ISSN 10637850, Technical Physics Letters, 2011, Vol. 37, No. 12, pp. 1172–1175. © Pleiades Publishing, Ltd., 2011. Original Russian Text © R.G. Sharafutdinov, E.A. Baranov, S.Ya. Khmel’, 2011, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2011, Vol. 37, No. 24, pp. 66–73.
Optical ElectronBeam Diagnostics of Free Supersonic Jet of Nitrogen Activated by ElectronBeamGenerated Plasma R. G. Sharafutdinov, E. A. Baranov*, and S. Ya. Khmel’ Institute of Thermophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *email:
[email protected] Received June 30, 2011
Abstract—Electronbeaminduced optical emission spectroscopy has been used for the first time to measure the rotational temperature and number density of gas in a free supersonic neutral nitrogen jet and in that acti vated by electronbeam plasma. The electronbeam plasma was generated by a lowenergy electron beam at a distance of 10 mm downstream from the edge of a conical supersonic nozzle. For diagnostics of the acti vated jet, the spectrum of intrinsic optical emission of plasma was subtracted from the spectrum of emission induced by the probing electron beam in the activated jet. It was established that the rotational temperature in the activated jet is increased as compared to that in the neutral jet, while the density at the jet axis is decreased. The electronbeam activation of nitrogen in the jet leads to a 35% increase in the rotational tem perature. DOI: 10.1134/S1063785011120297
Plasmaactivated free supersonic jets are used for deposition of thin films and coatings, etching and treatment of solid surfaces, and processing of various substances [1, 2]. In order to control processes in these jets, it is necessary to have information on the compo sition of plasma and the distribution of energy between its components. Diagnostics of activated gas jets and plasma is a topical task, solution of which requires the development of proper techniques. Nitrogen is a con venient model gas for this purpose. At the same time, nitrogen plasma jets are used for the deposition of nitrides of various elements and for the modification of properties of various surfaces [3]. The rotational temperature of nitrogen molecules and the density of gas in plasma jets and electricdis charge plasma can be measured using the methods of optical emission spectroscopy (OES) [4], laser induced fluorescence (LIF) [5], and electronbeam diagnostics (EBD) [6–9]. The OES is a nonperturb ing diagnostic method that is simple and employs readily available equipment. On the other hand, this method is characterized by comparatively low spatial resolution and complexity of interpretation of results. The results of LIFbased diagnostics, which is only mildly perturbing and ensures highly local probing, are simple to interpret and provide various data about the object studied. However, this method requires sophis ticated and expensive equipment. The EBD occupies an intermediate position between the first two meth ods. The EBD employing a response in the optical spec tral range is widely used in investigations of free super sonic jets of a neutral gas. In particular, it has been
employed for measurements of the local density and the rotational, vibrational, and translational tempera tures of gases [6–9]. As the gas temperature increases, application of this method encounters difficulties. Indeed, at a temperature of several thousand degrees, the emission excited by an electron beam in the optical spectral range is masked by the intrinsic emission from plasma. For this reason, the plasma jets are more fre quently studied by the EBD using Xray radiation. There are only several investigations where the EBD using optical response was applied to the free jets of thermal plasma generated by plasmatrons (see, e.g., [10]). Based on the analysis of optical emission spectra from Ar and N2–CO2 in neutral gases and plasma jets, it was shown that, at high temperatures, the EBD method of density measurement is inapplicable because the presence of intrinsic emission signifi cantly changes the intensity of spectral lines. This Letter reports on the use of optical EBD for the investigation of free supersonic nitrogen jets acti vated by electronbeam plasma. The method of density and rotational temperature measurement in flows of neutral gaseous nitrogen by means of optical EBD has been described elsewhere [6–9]. According to this, a beam of highenergy elec trons crosses the probed gas jet and interacts with nitrogen molecules, which leads to their dissociation, ionization, and excitation with the formation of mol ecules, atoms, and ions in various states. The major event in this diagnostics is the ionization of nitrogen molecules in the ground state by electrons with the formation of excited molecules:
