Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron
Dependences of the charge exchange (CX) fluxes of neutral are investigated via neutral particle analyzers (NPA) in the U-3M torsatron. Fast (≤0.5 ms) decay of the vertical and tangential CX fluxes has been observed after turning off RF heating power. According to these measurements, the U-3M energy...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2012
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irk-123456789-1090832016-11-21T03:02:07Z Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron Dreval, M. Slavnyj, A.S. Магнитное удержание Dependences of the charge exchange (CX) fluxes of neutral are investigated via neutral particle analyzers (NPA) in the U-3M torsatron. Fast (≤0.5 ms) decay of the vertical and tangential CX fluxes has been observed after turning off RF heating power. According to these measurements, the U-3M energy confinement time of the 0.5…4.5 keV ions is less than 0.5 ms in the low density (ne=(1…4)·10¹² cm⁻³) discharges. No difference between confinement of the ion energy component parallel to the magnetic field and confinement of the perpendicular to the magnetic field one was observed in U-3M. Evidently, an ion cooling through CX collisions with neutrals sustain the main channel of the 0.5…4.5 keV ion energy loss in the U-3M torsatron. Зависимости потоков нейтралов перезарядки измерены с помощью анализаторов нейтральных частиц в торсатроне У-3М. Быстрое (≤0.5 мс) время спада вертикального и тангенциального потоков нейтралов перезарядки наблюдалось после выключения мощности ВЧ-нагрева. Согласно этим измерениям время удержания энергии 0.5…4.5 кэВ ионов в У-3М меньше, чем 0.5 мс в низкоплотных (ne=(1…4)·10¹² cm⁻³) разрядах. В У-3М не было обнаружено разницы между удержанием компонентов ионной энергии параллельной и перпендикулярной магнитному полю. По-видимому, остывание ионов посредством актов перезарядки с нейтралами является основным каналом потери энергии 0.5…4.5 кэВ ионов в торсатроне У-3М. Залежності потоків нейтралів перезарядки виміряні за допомогою аналізаторів нейтральних частинок у торсатроні У-3М. Швидкий (≤0.5 мс) час спаду вертикального і тангенціального потоків нейтралів перезарядки спостерігався після виключення потужності ВЧ-нагріву. Згідно цим вимірам час утримання енергії 0.5…4.5 кеВ іонів в У-3М менший, ніж 0.5 мс у розрядах з малою густиною (ne = (1…4)·10¹² cm⁻³). B У-3М не було виявлено різниці між утриманням компонент іонної енергії паралельної та перпендикулярної магнітному полю. Мабуть охолодження іонів за допомогою актів перезарядки з нейтралами є основним каналом втрати енергії 0.5…4.5 кеВ іонів у торсатроні У-3М. 2012 Article Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron / M. Dreval, A.S. Slavnyj // Вопросы атомной науки и техники. — 2012. — № 6. — С. 13-15. — Бібліогр.: 9 назв. — англ. 1562-6016 PACS: 52.55.Hc; 52.70.Nc http://dspace.nbuv.gov.ua/handle/123456789/109083 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Магнитное удержание Магнитное удержание |
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Магнитное удержание Магнитное удержание Dreval, M. Slavnyj, A.S. Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron Вопросы атомной науки и техники |
| description |
Dependences of the charge exchange (CX) fluxes of neutral are investigated via neutral particle analyzers (NPA) in the U-3M torsatron. Fast (≤0.5 ms) decay of the vertical and tangential CX fluxes has been observed after turning off RF heating power. According to these measurements, the U-3M energy confinement time of the 0.5…4.5 keV ions is less than 0.5 ms in the low density (ne=(1…4)·10¹² cm⁻³) discharges. No difference between confinement of the ion energy component parallel to the magnetic field and confinement of the perpendicular to the magnetic field one was observed in U-3M. Evidently, an ion cooling through CX collisions with neutrals sustain the main channel of the 0.5…4.5 keV ion energy loss in the U-3M torsatron. |
| format |
Article |
| author |
Dreval, M. Slavnyj, A.S. |
| author_facet |
Dreval, M. Slavnyj, A.S. |
| author_sort |
Dreval, M. |
| title |
Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron |
| title_short |
Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron |
| title_full |
Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron |
| title_fullStr |
Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron |
| title_full_unstemmed |
Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron |
| title_sort |
confinement of the 0.5…4.5 kev plasma ions in low density discharges of the u-3m torsatron |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2012 |
| topic_facet |
Магнитное удержание |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/109083 |
| citation_txt |
Confinement of the 0.5…4.5 keV plasma ions in low density discharges of the U-3M torsatron / M. Dreval, A.S. Slavnyj // Вопросы атомной науки и техники. — 2012. — № 6. — С. 13-15. — Бібліогр.: 9 назв. — англ. |
