Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off
For the past decades the microwave radiometry is a routinely used diagnostics to obtain the information on temporal evolution and radial profile of the bulk electron temperature at Uragan-3M torsatron plasma experiments. However, in the case of low plasma density operation we observe the high leve...
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irk-123456789-1154722017-04-06T03:02:28Z Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off Zamanov, N.V. Pavlichenko, R.O. Kulaga, A.E. Plasma diagnostics For the past decades the microwave radiometry is a routinely used diagnostics to obtain the information on temporal evolution and radial profile of the bulk electron temperature at Uragan-3M torsatron plasma experiments. However, in the case of low plasma density operation we observe the high level of electron cyclotron emission at the frequencies that match the second and third harmonics of the extraordinary mode after RF heating pulse off. This effect could be explained with the production of the suprathermal electrons. Study of the behavior suprathermal electrons is of great importance because: (a) suprathermal electrons significantly distort or make it impossible to measure the thermal electron temperature; (b) such electrons influence the ionization process, the excitation of the plasma ions and may lead to the occurrence of several plasma instabilities. The present work describes the results of experimental studies of the behavior of the emission after turning off the RF heat pulse В течение последних десятилетий микроволновая радиометрия обычно используемая диагностика для получения информации о временной эволюции и радиальном профиле температуры электронов во время плазменных экспериментов на торсатроне Ураган-3М. Тем не менее, в случае низкоплотной плазмы при помощи этой диагностики наблюдается появление излучения после отключения импульса высокочастотного нагрева. Этот эффект можно объяснить существованием надтепловых электронов. Изучение поведения надтепловых электронов важно, поскольку: (а) надтепловые электроны существенно искажают или делают невозможным измерения электронной температуры; (б) такие электроны влияют на процессы ионизации, возбуждения ионов плазмы и могут приводить к возникновению целого ряда неустойчивостей. Описываются результаты экспериментальных исследований поведения излучения после выключения импульса ВЧ-нагрева. Протягом останніх десятиліть мікрохвильова радіометрія є діагностикою, що зазвичай використовується для отримання інформації про часову еволюцію та радіальний профіль температури електронів під час плазмових експериментів на торсатроні Ураган-3М. Тим не менш, у випадку низькощільної плазми за допомогою цієї діагностики спостерігається поява випромінювання після відключення імпульсу високочастотного нагріву. Цей ефект можна пояснити існуванням надтеплових електронів. Вивчення поведінки надтеплових електронів важливо, оскільки: (а) надтеплові електрони істотно спотворюють або унеможливлюють вимірювання електронної температури; (б) такі електрони впливають на процеси іонізації, збудження іонів плазми і можуть призводити до виникнення цілого ряду нестійкостей. Описуються результати експериментальних досліджень поведінки випромінювання після вимкнення імпульсу ВЧ-нагріву. 2016 Article Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off / N.V. Zamanov, R.O. Pavlichenko, A.E. Kulaga // Вопросы атомной науки и техники. — 2016. — № 6. — С. 317-320. — Бібліогр.: 7 назв. — англ. 1562-6016 PACS: 52.55.Hc, 52.70.Gw, 52.35.Hr, 52.25.Os, 42.60.Jf, 42.15.Eq http://dspace.nbuv.gov.ua/handle/123456789/115472 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 |
| collection |
DSpace DC |
| language |
English |
| topic |
Plasma diagnostics Plasma diagnostics |
| spellingShingle |
Plasma diagnostics Plasma diagnostics Zamanov, N.V. Pavlichenko, R.O. Kulaga, A.E. Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off Вопросы атомной науки и техники |
| description |
For the past decades the microwave radiometry is a routinely used diagnostics to obtain the information on
temporal evolution and radial profile of the bulk electron temperature at Uragan-3M torsatron plasma experiments.