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1
+
N2 ( X Σg ) + eb
+
2
+
N2 ( B Σu ) + eb + es ,
(1)
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+
where X Σ g is the ground state of nitrogen molecule, 2
+
2
+
B Σ u is the excited state of molecular nitrogen ion, and eb and es are the primarybeam and secondary electrons, respectively. Nitrogen ion in the excited state exhibits spontaneous transition to the ground level with photon emission, which corresponds to the 2 + 2 + B Σu X Σ g transition. A system of bands that corresponds to this transition is called the first negative + system of nitrogen ion (1NS N 2 ). Using the spectrum of (0–0) emission band of this system, it is possible to determine the rotational temperature of nitrogen ions 2 + in the B Σ u state, which differs by 12.5 K from the rotational temperature of the ground state of nitrogen molecules [11]. In the present study, the rotational temperature of nitrogen was determined by comparing the calculated and measured spectra. Calculation of the emission spectrum with allowance for the transmission function encounters no difficulties. The electronvibrational rotational spectrum of emission in the (0–0) band of 2 + 2 + + B Σu X Σ g N 2 transition has been simulated, e.g., in [4]. The gas density was determined using a method described in [6, 9], according to which the integral 2 + intensity of emission in the (0–0) band of B Σ u X Σ g transition in nitrogen ion is measured. This intensity is proportional to the gas density n, probing (diagnostic) electron beam current, and a coefficient that can be determined by calibration at a known gas density. As the gas density increases, the role of inter actions that lead to quenching of the electroninduced fluorescence (i.e., to nonradiative transitions due to the collisions of excited ions with neutral molecules) becomes clear. Therefore, the measurements should be performed with allowance for the fluorescence quenching. In this case, the (0–0) band intensity can be expressed as follows [12]: I0 ( n ) I ( n ) = , (2) 1 + nτ 〈 vQ qu〉 where I0(n) is the emission intensity without allow ance for the fluorescence quenching, τ is the lifetime 2 + of the B Σ u (v = 0) level [13], and 〈vQqu〉 is the fluo rescence quenching constant that can be calculated as described in [12]. The experiments were performed on a lowdensity gasdynamic setup (VS4) [14], in which technical purity nitrogen is supplied via a forechamber to a supersonic jet and flows out into a vacuum chamber. The nozzle had conical taper and horn with the angles α = 44.20° and β = 6.53°, respectively. The critical crosssection radius (rx = 2 mm) was constant along the capillary with a length of Lx = 1.2 mm at an expan TECHNICAL PHYSICS LETTERS
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sion factor of Sa/Sx = 10.81. The gas pressure in the forechamber (or the stagnation pressure) was 54 Torr. The gas efflux from the nozzle led to the formation of a free, lowdensity supersonic jet. The background pres sure PH in the vacuum chamber was 5.5 × 10–3 Torr. At a distance of 10 mm downstream from the nozzle edge, the jet was activated by a lowenergy (2 keV) electron beam with a total beam current of 100 mA. The pri mary beam with a width of about 4 mm was generated by an electron gun with plasma cathode. The EBD was performed using a probing high energy (14 keV) electron beam with a total beam cur rent of 10 mA, which was generated by an electron gun with hot cathode. The probing beam width did not exceed 1 mm. The optical emission induced by this diagnostic electron beam was collected by a lens onto the entrance slit of a monochromator, expanded into the spectrum, and detected by a photoelectron multi plier, the output signal of which was digitized and fed to a computer. The spectrum of emission was con structed and processed using a standard interface. The rotational temperature was determined using a partly resolved rotational spectrum of the (0–0) band + of 1NS N 2 , which was measured at a monochromator slit width of 0.05–0.1 mm at a dispersion of 0.65 nm/mm. The spectrum used to calculate the rotational temperature in the activated jet was obtained by subtracting the spectrum of intrinsic opti cal emission from plasma (measured with only the activating electron gun switched on) from the spec trum of emission induced by the diagnostic electron beam in the activated jet (measured with both activat ing and diagnostic electron guns switched on). The other conditions