ISSN 1063-7842, Technical Physics, 2006, Vol. 51, No. 10, pp. 1316–1319. © Pleiades Publishing, Inc., 2006. Original Russian Text © Yu.A. Burachevsky, V.A. Burdovitsin, A.S. Klimov, E.M. Oks, M.V. Fedorov, 2006, published in Zhurnal Tekhnicheskoœ Fiziki, 2006, Vol. 76, No. 10, pp. 62–65.
GAS DISCHARGES, PLASMA
Plasma Localization in an Extended Hollow Cathode of the Plasma Source of a Ribbon Electron Beam Yu. A. Burachevsky, V. A. Burdovitsin, A. S. Klimov, E. M. Oks, and M. V. Fedorov Tomsk State University of Control Systems and Radioelectronics, pr. Lenina 40, Tomsk, 634050 Russia e-mail:
[email protected] Received February 27, 2006
Abstract—The plasma density distributions in the slit aperture of the extended rectangular hollow cathode of the source of a ribbon electron beam are investigated experimentally. It is found that a local maximum whose parameters are determined by the discharge current appears in the density distribution when the slot width is less than a certain threshold value. This maximum results in an inhomogeneous current density distribution in the beam. It is shown that the appearance of the local maximum in the plasma density is related to the overlapping of the ion sheaths in the slit aperture of the hollow cathode. PACS numbers: 52.27.–h DOI: 10.1134/S1063784206100094
INTRODUCTION One of the methods for generating a plasma with an extended surface is gas ionization by a ribbon electron beam [1]. A fairly high gas pressure (above 1 Pa) required for implementing this method significantly complicates the application of electron sources with a heated cathode. An alternative approach is to use plasma sources, in particular, those with an extended hollow cathode [2]. In this case, however, the problem arises of achieving a sufficiently high (above 0.1 A/cm2) beam current density, which is the main parameter determining the properties of the generated plasma. Experiments with a cylindrical beam source have shown that the current density can be increased while preserving the extraction efficiency by simultaneously decreasing the size of the emission window in the anode and the exit aperture of the cathode cavity [3] due to an increase in the ratio of the emission window area to the area of the anode region onto which the electron current from the cathode cavity is collected. This allowed one to suppose that, in the case of a ribbon beam too, the emission current density can be increased in a similar way. The aim of the present study was to investigate the possibility of increasing the electron current density in the source of a ribbon electron beam by modifying the electrode configuration, namely, by decreasing the width of the slit aperture of the hollow cathode.
source consists of 310 × 60 × 30-mm rectangular hollow cathode 1; plane anode 2 with a 310 × 10-mm emission window; and dielectrics 3 and 4, on which the electrodes are mounted. The height h of the cathode cavity was adjusted with the help of insert 5, and the dimensions of the cathode opening aperture were varied by using inserts 6 with slits of different width. The discharge voltage Ud and the accelerating voltage Ua were applied to the source electrodes as is shown in Fig. 1. The plasma parameters were measured by single-pin probe 7, which was inserted in the cathode cavity and 5 6 1 7 3
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h
2
+ Ud – Ua
4
+ Ua 8
EXPERIMENTAL TECHNIQUES The experiments were performed with an electron source the schematic of which is shown in Fig. 1. The
– Ud
X
V R
Fig. 1. Schematic of the electron source.
Up
PLASMA LOCALIZATION IN AN EXTENDED HOLLOW CATHODE
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n, 1016 m–3 (a) 3 1 2
3 4
2 1 Fig. 3. Plasma glow in the slit aperture of the cathode cavity.
0 5
1 2
4
3 4
n, 1016 m–3 12
(b)
10
3
1 2 3
8 2
4 5
6
1 0
(a)
4 0
5
10
15
20 x, cm
Fig. 2. Distribution of the plasma density n in the aperture of the cathode cavity at a gas pressure of 6 Pa, discharge currents of (a) 400 and (b) 800 mA, and different slit widths: (1) 13, (2) 11, (3) 9, and (4) 8 mm.
2 0 (b) 10 8
could be moved along it. When performing probe measurements, the anode grid was removed and the voltage Ua was not applied to accelerating electrode 8. In order to record the plasma glow, the cathode aperture was photographed from the side of the emission window. The electron source was installed in a vacuum chamber, which was evacuated with the help of a mechanical forevacuum pump. The gas pressure was varied in the range 3–10 Pa by admitting atmospheric air. EXPERIMENTAL RESULTS The measurements have shown that the decrease in the width of the slit aperture of the cathode cavity leads to an increase in the plasma density (see Fig. 2). However, at a slit width of 9 mm and less, the plasma distribution along the cavity becomes nonuniform and one or several local maxima appear in the plasma glow (Fig. 3). The positions of these maxima can change suddenly during the experiment. In this series of experiments, the position of the region with an increased plasma density (IPD) was stabilized by slightly (by ~0.5 mm) increasing the width of the cathode slit in its middle part. This allowed us to investigate the observed phenomena in more detail. The local maximum was most pronounced at low discharge currents (Fig. 4a). TECHNICAL PHYSICS
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1 2
6
3 4
4 2 0
0
5
10
15
20
25 x, cm
Fig. 4. Plasma density distribution in the aperture of the cathode cavity at a gas pressure of 6 Pa, cavity heights h of (a) 60 and (b) 44 mm, and different discharge currents: (1) 0.2, (2) 0.4, (3) 0.6, (4) 1.0, and (5) 1.3 A.
