Physical processes in the negative ion source with reflected discharge
In this article the results of experimental and theoretical investigations of the physical processes in the negative ion source with reflected discharge are represented. This source was developed at the Institute of Physics of NAS of Ukraine. It is naturally, that for optimal utilization of the sour...
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irk-123456789-815152015-05-18T03:02:10Z Physical processes in the negative ion source with reflected discharge Goretskii, V.P. Ryabtsev, A.V. Soloshenko, I.A. Tarasenko, A.F. Tschedrin, A.I. In this article the results of experimental and theoretical investigations of the physical processes in the negative ion source with reflected discharge are represented. This source was developed at the Institute of Physics of NAS of Ukraine. It is naturally, that for optimal utilization of the source one should know the mechanisms of formation and destruction of negative ions, mechanisms of small admixture influence on emission characteristics of the source and space-time evaluation of source plasma in the direction of extracted slit. All these problems were studied in the present work in detail. 1999 Article Physical processes in the negative ion source with reflected discharge / V.P. Goretskii, A.V. Ryabtsev, I.A. Soloshenko, A.F. Tarasenko, A.I. Tschedrin // Вопросы атомной науки и техники. — 1999. — № 4. — С. 16-19. — Бібліогр.: 8 назв. — англ. 1562-6016 http://dspace.nbuv.gov.ua/handle/123456789/81515 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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In this article the results of experimental and theoretical investigations of the physical processes in the negative ion source with reflected discharge are represented. This source was developed at the Institute of Physics of NAS of Ukraine. It is naturally, that for optimal utilization of the source one should know the mechanisms of formation and destruction of negative ions, mechanisms of small admixture influence on emission characteristics of the source and space-time evaluation of source plasma in the direction of extracted slit. All these problems were studied in the present work in detail. |
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Article |
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Goretskii, V.P. Ryabtsev, A.V. Soloshenko, I.A. Tarasenko, A.F. Tschedrin, A.I. |
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Goretskii, V.P. Ryabtsev, A.V. Soloshenko, I.A. Tarasenko, A.F. Tschedrin, A.I. Physical processes in the negative ion source with reflected discharge Вопросы атомной науки и техники |
| author_facet |
Goretskii, V.P. Ryabtsev, A.V. Soloshenko, I.A. Tarasenko, A.F. Tschedrin, A.I. |
| author_sort |
Goretskii, V.P. |
| title |
Physical processes in the negative ion source with reflected discharge |
| title_short |
Physical processes in the negative ion source with reflected discharge |
| title_full |
Physical processes in the negative ion source with reflected discharge |
| title_fullStr |
Physical processes in the negative ion source with reflected discharge |
| title_full_unstemmed |
Physical processes in the negative ion source with reflected discharge |
| title_sort |
physical processes in the negative ion source with reflected discharge |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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1999 |
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http://dspace.nbuv.gov.ua/handle/123456789/81515 |
| citation_txt |
Physical processes in the negative ion source with reflected discharge / V.P. Goretskii, A.V. Ryabtsev, I.A. Soloshenko, A.F. Tarasenko, A.I. Tschedrin // Вопросы атомной науки и техники. — 1999. — № 4. — С. 16-19. — Бібліогр.: 8 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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| first_indexed |
2025-07-06T06:30:38Z |
| last_indexed |
2025-07-06T06:30:38Z |
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1836878076750856192 |
| fulltext |
PHYSICAL PROCESSES IN THE NEGATIVE ION SOURCE WITH
REFLECTED DISCHARGE
V.P.Goretskii, A.V.Ryabtsev, I.A.Soloshenko, A.F.Tarasenko, A.I.Tschedrin
Institute of Physics of NAS of Ukraine
INTRODUCTION
In this article the results of experimental and
theoretical investigations of the physical processes in
the negative ion source with reflected discharge are
represented. This source was developed at the Institute
of Physics of NAS of Ukraine. It is naturally, that for
optimal utilization of the source one should know the
mechanisms of formation and destruction of negative
ions, mechanisms of small admixture influence on
emission characteristics of the source and space-time
evaluation of source plasma in the direction of extracted
slit. All these problems were studied in the present work
in detail.
