Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron
Ion saturation current and floating potential fluctuations are recorded by movable array of 4 Langmuir probes near the boundary of the confinement region in the l = 3/m = 9 Uragan-3M torsatron with an RF produced and heated plasma. On the basis of these data main spectral and time characteristics...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2002
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| Цитувати: | Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron / E.L. Sorokovoy, V.V. Chechkin, L.I. Grigor’eva, A.V. Chechkin, V.Yu. Gonchar, E.D. Volkov, N.I. Nazarov, S.A. Tsybenko, A.V. Lozin, A.P. Litvinov, A.Ye. Kulaga, Yu.K. Mironov, V.S.Romanov, S. Masuzaki, K. Yamazaki // Вопросы атомной науки и техники. — 2002. — № 5. — С. 6-9. — Бібліогр.: 7 назв. — англ. |
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irk-123456789-778162015-03-07T03:01:40Z Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron Sorokovoy, E.L. Chechkin, V.V. Grigor’eva, L.I. Chechkin, A.V. Gonchar, V.Yu. Volkov, E.D. Nazarov, N.I. Tsybenko, S.A. Lozin, A.V. Litvinov, A.P. Kulaga, A.Ye. Mironov, Yu.K. Romanov, V.S. Masuzaki, S. Yamazaki, K. Magnetic confinement Ion saturation current and floating potential fluctuations are recorded by movable array of 4 Langmuir probes near the boundary of the confinement region in the l = 3/m = 9 Uragan-3M torsatron with an RF produced and heated plasma. On the basis of these data main spectral and time characteristics of the low frequency electrostatic turbulence have been derived. The existence of the radial electric field shear and reversal of poloidal phase velocity of the fluctuations at the plasma boundary have been confirmed. The time function of the turbulent E×B particle flux contains intermittent bursts with the amplitude multiply exceeding the average flux. Up to 70% of the total fluctuating flux is carried in these bursts. 2002 Article Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron / E.L. Sorokovoy, V.V. Chechkin, L.I. Grigor’eva, A.V. Chechkin, V.Yu. Gonchar, E.D. Volkov, N.I. Nazarov, S.A. Tsybenko, A.V. Lozin, A.P. Litvinov, A.Ye. Kulaga, Yu.K. Mironov, V.S.Romanov, S. Masuzaki, K. Yamazaki // Вопросы атомной науки и техники. — 2002. — № 5. — С. 6-9. — Бібліогр.: 7 назв. — англ. 1562-6016 PACS: 52.55.Hc http://dspace.nbuv.gov.ua/handle/123456789/77816 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| topic |
Magnetic confinement Magnetic confinement |
| spellingShingle |
Magnetic confinement Magnetic confinement Sorokovoy, E.L. Chechkin, V.V. Grigor’eva, L.I. Chechkin, A.V. Gonchar, V.Yu. Volkov, E.D. Nazarov, N.I. Tsybenko, S.A. Lozin, A.V. Litvinov, A.P. Kulaga, A.Ye. Mironov, Yu.K. Romanov, V.S. Masuzaki, S. Yamazaki, K. Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron Вопросы атомной науки и техники |
| description |
Ion saturation current and floating potential fluctuations are recorded by movable array of 4 Langmuir probes near
the boundary of the confinement region in the l = 3/m = 9 Uragan-3M torsatron with an RF produced and heated
plasma. On the basis of these data main spectral and time characteristics of the low frequency electrostatic turbulence
have been derived. The existence of the radial electric field shear and reversal of poloidal phase velocity of the
fluctuations at the plasma boundary have been confirmed. The time function of the turbulent E×B particle flux contains
intermittent bursts with the amplitude multiply exceeding the average flux. Up to 70% of the total fluctuating flux is
carried in these bursts. |
| format |
Article |
| author |
Sorokovoy, E.L. Chechkin, V.V. Grigor’eva, L.I. Chechkin, A.V. Gonchar, V.Yu. Volkov, E.D. Nazarov, N.I. Tsybenko, S.A. Lozin, A.V. Litvinov, A.P. Kulaga, A.Ye. Mironov, Yu.K. Romanov, V.S. Masuzaki, S. Yamazaki, K. |
| author_facet |
Sorokovoy, E.L. Chechkin, V.V. Grigor’eva, L.I. Chechkin, A.V. Gonchar, V.Yu. Volkov, E.D. Nazarov, N.I. Tsybenko, S.A. Lozin, A.V. Litvinov, A.P. Kulaga, A.Ye. Mironov, Yu.K. Romanov, V.S. Masuzaki, S. Yamazaki, K. |
| author_sort |
Sorokovoy, E.L. |
| title |
Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron |
| title_short |
Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron |
| title_full |
Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron |
| title_fullStr |
Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron |
| title_full_unstemmed |
Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron |
| title_sort |
density and potential fluctuations in the edge plasma of the uragan-3m torsatron |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2002 |
