ISSN 1063-7842, Technical Physics, 2006, Vol. 51, No. 6, pp. 786–790. © Pleiades Publishing, Inc., 2006. Original Russian Text © I.S. Zhirkov, V.A. Burdovitsin, E.M. Oks, I.V. Osipov, 2006, published in Zhurnal Tekhnicheskoœ Fiziki, 2006, Vol. 76, No. 6, pp. 106–110.
ELECTRON AND ION BEAMS, ACCELERATORS
Formation of Narrow-Focused Electron Beams Generated by a Source with a Plasma Cathode in the Forevacuum Pressure Range I. S. Zhirkov, V. A. Burdovitsin, E. M. Oks, and I. V. Osipov Tomsk State University of Control Systems and Radioelectronics, pr. Lenina 40, Tomsk, 634050 Russia e-mail:
[email protected] Received September 28, 2005
Abstract—Results are presented from experimental studies of the formation of focused electron beams produced by extracting electrons from the plasma of a steady-state discharge with a hollow cathode in the forevacuum pressure range. Based on the measurements of the energy spectrum and diameter of the electron beam, as well as of the emission parameters of the plasma produced in the course of beam–gas interaction, a conclusion is drawn about the excitation of a beam–plasma discharge that deteriorates the beam focusing conditions. The threshold beam current density for the excitation of a beam–plasma discharge is found to increase with accelerating voltage and gas pressure. PACS numbers: 41.75.Fr DOI: 10.1134/S1063784206060156
INTRODUCTION
EXPERIMENTAL TECHNIQUE
An advantage of plasma sources of focused electron beams is the absence of electrodes heated to thermoemission temperatures. This allows such sources to operate at gas pressures that are one to two orders of magnitude higher than those typical of hot-cathode guns and, therefore, makes them attractive for use in various technological processes, such as electron-beam melting, welding, and dimensional processing [1]. The ability of plasma cathodes to efficiently operate at elevated pressures substantially extends the range of the technological processes involving electron beams [2]. On the other hand, at elevated pressures, the probability of breakdown of the accelerating gap increases [3]; this circumstance limits the maximum accelerating voltage. Obviously, in this case, an increase in the beam power can only be achieved by increasing the beam current. However, in this approach, there are also limitations related to the beam–gas interaction [4, 5]. It was shown in [6, 7] that the operating pressure range of electron sources with a plasma cathode can be extended to the forevacuum range, which can be achieved with mechanical evacuation alone. Based on these studies, plasma electron sources capable of generating axisymmetric and ribbon beams with a current of 1 A and an electron energy of 10 kV at gas pressures of up to 10 Pa were developed [8, 9]. The aim of the present study is to investigate the specific features of the formation of narrow-focused electron beams under these conditions.
The experiments were conducted with a plasma electron source specially designed for operating in the forevacuum pressure range [10] (see Fig. 1). The plasma is generated by a discharge with hollow cathode 1. The electrons are extracted from plasma 2 along the system axis through the central opening in anode 3. Stable operation of the source at elevated pressures is ensured by screening the plasma generation region from the accelerating field and using a specially designed high-voltage insulator [11] to prevent peripheral breakdowns. The plasma electron source is mounted on the flange of the vacuum chamber. Accelerating electrode 5 and magnetic lens 6 are used to generate and focus electron beam 4. The energy of the beam electrons was measured by electrostatic analyzer 7—a 127° cylindrical Hughes–Rozansky capacitor [12] built according to the drawing kindly put at our disposal by Dr. R. Hollinger [13] from the GSI Accelerator Center (Darmstadt, Germany) in the framework of the Russian–German scientific cooperation program. The use of this type of analyzer instead of retardingpotential analyzers allowed us to avoid applying high voltages to the analytical part of the device. The beam diameter was measured by rotating probe 8 [14], made of a 0.2-mm tungsten wire. The signal generated by the probe when it crossed the electron beam was recorded by an oscilloscope. The plasma emission spectrum in the beam drift space was measured by a USB 2000 Ocean Optics spectrometer, the signal to which was fed
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FORMATION OF NARROW-FOCUSED ELECTRON BEAMS 1
Ip
2
– +
3
through an optical fiber. The density and temperature of the plasma electrons were measured with a double Langmuir probe. The vacuum chamber was evacuated by a mechanical forevacuum pump, and the residual gas was used as a working medium. When performing optical measurements, we used argon.