It was shown, that main mechanism of negative
hydrogen ion formation is a dissociative attachment of
electrons to oscillatory excited molecules H2:
e + H2 (v = 5 ÷ 8) → H— + H.
The destruction of H— ions is mainly determined
by detachment under the influence of H— ion collisions
with hot electrons of plasma:
e + H— → e + e + H.
The rate of the last process drops dramatically with
electron temperature decrease, this explains the pres-
ence of maximum for H— concentration in the region
between discharge column and anode, where electron
temperature is low.
The problem with mechanism of small admixture
influence of Cs on emission characteristics of H— ion
source was solved. It was shown that the increase of
extracted H— ion current with Cs vapor added, is not
due to volume processes with Cs participation, but due
to conversion of H to H— on the anode surface covered
with cesium having low work function.
EXPERIMENTS
The scheme of the ions source under study is
shown in Fig.1. Incandescent tungsten cathode 1 with a
diameter of 2 mm serves as an electron source. Before
anode chamber 3, aperture 2 is placed which limits the
radial size of plasma column 6. Reflection of electrons
providing gas ionization is realized by anticathode 7
with potential equal to that of the cathode. Gas feed is
performed through the holes in anode 5 placed at the
equal distance from each other. Magnetic field with the
strength of up to 2 kOe is directed along the system
axis. Extraction of ions is accomplished through the slit
by field electrode 4. Diameter of the discharge chamber
comprises 5 mm, diameter of the aperture comprises
2.5 mm. Exactly with this configuration, the maximum
ion yield is observed in a range of 2⋅10-2 ÷ 2⋅10-1 Torr;
discharge voltage – in a range of 100 ÷ 200 V; dis-
charge current – in a range of 1 ÷ 10 A; extracting volt-
age – in a range of 8 ÷ 14 keV. For a thorough compari-
son of experimental and calculated results, measure-
ments of the plasma parameters have been accom-
plished. Plasma density values, averaged along the
plasma column radius, which have been obtained from
measurements of the ions flux on the anticathode, grow
up proportionally to the discharge current and reach 1014
cm-3. (Temperature of electrons in the discharge column
varied from 2 up to 6 eV accordingly at a pressure
variation from 2⋅10-1 up to 2⋅10-2 Torr). Plasma
parameters outside the column can be measured by a
thin cylindrical probe introduced into plasma through
the emission slit perpendicularly to the magnetic field
direction. The plasma concentration on a rim is lower
approximately by an order than that in a column, and
the temperature of electrons at p ≈ 1⋅10-1 Torr is about 1
eV. It is visible already from these data that the re-
searched discharge is an analog of the known two-
chamber systems developed for injectors of neutral par-
ticles. In this discharge, fast electrons create dense
plasma and turn out oscillatory excited molecules H2.
Outside a column, there are optimum conditions for H-
ion formation. In this case, the cooling of electrons is
carried out during diffusion across a magnetic field.
Introduction of cesium was accomplished by
sputtering of a dichromate cesium tablet 8 (see Fig.1)
placed on the reflector 7. When the discharge was
glowing with the tablet installed, its voltage fell down to
50 V, and current of negative ions, extracted from the
source, decreased. After several hours of operation, the
voltage returned back to a value typical of the pure hy-
drogen regime and the current of H– ions rose dramati-
cally. Subsequent measurements were performed in this
regime.
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. № 4.
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aperture, 3 -anode chamber, 4 - extracting electrode, 5 - holes for
the gas supply, 6 - discharge column, 7 - anticathode,
8 - dichromate cesium tablet.
NUMERIC SIMULATION
For determining the current density of H– ions
and calculating the concentrations of both charged (
+++− sCHHHe N,N,N,N,n
2 ) and neutral (
Cs)v(HHH N,N,N,N
22 ) mixture components the system of
kinetic equations [1] was solved together with the
Boltzman equation [2, 3].
The plasma distribution along the discharge
chamber and radius was supposed to be homogenous.
