Metal-hydride Activation Of Hydrogen As A Tool Of Ions Energy Controlling In Plasma Sources
V.N. Borisko, Ye.V. Klochko, A.F. Korj, I.N. Sereda
Kharkov National University by V.N. Karazin, Kurchatov av. 31, 61108, Kharkov, Ukraine
Abstract. In this work the influence of metal-hydride activation of hydrogen on physical processes taken place in plasma
of the ion source based on Penning trap with metal-hydride cathodes (flat or hollow) has been investigated. The main
conclusion of the investigations is the decreasing of energy of extracted ions by the means of increasing of ionization
efficiency of hydrogen desorbed in vibrationally-excited state. Thus, on basis of carried out experimental data the new
method of energy distribution function of hydrogen ions controlling due to metal-hydride cathodes applying is suggested.
Keywords: PIG ion source, metal-hydride.
PACS: 52.40. Hf
For the different new technological application of hydrogen ion beams it is necessary to develop and create ion
sources that are often satisfied controversial demands. One of such demand is independent controlling of energy
distribution function of extracted ions, and emission capability of the device as a whole. The problem is partially
solvable if as cathodes of plasma-forming stage of the source are used getter hydride-forming alloys Zr-V system,
which are able reversibly absorb hydrogen at low working pressure . Successful solution of this problem is
conditioned on preliminary excitation of hydrogen molecules desorbed from such compounds under ion
bombardment to necessary working area of the ion source. Hydrogen activation is conditioned on vibrationall
excitation of H2 molecules which are formed as a result of atoms H recombination on the surface of such compounds
with the following transition into gas phase of discharge in thermodynamic nonequilibrium state . At that the
ionization potential of desorbed hydrogen decreases on 0.5 eV and ionization cross-section increases in 1.5 times
comparable with common molecular hydrogen filled from a balloon . In this work the experimental investigations
of influence of metal-hydride activation of hydrogen on physical processes taken place in plasma-forming stage of
the ion source based on Penning trap and on the characteristics of ejected ion beams has been presented.
The experiments were carried out on an installation that
is shown schematically in a fig. 1. The electrodes of
reflected discharge were placed inside quartz cylinder. The
anode (3) of the gas-discharge system was made from
stainless steel and represented the hollow cylinder 30 mm
in inner diameter and 30 mm in length. The total length of
a discharge cell was 70 mm. The metal-hydride cathode (4)
was performed in a form of a disk electrode with a
diameter of 20 mm and a width of 5 mm from Zr50V50
getter alloy saturated with hydrogen. It was pressed with
copper binding powder. The content of a copper filer
formed 40 % from the mass of hydride. The maximum
amount of the accumulated hydrogen in such electrode was
229 cm3 at standard conditions. The second cathode (5)
was made from copper. In verifying experiments two cathodes from copper were used. In the center of both cathodes
FIGURE 1. The scheme of experimental device.
1 – vacuum chamber, 2 – magnetic system, 3 – anode,
4 – metal-hydride cathode, 5 – cupper cathode,
6, 7 – energy analyzer.
were holes 6 mm in diameter. Behind them energy analyzers (6, 7) for energy spectrum of extracted ions
investigation were placed. The initial pressure in discharge chamber was established by the hydrogen filling from a
balloon through a leak. After the ignition of the discharge the working pressure in discharge cell was established at
the cost of ion-stimulated desorbtion of hydrogen from metal-hydride cathode. The magnitude of working pressure
linearly depended on current density of ions bombarded such a cathode . The working pressure of hydrogen in a
vacuum chamber varied from 2·10-5 torr up to 4·10-3 torr. The system of discharge electrodes was in longitudinal
exterior magnetic field formed by coils of solenoid (2). The range of changes of an exterior magnetic field was from
0.1 up to 1 kOe.
RESULTS AND DISCUSSION
Typical ion energy distribution functions extracted along the magnetic field from the direction of metal-hydride
cathode for the different pressure of working gas are presented on fig. 2. One can see, in case of metal-hydride using
as material of discharge cathode the widening of ion energy distribution functions takes place (fig. 2, curves 2, 3). At
that against a background of high-energy maximum in the energy range of 1.1-1.3 keV an additional low-energy
peak on distribution function with a maximum in 0.6-0.7 keV appears, which is observed for a whole range of
investigated pressures. At copper cathode using such a widening does not occur and on the distribution function is
the one pronounced maximum in the energy range of 1.1-1.2 keV. The position of high-energy maximum for the
The presence of low-energy peak on ion energy distribution functions in case of metal-hydride cathode applying
one cannot explain only by atomic hydrogen ion H+ increasing in discharges with metal-hydride electrodes, which
does not exceed the value of 0.01 % . The presence of low-energy peak on distribution functions of ejected ions in
case of metal-hydride cathode applying is conditioned on plasma potential in the discharge area where their
ionization realizes. In case of saturated metal-hydride cathode applying under ion bombardment the large amount of
activated hydrogen desorbed to paraxial region of the discharge cell, where the space potential is about 0.6 kV .
