagation of the
positive electric field increases the shear of plasma rotation
deeper in the core. Such a displacement of the shear pattern to
the core region might cause a transport barrier shift towards
the inner region of the plasma column.
1
2
3
4
6
-1
0
1
50 55 60 65 70
16
20
24
L-H
ne, 1013 cm-3
(a)
(b)D
α
, a
.u
.
(c)
MHD, a.u.
-1
0
1
2
3
4
5
electron diamagnetic drift(d)
time, ms
r c,
cm
ion diamagnetic drift
V θ, 1
05 cm
/s
V
MHD
(e)
Fig 2. Time evolution of the signals measured in a shot virtually
without MHD activity (dotted line) and in a shot with a sharp
MHD in TUMAN-3M:
a) line averaged plasma density measured along central chor;
b) D emission intensity; c) magnetic probe signal with MHD
burst; d) magnetic island poloidal velocity derived from magnetic
probe signal evolution (thick grey curve) and the Doppler
velocities; e) cut-off radii (dotted line, microwave frequency
23.5 GHz and solid line, microwave frequency 24.68 GHz)
3.TURBULENCE CHARACTERISTICS
3 .1. GEODESIC ACOUSTIC MODES (GAM)
Geodesic Acoustic Modes (GAM) were investigated on the
T-10 tokamak using the HIBP, correlation reflectometry (CR) and
multipin Langmuir probe diagnostics [1]. GAM are torsional
plasma oscillations with poloidal wavenumber m=0, a high
frequency branch of Zonal Flows (ZF). Regimes with ohmic
heating and with on- and off-axis ECRH were studied. It was
shown that GAM are mainly potential oscillations, but GAM are
pronounced enough in the density fluctuation to be detected by CR,
making the latter to an effective tool for further study of ZF/GAM.
Typically, the power spectrum (Fig.3) of the HIBP potential
oscillations exhibits a dominating solitary quasi-monochromatic
peak. The frequency of GAM changes in the region of
observation and decreases towards the plasma edge. After
ECRH switch-on, the frequency increases, correlating with
growth of the electron temperature Te. The GAM frequency (see
Fig.4) depends on the local Te as: fGAM~cs/R~Te
1/2 which is
consistent with a theoretical scaling for GAM, where cs is the
sound speed within a factor of unity.
Along with the above mentioned features, predicted for
ZF/GAM, some additional characteristics were found on T-10:
• GAM tend to be more excited near low-q magnetic
surfaces.
• Along with being mainly electrostatic, GAM also
have some magnetic component.
• GAM amplitude has an intermittent character.
• GAM exhibit a density limit.
Characteristics of GAM observed with O-mode CR in
OH discharges in the plasma edge of TEXTOR tokamak are
similar to those on T-10. The frequency of the observed
mode obeys the theoretically predict ed GAM scaling with
local temperature and ion mass. The poloidal distribution of
17
the amplitude of the GAM-induced density fluctuations was
studied. On the basis of measurements at several poloidal
positions a good qualitative agreement with theoretically
predicted sin (or m = 1) distribution was found.
Fig. 3.Top: Power spectrum of potential oscillations and
total beam current (proportional to the
plasma density) measured by HIBP in T-10;
Middle:Spectrum of plasma density oscillations measured
by the reflectometer;Bottom:Spectra of floating potential
and ion saturation current oscillations measured
by the Langmuir probe. Parameters of similar shots:
BT = 2.42 T, Ip = 290 kA, q(a) = 2.5, ne = 4 × 1019 m 3
Fig. 4. Evolution of spectrum measured at =0.83 during
ECRH switch-on in T-10. The frequency of peak increases
together with growth of electron temperature
The phase coherence over about 90° confirms the long-scale
nature of the observed density oscillations which is also
consistent with the predicted m = 1 structure. The level of the
oscillations of turbulence rotation related with the GAM is
found to be in the range 5-10% of the ambient turbulence
rotation, which result in an increase of shearing rate by a
factor of approximately 5. The resulting shearing rate is
comparable with the decorrelation rate of ambient
turbulence. It is shown that the fluctuations in the ambient
plasma turbulence level are strongly correlated with those of
the oscillations of turbulence rotation due to the GAM.
Doppler reflectometry has recently been employed to
detect GAM as oscillations of poloidal velocity in the
ASDEX Upgrade tokamak [15]. A similar diagnostic has
been used to reveal GAM oscillations in the TUMAN-3M
tokamak during transition to ohmic H-mode triggered by
impulse gas puffing [16]. The oscillations of poloidal
velocity in the GAM frequency region were evaluated via
the averaging of the Doppler frequency shift spectra.