However, in the case of low plasma density operation we observe the high level of electron cyclotron emission at the
frequencies that match the second and third harmonics of the extraordinary mode after RF heating pulse off. This
effect could be explained with the production of the suprathermal electrons. Study of the behavior suprathermal
electrons is of great importance because: (a) suprathermal electrons significantly distort or make it impossible to
measure the thermal electron temperature; (b) such electrons influence the ionization process, the excitation of the
plasma ions and may lead to the occurrence of several plasma instabilities. The present work describes the results of
experimental studies of the behavior of the emission after turning off the RF heat pulse |
| format |
Article |
| author |
Zamanov, N.V. Pavlichenko, R.O. Kulaga, A.E. |
| author_facet |
Zamanov, N.V. Pavlichenko, R.O. Kulaga, A.E. |
| author_sort |
Zamanov, N.V. |
| title |
Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off |
| title_short |
Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off |
| title_full |
Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off |
| title_fullStr |
Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off |
| title_full_unstemmed |
Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off |
| title_sort |
behavior of suprathermal electrons at the uragan-3m torsatron after rf heating off |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2016 |
| topic_facet |
Plasma diagnostics |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/115472 |
| citation_txt |
Behavior of suprathermal electrons at the Uragan-3M torsatron after RF heating off / N.V. Zamanov, R.O. Pavlichenko, A.E. Kulaga // Вопросы атомной науки и техники. — 2016. — № 6. — С. 317-320. — Бібліогр.: 7 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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AT zamanovnv behaviorofsuprathermalelectronsattheuragan3mtorsatronafterrfheatingoff AT pavlichenkoro behaviorofsuprathermalelectronsattheuragan3mtorsatronafterrfheatingoff AT kulagaae behaviorofsuprathermalelectronsattheuragan3mtorsatronafterrfheatingoff |
| first_indexed |
2025-07-08T08:50:04Z |
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| fulltext |
ISSN 1562-6016. ВАНТ. 2016. №6(106)
PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY. 2016, № 6. Series: Plasma Physics (22), p. 317-320. 317
BEHAVIOR OF SUPRATHERMAL ELECTRONS AT THE URAGAN-3M
TORSATRON AFTER RF HEATING OFF
N.V. Zamanov, R.O. Pavlichenko, A.E. Kulaga
Institute of Plasma Physics of the NSC KIPT, Kharkov, Ukraine
E-mail: zamanov@kipt.kharkov.ua
For the past decades the microwave radiometry is a routinely used diagnostics to obtain the information on
temporal evolution and radial profile of the bulk electron temperature at Uragan-3M torsatron plasma experiments.
However, in the case of low plasma density operation we observe the high level of electron cyclotron emission at the
frequencies that match the second and third harmonics of the extraordinary mode after RF heating pulse off. This
effect could be explained with the production of the suprathermal electrons. Study of the behavior suprathermal
electrons is of great importance because: (a) suprathermal electrons significantly distort or make it impossible to
measure the thermal electron temperature; (b) such electrons influence the ionization process, the excitation of the
plasma ions and may lead to the occurrence of several plasma instabilities. The present work describes the results of
experimental studies of the behavior of the emission after turning off the RF heat pulse.
PACS: 52.55.Hc, 52.70.Gw, 52.35.Hr, 52.25.Os, 42.60.Jf, 42.15.Eq
INTRODUCTION
The presence of ECE afterglow which was obtained
at the second harmonic 2ωce under certain operating
conditions torsatron Uragan-3M (U-3M) is known for a
long time [1]. However, other groups of diagnostics,
installed at U-3M, could not observed such phenomena
as seen by ECE diagnostics signals, thereby hampering
the interpretation of this. Installation of a new
radiometer, to receive radiation at frequencies that
correspond to the third harmonic 3ωce, helped to
confirm the existence of afterglow effect (Fig. 1). The
time trace of the received radiation is identical to the
standard radiometer signal (2ωce). It has been proposed
the presence of suprathermal electron beam. The
question of occurrence of suprathermal electrons in the
U-3M and the impact such electrons on the modification
of the distribution function, and hence, impact on the
definition of "thermal" electrons temperature Te arose
[2-4]. This work includes the first results of a detailed
study of the behavior of suprathermal electron radiation.
