Formation of light impurities in a hydrogen plasma at initial stage of a discharge
Analyzing the dynamics of density for atomic and molecular hydrogen ions, the values of atomic hydrogen and UV radiation fluxes to the walls of the plasma chamber were obtained, resulting in light impurities of carbon and oxygen at plasma start-up during the process of desorption from the walls unde...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
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| Цитувати: | Formation of light impurities in a hydrogen plasma at initial stage of a discharge / V.B. Yuferov, E.I. Skibenko, V.I. Tkachov, V.V. Katrechko, A.S. Svichkar // Problems of atomic science and technology. — 2019. — № 4. — С. 110-112. — Бібліогр.: 11 назв. — англ. |
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Yuferov, V.B. Skibenko, E.I. Tkachov, V.I. Katrechko, V.V. Svichkar, A.S. 2023-12-03T14:50:03Z 2023-12-03T14:50:03Z 2019 Formation of light impurities in a hydrogen plasma at initial stage of a discharge / V.B. Yuferov, E.I. Skibenko, V.I. Tkachov, V.V. Katrechko, A.S. Svichkar // Problems of atomic science and technology. — 2019. — № 4. — С. 110-112. — Бібліогр.: 11 назв. — англ. 1562-6016 PACS: 52.50.Qt, 52.55.Hc https://nasplib.isofts.kiev.ua/handle/123456789/195190 Analyzing the dynamics of density for atomic and molecular hydrogen ions, the values of atomic hydrogen and UV radiation fluxes to the walls of the plasma chamber were obtained, resulting in light impurities of carbon and oxygen at plasma start-up during the process of desorption from the walls under irradiation. The fluxes of impurity atoms associated with the fluxes of photons and hydrogen atoms in a discharge are determined. Recommendations are given to reduce the amount of impurities at the initial stage of discharge. При розгляді динаміки щільності водневих атомарних і молекулярних іонів отримані величини потоків атомарного водню і УФ-випромінювання на стінки плазмової камери, через що з'являються легкі домішки вуглецю і кисню на початковій стадії розряду в процесі десорбції зі стінок при опроміненні. Визначено величини потоків домішкових атомів, пов'язаних з потоками фотонів і атомів водню, що виникають у розряді. Надано рекомендації щодо зменшення кількості домішок на початковій стадії розряду. При рассмотрении динамики плотности водородных атомарных и молекулярных ионов получены величины потоков атомарного водорода и УФ-излучения на стенки плазменной камеры, в результате чего появляются легкие примеси углерода и кислорода на начальной стадии развития разряда в процессе десорбции со стенок при облучении. Определены величины потоков примесных атомов, связанных с потоками фотонов и атомов водорода, возникающих в разряде. Даны рекомендации по уменьшению количества примесей на начальной стадии разряда. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Gas and plasma-beam discharges and their applications Formation of light impurities in a hydrogen plasma at initial stage of a discharge Утворення легких домішок на початковій стадії розряду Образование легких примесей на начальной стадии разряда Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Formation of light impurities in a hydrogen plasma at initial stage of a discharge |
| spellingShingle |
Formation of light impurities in a hydrogen plasma at initial stage of a discharge Yuferov, V.B. Skibenko, E.I. Tkachov, V.I. Katrechko, V.V. Svichkar, A.S. Gas and plasma-beam discharges and their applications |
| title_short |
Formation of light impurities in a hydrogen plasma at initial stage of a discharge |
| title_full |
Formation of light impurities in a hydrogen plasma at initial stage of a discharge |
| title_fullStr |
Formation of light impurities in a hydrogen plasma at initial stage of a discharge |
| title_full_unstemmed |
Formation of light impurities in a hydrogen plasma at initial stage of a discharge |
| title_sort |
formation of light impurities in a hydrogen plasma at initial stage of a discharge |
| author |
Yuferov, V.B. Skibenko, E.I. Tkachov, V.I. Katrechko, V.V. Svichkar, A.S. |
| author_facet |
Yuferov, V.B. Skibenko, E.I. Tkachov, V.I. Katrechko, V.V. Svichkar, A.S. |
| topic |
Gas and plasma-beam discharges and their applications |
| topic_facet |
Gas and plasma-beam discharges and their applications |
| publishDate |
2019 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| format |
Article |
| title_alt |
Утворення легких домішок на початковій стадії розряду Образование легких примесей на начальной стадии разряда |
| description |
Analyzing the dynamics of density for atomic and molecular hydrogen ions, the values of atomic hydrogen and UV radiation fluxes to the walls of the plasma chamber were obtained, resulting in light impurities of carbon and oxygen at plasma start-up during the process of desorption from the walls under irradiation. The fluxes of impurity atoms associated with the fluxes of photons and hydrogen atoms in a discharge are determined. Recommendations are given to reduce the amount of impurities at the initial stage of discharge.