| topic_facet |
Magnetic confinement |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/77816 |
| citation_txt |
Density and potential fluctuations in the edge plasma of the Uragan-3m torsatron / E.L. Sorokovoy, V.V. Chechkin, L.I. Grigor’eva, A.V. Chechkin, V.Yu. Gonchar, E.D. Volkov, N.I. Nazarov, S.A. Tsybenko, A.V. Lozin, A.P. Litvinov, A.Ye. Kulaga, Yu.K. Mironov, V.S.Romanov, S. Masuzaki, K. Yamazaki // Вопросы атомной науки и техники. — 2002. — № 5. — С. 6-9. — Бібліогр.: 7 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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| first_indexed |
2025-07-06T02:01:25Z |
| last_indexed |
2025-07-06T02:01:25Z |
| _version_ |
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| fulltext |
DENSITY AND POTENTIAL FLUCTUATIONS IN THE EDGE PLASMA
OF THE URAGAN-3M TORSATRON
E.L. Sorokovoy, V.V. Chechkin, L.I. Grigor’eva, A.V. Chechkin, V.Yu. Gonchar, E.D. Volkov,
N.I. Nazarov, S.A. Tsybenko, A.V. Lozin, A.P. Litvinov, A.Ye. Kulaga, Yu.K. Mironov,
V.S.Romanov, S. Masuzaki∗, K. Yamazaki∗
National Science Center “Kharkov Institute of Physics and Technology”,
Akademicheskaya st. 1, 61108 Kharkov, Ukraine
∗National Institute for Fusion Science,
Oroshi 322-6, Toki 509-5292, Japan
Ion saturation current and floating potential fluctuations are recorded by movable array of 4 Langmuir probes near
the boundary of the confinement region in the l = 3/m = 9 Uragan-3M torsatron with an RF produced and heated
plasma. On the basis of these data main spectral and time characteristics of the low frequency electrostatic turbulence
have been derived. The existence of the radial electric field shear and reversal of poloidal phase velocity of the
fluctuations at the plasma boundary have been confirmed. The time function of the turbulent E×B particle flux contains
intermittent bursts with the amplitude multiply exceeding the average flux. Up to 70% of the total fluctuating flux is
carried in these bursts.
PACS: 52.55.Hc
1. INTRODUCTION
An increased attention is paid at present to find out the
temporal structure of the electrostatic turbulence of edge
plasma in various toroidal facilities [1-3]. Such an
approach largely facilitates understanding of the nature of
anomalous transport at the boundary of the confinement
region. When studying statistic characteristics of
fluctuations in different plasma devices, a lot of common
features has been revealed. In particular, an essentially
non-Gaussian character of the turbulence is observed [4].
This is a consequence of electric field and plasma density
fluctuations at the plasma boundary to contain
intermittent bursts of large amplitude. The nature of these
bursts has not been found out yet and needs further
studies. In the present work, some basic characteristics of
the edge electrostatic turbulence are studied in the l = 3
Uragan-3M (U-3M) torsatron with a natural helical
divertor. With an RF-produced and heated plasma, an
intermittent character of the fluctuation-caused particle
flux at the plasma boundary has been confirmed and first
results of flux studies have been given.
2. EXPERIMENTAL CONDITIONS AND
MEASUREMENT TECHNIQUES
In the U-3M torsatron with a helical divertor (l = 3, m
= 9, R0 = 1 m, a ≈ 0.1 m, Bφ = 0.7 T, ι( a ) ≈ 0.4) a
hydrogen plasma is produced and heated by RF field in
the ω ≤ ωci range of frequencies. In the chosen regime of
device operation, the RF power absorbed in the plasma
was 240 kW. The central chord averaged electron density
was en ∼ 1018 m-3. The temperatures of the main groups
of ions and electrons were Ti ≈ 80 eV and Te(0) ≈ 400 eV,
respectively. To study electrostatic turbulence at the
plasma boundary, a movable array of 4 Langmuir probes
is used, which record fluctuations of ion saturation current
(ISC) Is as density fluctuations and of floating potential
(FP) Vf as space potential fluctuations. The probes 1,2,3,4
(Fig. 1)are located in the angles of an 3 mm side square,
Fig. 1. Relative lay-out of helical coils I, II, III and
calculated edge structure of field lines in the poloidal
cross-section where measurements are made. The range
of probe displacement LP is indicated by bold straight
segment. The disposition of probes 1, 2, 3, 4 is shown in
the inset as seen from the center outward.
oriented normal to the major axis of the torus and can be
moved parallel to the major radius at the distance of 1 cm
from the torus midplane. As a recording facility, a 4-
channel 12-bit ADC is used with the sampling rate 1.6 µs
per channel. This allows to record fluctuations with the
frequency up to 300 kHz.