of spectral measurements (mono chromator slit width, signal amplifier gain, photomul tiplier voltage) were unchanged. The gas density was determined by measuring the integral intensity of emission in the (0–0) band of the + 2 + 2 + B Σu X Σ g N 2 transition in nitrogen ions. The monochromator slit was oriented perpendicular to the diagnostic electron beam. The size of the probed region was 2 mm across the beam and 0.03 mm along the beam. The monochromator entrance and ext slit widths were 0.1 and 2.2 mm, respectively. At a disper sion of 1.3 nm/mm, this allowed the entire spectral band width to be covered. For the experimental data processing, the setup was calibrated under static con ditions. For determining the gas density in the acti vated jet, a signal corresponding to the intrinsic emis sion was subtracted from that measured in activated jet. Figure 1 shows axial profiles of the rotational tem perature of nitrogen molecules in the neutral and acti vated jets. For illustration, a vertical dashed line indi cates the position of the zone of electronbeam activa tion. The EBD measurements in activated jets were performed at distances greater than 20 mm from the 2011
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0
n/n0 80 60 Activating electron bean
Tr, K 100 90 80 70 60 50 40 30 20 10
40 20
10−2 20
40
60
1 2 3 4
1 2
10−3
10 20 30 40 50 60 70 80 90 100 X, mm
0
Fig. 1. Profiles of the rotational temperature of nitrogen molecules along the jet axis in (1) neutral jet at P0 = 54 Torr, (2) activated jet at P0 = 54 Torr, (3) neutral jet at P0 = 6.5 Torr, and (4) activated jet at P0 = 6.5 Torr.
20
40 60 X, mm
80
100
Fig. 2. Axial profiles of the gas density in (1) neutral jet and (2) activated jet at P0 = 54 Torr.
nozzle edge, since at shorter distances the background of intrinsic emission from plasma significantly increased, and the method of simple subtraction of the background from the total signal was inapplicable. At short distances from the nozzle edge, the rotational temperature of nitrogen in the activated jet is about 20 K higher than in the neutral jet. The error of calcu lations for the rotational temperature was within ±3 K. As this distance increases, the temperature in the acti vate jet decreases faster than in the neutral jet, which reflects a local character of the gas heating by the acti vating jet. This can be readily explained, since the transverse electron beam size is 4 mm, while the jet diameter at a distance of 10 mm downstream from the nozzle is about 30 mm. In addition, Fig. 1 presents the data for P0 = 6.5 Torr. As can be seen, a decrease in the gas density leads to lower influence of the activating beam on the gas temperature. Figure 2 shows the axial profiles of the gas density in the neutral and activated jets. These EBD measure ments in activated jets were also performed at dis tances greater than 20 mm from the nozzle edge. The activated jet exhibits additional decrease in the density, which is caused by heating of the gas jet by the elec tronbeamgenerated plasma. A more complete pat tern is provided by Fig. 3, which shows the transverse profiles of the gas density in the neutral and activated jets. Since the gas flow rates in both jets were the same, a decrease in the gas density at the axis must be accom panied by its growth at the periphery. However, this effect is very small, being dependent on the ratios of crosssections, and is not manifested in the obtained experimental data. Thus, the optical EBD method has been used for the first time to measure the rotational temperature and density of gas in a free supersonic nitrogen jet acti
vated by electronbeam plasma. The influence of intrinsic optical emission of plasma in the activated jet was taken into account by merely subtracting this background spectrum from the spectrum of emission induced by the diagnostic electron beam. This method poses certain restrictions on the region of experimen tal parameters, since a sufficiently high accuracy of measurements can be achieved only provided that the intrinsic emission intensity is at least half as small as the total intensity. The obtained results revealed the activating effect of electronbeam plasma on the gas jet parameters. As expected, the rotational tempera ture of nitrogen in the activated jet is increased, while the gas density at the jet axis is decreased as compared to those in the neutral jet. n/n0 1 2
10−3
10−4 −40
−30 −20
−10
0 10 Y, mm
20
30
40
Fig. 3. Transverse profiles of the gas density in (1) neutral jet and (2) activated jet at P0 = 54 Torr.
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Translated by P. Pozdeev
2011