При розгляді динаміки щільності водневих атомарних і молекулярних іонів отримані величини потоків атомарного водню і УФ-випромінювання на стінки плазмової камери, через що з'являються легкі домішки вуглецю і кисню на початковій стадії розряду в процесі десорбції зі стінок при опроміненні. Визначено величини потоків домішкових атомів, пов'язаних з потоками фотонів і атомів водню, що виникають у розряді. Надано рекомендації щодо зменшення кількості домішок на початковій стадії розряду.
При рассмотрении динамики плотности водородных атомарных и молекулярных ионов получены величины потоков атомарного водорода и УФ-излучения на стенки плазменной камеры, в результате чего появляются легкие примеси углерода и кислорода на начальной стадии развития разряда в процессе десорбции со стенок при облучении. Определены величины потоков примесных атомов, связанных с потоками фотонов и атомов водорода, возникающих в разряде. Даны рекомендации по уменьшению количества примесей на начальной стадии разряда.
|
| issn |
1562-6016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/195190 |
| citation_txt |
Formation of light impurities in a hydrogen plasma at initial stage of a discharge / V.B. Yuferov, E.I. Skibenko, V.I. Tkachov, V.V. Katrechko, A.S. Svichkar // Problems of atomic science and technology. — 2019. — № 4. — С. 110-112. — Бібліогр.: 11 назв. — англ. |
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ISSN 1562-6016. ВАНТ. 2019. №4(122) 110
FORMATION OF LIGHT IMPURITIES IN A HYDROGEN PLASMA
AT INITIAL STAGE OF A DISCHARGE
V.B. Yuferov, E.I. Skibenko, V.I. Tkachov, V.V. Katrechko, A.S. Svichkar
National Science Center “Kharkov Institute of Physics and Technology”, Kharkiv, Ukraine
E-mail: v.yuferov@kipt.kharkov.ua
Analyzing the dynamics of density for atomic and molecular hydrogen ions, the values of atomic hydrogen and
UV radiation fluxes to the walls of the plasma chamber were obtained, resulting in light impurities of carbon and
oxygen at plasma start-up during the process of desorption from the walls under irradiation. The fluxes of impurity
atoms associated with the fluxes of photons and hydrogen atoms in a discharge are determined. Recommendations
are given to reduce the amount of impurities at the initial stage of discharge.
PACS: 52.50.Qt, 52.55.Hc
When considering the dynamics of the density of
hydrogen atomic and molecular ions [1], the fluxes of
atomic hydrogen and UV radiation on the walls of the
plasma chamber were obtained. In the general case, at
the initial stage of discharge, the flow of impurities into
the plasma Fz determined by the sum of the products of
various radiation fluxes Fi, falling on the wall, by the
value of the erosion coefficient for this type of radiation
Ki: Z i ii
F Fκ= ∑ .