EXPERIMENTAL CONDITIONS
Uragan-3M is a l=3, m=9 small size torsatron with
major radius, R=1 m average plasma radius apl=0.12 m
and toroidal magnetic field B0=0.72 T. The whole
magnetic system is enclosed into large five meters
diameter (volume of V=70 m
3
) vacuum tank, so that an
open natural helical divertor is realized.
Current to the magnetic field coils is supplied in a
pulsed mode. Magnetic field pulse duration was about
5s. Wherein, pulse has the following parameters: pulse
raise time, pulse fall times are of the order of ~1 s, pulse
width ~3s with at least flat top pulse time of ~2 s.
Fig. 1. Electron cyclotron emission at the frequencies
that match the second and third harmonics of the
extraordinary mode after RF heating pulse off
Producing and plasma heating was carried out by RF
method, wherein the power is introduced into the
confinement volume at the frequencies (8.6...8.8) MHz,
close to the Alfvén resonance frequency ω ≤ ωci (on a
stationary part of the magnetic field pulse). The pulse
duration of the RF power was varied in the range
(50…70) ms. Input RF power in confinement volume
was carried out using two antennas located on the part
of a low magnetic field. In the presented experiments,
the value of the input power reached 140 kW. The
working gas is hydrogen.
ECE RADIOMETRY SYSTEM
Electron cyclotron emission (ECE) diagnostics is a
standard tool that routinely used for electron
temperature profile measurement of high temperature Te
plasmas at U-3M. The advantage of this method lies in
∙
318 ISSN 1562-6016. ВАНТ. 2016. №6(106)
the fact that in inhomogeneous magnetic field, region
of emission is close to the resonant surfaces (Fig. 2) and
the local temperature can be calculated with good
spatial resolution.
Fig. 2. Radial distribution of the first three harmonics of
the electron cyclotron frequencies for the central
magnetic field 0.72 T, operational frequencies for the
second and third harmonics depicted as filled circles
(upper); and Poincaré plot of the corresponding
poloidal cross-section U-3M magnetic fluxes (lower)
At the U-3M ECE diagnostics utilize the emission
from the plasma at frequencies, which corresponds to
the second 2ωce and the third 3ωce harmonics X-mode
extraordinary wave. Conical horn antenna is oriented
perpendicular to the magnetic field lines and is located
on the part of a low magnetic field [5].
The frequency range (33…37) GHz for 2ωce and
(57…75) GHz for 3ωce was chosen according to the
value of the toroidal magnetic field of (0.68…0.72) T.
Intensity emission of thermal electron depends on
the density, temperature and the average optical depth
(τavg) of the plasma: I(ECE)∝ne Te τavg. Assuming
parabolic density profile, with an average plasma
density =1.2·10
18
m
-3
, optical depth of the plasma
τavg~0.4. On this basis, it is possible to estimate value of
the electron temperature in the plasma column center:
Te0~0.5 keV.
RADIATION OF THE SUPRATHERMAL
ELECTRONS AT THE URAGAN-3M
TORSATRON AFTER RF HEATING OFF
During previous experiments afterglow effect (see
Fig. 1.) suggest that main driver of this phenomena is
electric field [6], which produced by time varied
magnetic field during plasma discharge. The operational
time was extended to full time of the major magnetic
pulse (5 s).
+
Fig. 3. Maximum intensity of the ECE observed on the
rising edge of the magnetic field pulse
It was found that maximum intensity of the ECE
observed at the rising edge of the magnetic field pulse
(Fig. 3). For this condition electrons go over to a state of
continuous acceleration since their dynamic friction
force is less than the force exerted by the electric field,
arising due to temporal changes of the magnetic field.
