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Spectral measurements of inductively coupled and helicon discharge modes of a laboratory argon plasma source
Spectrochimica Acta Part B 66 (2011) 149–155
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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Spectral measurements of inductively coupled and helicon discharge modes of a laboratory argon plasma source Murat Celik ⁎ Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
a r t i c l e
i n f o
Article history: Received 26 August 2010 Accepted 12 January 2011 Available online 19 January 2011 Keywords: Helicon plasma Inductively-coupled plasma Spectroscopic measurement Electric propulsion
a b s t r a c t An experimental study was conducted to investigate the effects of several operational parameters in the emission spectra, in the 400–850 nm wavelength region, of a laboratory Argon plasma source. In particular, the emission spectra of the inductively coupled plasma and the Helicon plasma modes of operation were compared. Comparisons of spectra point to a significant increase in the ionization fraction of the plasma for the Helicon mode of operation. The spectral measurements allow one to determine the major trends in the plasma electron density for various parameters such as power delivered to the helical antenna, propellant mass flow rate, and applied external magnetic field intensity. Analysis of a prominent Argon single ion line, at 434.8 nm, points out that the plasma electron density increases linearly with the power delivered to the helical antenna, and that there is an optimum propellant mass flow rate for maximum ionization fraction. Additional analysis of the same line shows that above a minimum applied axial magnetic field intensity, the variation in the magnetic field strength has little effect on the plasma electron density. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The use of helical shaped Radio Frequency (RF) antennas to create high density plasmas (n N 1018m− 3) has been widely studied. In the inductively coupled plasma (ICP) sources, typically, the region of plasma generation is surrounded by a helical shaped coil that creates a time varying magnetic field around it when supplied with RF currents. The time varying magnetic field induces a solenoidal RF electric field which accelerates the free electrons and creates the plasma [1,2]. In the helicon plasma sources, similar to the ICP sources, a radio frequency driven helical antenna is placed around a dielectric cylinder but with a direct current (DC) axial magnetic field applied in the region of the plasma generation allowing the excitation of a helicon wave within the source of the plasma [1]. Because of their efficient high density plasma production, the helicon plasma sources are getting increased attention over the past few decades [3–6]. However, the detailed mechanism of the helicon mode plasma generation is still an ongoing scientific debate [7–10]. The mini Helicon Thruster Experiment, mHTX, has been built to characterize the helicon plasma source in order to gain a better understanding of the plasma generation by helical shaped RF antenna, and identify methods by which plasma parameters can be tuned to
⁎ MIT Space Propulsion Laboratory, currently Assistant Professor at Bogazici University, Istanbul, Turkey. Tel.: + 90 212 359 7372; fax: + 90 212 287 2456. E-mail address: [email protected]. 0584-8547/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2011.01.003
accelerate the obtained high density plasma in order to achieve an efficient propulsive system [11–13]. In electric thrusters external energy is used to ionize gas and then accelerate the resulting plasma using electric and magnetic field forces. In thruster concepts such as the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), a helicon source is used to produce high density plasma, while a secondary stage is used to heat the ions by ion cyclotron resonance heating using radio frequency waves and a magnetic nozzle is used to convert azimuthal momentum into axial momentum to accelerate the gas particles [14–16]. For the mHTX concept, the goal is to obtain high density plasma using a helicon discharge and then accelerate it through thermal pressure which creates ambipolar potential gradients. The power is delivered to the particles through wave–particle coupling using the helicon waves. In the current study, emission spectroscopy is used as a means to deduce information about the plasma through the measurement of line radiation emitted from the plasma particles [17]. It is shown that change in the operational parameters significantly affects the ionization fraction of the plasma. 2. Experimental setup and procedures All spectral measurements were conducted at the MIT Space Propulsion Laboratory. The plasma source was placed inside the 1.5 m diameter 1.6 m long vacuum chamber that is equipped with a mechanical roughing pump and two cryogenic pumps with a total pumping capacity of 7000 L/s for Xenon [18].
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2.1. The plasma source The mHTX Helicon plasma source consists of a pair of cylindrical electromagnets surrounding a long 2 cm diameter cylindrical quartz tube where the propellant gas flows through. A 9.86 cm long 2 cm diameter right hand polarized half-helical antenna made of copper is placed over the quartz tube between the electromagnets [11]. The length of the antenna, 9.86 cm, was chosen to be the half wavelength of the 13.56 MHz frequency m = 1 Helicon waves in a plasma of 1020 m− 3 in density for 20 eV electron energy [11]. A schematic of the experimental Helicon plasma source is shown in Fig. 1. One end of the quartz tube is attached to a propellant gas flow line, and the other is open to the vacuum of the chamber. The gas flow is controlled by a digital flow meter located outside of the vacuum chamber. Even though in this study only the tests on Argon gas are presented, the plasma source was run on xenon, nitrogen, neon and air as well as nitrogen–argon mixture of varying ratios. The helical antenna is powered by a 1200 W RF power supply, Advanced Energy RFPP-10, operating at 13.56 MHz. The antenna is connected to the RF power supply by an in-house built coaxial transmission line located inside the chamber, a 13.56 MHz vacuum RF power feedthrough, and an impedance-matching network attached to the vacuum port on the outside of the chamber. The impedancematching network has a classic L-network circuit structure that employs two adjustable vacuum capacitors [11]. In order to produce the helicon discharge, the external magnetic field is generated through the use of a pair of electromagnets in Helmholtz configuration. The magnet system produces a maximum axial magnetic field intensity of 210 mT at the center, axial region between the electromagnets for 35 A of current to each coil. The helicon antenna is located in this high, axial magnetic field region. During the experiments the magnet current is modified for the desired magnetic field intensity. The plasma source was located in the center of the vacuum tank on a metal platform, and the antenna region was aligned with one of the side windows.