3. SPECTRAL CHARACTERISTICS OF
POTENTIAL AND DENSITY FLUCTUATIONS
For the chosen RF discharge regime, the plots of rms
values of ISC fluctuations δIs (probe 2) and FP δVf (probe
3) against the distance ∆ from the vertical axis of poloidal
cross-section along the line of probe array displacement
are shown in Fig. 2 together with profiles of time-
averaged ISC and plasma potential (the latter is estimated
6 Problems of Atomic Science and Technology. 2002. № 5. Series: Plasma Physics (8). P. 6-8
LP
0.8 1.0 1.2
0.2
0.0
- 0.2
R, m
Z, m I
II
III
probe head
1
23
4
10
20
, cm
0
2
I
, m
A
s
V
,
V
ol
ts
p
0
100
200
300
10 11 12 13
0
40
(a)
(b)
δ I
,
m
A
s
δ V
,
V
ol
ts
f
∆
0
1
0
1
0 100 200 300
13 cm
0 100 200 300
12.25 cm
11.25 cm
0
1
0 100 200 300
kHz
V , 10 m/s
, cm
10
5
0
-5
3
11 13
ph
∆
as Vp ≈ Vf + 2.5(Te/e)). The plasma potential possesses a
maximum at ∆ = 11 cm, thus indicating existence of
radial electric field shear. Also, a maximum of potential
fluctuation amplitude is located in this region (∆ = 11.25
cm).
Fig. 2. (a) average ISC Is (◊) and its rms fluctua-tions δIs
(●) as functions of distance ∆; (b) same for plasma
potential Vp (◊) and its fluctuations δVf (●)
Fig. 3. Normalized power spectra of ISC fluctuations for
various distances ∆
The power spectra of density fluctuations are shown
in Fig. 3 for three probe positions in the edge region. The
major part of the power is concentrated in the 0 - 100 kHz
range of frequencies. Close to the boundary of the
confinement region, the spectrum becomes more broad,
and an appreciable part of it is formed by 100 - 200 kHz
fluctuations.
Using cross-spectral characteristics of potential
fluctuations, recorded by the probes 3 and 4, the poloidal
phase velocity of fluctuations is plotted against ∆ in Fig.
4. This dependence confirms the effect of phase velocity
reversal in the vicinity of radial electric field shear near
the plasma boundary[5].
Fig. 4. Poloidal phase velocity of fluctuations Vph as a
function of distance∆
4. CHARACTERISTICS OF TURBULENT
PARTICLE FLUX
It has been shown earlier that the particle flux through
the plasma boundary is largely associated with density
and electric field fluctuations in the U-3M torsatron [6,7].
In particular, this infers from consideration of spectral
function of flux. In the present work, to understand better
the structure of the turbulent flux, it is considered in the
real time. To create the time function of the turbulent E×B
flux, φθ BEnT /~~~ =Γ , fluctuating signals are taken from the
probes, probe 2 (ISC) and 3,4 (FP), similar to what has
been done in [1]. As a quantity proportional to the
poloidal electric field fluctuations, the difference between
FP fluctuations recorded by the probes 4 and 3 is taken,
34 ff VVE −∝θ , and as a quantity proportional to the
density fluctuations n~ , ISC fluctuation sI~ is used. The
time traces of local turbulence flux are presented in Fig. 5
Fig. 5. Time function of turbulent E×B particle flux for
various distances ∆
for the same three distances ∆ as in Fig. 3. Proceeding
from the form of flux function, we may conclude that the
flux is mainly positive, that is, it is directed outward. Its
time structure is characterized by the presence of
relatively rare intense short time bursts, similar to what
have been observed in [1-3]. The amplitude of these
bursts can exceed the averaged flux by an order of
magnitude. The largest bursts are observed in the vicinity
of poloidal phase velocity shear, ∆ = 12.25 cm. It is of
7
0
10
20
30
0
10
20
30 13 cm
12.25 cm
11.25 cm
0
10
20
30
0 1 2
0 1 2
0 1 2ms
interest to estimate which part of the flux is transported
by bursts with the amplitude exceeding some given level
[1]. At first, the flux is normalized so that its average
value is 1, ∑ =
Γ
N
n n1
/N =1, where Γn ≡ ΓT(t)/<ΓT(t)>, n is
the point number in the time function of the flux, 1 ≤ n ≤
N. In the case considered, N = 1250, which corresponds to
the 0 ÷ 2 ms interval. For the normalized time flux, the
probability density function (PDF) p(Γn) = NN
nΓ can
be created, where for each set of data n
N Γ is the number
of Γn values, which fall into the Γn ± W/2 range, W being
the width of the interval centered at Γn. Setting Γn to lie
in the -40 ÷ +40 interval, we divide this interval into 160
sells of W = 0.5 width. The number of points falling into
each sell is divided by the total number of points N. Also,
the flux fraction function (FFF) FF(Γn) = p(Γn)∗Γn is
calculated, which allows to estimate fractions of the total
turbulent flux carried by bursts of various amplitude in
the selected time interval. To estimate which fraction of
the flux is carried by the bursts, whose amplitude exceeds
some given value, the FFF should be integrated from this
value to the highest defined one, ( ) '''
n
n
nn pdF
n
ΓΓΓ= ∫
∞
Γ> .