In the case of low operating pressure pH2<3·10
-6
Torr
the level of radiation intensity increases and registered
during full magnetic field pulse length. With an
operating pressure close to pH2=1.04·10
-5
Torr intensity
bursts are observed only at the magnetic field ramp up,
ramp down phases, and a short period after the RF-pulse
off (supplied at the stationary part of the magnetic
field).
Fig. 4. Time dependences of the hydrogen pressure in
the vacuum chamber during and after RF-pulse in the
RF heating mode
The temporal behavior of the working gas pressure
let us utilize the model which imply that occurrence of
the suprathermal electrons, could be described as
follows. At the active phase of the plasma discharge
(during RF-pulse) the pressure of the working gas are
falls and then inertially recovers to the initial level [7]
(Fig. 4) The pressure (electron density) cross
RF-pulse
∙
ISSN 1562-6016. ВАНТ. 2016. №6(106) 319
threshold level which is sufficient for initiation of the
electrons to accelerate and its emission is clearly
observed.
Fig. 5. Experiment of the shift the RF heating pulse to
the beginning of decay stage. Here are shown signals of
different groups of diagnostics
To investigate of this dependence on varied
magnetic field we deliberately shift the RF heating pulse
startup time to the beginning of decay stage (Fig. 5).
Thus the parallel electric field will be significantly
increased, which led to a stronger flow suprathermal
electrons and will finally to significant increasing in the
afterglow radiation, observed by ECE diagnostic.
Moreover finally it was found a clear correlation in the
behavior of signals from ECE detectors
(36.5…75) GHz, 140 GHz interferometer (line electron
plasma density, ), soft and hard X-ray (SXR+HXR)
emission, spectroscopy Hα and plasma current Itor
(Rogowski coil).
The temporal traces of the different diagnostics
show clear correlated peaks which could attribute as the
impact of the suprathermal electrons on construction
elements with subsequent ionization ejected impurities.
The effect of working gas ionization by the
suprathermal electrons has been registered as well
(Fig. 6).
Fig. 6. Detailed analysis of the data leads to the conclusion about the interaction of the suprathermal electron beam
with elements of construction (red dot); and of the ionization of the working gas (green dot)
CONCLUSIONS
In torsatron U-3M continued detailed experimental
study of driving of the suprathermal electrons after turn-
off RF-pulse, for the low plasma density case
=(0.5…1.2)·10
18
m
-3
and different initial pressure
values pH
2
=(0.85…1)·10
-5
Torr, shows clear and
simultaneous response at the main plasma parameters.
The main driver for the suprathermal electrons
appearing is the time variation of the local magnetic
field, which led to increasing of the accelerating electric
field.
This phenomenon indicates that the suprathermal
electrons in torsatron U-3M can play an active role in
the breakdown of the working gas in the containment
region (inside the helical coils).
It shown that the presence of suprathermal electrons
may lead to their interaction with the elements of
construction torsatron and additional ionization. Finally
we have to point out about the drawback of such
operational scenario. It is almost impossible to avoid
that undesirable high level HXR radiation which is
attributed with interaction of the high energetic particles
with inner vessel mechanical components.
To assess the value of ECE depending on the
prehistory of the rise of the magnetic field, further
detailed studies of suprathermal electrons will be held in
torsatron U-3M on the magnetic field phase of growth.
ACKNOWLEDGEMENTS
The authors thank the technical staff torsatron
U-3M, ensuring reliable operation of the device. Also
individually thank to V.G. Konovalov, M.N. Makhov,
Yu.K. Mironov, V.K. Pashnev, A.A. Petrushenya,
V.S. Romanov, A.N. Shapoval, E.L. Sorokovoy,
I.K. Tarasov for providing corresponding experimental
data and I.M. Pankratov for fruitful discussions.