In this case, we consider the entry of light impurities
through two processes: the supply of atomic hydrogen
and UV radiation on the wall. As can be seen from the
values of the effective cross sections of the processes,
the formation of atomic hydrogen is especially effective
for small Te, wherein the number of atoms in the elec-
tron-ion pair for the Te (Fig. 1) defined as
( ) ( )2 3 2 4 6 3 9 10 2 2 8
2 1 2 4 1 7 3 9
2 2 2 3 /
.eh
e
n n n n n ndF
dn n n n n
σ σ σ σ σ σ
σ σ σ σ
+ + +
+ +
+ + + + +
=
+ +
(1)
And as the electron plasma temperature changes from
4 to 12 eV, the number of generated hydrogen atoms
drops from ~18 to ~2 atoms (Fig. 1). The time depend-
ence of the averaged density of oxygen and carbon impu-
rities for two values of the desorption coefficient and the
electron temperature ~6 eV is shown in Fig. 2
Fig. 1. Dependence of the number of hydrogen atoms
per electron-ion pair on the temperature of electrons
at plasma density 1·1012 cm-3
1 2 2
10 5 3 ;
ph
e L H L
Z ph ph phZ
dF
n n v n v n v
dt
n v dF dF dF
α α α
σ σ σ
σ
+ = + + +
+ = + +
(2)
;
O
phO O h
ph h
dF dFdn K K
dt dt dt
= + (3)
;
C
phC C h
ph h
dF dFdn K K
dt dt dt
= + (4)
1 2 3 0C O
en n n n n n+ + + + − = . (5)
In Fig. 2 show the time dependencies of the values
nc and no for Te=6 eV, we have chosen, for the reason
that the time of generating plasma satisfy the conditions
set in [2] (0.5...1 ms).
t, µs
Fig. 2. Time dependence of the average density
of oxygen and carbon impurities for two values
of the desorption coefficient and the electron
temperature ~ 6 eV. For oxygen 1·10-3 and 2·10-3;
carbon – 1·10-2 and 2.5·10-2
SURFACE CONDITIONS THE AND THE
SELECTION OF EROSION RATIO
Selection of plausible desorption coefficient values
Kph and Kh is a very important operation, since their
value significantly depends on the composition of the
surface of the discharge chamber, which, in turn, deter-
mines the flow of impurities into the plasma. There is an
extensive literature on the values of the sputtering coef-
ficients of various materials by different ions, but there
are very few data in the area of interest to us. In [3]
shows the dependence of the coefficient of oxygen ex-
cretion Kh
o in the form of water vapor from the surface
of stainless steel when hydrogen atoms with an energy
of 0.2 eV interact with it. For temperature range
100...400°С the value lies within 3·10-3…0.2. Since in
this work there is no data on the values Kh
o for room
temperature and surface oxygen concentration, magni-
tude Kx
o was obtained by extrapolation, and the desorp-
tion cross section was estimated for the relative concen-
mailto:v.yuferov@kipt.kharkov.ua
ISSN 1562-6016. ВАНТ. 2019. №4(122) 111
tration of oxygen on the surface θ = 0.15, selected on
the basis of the data of [3 - 8] for similar experimental
conditions. Thus, the value we take Kh
o =1·10-5 for
T = 300 K and σx
o=6.6·10-18 сm-2. The value of the cross
section for chemical removal of carbon from the inconel
surface at temperature T = 470 K for the heat fluxes of
atomic hydrogen is given in [8] σh
o=2.3·10-17 сm-2 for
filling 0.05≤θ≤0.4. Extrapolation to room temperature
gives σh
c≈1·10-20 сm-2 – a value that is rather small and
therefore not taken into account, although, as in the case
of oxygen, σh
c may increase with increasing energy of
hydrogen atoms EH. The magnitudes of the photode-
sorption cross sections strongly depend on the energy of
the quanta, are maximum in the region of ultraviolet and
soft X-ray radiation and lie at level 10-17…10-20 сm-2
[1]. The measured value Kph for CO2 for quanta with
energy from 2 to 10 eV varies from 1.5·10-4 to 1·10-2
[6]. In [9] for radiation with λmin= 1300 A, Kph= 2.5·10-2
(for СО). It should be noted that during the irradiation
of oxygen- and carbon-containing surfaces simultane-
ously with electromagnetic (ultraviolet) radiation and
atomic hydrogen, the yield of carbon and oxygen from
the surfaces may increase. This, in our opinion, is due to
the fact that intermediate products of chemical reactions
are radicals CH, CH2, CH3, OH on the surface can be
desorbed by ultraviolet radiation, which will increase
the coefficients of Kh and Kph. In the calculations, the
Kph values are taken for quanta with an energy of 10.5
and 3 eV, respectively 1·10-2, 2·10-2 and 1·10-3.
