ФИЗИКА ПЛАЗМЫ, 2010, том 36, № 3, с. 295-302
НИЗКОТЕМПЕРАТУРНАЯ ПЛАЗМА
УДК 533.9
DOUBLE LAYER IN A CYLINDRICAL HOLLOW-CATHODE DISCHARGE
© 2010 г. A. Abu-Hashem*, M. A. Abd Al-Halim*, **, M. A. Hassouba*, and M. M. Masoud***
*Benha University, Benha, Egypt **North Carolina State University, Raleigh, USA ***Nuclear Research Center, Atomic Energy Authority, Cairo, Egypt Поступила в редакцию 18.02.2009 г.
Окончательный вариант получен 12.05.2009 г.
A dc cylindrical coaxial glow discharge with an inner grid anode has been studied. The region between the two electrodes is seen dark, while a brightly glowing region forms inside the grid anode up to the center. The current—voltage characteristic of a dc cylindrical glow discharge in nitrogen is similar to that of a normal glow discharge, while the normal glow discharge voltage decreases with increasing pressure. The minimum plasma potentials are observed in the hollow cathode region due to the accumulation of electrons at the back of the grid anode. At the center, some of the passed electrons are converged, so their potential is decreased. These electrons have a sufficient time to be redistributed to form one group with a Maxwellian electron energy distribution function. The electron temperature measured by electric probes varies from 1.6 to 3.6 eV, while the plasma density varies from 3.9 x 1016 to 7 x 1013 m-3, depending on the discharge current and probe position. The plasma density increases as the electrons move radially from the grid toward the central region, while their temperature decreases.
1. INTRODUCTION
The cylindrical glow discharge consists of two coaxial cylindrical electrodes. The aim of the cylindrical or spherical structure, with grid anode, is to provide inertial electrostatic confinement of electrons [1]. When the outer electrode is the cathode and the inner grid electrode is the anode and a suitable voltage is used to operate the system, gas breakdown occurs and the negative glow region forms at the center of the grid anode. The electrons are rapidly emitted and accelerated from the cathode toward the grid anode. The electron beam passes through the anode and is injected toward the grid center. When the beam current is higher than the space-charge-limited current of this area, these electrons will be confined at the center of the spherical or cylindrical grid anode, causing the so-called "cathode". In addition, under proper conditions, the ion and electron flows create a space-charge-induced "double potential" well (a negative and a positive potential well) [2]. This structure traps high-energy electrons at the center of the inner electrode.
The coaxial virtual cathode oscillator consists of two metallic coaxial cylindrical electrodes. The inner electrode is positively biased grid, while the outer electrode is negatively biased discharge vessel. It has been shown that the forward current increases significantly as the gap distance between the cathode and the anode increases [3].
Rozsa et al. [4, 5] investigated laser oscillation on Al II transitions in a large-diameter hollow cathode
discharge tube, where Al vapor was produced by cathode sputtering. A special internal anode system considerably increased the tube voltage, which resulted in a low threshold current and an increased output power [4]. This effect was also observed on the 469.4-nm Kr and the 476.5-nm Ar ion transitions. A continuous-wave laser oscillation was observed for the first time on the 531.4- and 486.3-nm transitions of Xe II [5]. A higher voltage is required to increase the rate of volumetric ionization, thereby compensating ion loss by diffusion from the negative glow. The focusing of fast electrons toward the tube axis increased laser excitation in this region. Strong laser action with a low threshold current was achieved on the 283-nm Au II transition [6]. The threshold current was 0.4 A for the strongest 780.8-nm laser line near IR transitions of Cu II. At a discharge current of 2.4, a multiline output power of 30 mW was obtained on six transitions between 740.4 and 789.6 nm [7].
Iijima [8] studied a high-voltage hollow cathode discharge tube with an internal helical anode as an excitation source of metal vapor lasers. He found that the intensity ratio of the HeII to HeI lines emitted from this hollow cathode tube increased with increasing operating voltage, which in turn increased with increasing axial magnetic field. The radial distribution of the emitted luminous intensity is symmetric with respect to the axis. The current—voltage (I—V) characteristic was controlled by changing the distance between the threads of the helical anode [9].
Fig. 1. Schematic diagram of the experiment.
Fujii studied the confining process which attracts a negative glow within a hollow cathode of cylindrical geometry. The confining was interpreted as a transition process from a Townsend discharge to a normal glow discharge. The efficiency of the laser tube increases as the product Pd (the pressure multiplied by the interelectrode distance) decreases and also as the hole diameter of the cathode wall approaches the inner diameter of the hollow cathode [10].