2.2. Spectral measurement setup The radiation collection, from the plasma region of interest, is accomplished by a pair of 2.54 cm diameter collimating–focusing lenses with 100 cm and 10 cm focal lengths, respectively. The whole optical setup is placed on a metal shelf attached to the vacuum tank window as shown in Fig. 2. The end of a 91.44 cm long Oriel fiber bundle is placed at the focal point of the focusing lens. In order to reduce the stray light from inside the vacuum tank, the window is covered with a black optical card board with a 2.54 cm hole cut in the middle, in line with the lens axis. A mechanical diaphragm is placed before the collimating lens in order to adjust the intensity of the
collected light. The shelf is covered with an optically opaque black cloth during the data acquisition. Mechanical translation stages were used to adjust the exact location and the proper alignment of the optical components. A fiber adapter is used to hold the fiber bundle directly at the entrance slit of the spectrometer. A Thermo Jarrel Ash Monospec-18 spectrometer was used as the dispersive instrument. This 15.6 cm focal length, f/3.8 aperture Czerny turner type spectrometer provides a resolution of ∼0.7 nm for 1200 g/mm grating. An Andor iDus DU420A CCD detector was attached to the exit port of the spectrometer for the presented spectral measurements. The intensity calibration of the measured spectra was achieved by using a tungsten lamp with known continuum emission intensity. The tungsten lamp was placed in the same location as that of the helicon plasma source inside the vacuum chamber. The emission intensity produced by the tungsten lamp was measured with the same optical path and exposure time for each spectrometer dial setting that a helicon plasma emission data was taken. 2.3. Spectral measurement procedure A vacuum pressure level of 1.2 × 10− 7 Torr was obtained before the plasma source started operating. First, the propellant flow was turned on by digitally setting the flow rate on the flow-meter located outside of the vacuum chamber. For 20 sccm of argon flow, the background pressure inside the vacuum tank stabilized around 3.2 × 10− 5 Torr. Second, the magnet power supplies were turned on and currents of up to 35 A were delivered to the electromagnet coils to create the desired magnetic field intensity in the antenna region between the electromagnets. Next, the RF power supply was turned on and was set to a desired power level up to the maximum 1200 W. As the RF power was delivered to the antenna, a plasma discharge was observed. The capacitance of the impedance-matching network circuit was adjusted by changing the dial settings on the two capacitors until the best impedance match for the plasma discharge was obtained. The best impedance-match was verified by monitoring the total RF power delivered to the plasma on the RF power supply display. The match was also confirmed by visibly observing the brightness and stability of the obtained plasma from a vacuum window port. Spectral measurements were then taken for varying operational parameters. 3. Results and discussion During the spectral measurements, several operational parameters were varied and the plasma emission spectra were recorded. The RF power delivered to the plasma was varied from 400 W to 1200 W. The magnet currents were varied from 0 to 35 A. An ampere of magnet coil current corresponds to ∼ 6 mT maximum axial magnetic field intensity in the antenna region. Thus the maximum axial magnetic field strength of up to 210 mT was obtained. The argon propellant flow rate was varied from 10 to 100 sccm. Spectral data were taken for the wavelength range of 400– 850 nm as this was the high sensitivity range for the available CCD detector. A study of the prominent argon neutral and single-ion emission lines in this wavelength region shows that the spectrum is dominated by the argon single-ion lines in the 400–550 nm region, however in the 700–850 nm region all the prominent lines are those of the neutral argon atom according to NIST Atomic Spectra Database [19]. 3.1. ICP vs. helicon regimes
Fig. 1. Schematic of the Helicon Plasma Source.