The spatial distribution of neutral components
was assumed to be uniform, since their free run paths
were comparable or more than the chamber radius under
experimental conditionsThe electron energy distribution
function (EEDF) in the discharge column was
determined from the Boltzman equation with the
assumption of weak dependencies of the charged
component concentrations on a radius. The last was
proved by probe measurements. As it was shown in the
previous proceedings [4-7], EEDF in the discharge
column region possesses a flat plateau shaped
dependence on the input beam energy ranging from
∼ 100 eV down to ∼ 20 eV, and has practically the
Maxwellian shape at low energies (Te = 3÷5 eV). A
high-energy part of EEDF was absent between the
cathode and anode due to the strong magnetizing of
electrons along a radius, and the electron temperature
was essentially smaller than that in the column region
(Te ≤ 1 eV). Due to this, EEDF was assumed to be
practically the Maxwellian f0m with radial dependent
temperature, which was determined from the equation
of energy balance.
Mobilities and diffusion coefficients were cal-
culated taking magnetizing into account. It was assumed
that, in the whole discharge chamber, ions possessed the
Maxwellian distribution. The principal mechanism
determining the transfer coefficients at a pressure
p ∼ 0.1 Torr and the plasma concentration
n ∼ 1013 - 1014 cm -3 for electrons consisted in scattering
on ions. It was assumed, like in [5], that the temperature
of atomic hydrogen TH and that of negative ions TH
–
were equal to 4⋅103 K.
While solving the system of kinetic equations
and the Boltzman equation elementary processes listed
in Table were taken into consideration.
RESULTS OF NUMERICAL SIMULATIONS AND
COMPARISON WITH EXPERIMENTS
Fig.2 represent calculated radial distributions of
charged components in the discharge chamber of the
negative ion source scheme of which is shown on Fig.1.
As one can see from Fig.2, the gradients of all com-
ponent concentrations are negligible in the central part
of the plasma column. The gradients increase by abso-
lute value at the column boundary and in the drift part
of the source. While the electron and positive ion con-
centrations are almost monotonically decrease with
radius, the radius distribution of negative ion concen-
tration has the pronounced non-monotone character. In
addition, the concentration NH- in the maximum region
is more then three orders high then NH- in the region of
the plasma column. This important for experiments fact
may be explained as follow. As it was mention above,
the processes of H-- formation and destruction, deter-
mine the value of NH- in the column region, where the
main mechanism of destruction is electron detachment
in collisions of H— with plasma fast electrons (see Ta-
ble, reaction 17). The cross-section for this process has
threshold character: σ−−0 equals zero when electron
energy εth ≤ 1.25 eV and it reaches value of σ−−0 ≈ 2⋅
10-15 cm2 when electron energy ε ≈ 10 eV. As the
electron temperature beyond column decreases along a
radius, the rate of the process slumps, which leads to
increase of H— concentration despite of some decrease
of their formation. The same reason causes the presence
of a maximum of negative ion current density jH- to the
anode depending on the distance between the discharge
column and the anode (see Fig.2) (the maximum
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. № 4.
Серия: Ядерно-физические исследования (35), с. 16-19.
16
1,01,52,02,53,0510152025jH--, mA/cm2
R / Rk
Fig.3. Dependence of H— ions current density on the distance between
the discharge chamber and the column.
Table.