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2025-07-07T22:32:49Z |
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ISSN 1562-6016. ВАНТ. 2012. №6(82) 13
CONFINEMENT OF THE 0.5…4.5 keV PLASMA IONS IN LOW DENSITY
DISCHARGES OF THE U-3M TORSATRON
M. Dreval, A.S. Slavnyj
Institute of Plasma Physics NSC “Kharkov Institute of Physics and Technology”,
Kharkov, Ukraine
Dependences of the charge exchange (CX) fluxes of neutral are investigated via neutral particle analyzers (NPA)
in the U-3M torsatron. Fast (≤0.5 ms) decay of the vertical and tangential CX fluxes has been observed after turning
off RF heating power. According to these measurements, the U-3M energy confinement time of the 0.5…4.5 keV
ions is less than 0.5 ms in the low density (ne=(1…4)·1012 cm-3) discharges. No difference between confinement of
the ion energy component parallel to the magnetic field and confinement of the perpendicular to the magnetic field
one was observed in U-3M. Evidently, an ion cooling through CX collisions with neutrals sustain the main channel
of the 0.5…4.5 keV ion energy loss in the U-3M torsatron.
PACS: 52.55.Hc; 52.70.Nc
INTRODUCTION
Neutral atoms and molecules substantially affect
plasma confinement in small and medium size fusion
devices. Neutrals can introduce plasma cooling and
additional viscosity which dominates the neo-classical
ion viscosity in the gas fueling region [1, 2].
Furthermore, according to those studies, the neutral flux
can modify or even determine the edge radial electric
field, plasma rotation and plasma confinement. An
influence of neutrals can determines ion energy
confinement in the small-size torsatron U-3M. The
U-3M plasma volume is about 0.3 m3 and its chamber
volume is about 70 m3. The hydrogen pressure feeding
level used in U-3M discharges is about 10-5 Torr. In the
case when all hydrogen will be ionized and accumulate
in the confinement region the plasma density should
achieve a value about 1014 cm-3. This density is two
orders of magnitude higher than practically achieved
densities in U-3M discharges. Evidently, substantial
flux of the neutrals with mean free path longer than
U-3M plasma size from large U-3M vacuum vessel
should sustain main channel of the ion energy loss via
the charge exchange (CX) collisions in the low density
discharges of U-3M. According to set of previous
articles [3-6], confinement of fast ions in U-3M was
considered as collisionless. The electron-ion and ion-ion
collision times are longer than the neoclassical
confinement time of these ions, thus the confinement of
the 0.5…4.5 keV ions is collisionless, if we are not
taking into account CX collisions. Due to substantial
difference in confinement of trapped and passing ions
the confinement time of energetic ions with major part
of the energy perpendicular to the magnetic field lines is
significantly shorter than the confinement time of the
ions with major part of the energy parallel to the
magnetic field in the collisionless stellarator plasma. We
can estimate the role of the CX ions cooling by the
comparison of the parallel ion energy and perpendicular
ion energy confinement times. If these times are
comparable, then CX cooling is dominant channel of the
ion energy loss and U-3M confinement cannot be
considered as the collisionless.
In this paper, the evidence of extremely short
parallel ion energy and perpendicular ion energy
confinement times is shown in low density U-3M
discharges. These times are shorter that temporal
resolution of the NPAs and less than 0.5 ms.
1. EXPERIMENTAL SET-UP
“Uragan-3M” is a small size torsatron with l/m=3/9,
R0=1 m major radius, ā≈0.12 m average plasma radius
and toroidal magnetic field B0≤1 T. The whole magnetic
system is enclosed into 5 m diameter vacuum tank. The
low density discharges (ne=(1…4)·1012 cm−3) under
consideration are induced by RF antennas [7, 8]. The
U-3M is equipped with two CX neutral particle
analyzers (NPA) [8]. Magnetic mass separators,
reflection dumpers have been added in present NPAs
set-up in contrast to the used in previous work [8]. A
tangential neutral flux from almost whole U-3M cross-
section is measured by one of the NPA for parallel ion
energy studies. The perpendicular ion energy
distribution is measured along the vertical lines by the
other NPA. A radial shift of the NPA line of sight was
varying from the inner conductor wall R=85…86 cm to
the geometrical centre of the U-3M winding of
R=100…101 cm, corresponding to the half-plasma
radius ρ≈0.5 [8]. Due to strongly three-dimensional
geometry of the torsatron magnetic field lines, parallel
and perpendicular CX fluxes are coupled. However,
present set-up is reasonable for qualitative comparison
of these fluxes behaviors. The NPA signals integration
time is 0.1 ms, and its sampling rate is about 50 kS/s.
