Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas
The response of TJ-II plasmas to an induced toroidal electric field, Eǿ , was studied in past experiments with the aid of Ohmic inducted current. It was found that positive induced plasma current, Ip, degrades the confinement while negative Ip improves it. Candidates to provide a physical explanatio...
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2002
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| Cite this: | Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas / D. López-Bruna, J. A. Romero, F. Castejón, T. Estrada, E. Ascasíbar // Вопросы атомной науки и техники. — 2002. — № 4. — С. 51-53. — Бібліогр.: 6 назв. — англ. |
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López-Bruna, D. Romero, J. A. Castejón, F. Estrada, T. Ascasíbar, E. 2015-04-14T05:33:09Z 2015-04-14T05:33:09Z 2002 Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas / D. López-Bruna, J. A. Romero, F. Castejón, T. Estrada, E. Ascasíbar // Вопросы атомной науки и техники. — 2002. — № 4. — С. 51-53. — Бібліогр.: 6 назв. — англ. 1562-6016 PACS: 52.55.Fa https://nasplib.isofts.kiev.ua/handle/123456789/80264 The response of TJ-II plasmas to an induced toroidal electric field, Eǿ , was studied in past experiments with the aid of Ohmic inducted current. It was found that positive induced plasma current, Ip, degrades the confinement while negative Ip improves it. Candidates to provide a physical explanation are Eǿ itself and the plasma current via magnetic shear modification. A movable mirror in an electron Cyclotron Heating line of TJ-II permits varying the refraction index of the heating wave, N||, thus allowing for non inductive electron cyclotron current drive (ECCD) up to Ip ≈ ±1 kA. Comparing discharges without and with ECCD (changing shear but not Eǿ ) but sharing the same transformer action (changing both shear and Eǿ ), magnetic shear and electric field effects can be discriminated in time. It has been found that plasma response with ECCD is delayed respect to the transformer switch-on time, clearly pointing to shear effects. Effects related to Eǿ alone, if they exist, are weaker. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Magnetic confinement Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas Article published earlier |
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Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas |
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Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas López-Bruna, D. Romero, J. A. Castejón, F. Estrada, T. Ascasíbar, E. Magnetic confinement |
| title_short |
Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas |
| title_full |
Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas |
| title_fullStr |
Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas |
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Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas |
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discrimination between magnetic shear and toroidal electric field effects in tj-ii plasmas |
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López-Bruna, D. Romero, J. A. Castejón, F. Estrada, T. Ascasíbar, E. |
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López-Bruna, D. Romero, J. A. Castejón, F. Estrada, T. Ascasíbar, E. |
| topic |
Magnetic confinement |
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Magnetic confinement |
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2002 |
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English |
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Вопросы атомной науки и техники |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Article |
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The response of TJ-II plasmas to an induced toroidal electric field, Eǿ , was studied in past experiments with the aid of Ohmic inducted current. It was found that positive induced plasma current, Ip, degrades the confinement while negative Ip improves it. Candidates to provide a physical explanation are Eǿ itself and the plasma current via magnetic shear modification. A movable mirror in an electron Cyclotron Heating line of TJ-II permits varying the refraction index of the heating wave, N||, thus allowing for non inductive electron cyclotron current drive (ECCD) up to Ip ≈ ±1 kA. Comparing discharges without and with ECCD (changing shear but not Eǿ ) but sharing the same transformer action (changing both shear and Eǿ ), magnetic shear and electric field effects can be discriminated in time. It has been found that plasma response with ECCD is delayed respect to the transformer switch-on time, clearly pointing to shear effects. Effects related to Eǿ alone, if they exist, are weaker.
