Low frequency oscillations in U-2M conditioning RF discharges
MHD oscillations were studied in low magnetic field (B₀ = 0.01 T) radio frequency discharges of the URAGAN-2M (U-2M) stellarator. Coherent oscillations are observed by bolometers, triple Langmuir probe microwave interferometer (140 GHz) and magnetic pick-up coils in different toroidal cross-sectio...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
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irk-123456789-1488252019-02-19T01:27:34Z Low frequency oscillations in U-2M conditioning RF discharges Dreval, M.B. Shapoval, A.M. Beletskii, A.A. Pavlichenko, R.O. Ozherel’yev, F.I. Makhov, M.M. Магнитное удержание MHD oscillations were studied in low magnetic field (B₀ = 0.01 T) radio frequency discharges of the URAGAN-2M (U-2M) stellarator. Coherent oscillations are observed by bolometers, triple Langmuir probe microwave interferometer (140 GHz) and magnetic pick-up coils in different toroidal cross-sections of U-2M. Modified diagnostics show, that the plasma beta (β = nT/(μ₀B² )) is rather high in these discharges in spite of the low temperature (Te = 10…50 eV) and density (ne = (0.5…1)∙10¹² cm⁻³ ) due to very low magnetic field. Variation of the magnetic configuration of U-2M substantially modifies fluctuations amplitude, oscillating modes type and frequency. Rotated m = 2 mode is transformed into the Sawtooth-like oscillations in different magnetic configurations. Strong dependence on the magnetic configuration indicates that observed in U-2M phenomena can have similar nature as in stellarator high temperature plasmas. МГД-осциляції плазми вивчалися у високочастотних розрядах стеларатора У-2М при малому магнітному полі (B0 = 0,01 Tл). Когерентні осциляції спостерігаються болометрами, потрійним ленгмюрівським зондом, НВЧ-інтерферометром (140 ГГц) і магнітними зондами в різних тороїдальних перерізах У-2М. Оновлені діагностики показують, що β плазми (β = nT/(μ₀B ² )) досить висока в цих розрядах завдяки низькому магнітному полю, незважаючи на низькі температуру (Te = 10…50 еВ) і густину (ne = (0,5…1)∙10¹² cм⁻³ ). Варіювання магнітної конфігурації У-2М значно змінює амплітуду флуктуацій, тип моди і частоту. Мода m = 2, що обертається, трансформується в пилкоподібні коливання в різних магнітних конфігураціях. Сильна залежність від магнітної конфігурації вказує, що спостережуваний на У-2М ефект може мати схожу природу з процесами, що і аналогічна поведінка осциляцій в гарячій плазмі стелараторів. МГД-осцилляции плазмы изучались в высокочастотных разрядах стелларатора У-2М при малом магнитном поле (B0 = 0,01 Tл). Когерентные осцилляции наблюдаются болометрами, тройным ленгмюровским зондом, СВЧ-интерферометром (140 ГГц) и магнитными зондами в различных тороидальных сечениях У-2М. Обновленные диагностики показывают, что β-параметр (β = nT/(μ₀B ² )) достаточно высок в этих разрядах, несмотря на низкие температуру (Te = 10…50 эВ) и плотность (ne = (0,5…1)∙10¹² cм⁻³ ). Благодаря низкому магнитному полю вариация магнитной конфигурации У-2М значительно изменяет амплитуду флуктуаций, тип моды и частоту. Вращающаяся мода m = 2 трансформируется в пилообразные колебания в различных магнитных конфигурациях. Сильная зависимость от магнитной конфигурации указывает, что наблюдаемый на У-2М феномен может иметь схожую природу с осцилляциями в горячей плазме стеллараторов. 2018 Article Low frequency oscillations in U-2M conditioning RF discharges / M.B. Dreval, A.M. Shapoval, A.A. Beletskii, R.O. Pavlichenko, F.I. Ozherel’yev, M.M. Makhov // Вопросы атомной науки и техники. — 2018. — № 6. — С. 42-45. — Бібліогр.: 6 назв. — англ. 1562-6016 PACS: 52.55.Hc; 52.70.La http://dspace.nbuv.gov.ua/handle/123456789/148825 en Вопросы атомной науки и техники 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 |
Магнитное удержание Магнитное удержание |
| spellingShingle |
Магнитное удержание Магнитное удержание Dreval, M.B. Shapoval, A.M. Beletskii, A.A. Pavlichenko, R.O. Ozherel’yev, F.I. Makhov, M.M. Low frequency oscillations in U-2M conditioning RF discharges Вопросы атомной науки и техники |
| description |
MHD oscillations were studied in low magnetic field (B₀ = 0.01 T) radio frequency discharges of the
URAGAN-2M (U-2M) stellarator. Coherent oscillations are observed by bolometers, triple Langmuir probe
microwave interferometer (140 GHz) and magnetic pick-up coils in different toroidal cross-sections of U-2M.