Up
– 5
787
U
+
EXPERIMENTAL RESULTS AND DISCUSSION
6 To oscilloscope
Visual observations show that, after the beam current exceeds a certain threshold value, a bright white glow appears in the beam focal plane, the rest of the beam remaining violet in color. In this case, the visible plasma emission becomes enriched with spectral lines with low excitation energies (Fig. 2) and the beam energy spectrum widens (Fig. 3). The beam diameter in the focal plane also increases with increasing beam current (see Fig. 4). Note that, starting from a certain value of the electron beam current, a sharp increase in both the beam plasma density (Fig. 5) and the plasma electron temperature (Fig. 6) is observed.
4
8
7
Fig. 1. Schematic of the experimental setup.
I, arb. units 4 (a)
2 C 391.13 C 427.60 14 12
0 300
500
700
λ, nm
I, arb. units C 391.13 58
(b)
C 427.60 58
4
Ar 750.59 49
2
Ar 738.28 19
Ar 763.46 42 Ar 771.57 20 Ar 811 29
0 300
500
700
λ, nm
Fig. 2. Emission spectra of the beam plasma for an acceleration voltage of 5 kV and beam currents of (a) 20 and (b) 60 mA. The working gas is argon at a pressure of 3 Pa. TECHNICAL PHYSICS
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d, mm
f(W) 1.2
15 0.8
3 2
1
9
0.4 3 3.7
3.9
4.1 W, keV
20
Fig. 3. Electron energy distribution function for an accelerating voltage of 4 kV, a gas pressure of 3 Pa, and beam currents of (1) 30, (2) 42, and (3) 64 mA.
40
60
80 Ib, mA
Fig. 4. Beam diameter d vs. beam current Ib for the same parameters as in Fig. 3.
ni , 10–9, cm–3
Te, eV 2
5
50
1
30
3
10 0
40
1
80 Ib, mA
Fig. 5. Calculated (curve 1) and measured (curve 2) density of the beam plasma ni vs. beam current Ib for the same parameters as in Fig. 3.
40
60
80 Ib, mA
Fig. 6. Plasma electron temperature Te vs. beam current Ib for the same parameters as in Fig. 3.
Assuming that the interaction of the beam electrons with gas particles is binary in character, the one-dimensional equation of the ion balance can be written as j 2 ------ πr = 2πrn i v , eλ
20
(1)
where j is the beam current density, r is the beam radius, λ is the electron mean free path with respect to ionization, ni is the ion density, and v is the ion thermal velocity. It follows from this equation that, if r is constant, then the ion density ni depends linearly on the beam current (Fig. 5). A drastic deviation of the experimental curve from the theoretical one after the beam current reaches a threshold value indicates a change in the character of the interaction between the electron beam and the plasma. This deviation, along with abrupt changes in the plasma emission spectrum (Fig. 2), the energy
distribution function of the beam electrons (Fig. 3), and the plasma electron temperature (Fig. 6), allows us to conclude that the beam–plasma interaction becomes collective in character and a beam–plasma discharge (BPD) is excited in the beam focal plane [15]. The threshold electron beam current required for the excitation of a BPD was found to increase with accelerating voltage and gas pressure (Fig. 7). The threshold current density can be calculated from the criterion for the excitation of a BPD [15], n 1/3 ω ⎛ ----e⎞ = 5ν en , ⎝ ni⎠
(2)
where ω is the electron plasma frequency; ne and ni are the densities of the beam electrons and the plasma, respectively; and νen is the collision frequency between the plasma electrons and neutral gas particles. TECHNICAL PHYSICS
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FORMATION OF NARROW-FOCUSED ELECTRON BEAMS Jb, mA/mm2 13
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Jb, mA/mm2 50 3 3 2
11
30
2
1 1 9 10 2
4
6
8
10 U, kV
0
In order to make use of this criterion, let us consider the case of subthreshold currents. In this case, the gas particles are ionized via binary collisions with the beam electrons and we can use Eq. (1) for the ion balance. Taking into account that the thermal electron velocity is 8kT equal to --------- , from Eq. (1) we obtain πM (3)
Substituting the expressions for the beam electron j m density ne = -- ---------- , the electron plasma frequency ω = e 2eU 2
e n ---------i , and the collision frequency between the plasma ε0 m electrons and neutral gas particles νen = σenvepNn (where vep is the thermal velocity of the plasma electrons, σen is the effective cross section for electron–neutral collisions, Nn is the neutral particle density, and U is the accelerating voltage) into Eq. (2), we obtain the expression for the threshold current density j = 25
kT 2 2ν en ε 0 ⎛ --------⎞ ⎝ ⎠ πM
1/6
2
λ ⎛m ----------⎞ ⎝ re 2 ⎠
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Eq. (4) that the threshold current density increases with increasing gas pressure and accelerating voltage, as is actually observed in our experiments. CONCLUSIONS A specific feature of the formation of a focused electron beam at elevated (forevacuum) pressures is the excitation of a beam–plasma discharge in the focal plane at beam current densities exceeding a certain threshold value. This leads to the widening of the beam energy spectrum and additional electron scattering. As a result, the beam diameter increases abruptly and the beam current density decreases accordingly. An increase in the accelerating voltage allows one to increase the threshold current density to a level acceptable for use in material processing technologies. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 05-02-98000. REFERENCES
1/3
1/3
U .