Position of high-energy maximum on distribution functions of ejected ions for metal-hydride and copper cathodes
corresponds to plasma potential in anode layer.
As long as hydrogen desorbtion from metal-hydride cathode is proportional to ion current density bombarded
such a cathode then at low pressure of working gas the quantity of desorbed hydrogen from metal-hydride is small.
Therefore at low working pressures sufficient increase of its concentration on the system axis does not observe .
As a consequence the low-energy maximum on ion energy distribution functions in this case small as well (fig. 2 (а),
curve 2). At increasing of pressure of working gas current of bombarded the cathode particles increases, hydrogen
desorbtion from such a cathode increases that leads to increasing of its concentration on the axis of the system.
Therefore at working pressure increasing sufficient increasing of low-energy maximum on the curve of ion energy
distribution function is observed as well (fig. 2 (b), curve 2). Position and size of high-energy maximum (1.1-
1.2) keV at that does not change and coincides with the position of maximum on ion energy distribution function for
copper cathode (fig. 2 (a), curve 1). At further increasing of hydrogen pressure the first maximum on distribution
function increases and becomes sufficiently higher than the second one (fig. 2 (b), curve 3).
The investigations of metal-hydride activation of hydrogen on ion energy distribution function extracted from
penning type plasma source with hollow metal-hydride cathode has been carried out as well. Such target setting is
conditioned that hollow cathode effect applying in plasma-forming stage of a source allows sufficiently increase the
magnitude of discharge current at simultaneous decrease of discharge voltage drop. With this aim the metal-hydride
flat cathode 4 (fig. 1) was changed on hollow one. The length of the hollow was 29 mm and 6 mm in diameter.
Extraction of positive ions was realized along the discharge axis through the hollow 6 mm in diameter in the center
of the copper anticathode 5 (fig 1). In check experiments copper hollow cathode with the same dimensional and
construction was applied.
Investigations of ion energy distribution function
when the both cathodes are at ground potential have
shown that the most probable ion energy is within a rather
narrow maximum in the energy range close to discharge
potential (fig. 3). Such a behavior of distribution function
was observed in whole range of working pressure and was
the same both in case of metal-hydride cathode applying
and copper one. This is concerned with that ions in such
discharges generates mainly in negative glow. The
potential in negative glow close to anode potential and
therefore generated ions without sufficient energy loss at
the expense of diffusion along the discharge axis reaches
the anticathode .
With the purpose of possibility to operate of ion
energy distribution function the negative voltage supply
on the hollow cathode relatively to ground potential was
realized. It led to discharge voltage drop decreasing. At
that the most probable ion energy for discharge with
metal-hydride hollow cathode (fig. 4, (a)) and for check
discharge with copper hollow cathode (fig. 4, (b))
decreases as the value of cathode negative shift increases. However if in case of check discharges such a behavior is extracted ions was less than anode potential and was the value of (0.7-0.8)·eUa. At increasing of negative shift value
on metal-hydride hollow cathode the difference between most probable ion energies and anode potential increased.
Such an influence of negative glow shift on ion energy distribution function one can explain by following. Negative
voltage supply on hollow cathode leads to current density of ions bombarded the surface of the cathode increasing.
As a result the quantity of activated hydrogen in the hollow which has reduced ionization potential increases. It leads
to increasing of ionization efficiency in the hollow. As a result space potential in this area decreases and as a
consequence the most probable ion energy lesser than discharge voltage drop value.
It should be pointed out that such an effect is typical only for discharge with metal-hydride hollow cathode as
long as at hollow cathode changing on flat one the shift of maximum on distribution function at negative shift
supplying practically was not observed.
Thus as a result of carried out experimental investigations it has been shown that at activated hydrogen
desorbtion from the surface of metal-hydride cathode the possibility to form hydrogen ion beams with the energy
lesser than in case of cathode materials applying that does not content hydride phases appears. It is conditioned on
reduced ionization potential of activated hydrogen. It was shown that flat metal-hydride cathode applying leads to
addition low-energy maximum on ion energy distribution function appearing. Metal-hydride hollow cathode
applying allows to widen the range of ion energy distribution function controlling aside less energy values at the
expense of negative shift supplying on such a cathode.
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