Modified diagnostics show, that the plasma beta (β = nT/(μ₀B²
)) is rather high in these discharges in spite of the low
temperature (Te = 10…50 eV) and density (ne = (0.5…1)∙10¹² cm⁻³
) due to very low magnetic field. Variation of the
magnetic configuration of U-2M substantially modifies fluctuations amplitude, oscillating modes type and
frequency. Rotated m = 2 mode is transformed into the Sawtooth-like oscillations in different magnetic
configurations. Strong dependence on the magnetic configuration indicates that observed in U-2M phenomena can
have similar nature as in stellarator high temperature plasmas. |
| format |
Article |
| author |
Dreval, M.B. Shapoval, A.M. Beletskii, A.A. Pavlichenko, R.O. Ozherel’yev, F.I. Makhov, M.M. |
| author_facet |
Dreval, M.B. Shapoval, A.M. Beletskii, A.A. Pavlichenko, R.O. Ozherel’yev, F.I. Makhov, M.M. |
| author_sort |
Dreval, M.B. |
| title |
Low frequency oscillations in U-2M conditioning RF discharges |
| title_short |
Low frequency oscillations in U-2M conditioning RF discharges |
| title_full |
Low frequency oscillations in U-2M conditioning RF discharges |
| title_fullStr |
Low frequency oscillations in U-2M conditioning RF discharges |
| title_full_unstemmed |
Low frequency oscillations in U-2M conditioning RF discharges |
| title_sort |
low frequency oscillations in u-2m conditioning rf discharges |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2018 |
| topic_facet |
Магнитное удержание |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/148825 |
| citation_txt |
Low frequency oscillations in U-2M conditioning RF discharges / M.B. Dreval, A.M. Shapoval, A.A. Beletskii, R.O. Pavlichenko, F.I. Ozherel’yev, M.M. Makhov // Вопросы атомной науки и техники. — 2018. — № 6. — С. 42-45. — Бібліогр.: 6 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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AT drevalmb lowfrequencyoscillationsinu2mconditioningrfdischarges AT shapovalam lowfrequencyoscillationsinu2mconditioningrfdischarges AT beletskiiaa lowfrequencyoscillationsinu2mconditioningrfdischarges AT pavlichenkoro lowfrequencyoscillationsinu2mconditioningrfdischarges AT ozherelyevfi lowfrequencyoscillationsinu2mconditioningrfdischarges AT makhovmm lowfrequencyoscillationsinu2mconditioningrfdischarges |
| first_indexed |
2025-07-12T20:22:20Z |
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| fulltext |
ISSN 1562-6016. ВАНТ. 2018. №6(118)
42 PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY. 2018, № 6. Series: Plasma Physics (118), p. 42-45.
LOW FREQUENCY OSCILLATIONS IN U-2M CONDITIONING RF
DISCHARGES
M.B. Dreval, A.M. Shapoval, A.A. Beletskii, R.O. Pavlichenko, F.I. Ozherel’yev,
M.M. Makhov
National Science Center “Kharkov Institute of Physics and Technology”,
Institute of Plasma Physics, Kharkiv, Ukraine
MHD oscillations were studied in low magnetic field (B0 = 0.01 T) radio frequency discharges of the
URAGAN-2M (U-2M) stellarator. Coherent oscillations are observed by bolometers, triple Langmuir probe
microwave interferometer (140 GHz) and magnetic pick-up coils in different toroidal cross-sections of U-2M.