320 ISSN 1562-6016. ВАНТ. 2016. №6(106)
REFERENCES
1. V.S. Voitsenya et al. Progress in stellarator research
in Kharkov IPP // Physica Scripta. 2014, v. T161,
p. 014009.
2. T. Kudyakov et al. Spatially and temporally resolved
measurements of runaway electrons in the TEXTOR
tokamak // Nuclear Fusion. 2008, v. 48, p. 122002.
3. P. Helander et al. Runaway acceleration during
magnetic reconnection in tokamaks // Plasma Physics
and Controlled Fusion. 2002, v. 44, p. B247-B262.
4. V.V. Parail et al. Diffusion of runaway electrons in a
tokamak // Nuclear Fusion. 1978, v. 18, p. 9371982.
5. R.O. Pavlichenko et al. Peculiarities of the
radiometric measurements on Uragan-3M torsatron for
RF heated plasma // Problems of Atomic Science and
Technology. Series “Plasma Physics”. 2011, № 1,
p. 191-193.
6. R.O. Pavlichenko. Influence of suprathermal
electrons on ECE measurements in the URAGAN-3M
torsatron // Problems of Atomic Science and
Technology. Series “Plasma Physics”. 2015, № 1,
p. 293-296.
7. V.K. Pashnev, A.A. Petrushenya et al. Hydrogen
recycling during rf plasma heating in the U-3M
torsatron // Problems of Atomic Science and
Technology. Series “Plasma Physics”. 2014, № 6,
p. 272-274.
Article received 26.10.2016
ПОВЕДЕНИЕ ИЗЛУЧЕНИЯ НАДТЕПЛОВЫХ ЭЛЕКТРОНОВ
В ТОРСАТРОНЕ УРАГАН-3М ПОСЛЕ ВЫКЛЮЧЕНИЯ НАГРЕВА
Н.В. Заманов, Р.О. Павличенко, А.Е. Кулага
В течение последних десятилетий микроволновая радиометрия обычно используемая диагностика для
получения информации о временной эволюции и радиальном профиле температуры электронов во время
плазменных экспериментов на торсатроне Ураган-3М. Тем не менее, в случае низкоплотной плазмы при
помощи этой диагностики наблюдается появление излучения после отключения импульса высокочастотного
нагрева. Этот эффект можно объяснить существованием надтепловых электронов. Изучение поведения
надтепловых электронов важно, поскольку: (а) надтепловые электроны существенно искажают или делают
невозможным измерения электронной температуры; (б) такие электроны влияют на процессы ионизации,
возбуждения ионов плазмы и могут приводить к возникновению целого ряда неустойчивостей.
Описываются результаты экспериментальных исследований поведения излучения после выключения
импульса ВЧ-нагрева.
ПОВЕДІНКА ВИПРОМІНЮВАННЯ НАДТЕПЛОВИХ ЕЛЕКТРОНІВ
У ТОРСАТРОНІ УРАГАН-3М ПІСЛЯ ВИМКНЕННЯ НАГРІВУ
М.В. Заманов, Р.О. Павліченко, А.Є. Кулага
Протягом останніх десятиліть мікрохвильова радіометрія є діагностикою, що зазвичай використовується
для отримання інформації про часову еволюцію та радіальний профіль температури електронів під час
плазмових експериментів на торсатроні Ураган-3М. Тим не менш, у випадку низькощільної плазми за
допомогою цієї діагностики спостерігається поява випромінювання після відключення імпульсу
високочастотного нагріву. Цей ефект можна пояснити існуванням надтеплових електронів. Вивчення
поведінки надтеплових електронів важливо, оскільки: (а) надтеплові електрони істотно спотворюють або
унеможливлюють вимірювання електронної температури; (б) такі електрони впливають на процеси іонізації,
збудження іонів плазми і можуть призводити до виникнення цілого ряду нестійкостей. Описуються
результати експериментальних досліджень поведінки випромінювання після вимкнення імпульсу
ВЧ-нагріву.
|