This is illustrated in Fig. 6. Here, the cumulative
probability of a flux event is plotted along the Y axis, and
the flux fraction carried by the given flux events, i.e., by
bursts whose amplitude exceeds some given value, is
plotted along the X axis.
Fig. 6. Cumulative probability of flux events (axis Y),
which carry the given fraction of the average particle flux
(axis X).
It follows from Fig. 6 that in the vicinity of phase velocity
shear, ∆ = 12.25 cm, 2.5% of the largest flux events carry
50% of the total turbulent flux. For ∆ = 11.25 cm and ∆ =
13 cm this percentage amounts 7.5% and 8.5%
respectively. In Fig. 7 the total turbulent flux and its
fraction, which is carried by bursts with 5-fold excess
over the averaged level, are plotted against the distance ∆.
The total flux has a local minimum in the vicinity of
phase velocity shear. In this position, the flux fraction
carried by bursts with 5-fold excess attains 70%. It can
be concluded from Fig. 7 that presumably a dominating
fraction of plasma objects transported across the magnetic
field lines in the form of intermittent bursts does not leave
the stochastic region and finally enters the divertor.
Fig. 7. ∆ - dependence of total turbulent E×B particle flux
(◊) and of its fraction, carried by bursts with 5-fold
excess of the amplitude over the average level (●).
5. CONCLUSIONS
Main spectral and temporal characteristics of the low
frequency electrostatic turbulence have been studied in
the edge plasma of the Uragan-3M torsatron. In particular,
the reversal of the poloidal phase velocity of the
fluctuations occurring near the radial electric field shear
has been confirmed. Similar to what has been observed in
some tokamaks and stellarators, the time function of the E
×B turbulent flux possesses short time intermittent bursts,
their amplitude multiply exceeding the average level.
Taking only a small part of the flux in time (2.5 - 9 %),
these bursts can carry up to 70 % of the total turbulent
flux. The character of the burst amplitude - distance from
the boundary plots allows to suppose that the major part
of the plasma objects carried by the bursts into the
stochastic layer enters the divertor.
8
0.0
0.5
1.0
0.0
0.5
1.0
0.0 0.4 0.8 1.2
13 cm
0.0 0.4 0.8 1.2
12.25 cm
11.25 cm
0.0
0.5
1.0
0.0 0.4 0.8 1.2
11 12 13
1
10
100
, cm∆
This work was performed in collaboration with
National Institute for Fusion Science (Toki, Japan) by the
Program LIME.
REFERENCES
[1] Carreras B.A. et al 1996 Phys. Plasmas 3 (7) 2664.
[2] Sanchez E. et al 2000 Phys. Plasmas 7 (5) 1408.
[3] Rudakov D.L. et al 2002 Plasma Phys. Control.
Fusion 44 717.
[4] Gonchar V.Yu. et al "Stable Lévy distributions for
plasma density and potential fluctuations in the edge
plasma of a torsatron" submitted to Rus. J. Plasma
Phys.
[5] Endler M. 1999 J. Nucl. Mater. 266-269 84
[6] Sorokovoj E.L. et al 1996 Controlled Fusion and
Plasma Physics (Proc. 23rd Eur. Conf. Kiev, 1996),
vol. 20C, part II (Geneva: European Physical Society),
p. 523.
[7] Chechkin V.V. et al 1996 Nucl. Fusion 36 133.
9
National Science Center “Kharkov Institute of Physics and Technology”,
Akademicheskaya st. 1, 61108 Kharkov, Ukraine
Oroshi 322-6, Toki 509-5292, Japan
References
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