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1562-6016 |
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https://nasplib.isofts.kiev.ua/handle/123456789/80264 |
| citation_txt |
Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas / D. López-Bruna, J. A. Romero, F. Castejón, T. Estrada, E. Ascasíbar // Вопросы атомной науки и техники. — 2002. — № 4. — С. 51-53. — Бібліогр.: 6 назв. — англ. |
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2025-11-26T02:45:06Z |
| last_indexed |
2025-11-26T02:45:06Z |
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1850609171587661824 |
| fulltext |
DISCRIMINATION BETWEEN MAGNETIC SHEAR AND TOROIDAL
ELECTRIC FIELD EFFECTS IN TJ-II PLASMAS
D. López-Bruna, J. A. Romero, F. Castejón, T. Estrada, E. Ascasíbar and the TJ-II Team
Laboratorio Nacional de Fusión por Confinamiento Magnético
Asociación EURATOM-CIEMAT. Av. Complutense 22, 28040 Madrid, Spain
The response of TJ-II plasmas to an induced toroidal electric field, Eφ, was studied in past experiments with the aid of
Ohmic inducted current. It was found that positive induced plasma current, Ip, degrades the confinement while negative
Ip improves it. Candidates to provide a physical explanation are Eφ itself and the plasma current via magnetic shear
modification. A movable mirror in an electron Cyclotron Heating line of TJ-II permits varying the refraction index of
the heating wave, N||, thus allowing for non inductive electron cyclotron current drive (ECCD) up to Ip ≈ ±1 kA.
Comparing discharges without and with ECCD (changing shear but not Eφ) but sharing the same transformer action
(changing both shear and Eφ), magnetic shear and electric field effects can be discriminated in time. It has been found
that plasma response with ECCD is delayed respect to the transformer switch-on time, clearly pointing to shear effects.
Effects related to Eφ alone, if they exist, are weaker.
PACS: 52.55.Fa
1. INTRODUCTION
The TJ-II facility[1] is very well suited for investigating
confinement properties in different magnetic
configurations, as it covers an ample range of rotational
transform profiles, ι. In addition to this, a set of coils
designed to induce toroidal plasma currents, Ip, can be
used to sweep ι profiles during a discharge. This has been
used recently to study the confinement in TJ-II under
positive and negative induced plasma currents[2]. Since
the technique is essentially the induction of a toroidal
electric field via transformer action, which gives the
ohmic heating (OH) mechanism in tokamaks, we shall
refer to it as OH induction. The main result was found
that Ip>0 causes a degradation of confinement and vice
versa in a very reproducible way. Two global magnitudes
can change sign under OH induction: the toroidal electric
field and, being TJ-II vacuum configurations almost
shearless, the magnetic shear defined as
( ) ( )ριιρ dds −=ˆ , where ρ is a flux surface radial
coordinate. The symmetry of the effect on confinement
suggests that either magnitude can be explicitly invoked
as the main cause of the observations. To discriminate
among them, new experiments were designed based on
non inductive techniques to drive Ip. It is worth
investigating the physics behind these observations
because the possibility of improving (degrading)
confinement with external means opens a convenient way
to control confinement in stellarator plasmas. Moreover,
the knowledge of the effect on transport of magnetic
shear, current and electric field that can be studied on
stellarators maybe extensible to tokamaks and other
confinement devices as well.
2. DESCRIPTION OF THE EXPERIMENTS
A movable mirror in the Electron Cyclotron
Heating (ECH) line of TJ-II allows varying the refraction
index of the heating wave, N||, thus forcing currents of
either sign without electric field induction. Here we have
launched the waves at N||=0.0 and N||=±0.2, i.e., no ECCD
and a drive of approximately –1 kA for N||=0.2 and 0.3 kA
for N||=-0.2. A typical discrimination experiment consists
of a set of three discharges: a reference one without OH
and two more with the same prescription for OH coils but
different ECCD level. Fig. 1 shows typical prescriptions
for the current in the OH coils.
