The magnetic hollow cathode is an element for the construction of discharge arrangements for different purposes, which are still functional with relatively low gas pressures.
It can be widely used for the ionization of gas flows, the production of high plasma densities, Magnetron arrangements for thin films deposition (Pessoa et al., 2006), spectra light sources, low-vacuum electron beam sources and gas lasers.
The construction by plasma beams for the material denudation or to the building by cold cathode ion source.
Hollow cathodes discharge, in their various forms, are very suitable in a large variety of applications where high current densities, low cathode-fall voltages and extended lifetimes are required (Rawlin and Pawlik, 1968; Kirkici and Bruno, 1995; Fradkin et al., 1970; Ferreira and Delcroix, 1978). They are capable of operation at a fraction of an Ampere up to several hundred Amperes, all in arc discharge.
Although there could be various technical applications of the magnetic hollow
cathode, essentially two application possibilities were tested in this study:
||Sputtering plant with magnetic hollow cathode for layer demolition
||Cold cathode ion source with magnetic hollow cathode
MATERIALS AND METHODS
Two differently ion source types were developed and their characteristics are
examined (Boubetra, 2007). The description of the sources is shown in Fig.
1 and 2.
||The cold cathode Ion source with magnetic hollow cathode in
||Cold cathode Ion source with magnetic hollow cathode in anode
COLD CATHODE SOURCE IN CATHODE CIRCUIT
The ion source is similar to a Penning ion source in its structure (Kerkow
et al., 1992). But, the cathode is replaced by the magnetic hollow cathode
as shown in Fig. 1. A high magnetic field with similar characteristics
to that used in Penning ion source is applied to the anode region.
At low pressure the gas enhancement in the hollow cathode is not sufficient for the maintenance of discharge.
In this case, only the pendulum effect of electrons ensures low discharge current of about 1 mA.
Increasing the pressure to around 0.1 Pa leads to the enhancement of gas and discharge current with the magnitude of about 2 within the range of some 100 mA.
Under these pressure conditions, the discharge is better collimated as that observed in Penning-arrangement where discharge is distributed over the entire cathode surface. According to higher unloading pressure the current density is therefore higher in a discharge with magnetic hollow cathode than in the Penning ion source. Bethge and Baumann (1974) tried to limit discharge by the reduction of the cross sections and by drilling in the magnetic pole with small surface ranges. However the volume of the small hollow cathode is not sufficient to ensure the necessary Gas reinforcement for discharge. The importance of the developed ion source is not only apparent in the production of ions using the oscillating electrons but it raises in directing plasma through an inhomogeneous magnetic field towards the counter cathode so that the appearance of ions opening takes place.
With following reflection the electrons and the plasma become in opposite directions by the magnetic field absent-minded. The pendulum effect of the electrons is small thereby. The ion source reacts very flexibly to pressure changes i.e., the discharge current can be adjusted over a pressure control between the smallest and the greatest possible river continuously. A substantial advantage is that discharge in the Penning arrangement themselves with vacuums of 0.01 Pa still burns.
RESULTS AND DISCUSSION
A special execution of the source for a 350 kV-ion accelerator was tested.
Typical ion stream, which were based on the target system of the 350 kV-ion
accelerator, are arranged in Table 1 for different feed gases.
The ion current were measured with a hole diameter of about 0.7 mm at the output slit of the ion source.
More than 100 μA can be reached for all permanent gases according to the surface increase and enlargement of the output slit.
The use of another design for the magnetic hollow cathode e.g., as shown in
Fig. 2, permits to obtain very handy and productively ion source
with small dimensions.
This type of particularly ion source should be suitable to produce ions by atomization of difficulty evaporable materials.
By the conversion achievement in the ion source the parts for the magnetic
field generation must separate then from the ion source interior. In this case
the material in the hollow cathode is sputtered or evaporated by diffusion process
into the anode region.
||Ion current in μA with corresponding gas
For geometry reasons, the largest part of the steam cloud is condensed on the
cathode. While this electrode is also met by ions a sputtering process take
place. On the other hand it contributes to the increase of the density within
the plasma range. A substantial disadvantage of the ion source is that the ion
emission current density is determined by the internal field gradient between
plasma and output. It determines the number of the emitted ions. Since the magnetic
field hardly affects the ion movement, one must steer the ion emission from
the plasma by suitable deformation of the output. In addition, the atomization
of the screen material at the output limits the life span of the ion source.
COLD CATHODE ION SOURCE IN ANODE CIRCUIT
In anode circuit the face with the output of the ions is on anode potential
in accordance with Fig. 2 held together of the magnetic field
in the anode region. Only the anode drop as characteristic of the anode-lateral
polarization of the plasma separate the ions at the output slit.
The output slit serves as gas throttle for the neutral gas atom. When it stays before the magnetic disk likewise anode potential, then a part of the plasma is transferred of the ion source by the magnetic field into the outside space.
The ions were aspired and by the electrode and the emission amperage of ion source depends then on the plasma volume, which is seized by the suction electrode and on the charge carrier density.
Therefore, the arrangement of electrode geometry is crucial with this switching type for the ion yield of the ion source. Technically these criteria could be considered with the testing of the ion source in anode circuit only insufficiently, because the cover plate for constructional reasons at cathode potential and be switched only one sheet metal screen with the output had to be put as anode could.
Nevertheless ion current could be obtained, which were comparable with the
source in cathode circuit. The advantage of the ion source in anode circuit
consists above all of the fact that one can vary the suction conditions independently
of the fuel conditions. During the cathode circuit the plasma is separate by
an outer zone from the output. The thickness of this outer zone depends on the
plasma density, i.e., on the discharge current and on the magnetic focusing
conditions. Also still another small portion of a cathode-falls becomes effective;
however the source of discharge is the magnetic hollow cathode and not the back
plate electrode as with a Penning source. Since the potential difference between
plasma and back plate electrode amounts to for instance 300 V (Burning tension),
the distance between plasma and back plate electrode 1 to 2 mm amounts to, field
strengths of 103 V cm-1 arise, which affects however diverging
the emission direction of the ions. In the anode circuit geometry of the suction
field is determined by the shape of the suction electrode and the output and
can contribute to the focusing of the ion beam. Since the energy distribution
of the ions lies within the range of 10 eV, a better jet forming should be possible
in anode circuit. A disadvantage of the ion source in anode circuit is however
that the ion source must be operated with higher gas pressure than in cathode
circuit and the combustion behaviour of the ignition characteristic of the magnetic
hollow cathode is determined. This disadvantage becomes balanced by the fact
that the atomization is small only in the range of the output and can only occur,
if negative ions are present in discharge.
We have proposed a simple technological construction of cold cathode in order to compare it with glow cathode or HF-ion sources. The disadvantage is however the high gas pressure for an discharge and the smaller ion yield. By using a magnetic hollow cathode it is well possible to realize an homogeneous transfer of discharge by inhomogeneous magnetic fields within the plasma range to the anode. The plasma follows the magnetic field lines in its density and direction.
This research was supported in part by the Department of Physics, Humboldt University Berlin and by the Company Antar Trade Condor, BBA, Algeria.