An increase in the discharge current leads to an increase in the plasma density outside the IPD region. At certain values of the discharge current, the local maximum of the plasma density becomes virtually indistinguishable and the bright glow region spreads over the entire cathode cavity. As the cathode cavity height h (Fig. 1) is reduced and, accordingly, the volume of the cathode cavity and the area of its wall decrease, the threshold discharge current at which the local maximum disappears also decreases (Fig. 4b). An increase in the pressure or the width of the slit aperture of the cathode cav-
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under the condition
Id, A
S a /S c < m/M,
1.4 3
1.2 1.0
2 0.8 0.6
1
0.4 0.2 6
7
8
9
10 b, mm
Fig. 5. Discharge current Id at which plasma inhomogeneity disappears as a function of the slit width b for different gas pressures: (1) 10, (2) 6, and (3) 4 Pa.
ity also leads to a decrease in the threshold current (Fig. 5). Probe measurements of the plasma density distribution in the cathode cavity indicate that the density maximum is most pronounced in the plane of the slit aperture. DISCUSSION A local glowing region in the exit aperture of an extended cathode cavity was previously observed in [4], where this effect was attributed to the discharge contraction; however, no experimental data on the potential drop were presented. In order to explain the phenomena observed in our study, one would make use of the model proposed in [5], where a double layer was assumed to form in the aperture of the cathode cavity Measured plasma density and estimated ion sheath thickness for different dimensions of the cavity and different discharge currents Id Density, 1016 m–3
Sheath thickness, mm
Id, mA
Sa/Sc
0.7
9.0
200
0.046
2.1
5.4
400
5.3
3.6
600
2.8
1000
3.6
4.1
200
7.0
3.0
400
9.3
2.8
600
10.5
2.7
1000
10
0.057
(1)
where Sa and Sc are the areas of the aperture and the cavity wall, respectively, and m and M are the electron and ion masses. However, probe measurements of the floating potential did not reveal any significant potential difference between the IPD region and the rest of the plasma. On the other hand, the values of the ratio Sa/Sc that correspond to the distributions shown in Figs. 4a and 4b are equal to 0.046 and 0.057. These values are one order of magnitude larger than the value of m/M for nitrogen, which is the main gas constituent in the vacuum chamber after admitting atmospheric air. This means that, in our experiments, the IPD region appeared under the conditions such that criterion (1) for the formation of a double layer was not satisfied. This fact made us search for an alternative explanation of the phenomena observed. It follows from the experimental results that the decisive parameter determining the appearance of the IPD region is the slit width. This allowed us to propose a mechanism based on the concept of spontaneous contraction of the discharge. The scenario of this process may be as follows. At low discharge currents and, accordingly, low plasma densities, the slit aperture of the cathode cavity is overlapped by the ion sheaths. A random deviation from the established value of the plasma density or the plasma potential results in a decrease in the thickness of the ion sheaths and an increase in the electron current in this region. This in turn leads to the intensification of the ionization processes and an increase in the plasma density. As a result, the thickness of the ion sheaths decreases further. The process develops in an avalanche manner and terminates with a formation of a local region through which almost the entire electron current flows. As the discharge current increases, the plasma density in the local region increases and the ion sheaths outside this region turn out to be open, which leads to a change in the plasma density distribution along the cavity. Estimates of the ion sheath thickness for the measured plasma densities give values comparable to the width of the slit aperture (see table). This is an additional argument in favor of the mechanism proposed. It is also obvious that a decrease in the area of the cavity wall at a constant discharge current results in an increase in the plasma density, which manifests itself as a decrease in the threshold current (Fig. 4). In [6], we pointed out the effect that can disturb the homogeneity of a ribbon electron beam, namely, the existence of a positive feedback between the plasma density and the return ion current from the accelerating gap to the discharge region. The results of the present study indicate another mechanism for the appearance of such inhomogeneities. TECHNICAL PHYSICS
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PLASMA LOCALIZATION IN AN EXTENDED HOLLOW CATHODE
CONCLUSIONS In a discharge with an extended hollow cathode, an IPD region forms in the slit aperture when the slit width is smaller than a certain threshold value determined by the area of the cavity wall. As the discharge current increases, the IPD region expands and can spread over the entire slit length. A possible mechanism for the formation of the IPD region is the localized avalanche-like opening of the ion sheaths in the slit aperture of the cathode cavity and the accompanying contraction of the discharge. The formation of an IPD region can be prevented by decreasing the thickness of the ion sheaths in the aperture of the cathode cavity, which in turn can be achieved by increasing the discharge current or decreasing the area of the cavity wall. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, projects nos. 05-02-98000 and 0508-01319.
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Translated by B. Chernyavskiœ