-8
-6
-4
-2
0
2
4
6
8
1100 1150 1200 1250 1300
I O
H, k
A
tiempo, ms
vй
rti
ce
Fig. 1. Typical prescription of the current in the OH
coils. The discharges start with null or very low dIOH/dt
and, at a prescribed time (vertical line), IOH is ramped up
or down to induce a toroidal electric field.
The discharges are taken to equilibrium and then the
toroidal electric field is induced.A transition in the line
density evolution can be easily identified after the time at
which OH starts (vertical dashed line in Fig. 1).
Discharges with different ECCD have different Ip, but
share the same loop voltage. Therefore, a delay in the
transition in line density (or any other reproducible
timetraces) is an indicator that Ip, but not the electric field,
provokes the plasma response.
0.0 0.5 1.0
1.4
1.5
1.6
1.7
ρ
t=1.13 s
t=1.20 s
Fig. 2. Simulation of ι evolution from t=1.13 s (Ip=-1.5
kA) to t=1.20 s (Ip=2.3 kA). The vacuum profile is shown
with dots.
Problems of Atomic Science and Technology. 2002. № 4. Series: Plasma Physics (7). P. 51-53 51
3. EXPERIMENTAL RESULTS AND
DISCUSSION
The induced current in these discharges is enough to
cause notable changes in the magnetic shear along the
discharges. To ilustrate this, in Fig. 2 we plot the
evolution of ι calculated for a ramp up rate dIOH/dt=25
kA/s, starting from negative current, in toroidal geometry
and conditions similar to the experiments.
Fig. 3 shows a 200 ms time window for a typical OH
experiment with positive induction. From top to bottom
and left to right the following raw data from several
diagnostics are shown: plasma current, line average
density, electron temperature near the plasma center, Hα
emission line at the limiter, loop voltage, soft X-ray
emission, charge exchange neutral emission and Mirnov
coil signals. It can be observed that the levels of Ip are
different for the three discharges but the loop voltages are
experimentally the same except for the control case (no
OH induction). In despite of this, some distinguishable
signatures in the time traces for the case with lower Ip are
delayed with respect to the case of higher Ip. This is what
was expected in the case that the main cause for plasma
changes is Ip instead of the toroidal electric field. Note
that the larger MHD events (see Mirnov coil signals in
Fig. 3) are correlated with brief transients in Hα and other
diagnostics (density, electron temperature, soft X-rays)
but not with the time at which confinement changes trend.
Fig. 3. Time traces of several TJ-II diagnostics in a case of positive OH induction at several levels of plasma current.
As a general rule, the same patterns as in previous
experiments have been found: improvement (degradation)
of the confinement at negative (positive) plasma current.
It is worth noting, however, that when the positive plasma
current is high enough the confinement seems to recover.
This fact is illustrated in Fig. 4 where the evolution of line
density is plotted versus the plasma current for three
discharges with different line densities. The apparent
dependence of the density on Ip is stronger at higher
density. In the high density case it is clear how the density
drops but recovers as of Ip≈6 kA. In cases (not shown) of
negative plasma current induction, the density increases
monotonically with the magnitude of the current up to the
values reached of ≈-10 kA.
These experiments show that, for the present plasma
conditions (Electron Cyclotron Heating, average density
below 1019 m-3, peaked electron temperature with Te≈1
keV at the magnetic axes and ≈0.1 keV ions), the plasma
current is the main responsible for the changes in
confinement. Toroidal electric field effects per se are, if
any, much weaker. An important result to consider in the
passing is that, if the global magnetic shear is a true knob
in these experiments via plasma current, then not only its
magnitude, but also its sign, plays a role in the
confinement.
Fig. 4. Relation between line density and plasma current
in three discharges with low, middle and moderate
density.
52
Possible explanations, still to be investigated, rely on:
i-Dissipative trapped electron modes. These are
expected at Te>>Ti, low collisionality, low or negative
density gradient and large fraction of trapped particles.
All of these conditions are met in the present plasmas.