1 H2(v) + e → H2
+ + e + e, v = 0, …, 14
2 H + e → H+ + e + e
3 H2 + e → H + H + e
4 H + H (wall) → H2
5 H2
+ + e → H + H
6 H2 + e ↔ H2(v) + e, v = 1, 2, 3
7 H2 + e → H2
*(Β1Σu
+, c1Πu) +e → H2(v) + e + ηω,
v = 0, …, 14
8 H2(v) + H → H2(v’) + H
9 H2(v) + wall → H2(v’), v = 1, …, 14,
v’ = 0, …, v
10 H2(v) + e → H- + H, v = 1, …, 14
11 H + e → H-
12 H2
+ + e → H- + H+
13 H- + H2
+ → H + H2
14 H- + H+ → H + H
15 H- + H2 → H + H2 + e
16 H- + H → H + H + e
17 H- + e → H + e + e
18 H2
+ + e (wall) → H2
19 H+ + e (wall) → H
20 Cs + e → Cs+ + e
21 Cs+ + Cs + M → Cs2
+ + M
22 Cs2
+ + e → Cs + Cs
23 Cs+ + H- → Cs + H
24 Cs2
+ + H- → Cs + Cs + H
25 Cs+ + e (wall) → Cs
26 H + e (wall) → H-
0.05 0.10 0.15 0.20 0.25
0
5
10
15
20
25
30
35
IH- mA
2
1
p (Torr)
Fig.4. Dependence of extracted current of H– ions on pressure in the
source chamber at U0 = 14 kV (1 - with Cs, 2 - without Cs, solid
curve for emission slit 1.5x40 mm2, dash curve - 0.7x40 mm2).
disappear if the rate of the 17th process is artificially
supposed to be constant along a radius). As one can see
from Fig.4, for the discharge column radius Rc = 0.125
cm, an optimum reaches when R/Rc = 1.8. The optimal
value of jH- for this case exceed that for R/Rc = 1 in 7.5
times. The results of experiments carried out with
different diameters of the anode chamber are in good
agreement with calculated results: the optimum is
reached when R/Rc ≈ 2; the optimal value of extracted
current is approximately in 6 times higher than for
R = Rc.
Fig.4 exhibits the dependence of the H– ion cur-
rent, extracted from the source, on pressure with the
extracting voltage of 14 kV. One can see from the figure
that the introduction of cesium results in significant
changes of emission properties of the source. Principal
changes are the following: 1) increase of the maximum
value of H– ions current density; 2) increase of a gas
economy, especially at a low pressure (Fig.5);
3) influence of the cesium adding decreases with an
increase of the slit width. Attention should be paid to
changes in the mentioned dependencies: in the pure
hydrogen regime, the optimal pressure value exists pro-
viding the best gas economy of the source
(p ≈ 10-1 Torr), whereas the economy is inversely
proportional to H2 pressure in the regime with cesium
added. The maximum achieved value of the gas econ-
omy comprises ∼ 2 %, which exceeds the value in the
pure hydrogen regime by one order of magnitude.
It should be noted that even these experimental
results point to the mostly surface character of the ce-
sium influence. First and foremost, it is evident from the
fact that, in the cesium regime, an increase of the slit
width dos not lead to a proportional rise of the extracted
H– current (see curves 1). While without cesium, the
magnitude of the extracted current is always pro-
portional to the slit width (see curves 2). Two more
facts indicate the surface character of the effect: 1) ce-
sium influence is most effective at a low gas pressure,
when the role of volume processes is less important; 2)
in the cesium regime, the current of extracted H– ions
depends more weakly on pressure, than in the case of
pure hydrogen discharge.
Fig.6 shows the dependencies of the current
density for hydrogen negative ions jH
– at the anode on
pressure for three cases: 1 - pure hydrogen discharge
(considered processes are 1 - 19 in Table); 2 - cesium is
present only in a volume of the discharge chamber
(processes 1 - 25); 3 - cesium is present only at the
surface (processes 1 - 19, 26). One can see from Fig. 4
(curve 2) that introducing Cs vapor into the discharge
chamber volume, under the condition of an artificial
maintaining of the discharge voltage, results only in an
inessential increase of jH
– at high pressure as compared
to the case of the pure hydrogen regime (curve 1). The
principal mechanism of the hydrogen negative ions
formation in this case, as well as in the case of the pure
hydrogen discharge, consists in the dissociate attach-
ment of electrons to vibrationally excited hydrogen
molecules (process 10). A weak increase of the negative
ions current with addition of cesium is caused by the
following reasons. Cesium performs both appropriate
and inappropriate roles in volume processes. Due to low
energies of electron excitation and ionization of Cs, the
number of electrons with energy exceeding the Cs ex-
citation energy is decreased. The electron temperature
Te (Te = -1/(d ln f0/dε) makes sense only for energy
≤ 20 eV) falls down in agreement with the experimental
data. On the one hand, a decrease of the amount of fast
electrons and the temperature of the electrons majority
causes a decrease of the rate of formation of vibration-
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. № 4.