The composition of the surfaces of the discharge
chambers of thermonuclear installations, according to
the data of [4 - 8], substantially depends on the working
and cleaning conditions; immediately after purification
with atomic hydrogen, the surface concentration of car-
bon, oxygen and sulfur lies at about 2...5% of each of
the elements, respectively, the concentration of the sub-
strate metal is about 85...90%. In the process of long-
term operation, the composition of the surface can vary
greatly in such a way that the concentration of carbon
increases to 30...95%, oxygen to 10...30%, metal to
1...35%. (Unfortunately, the Kh and Kh
o values for films
of this composition were not measured directly in the
discharge chambers). In addition, in the intervals be-
tween the plasma pulses, i.e. peculiar cleaning, the sur-
face composition may also change as a result of adsorp-
tion of water vapor, CO, CH4, CO2, are always present
in the discharge chamber and the divertor, and coming
from the associated volumes. According to the data of
[10, 11], we consider the composition of the atmosphere
and the filling of the surface with oxygen θ in installa-
tions (Table 1).
Table 1
Atmospheric
composition
Partial pressure, Pa
Installations
JET-2 TFR-600 ASDEX
filling of the surface with oxygen
No data 0.02 1.0 No data
HO
CH
CO
(2…3.3)·10-5
(1.3…4)·10-6
2.7·10-6…1·10-5
1.3·10-6
-
-
4·10-6
-
-
6.7·10-5
2.7·10-6
1.4·10-5
It should be noted that in JET-2 and TFR-600 Spe-
cial vacuum technologies for cleaning materials were
used, including heating the chambers to 300...400°С,
and in the case of TFR, a special initial heating of mate-
rials up to 800°С before installation of the system.
Camera installation ASDEX could be heated only to
120°С, in addition, its divertor was weakly purified dur-
ing plasma purification. Therefore, in the period be-
tween plasma pulses, as mentioned above, a weakly
coupled adsorption layer will increase on the cleaned
surfaces of the discharge chambers, the concentration of
which surf
Ln will vary:
( ) ( ) 10surf surf ds
L L i
dc
A
n n const A tp
A
θ d −= + × × , (6)
where A(θ) – coefficient of capture of particles in the
discharge chamber; Ads⁄Adc – the area ratio of the di-
vertor slits and the discharge chamber; δ – coefficient of
surface roughness; pi – pressure in the diverter chamber.
Coefficient A(θ) determined by the ratio Ads⁄Adc and the
local coefficient of adhesion (capture) of gas on the sur-
face α(θ).
For most pure metals α(θ) for room temperature, the
substrates lie at a level of 0.1...0.05 for CO, CH4, CO2;
0.5...0.2 for H2O. For metal carbides, these values are
about 0.5...0.03 [3]. Thus, in a wide range of variation
of θ, setting the pressure values Pi, characteristic of the
case [2] (Table 2), and also taking into account the ge-
ometry of the system Ads⁄Adc=5·10-2 and δ = 1, at initial
concentrations of oxygen and carbon on the surface,
equal to 5%, At initial concentrations of oxygen and
carbon on the surface, equal to no and nc for θ < 0.2:
13 125 10 3.9 10 t;O
surfn = × + × (7)
13 125 10 2.9 10 t,C
surfn = × + × (8)
where t – is the time of accumulation of impurities after
cleaning the surface before filling with θ≈0.05. As can
be seen Kh and Kph taken by us for fillings θ≈0.15, will
be achieved in a few tens of seconds, i.e. our choice is
understated, since in the case of [2] the impulses usually
go in 2...3 minutes.
Table 2
Impurity
gases H2O CO CO2 CH4
Pressure, Pa 1.33·10-5 6.6·10-6 4·10-6 4·10-6
CONCLUSIONS
Thus, as can be seen from the results of the calcula-
tions in papers [1] and [3], the concentration of light
impurities under equal conditions on the surface is de-
termined by the temperature of the plasma electrons,
which, in turn, affects the values of the fluxes Fh and Feff
and the ratios of the number of quanta or atoms on the
ion-electron pair formed in the discharge, which change
during the plasma generation process.