The discharge characteristics were discussed morphologically with a view to applying this tube to a metal vapor laser [11]. In the negative glow of a low-pressure He and a He—Cd discharge, some electrons have energies more than 100 eV and the density of low-energy (30—34 eV) electrons is twice as high as that in the positive column [12]. It was found that a hollow-cathode metal ion lasers provided more than twenty wavelengths with a significant output. They measured the density of sputtered metal atoms as a function of the discharge current and gas pressure. Excitation of the upper laser levels occurred via charge transfer reaction, and the required ground state metal density was generated via sputtering [13].
The aim of present work is to operate a dc cylindrical glow discharge with coaxial electrodes, where the
inner grid anode is a grid, which forms a negative layer inside the real grid anode using nitrogen gas. The electrical characteristics and the plasma parameters (the plasma potential, electron temperature Te, plasma density N, and electron energy distribution function (EEDF)), which vary radially from the grid anode toward the center of the grid, are determined.
2. EXPERIMENTAL SETUP
A schematic diagram of the experimental setup is shown in Fig. 1. The discharge vessel is a cylindrical stainless-steel tube 15 cm in length and 5 cm in diameter. A 10-cm-long 4-cm-diameter high-transparency stainless-steel cylindrical grid is enclosed inside the discharge vessel. The two cylinders are hollow and coaxial. The outer cylinder (discharge vessel) and the inner grid cylinder represent the cathode and the anode, respectively. The two cylindrical electrodes are insulated from one another by two glass discs.
The discharge vessel is evacuated and filled with nitrogen, and the pressure is controlled by dynamic continuous flow. The distance between the two electrodes is fixed at 0.5 cm. The discharge vessel has three side ports: the first port is used for the working nitrogen gas
Vd, V 330 r
320
310
300
290
280
A A
a P = 1 mbar □ P = 3 mbar x P = 5 mbar
x *
□ □ u X * x
X X X X X X X
X X
0
8 10 Id, mA
V V
r n' v
330 320 310 300 290 280
0
10
12 14 P, mbar
Fig. 2. I—V characteristic of a cylindrical hollow-cathode discharge at different nitrogen pressures.
flow from the gas cylinder through a needle valve, the second one is used to insert an electric probe (single or double) inside the plasma, and the third one is connected to the vacuum system.
A single Langmuir probe is used to measure the plasma potential, the electron temperature, and the plasma density and estimate the EEDF. The single probe is made of a tungsten wire with a radius of 0.14 mm and length of 3 mm. Also, a double electric probe is used to estimate the electron temperature and plasma density. It consists of two tungsten wires, each had a radius of 0.14 mm and length of 2 mm, which are separated from one another by a distance of 2 mm. Both the single and double probes were fitted inside the inner grid electrode.
3. RESULTS AND DISCUSSION
In the present work, a dc cylindrical coaxial glow discharge with an inner grid anode is studied using nitrogen as a working gas. When breakdown occurs between the two coaxial electrodes, the region between them is seen dark, while a brightly glowing region forms inside the grid anode up to the center. In this paper, we consider the I—V characteristic, the radial distributions of the potential and electric field, the EEDF, the electron temperature, and the plasma density.
3.1. The Current—Voltage Characteristic of the Discharge
The I—V characteristic of a dc cylindrical glow discharge in nitrogen was measured. Figure 2 shows the I—V characteristics of the discharge at nitrogen pressures of 1, 3, and 5 mbar. At a nearly constant discharge voltage Vd, the discharge current increases with
Fig. 3. Relation between the normal glow voltage as a function of the nitrogen pressure.
pressure. These curves are similar to that of a normal glow discharge [14, 15, 16]. The figure also shows that the discharge voltage increases with decreasing gas pressure at a constant discharge current. Since, at low gas pressures, collisions between electrons and gas atoms decrease and, thus, the mean free path increases, large values of the discharge voltage will be required to maintain the discharge [17].
Figure 3 shows the relation between the normal glow discharge voltage Vn (the voltage at which the normal glow region starts) as a function of the gas pressure. It is found that the normal glow discharge voltage Vn decreases sharply in the pressure range 0.5—3 mbar and decreases slowly at higher pressures. The decrease in the normal glow discharge voltage Vn with increasing gas pressure can be explained according to the relat
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