(4)
The calculated curves (see Fig. 8) coincide qualitatively with the experimental ones. In our opinion, this also indicates the excitation of a BPD under our experimental conditions. The difference in the absolute values of the current density can be attributed to either assumptions adopted in our simplified model or the averaging of the measured current density over the beam cross section. Equation (4) allows us to analyze the relation between the beam current density j and the experimental parameters. In particular, it follows from TECHNICAL PHYSICS
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Fig. 8. Calculated threshold beam current density jb vs. accelerating voltage U for gas pressures of (1) 3, (2) 5, and (3) 7 Pa.
Fig. 7. Threshold beam current density jb vs. accelerating voltage U for gas pressures of (1) 2, (2) 3, and (3) 4 Pa.
4πrj πM n i = ------------- ---------. eλ2π 8kT
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1. E. M. Oks, Plasma-Cathode Electron Sources (Izd. Nauch.-Tekhn. Lit., Tomsk, 2005). 2. S. I. Belyuk, Yu. E. Kreœndel’, N. G. Rempe, et al., Avtomat. Svarka, No. 3, 61 (1979). 3. V. A. Burdovitsin, Yu. A. Burachevskiœ, E. M. Oks, et al., Zh. Tekh. Fiz. 71 (2), 48 (2001) [Tech. Phys. 46, 179 (2001)]. 4. E. A. Abromyan, B. A. Alterkop, and G. D. Kuleshov, High-Intensity Electron Beams (Energoatomizdat, Moscow, 1983) [in Russian]. 5. V. A. Gruzdev, Yu. E. Kreœndel’, and Yu. M. Larin, Teplofiz. Vys. Temp. 11, 482 (1973).
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6. S. I. Belyuk, Yu. E. Kreœndel', and N. G. Rempe, Zh. Tekh. Fiz. 50, 203 (1980) [Sov. Phys. Tech. Phys. 25, 124 (1980)]. 7. V. A. Burdovitsin, Yu. A. Burachevskiœ, E. M. Oks, et al., Izv. Vyssh. Uchebn. Zaved., Fiz., No. 9, 85 (2001). 8. V. Burdovitsin and E. Oks, Rev. Sci. Instrum. 70, 2975 (1999). 9. V. A. Burdovitsin, Yu. A. Burachevskiœ, E. M. Oks, et al, Prib. Tekh. Éksp., No. 2, 127 (2003). 10. V. Burdovitsin, I. Zhurkov, E. Oks, et al., in Proceedings of the 13th International Symposium on High Current Electronics, Tomsk, 2004, pp. 68–69. 11. V. A. Burdovitsin, I. S. Zhurkov, E. M. Oks, et al., Prib. Tekh. Éksp., No. 6, 1 (2005).
12. V. P. Afanas’ev and S. Ya. Yavor, Electrostatic Energy Analyzers of Charged Particle Beams (Nauka, Moscow, 1978) [in Russian]. 13. M. Galonska, R. Hollinger, and P. Spaedtke, Rev. Sci. Instrum. 75, 1592 (2004). 14. S. I. Belok, Yu. E. Kreœndel, N. G. Rempe, et al., PlasmaEmitter Electron Sources (Nauka, Novosibirsk, 1983), pp. 80–91 [in Russian]. 15. A. A. Ivanov and V. G. Leiman, Fiz. Plazmy 3, 780 (1977) [Sov. J. Plasma Phys. 3, 440 (1977)].
Translated by B. Chernyavskiœ
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