Reduced Te leads to an increase in income of impuri-
ties into the plasma by increasing the ratio of the num-
ber of photons and atoms in the ion-electron pair.
Changes in temperature also leads to a redistribution
contribution of photons and atoms in the impurity con-
centration. Moreover, at Te<6 eV the contribution of Fh
is significant, at Te>6 eV – are photons, since for the
conditions under consideration Kph>>Kh.
ISSN 1562-6016. ВАНТ. 2019. №4(122) 112
It should also be noted that at the gas-kinetic rates of
the desorbed molecules H2O, CH2, CO the lengths be-
fore ionization at ne=1012…1013 сm-3 lie in the range
10...1 сm. Moreover, as the density increases, the depth
of penetration of impurities decreases. Here we do not
consider elementary processes leading to a decrease in
concentration gradients, nor processes of impurity trans-
fer in plasma. Those at small Te impurities penetrate to a
considerable depth, but still at ne≈1×1013 сm-3 the cap-
ture of impurities goes to the outer plasma regions
~1…3 сm. Such a distribution can be maintained long
enough for small Te and Ti due to very low values of the
diffusion coefficients and large ionization rates. How-
ever, at the stage of plasma heating, due to an increase
in the diffusion coefficients of the impurity for
1…5 ms equalize radial distribution.
Fig. 3. Distribution of energy lost by electrons in CO2
(dashed line) and H2O (solid line) through various exci-
tation channels: 1 – Oscillations; 2 – Electronic excita-
tion (dissociation, dissociative adhesion); 3 – Electronic
excitation; 4 – Ionization; 5 – Elastic collision loss
It should also make some comments regarding the
impurities. In Fig. 3 they are represented in the atomic
state, whereas desorption from the surface occurs in the
form of molecules, which in this case will be represented
in the form of water molecules H2O and hydrocarbons,
i.e. fragments of CnHm, in particular, methane CH4, CH3,
CH, CO2 clearly visible in the mass spectrograms plasma
discharges. On the other side all these compounds have a
radiating vibrational levels significantly increase the en-
trainment of the energy from the plasma.
In Fig. 3 shows the energetics of the vibrational lev-
els for the H2O and CO2 molecules which indicates that
in spite of the relatively small percentage of impurities
H2O and CO2 ~10% in the hydrogen plasma, they will
significantly increase the energy consumption of such
hydrogen plasma discharges.
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Article received 29.05.2019
ОБРАЗОВАНИЕ ЛЕГКИХ ПРИМЕСЕЙ НА НАЧАЛЬНОЙ СТАДИИ РАЗРЯДА
В.Б. Юферов, Е.И. Скибенко, В.И. Ткачев, В.В. Катречко, А.С. Свичкарь
При рассмотрении динамики плотности водородных атомарных и молекулярных ионов получены вели-
чины потоков атомарного водорода и УФ-излучения на стенки плазменной камеры, в результате чего появ-
ляются легкие примеси углерода и кислорода на начальной стадии развития разряда в процессе десорбции
со стенок при облучении. Определены величины потоков примесных атомов, связанных с потоками фотонов
и атомов водорода, возникающих в разряде. Даны рекомендации по уменьшению количества примесей на
начальной стадии разряда.
УТВОРЕННЯ ЛЕГКИХ ДОМІШОК НА ПОЧАТКОВІЙ СТАДІЇ РОЗРЯДУ
В.Б. Юферов, Є.І. Скібенко, В.І. Ткачов, В.В. Катречко, О.С. Свічкар
При розгляді динаміки щільності водневих атомарних і молекулярних іонів отримані величини потоків
атомарного водню і УФ-випромінювання на стінки плазмової камери, через що з'являються легкі домішки
вуглецю і кисню на початковій стадії розряду в процесі десорбції зі стінок при опроміненні. Визначено ве-
личини потоків домішкових атомів, пов'язаних з потоками фотонів і атомів водню, що виникають у розряді.
Надано рекомендації щодо зменшення кількості домішок на початковій стадії розряду.
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