Modified diagnostics show, that the plasma beta (β = nT/(μ0B2)) is rather high in these discharges in spite of the low
temperature (Te = 10…50 eV) and density (ne = (0.5…1)∙1012 cm-3) due to very low magnetic field. Variation of the
magnetic configuration of U-2M substantially modifies fluctuations amplitude, oscillating modes type and
frequency. Rotated m = 2 mode is transformed into the Sawtooth-like oscillations in different magnetic
configurations. Strong dependence on the magnetic configuration indicates that observed in U-2M phenomena can
have similar nature as in stellarator high temperature plasmas.
PACS: 52.55.Hc; 52.70.La
INTRODUCTION
The magnetic topology is a very important ingredient
of plasma confinement in magnetic traps, having a
greater influence than the magnetic configuration on the
transport in the several collisionality regimes.
Stellarators rely on currents in external coils to provide
the magnetic fields with the rotational transform needed
for plasma confinement and stability. The radial profile
of the rotational transform, ι(r), is one design parameter
of these configurations and it can be chosen to exclude
low order rational surfaces inside the plasma.
Nonetheless, the current in the plasma of stellarators can
be substantial, as well as finite plasma beta effects
cannot be completely compensated. These factors can
modify designed vacuum rotational transform profile
and introduce rational surfaces. The presence of rational
surfaces that can break the magnetic topology of nested
flux surfaces does affect the particle and heat fluxes as
well as the turbulence and electric fields. Strong plasma
current can cause deterioration of stellarator discharge.
Disruptive-like events accompanied by a fast energy
loss were observed as low order rational resonances
appear in the outer region of the W7-AS [1] plasma in
discharges with a significant contribution of net toroidal
current densities. The m = 2 MHD mode (at rational
surface ι = 1/2) was observed as the precursor of the
observed energy loss in W7-AS discharges. The MHD
mode properties depend on the rational surface. In the
case of ι = 1/2 rotated m = 2 mode is observed. In the
case of ι = 1, Sawtooth-like oscillations, similar to
tokamaks Sawtoothes are observed in stellarators [2, 3].
The rational surfaces can deteriorate plasma
confinement, but in can improve plasma confinement as
well via ITB formation [2]. The role of rational surfaces
in the transport barrier formation accompanied by the
sawtooth-like oscillations was discussed in the case of
hot plasma of TJ-II [1]. Similar Sawtooth-like
oscillations were recently observed in W7-X discharges
with rational surfaces [3]. Various MHD oscillations of
the U-2M plasma were observed recently. In particular,
MHD instabilities are observed in the low magnetic
field low temperature radio frequency (RF) discharges.
These types of RF discharge are routinely used for the
U-2M wall conditioning [4]. Main advantage of these
discharges is related to the low temperature plasma
conditions. It is possible to use simple Langmuir probe
and magnetic probe diagnostic techniques for the
measurement of electron temperature, density, radial
electric field, and plasma current profiles. It is
substantially more difficult to measure these set of
profiles in high temperature plasmas (and impossible in
U-2M due to luck of complicate diagnostics). First
demonstration of MHD activity in various magnetic
configurations of the low magnetic field discharge
of U-2M is present in our work.
CONDITIONS AND DIAGNOSTICS
In our experiments, the frame-type antenna (FTA)
and three half turn (THT) antenna simultaneously were
used in low magnetic field B = 0.01 T (ω >> ωci)
hydrogen discharges. Top view of the U-2M torus with
involved diagnostics is shown in Fig. 1.