To study an influence of the GAM oscillations on the RMS
level of plasma scattering fluctuations cross-correlation spectral
analysis has been employed. Quasi-coherent oscillations of the
poloidal velocity in the GAM frequency region between 20 and
40 kHz were observed. The oscillations were detected for cut-
off locations near the edge transport barrier occurring during the
H-mode. There is no quasi-coherent oscillation of the poloidal
velocity if the cut-off is located in the SOL. The discovered
correlation between the plasma fluctuation level and the poloidal
velocity oscillation indicates an impact of the GAM oscillations
on the plasma turbulence.
3.2. CORE TURBULENCE IN T-10
Turbulence characteristics were investigated in detail in OH
and ECRH discharges in T-10 using correlation reflectometry,
HIBP and Langmuir probe arrays [2]. The OH and ECRH
discharges show a distinct transition from the core turbulence,
having a complex spectral structure, to the unstructured one at
the plasma boundary.
The core turbulence includes the “Broad Band” (BB),
“Quasi-Coherent” (QC) features, arising due to the excitation of
rational surfaces with high poloidal m-numbers, “Low
Frequency” (LF) near zero frequency, and the GAM oscillations
at 20…30 kHz. All experimentally measured properties of LF
and HF QC are in a good agreement with the behaviour of the
linear increments of Ion Temperature Gradient/Dissipative
Trapped Electron Mode (ITG/DTEM) instabilities. Significant
local decrease of the turbulence amplitude and coherency was
observed at the edge velocity shear layer and in the core near
q =1 radius at 5…15 ms after ECRH switch-off.
Reflectometry at half minor radius shows that long
wavelength turbulence is replaced by shorter wavelength
turbulence when the density increases up to half of the
Greenwald density. The shorter wavelength turbulence is
dominant at higher densities. This observation offers the
possibility to explain the confinement rise at low and its
saturation at higher densities. The second factor, which may
influence the confinement, is the strong decrease of the Te/Ti
ratio with the density increase which could also lead to the
confinement rise.
3.3. TURBULENCE CHARACTERISTICS IN FT-2
The new highly localized correlative enhanced scattering
diagnostics is capable of determinating the small scale
turbulence wave number spectra developed at the FT-2
tokamak and provided already the following results [5]:
• Two modes are found in the UHR BS spectra under
conditions when the threshold for the Electron Temperature
Gradient (ETG) mode instability [17] is exceeded. The ETG
mode is a possible candidate to explain anomalous electron
energy transport.
18
• The first mode has a frequency less than 1 MHz and radial
wave number 25 m-1<q<150 m-1, and is localized at the
plasma edge and associated with the ITG mode. Its wave
number spectrum is quickly decaying in a way similar to
that observed on Tore Supra.
• The second mode has a frequency higher than 2 MHz and
radial wave number q>150 m-1, and is associated with the
ETG mode. Its phase velocity is twice as high and its
amplitude is growing towards the centre. In the region of
observations its level is comparable to that of the ITG mode,
but is however much smaller than that of the latter mode at
the edge.
• The possibility of the poloidal rotation profile determination
with the UHR BS technique is demonstrated.
4. CONCLUSION AND OUTLOOK
The strong innovation potential of this INTAS project lies
in the field of tokamak physics and tools to control plasma
turbulence and electric fields, as well as in the field of
advanced plasma diagnostics. This project led to an improved
understanding of the relation between the global confinement
properties of tokamak plasmas and the physics of the
electrostatic and magnetic turbulence.
The main goal of the coherent approach was to identify the
major physical laws and instabilities ruling the transport in
tokamak plasmas in order to incorporate them into theoretical
turbulence models as well as in analytical transport models.
This is of crucial importance, because the ITER project relies
mostly on scaling laws. A thorough understanding can pave
new ways towards advanced scenarios and their external
control, and hence lead to an optimized construction of next
generation tokamaks. Any new ideas on external control of
transport barriers by means of magnetic and electrostatic
perturbation on the plasma edge can be easily tested since the
tokamaks in the project can be relatively easily modified
according to new ideas.
A new INTAS project has started in October 2006 with the
same partners plus four other institutions. This will in addition
provide advanced theoretical models and numerical
simulations, access to the long pulse tokamak Tore Supra, and
the availability of advanced magnetic sensors to study
magnetic turbulence inside the plasma.
ACKNOWLEDGEMENT
The authors are grateful to INTAS (International
Association for the promotion of co-operation with
scientists from the New Independent States of the former
Soviet Union) which supported the research activities in
the framework of project INTAS 2001-2056.
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