2. NPA SIGNALS IN OLD NPA SET-UP
In previously used NPA set-up, long enough decay
of measured signal was observed after turning the RF
power off. This long decay was improperly qualified as
a long enough confinement time of the energetic ions
[4, 7]. An example of such CX flux decay in old set-up
in the sweeping NPA mode of operations [8] is shown
in Fig.1. The 0.1 ms integration time was used in this
experiments [8] in contrast to results of Ref. [4, 7]
where long integration time in addition to NPA
diagnostic error cause the long confinement time. Due
to extremely low cross-section of the hydrogen atoms
striping by the nitrogen in a energy range below 100 eV
[8], the NPA signal in minima of the analyzing energies
should be equal to zero.
14 ISSN 1562-6016. ВАНТ. 2012. №6(82)
Fig. 1. NPA signal (top frame) and analising energy
(bottom frame) in old NPA set-up. Dashed line marks
the RF generator switching off time
Observed in these minima NPA signal is not related
to the CX flux and represent an NPA diagnostic error.
During U-3M discharge a level of the NPA signal in
energy minima is low and this level is order of
magnitude lower the NPA signal level in medium
energies. It allows us to analyze CX energy distributions
in the main U-3M discharge [8]. In contrast to the main
discharge the signal after RF power switching off is not
correlated with the analyzing energy and its level is high
in the zero energy stage. This is direct evidence that the
NPA signal after RF switching off was not related with
the CX flux in pervious NPA set-up.
3. CX FLUXES DECAY
Vertical CX flux decays in present NPA set-up and
NPA mode with fixed energy, are shown in Fig. 2.
Fig. 2. Normalized CX fluxes from plasma periphery
and geometrical chamber centre (top frames); NPA line
of sight positions and the energy of CX flux are marked
in legends. RF power (bottom frames)
Such fast decay of the CX flux is always observed in
all low density discharge conditions and all CX energies
(0.4…4.5 keV) and in all NPA line of sight positions
(ρ=0.5…1.2 ). Same fast decay of the parallel to the
magnetic field CX flux was observed in all discharge
conditions and for all measurable CX energies. The
675 eV CX flux decay in discharges with different
magnetic fields is demonstrate (Fig. 3) this fast decay.
Fig. 3. Tangential 675 eV CX fluxes in discharges with
different magnetic field; doted line is marked end of RF
A level of parallel CX flux is at least order of magnitude
lower than the level of the perpendicular one. It explains
a noisy character of the observed signal in 0.1 ms
integration time set-up, as well as non-systematic noise
due to the NPA photomultipliers noise. Absence of any
correlations of the parallel NPA signal bursts after the
decay stage in set of similar U-3M pulses demonstrates,
that these bursts can-not be associated with CX flux.
The CX flux from a plasma
∫=Γ dlzREREcx ),,(),( ξ depends on molecular and
atomic hydrogen concentrations n0 and nH2 respectively:
( )2
0 1
0( ) ( ) ( ) ,i cx H cxn E n E n Eξ σ σ≈ + where 0
cxσ , 1
cxσ are
cross sections of the hydrogen ion charge exchange on
hydrogen atom and molecule respectively. In general,
CX flux decay can be determined by ions cooling and
by a modification of the concentration of the neutrals
(conversion to molecules in particular). Energy
dependence of 0
cxσ , 1
cxσ [9] and ratio of this cross-
sections are shown in Fig. 4.
Fig. 4. Atomic and molecular CX cross-sections and
their ratio (in a central inset)
According to these data, the ratio of atomic and
molecular cross-sections is order of magnitude lower at
225 eV than at 2.25 keV. If substantial modification of
ISSN 1562-6016. ВАНТ. 2012. №6(82) 15
the molecular and atomic concentrations ratio takes
place during the decay stage of the discharge, then
decay times of CX flux of these two energies should be
significantly different. Experimental comparison of the
CX fluxes under consideration (Fig. 5) does clearly
show that no difference in their decay times is
recognizable.