Fig. 1. Toroidal location of bolometric arrays,
Triple probe, 2 mm interferometer , THT and
FTA RF antennas
The Langmuir probe (LP) is used for measurement
of complete profiles up to plasma center (Fig. 2) in cold,
ISSN 1562-6016. ВАНТ. 2018. №6(118) 43
low density RF discharges under consideration. New
electronics was designed and manufactured for Te, n, Vp
measurements with high time resolution via triple
Langmuir probe (TP) technique.
A comparison of the TP measurements and
conventional single pin scanned LP measurements
shows good agreement (Fig. 3). Two multichannel
pinhole cameras were recently installed in U-2M for
monitoring the oscillations of visible light emission
from two positions in the same plasma cross-section
(Fig. 4). Strong noise suppression technique was used in
hardware design [5].
Fig. 2. LP manipulator positions
Fig. 3. TP (blue crosses) and conventional single pin
scanned LP (red squares) data of ion saturation
current, electron temperature and floating potential
Fig .4. Lines of sights of two arrays of bolometers
A magnetic pick-up coil was installed outside of the
vessel. The thickness of bellow section of U-3M
vacuum chamber is about 1mm. It is thin enough for the
frequency range of 0.5…20 kHz under consideration.
EXPERIMENTAL RESULTS
Fluctuations in conditioning discharges (B=0.01 T)
were observed recently [6]. These coherent fluctuations
are observed in set of diagnostics, as it is shown in
Fig. 5.
Fig. 5. Signals of interferometer, probe ion saturation
current and floating potential, magnetic pick-up coil
This is the discharge in one of the magnetic
configurations. Variation of the magnetic configuration
of U-2M substantially modifies the features of the
oscillations in the plasma of such low magnetic field
discharges (Fig. 6).
Fig. 6. Signals from 20-channals bolometer in two
magnetic configurations (r1, and r2)
Significant dependence of the fluctuations amplitude
is clearly observed in this figure. In present work three
representative configurations are considered, as it is
shown in Table. In addition to significant variation of
44 ISSN 1562-6016. ВАНТ. 2018. №6(118)
the fluctuations amplitude, the modification of the
oscillating modes types and frequency ranges were also
observed.
Representative configurations
Kφ B Gs Ih A It A Icorr A P·10-5 Torr
r1 0.32 100 340 150 - 9
r2 0.26 122 340 200 150 19
r3 0.22 145 340 250 - 19
An oscillating part of the 20-channals bolometer
signals is shown in Fig. 7 (see data of r2 discharge from
Fig. 6). The phase delay between channels demonstrates
that the poloidal mode number is m = 2 and it is rotating
in the electron diamagnetic direction.
Fig. 7. Fluctuations observed by 20-channals bolometer
Fig. 8. Profiles of TP density, temperature and plasma
potential in two instances of r1 and in r2 pulses
Different shapes of the plasma profiles are observed
in different configurations (Fig.8). In particular, one can
see a shear of electric field in configurations with
fluctuations. This shear can be caused by presence of
rational magnetic surfaces in magnetic configurations
with oscillations. Due to variation of magnetic
configuration rotating m = 2 mode in r2 is transformed
into the Sawtooth-like oscillations in r3.
Clear phase inversion of the Sawtooth-like
oscillations is observed both in 12-channal and
20-channal bolometers (as it is seen from Fig. 9). In
spite of different conditions, i.e., low temperature and
partially ionized plasma in RF conditioning discharges
of U-2M in comparison with hot plasma of TJ-II [2] and
W7-X [3], a qualitative similarity of the oscillations
behavior was observed. Strong dependence on the
magnetic configuration indicates that observed
phenomena can have similar origin in different plasma
conditions.
Fig. 9. Signals from 20-channals bolometer r3
SUMMARY AND CONCLUSIONS
An appearance of various 1…20 kHz oscillations
was observed in URAGAN-2M (U-2M) low magnetic
field (B=0.01 T) discharges. New measurements by
modified diagnostics show, that the plasma beta
parameter is rather high in these discharges in spite of
the low temperature and density. Variation of the
magnetic configuration of U-2M substantially modifies
the features of the oscillations (amplitude, frequency,
mode type) in the plasma of such discharges. Rotated
m = 2 mode is transformed into the Sawtooth-like
oscillations in different magnetic configurations.