They are sensitive also to global magnetic shear[3]. A
calculation[4] of trapped fraction for TJ-II in the
vacuum configuration pertinent to these experiments
is ≈35%.
ii-Electron temperature gradient modes. The
corresponding growth rates may be affected by both
the sign and the magnitude of the magnetic shear.
iii-Modification of particle orbits due to changes in the
magnetic topology. This may affect considerably the
fraction of direct particle losses in TJ-II. Three-
dimensional calculations[5] suggest that these losses
can affect very differently to the populations of
passing and trapped particles depending on the
rotational transform of the configuration.
Numerical calculations of drift wave growth rates in
stellarator geometry point to the local magnetic shear as
an important magnitude to consider[6]. However, the
very low values of plasma β in these experiments should
exclude any changes in this parameter.
REFERENCES
1. Alejaldre, C., et al., Fusion Technol. 17, 131 (1990)
2. J. A. Romero et al., Controlling confinement with
induced toroidal current in the flexible Heliac TJ-II,
Submitted to Nuc. Fusion (2002).
3. N. Domínguez, B. A. Carreras, V. E. Lynch, P. H.
Diamond, Phys. Fluids B4, 2894 (1992).
4. J. Guasp, M. Liniers, Informes Técnicos CIEMAT
946, Madrid (2000).
5. J. Guasp, M. Liniers, Informes Técnicos CIEMAT
951, Madrid (2000).
6. N. Nadeem, T. Rafiq, M. Persson, Phys. Plasmas 8,
4375 (2001).
TEMPERATURE OF IMPURITY IONS IN A RF HEATED PLASMA OF THE
U-3M TORSATRON AS MEASURED BY MEANS OF THE DOPPLER
SPECTROMETRY
V.G.Konovalov, V.N.Bondarenko, A.N.Shapoval, A.S.Slavny, V.S.Voitsenya, E.D.Volkov
Institute of Plasma Physics, NSC KIPT, 61108 Kharkov, Ukraine
PACS: 52.55.Hc; 52.70.Kz
1. INTRODUCTION
The spectroscopic methods of measurements of ion
temperature, Ti, are routinely used in magnetic fusion
experiments. However some strong requirements must be
satisfied as for the quality of spectroscopic equipment and
reproducibility of plasma parameters when spectral line
profile is obtained shot by shot. Besides, the radial
location of ions of the given ionization state in the plasma
volume should be known. The correctness of Ti
measurement results has to be controlled by comparing
them with temperature Ti of the main plasma ion
component (H+, D+) by calculation of energy balance for
impurity ions. Important is also the requirement to
estimate the role of other possible mechanisms of
broadening of the impurity ion line profiles.
The goal of this paper is to determine the plasma ion
temperature using data on profiles of spectral lines of
intrinsic impurity, i.e., carbon ions (lines CV 227.1 nm
and CIII 229.6 nm) in the plasma confinement volume of
U-3M torsatron and to compare these results with data of
Ti measurements based on the energy distribution of
charge exchange atoms (CXA) obtained by neutral
particle analyzer (NPA).
2. EXPERIMENTAL CONDITIONS AND
RESULTS
The experiments were carried out on the fusion device
Uragan-3M (U-3M) which is a l=3, 9 magnetic field period
torsatron with major radius 100 cm and mean plasma
radius 12.5 cm. The hydrogen plasma was produced and
maintained during ~50 ms by RF fields in the frequency
range ω≤ωci corresponding to a multimode Alfven
resonance regime. The spectroscopical measurements were
provided for the following conditions: toroidal magnetic
field 0.7T, PRF ≤ 240kW, mean plasma density ~1⋅1018 m-3.
55
Asociación EURATOM-CIEMAT. Av. Complutense 22, 28040 Madrid, Spain
REFERENCES
V.G.Konovalov, V.N.Bondarenko, A.N.Shapoval, A.S.Slavny, V.S.Voitsenya, E.D.Volkov
REFERENCES
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