Fig. 5. Normalized 225 eV and 2.25 keV CX fluxes (top
frames); RF power (bottom signal)
Therefore, the ratio of the molecular and atomic
concentrations is constant during the 0.5 ms decay time
of the CX fluxes and the decay is caused by the fast ion
cooling.
Thus, the decay time of perpendicular (ρ=0.5…1)
and tangential (from almost whole cross-section) CX
fluxes after switching off RF power is shorter that
0.5 ms. The U-3M energy confinement time of the
0.5…4.5 keV ions is less than 0.5 ms in the low density
(ne=(1…4)·1012 cm-3) discharges. This time is
substantially shorter than the energy confinement time
according to U-3M stellarator scaling [7]. Evidently, an
ion cooling through CX collisions with neutrals sustain
the main channel of the 0.5…4.5 keV ion energy loss in
the U-3M torsatron.
ACKNOWLEDGEMENTS
The authors thank the U-3M Team for cooperation.
REFERENCES
1. T. Fülöp, P. Helander, and P.J. Catto // Phys. Rev.
Lett. 2002, v. 89, p. 225003.
2. B.A. Carreras, P.H. Diamond, and G. Vetoulis //
Phys. Plasmas. 1996, v. 3, p. 4106.
3. I.M. Pankratov et al. // Contrib. Plasma Phys. 2010,
v. 50, p. 520.
4. V.V. Chechkin et al. // Plasma Devices Ops. 2008,
v. 16, p. 299.
5. V.V. Chechkin et al. // Plasma Phys. Control. Fus.
2006, v. 48, p. A241.
6. V.V. Chechkin et al // Nucl. Fusion. 2003, v. 43,
p. 1175.
7. V.E. Moiseenko et al // Nucl. Fusion. 2011, v. 51,
p.83036.
8. M. Dreval and A.S. Slavnyj // Plasma Phys. Control.
Fusion. 2011, v. 53, p. 065014.
9. Data from NIFS DATABASE, http://dbshino.nifs.ac.jp.
Article received 16.10.12
УДЕРЖАНИЕ ИОНОВ ПЛАЗМЫ 0.5…4.5 кэВ В МАЛОПЛОТНЫХ РАЗРЯДАХ
ТОРСАТРОНА У-3М
М. Древаль, А.С. Славный
Зависимости потоков нейтралов перезарядки измерены с помощью анализаторов нейтральных частиц в
торсатроне У-3М. Быстрое (≤0.5 мс) время спада вертикального и тангенциального потоков нейтралов
перезарядки наблюдалось после выключения мощности ВЧ-нагрева. Согласно этим измерениям время
удержания энергии 0.5…4.5 кэВ ионов в У-3М меньше, чем 0.5 мс в низкоплотных (ne=(1…4)·1012 cm-3)
разрядах. В У-3М не было обнаружено разницы между удержанием компонентов ионной энергии
параллельной и перпендикулярной магнитному полю. По-видимому, остывание ионов посредством актов
перезарядки с нейтралами является основным каналом потери энергии 0.5…4.5 кэВ ионов в торсатроне
У-3М.
УТРИМАННЯ ІОНІВ ПЛАЗМИ 0.5…4.5 кеВ У РОЗРЯДАХ ТОРСАТРОНА У-3М
З МАЛОЮ ГУСТИНОЮ
М. Древаль, А.С. Славний
Залежності потоків нейтралів перезарядки виміряні за допомогою аналізаторів нейтральних частинок у
торсатроні У-3М. Швидкий (≤0.5 мс) час спаду вертикального і тангенціального потоків нейтралів
перезарядки спостерігався після виключення потужності ВЧ-нагріву. Згідно цим вимірам час утримання
енергії 0.5…4.5 кеВ іонів в У-3М менший, ніж 0.5 мс у розрядах з малою густиною (ne = (1…4)·1012cm-3). B
У-3М не було виявлено різниці між утриманням компонент іонної енергії паралельної та перпендикулярної
магнітному полю. Мабуть охолодження іонів за допомогою актів перезарядки з нейтралами є основним
каналом втрати енергії 0.5…4.5 кеВ іонів у торсатроні У-3М.
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