Серия: Ядерно-физические исследования (35), с. 16-19.
16
0,05 0,10 0,15 0,20
0
2
4
6
8
10
12
Q/IH-, cm3/mA min
3 2
1
p (Torr)
Fig.5. Dependence of a gas flow per the H— ions extracted current on
pressure.(1, 2 source with Cs for the emission slit of 0.7x40 and
1.5x40 mm2, respectively; 3 – source without Cs , emission slit of
0.7x40 mm2)
0.050.100.150.200.250510
152025
30354045
50jH- mA/cm2
pH2 (Torr)
1 2 3 4
Fig.6. Calculated dependence of H– ions current density on pressure.
1 - pure hydrogen discharge; 2,4 - discharge in mixture H2/Cs,
NCs = 1.6⋅1013 cm-3, processes 1-25 are considered; 3 - discharge in
hydrogen with taking into account conversion H to H– at cesium
doped surface. Discharge voltage 120 V (1-3); 50 V (4).
ally excited molecules and, consequently, H– ions. On
the other hand, a decrease of Te leads to lowering the
rate of electrons detachment (process 17; cross section
of this process depends on temperature) and, as well, to
lowering the H2 dissociation rate and, respectively, to a
decrease of the atomic hydrogen concentration, which
participates actively in taking off H2 vibration excita-
tion. The last two factors promote an increase of the H–
ions concentration. The mentioned appropriate and in-
appropriate effects resulting from the Cs introduction
into discharge practically compensate each other
(Fig.6). However, under the condition of an artificial
maintaining of the discharge voltage at the same level,
appropriate effects slightly overcome inappropriate
ones. Decrease of a calculated value of the discharge
voltage down to 50 V (which is practically observed in
experiments with Cs introduced into the volume) leads
to a diminishing of the H– ions current. Thus, in accor-
dance with the experiment, the theory shows that Cs in
the volume cannot increase the current density of H–
ions extracted from the source.
On the contrary, considering the H to H– conver-
sion at the anode with adsorbed cesium leads to an in-
crease of the H– ions current (Fig.6, curve 3) and vol-
ume concentration of H– by a factor of 2÷4 even with
the conversion coefficient γ = 10-3. We shall not proceed
with a detailed comparison now, but mention that this
increase is in agreement with an increase of the H–
current observed experimentally (Fig.4). We also men-
tion that, under conditions of experiments, no special
care was taken for a checking or optimization of the Cs
coating. However, as it was mentioned in [8], conver-
sion coefficients under conditions of ion sources are
usually in a range of 10-3 to 10-1, depending on the
hydrogen atom temperature and a degree of the surface
coating by cesium. One can see from the calculation,
that already at a rather low conversion coefficient, an
essential increase of the negative ion yield at source
surface is observed.
CONCLUSIONS
Based on the theoretical and experimental results
obtained in the present work we may draw the follow
conclusions.
The optimal conditions for obtaining of the re-
cord value of the current density are realized in the sta-
tioner plasma source of negative hydrogen ions with
reflected discharge. It was achieved at the expense of
the source geometry, which assume two zones – a
plasma column and a drift region where efficient
cooling of plasma electrons is realized.
In the plasma sources, the limiting value for spe-
cific power which allow the maximum output of the H—
ions, is about 500 W/cm3.
The addition of cesium to initially plasma source
of H— ions may lead to a significant increase of the gas
economy of the source and the current density. This
effect is connected to the fact that the recharge of posi-
tive ions and fast atoms on the edge of the emission slit
became the main mechanism for the H— ions formation.
That is, the cesium addition changes the plasma source
to the surface-plasma source with the anode generation
of ions.
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3. V.P.Goretskii, A.V,Ryabtsev, I.A.Soloshenko et al.. //
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ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. № 4.
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