ISSN 1562-6016. ВАНТ. 2018. №6(118) 45
ACKNOWLEDGEMENTS
The authors thank the U-2M Team and especially to
A.V. Lozin, and V.E. Moiseenko for providing the
U-2M experimental conditions. This work has been
supported in part by National Academy Science of
Ukraine Project № X-4-1/2016.
REFERENCES
1. E. Sallander et al. // Nucl. Fusion. 2000, v. 40,
p. 1499.
2. T. Estrada et al. // Plasma Phys. Control. Fusion.
2005, v. 47, p. L57.
3. H.P. Laqua et al. // Proc. of 44th EPS Conference on
Plasma Physics. 2017, O2, p. 101.
4. G.P. Glazunov et al. // Problems of Atomic Science
and Technology. Ser. “Plasma Physics”.2012, № 6(82),
p. 117.
5. M.B. Dreval et al. // Fusion Engineering and Design
2018, v. 129, p. 345.
6. M.B. Dreval et al. // Problems of Atomic Science and
Technology. Ser. “Plasma Physics”. 2016, № 6(22),
p. 15.
Article received 15.10.2018
НИЗКОЧАСТОТНЫЕ ОСЦИЛЛЯЦИИ В ЧИСТЯЩИХ ВЧ-РАЗРЯДАХ У-2М
М.Б. Древаль, А.Н. Шаповал, А.А. Белецкий, Р.О. Павличенко, Ф.И. Ожерельев, М.М. Махов
МГД-осцилляции плазмы изучались в высокочастотных разрядах стелларатора У-2М при малом
магнитном поле (B0 = 0,01 Tл). Когерентные осцилляции наблюдаются болометрами, тройным
ленгмюровским зондом, СВЧ-интерферометром (140 ГГц) и магнитными зондами в различных
тороидальных сечениях У-2М. Обновленные диагностики показывают, что β-параметр (β = nT/(μ0B2))
достаточно высок в этих разрядах, несмотря на низкие температуру (Te = 10…50 эВ) и плотность
(ne = (0,5…1)∙1012 cм-3). Благодаря низкому магнитному полю вариация магнитной конфигурации У-2М
значительно изменяет амплитуду флуктуаций, тип моды и частоту. Вращающаяся мода m = 2
трансформируется в пилообразные колебания в различных магнитных конфигурациях. Сильная зависимость
от магнитной конфигурации указывает, что наблюдаемый на У-2М феномен может иметь схожую природу с
осцилляциями в горячей плазме стеллараторов.
НИЗЬКОЧАСТОТНІ ОСЦИЛЯЦІЇ В ЧИСТЯЧИХ ВЧ-РОЗРЯДАХ У-2М
М.Б. Древаль, А.М. Шаповал, О.О. Белецький, Р.О. Павліченко, Ф.І. Ожерел'єв, М.М. Махов
МГД-осциляції плазми вивчалися у високочастотних розрядах стеларатора У-2М при малому магнітному
полі (B0 = 0,01 Tл). Когерентні осциляції спостерігаються болометрами, потрійним ленгмюрівським зондом,
НВЧ-інтерферометром (140 ГГц) і магнітними зондами в різних тороїдальних перерізах У-2М. Оновлені
діагностики показують, що β плазми (β = nT/(μ0B2)) досить висока в цих розрядах завдяки низькому
магнітному полю, незважаючи на низькі температуру (Te = 10…50 еВ) і густину (ne = (0,5…1)∙1012 cм-3).
Варіювання магнітної конфігурації У-2М значно змінює амплітуду флуктуацій, тип моди і частоту. Мода
m = 2, що обертається, трансформується в пилкоподібні коливання в різних магнітних конфігураціях.
Сильна залежність від магнітної конфігурації вказує, що спостережуваний на У-2М ефект може мати схожу
природу з процесами, що і аналогічна поведінка осциляцій в гарячій плазмі стелараторів.
|