From the visual observation of the plasma, the increased magnetic field makes a significant difference in the color and the intensity of the radiation emanating from the discharge. Also, for the case with no magnetic field, which can be called Inductively Coupled Plasma mode,
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
151
Fig. 2. Schematic of the experimental setup for spectroscopic measurements.
there was no visible plume coming from the plasma source. However, for the high magnetic field case, the Helicon mode, a very bright plume formed at the open end of the quartz tube. Fig. 3 shows the comparison of the high resolution emission spectra for the Helicon and the ICP modes of operation for the argon discharge in the 400–850 nm wavelength range. The compared two spectra constructed from data were taken with the same exposure time and spectrometer dial settings. For both modes the argon propellant flow rate was set at 20 sccm. For the ICP mode of operation, the current to the magnets was turned off and for the Helicon mode of operation the magnetic field intensity in the antenna region was 180 mT (with 30 A of current to each magnet coils). In the Helicon mode operation the total power delivered to the helical antenna was set at 1000 W. As seen from the inset pictures in Fig. 3, for the Helicon mode of operation a bright blue plasma is observed, whereas for the ICP mode the observed plasma is dimmer and more reddish. This observation is substantiated by the measured spectra of the two modes of the operation. As seen from the spectral graphs when the magnetic field is turned on, the intensity of the plasma ionic emission lines increases as observed from the increase in the emission intensity
of the blue portion of the spectrum where the ion emission lines are dominant. Focusing only on the 800–850 nm portion of the spectrum where there are only strong argon neutral emission lines, Fig. 4 shows the comparison of spectra for the ICP and Helicon modes of operation. As observed from this spectral graph, the neutral emission intensity drops as the mode changes from ICP to Helicon. As discussed, the magnetic field effects on the discharge are profound. Spectral measurements of the antenna region emission spectrum are presented in Fig. 5 for the blue portion of the electromagnetic visible spectrum. In Fig. 5, the ICP emission intensity is 50 times amplified to make the comparison of the spectrum with that of the helicon discharge emission more clear. A few prominent argon neutral and single ion emission lines are identified with red and blue labels, respectively. As observed from the presented spectra, prominent argon ion lines appear in the Helicon mode of operation. In the ICP mode of operation the emission due to argon neutral lines are more prominent. In the Helicon mode, the neutral emission intensity seems to drop, as the argon ion emission lines in the 405–485 nm wavelength range
Fig. 3. Comparison of the Argon Plasma Spectra for the ICP and Helicon modes of operation.
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6 ICP Helicon
811.5nm
5
Intensity [a.u.]
4 800.6nm
3 801.5nm
2
842.5nm
810.4nm
840.8nm 826.5nm
1
0 800
810
820
830
840
850
Wavelength [nm] Fig. 4. Comparison of the Argon Plasma Spectra for Helicon and ICP modes of operation in 800–850 nm region.
become visible. This also explains why the helicon discharge plasma looks bright blue as opposed to reddish ICP plasma for the argon gas. The decrease in the neutral emission can be associated with the increase in the ionization fraction of the gas due to the more efficient heating of the electrons through wave–particle interactions of the Helicon mode.
ionization fraction varies between the two modes of operation. For a given ion line, assuming that the plasma is uniform, optically-thin and quasi-neutral, and that the excitation processes are dominated by electron induced collisions, the emission intensity will be given by
3.1.1. Change in ionization fraction for the ICP and Helicon modes of operation As discussed, the dramatic change in the observed emission spectra of the ICP and the Helicon modes of operation indicates a sharp increase in the ionization fraction of the plasma when the external magnetic field of 180 mT is provided. It is observed from Fig. 4 that when compared with the ICP mode of operation, the emission intensity of the argon neutral lines at 800.6 nm, 801.5 nm, 810.4 nm, 811.5 nm, 826.5 nm, 840.8 nm and 842.5 nm drops by a factor of 3.7 ± 0.6 for the Helicon mode of operation. Similarly, as observed from the spectra shown in Fig. 5, the emission intensity of the prominent argon ion lines at 434.8 nm, 473.6 nm, and 480.6 nm increases by a factor of 105.4 ± 9.2, as the magnetic field is turned on, indicating a sharp increase in the ionization fraction of the propellant gas. Using these ratios, it is possible to obtain a quantitative estimate on how significantly the
similarly for a neutral line, the emission intensity will be
2
Ii = ne ni hσi ve i = ne Ri ðTe Þ
ð1Þ
In = ne nn hσn ve i = ne nn Rn ðTe Þ
ð2Þ
where ne, ni and nn represent the electron, ion and neutral densities respectively, Te is the electron temperature, σi and σn are the total excitation collision cross section for given ion and neutral emission lines respectively and ve is the electron velocity. The respective excitation collision rate functions Ri(Te) = 〈σive〉 and Rn(Te) = 〈σnve〉 are only functions of the plasma electron temperature. For a given background density no, assuming single ionization only, the sum of ion and neutral particle densities would be a constant such that ni + nn = no. The plasma ionization fraction, η, can be defined as the ratio of the ion number density to the total particle density such n that η = i . Thus for a given plasma, the ratio of the electron and no
6 Helicon ICP*
480.6nm
Intensity [a.u.]
5
4
420.0nm
434.8nm
3
415.9nm 473.6nm
2
. 4259nm 430.0nm
442.6nm
451.1nm 461.0nm
1
0 405 410 415 420 425 430 435 440 445 450 455 460 465 470 475 480 485
Wavelength [nm] Fig. 5. Comparison of the Argon Plasma Spectra for Helicon and ICP modes of operation in 405–485 nm region. *The ICP spectrum is amplified 50 times.
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
ne η . The ratio of the ICP = 1−η nn and helicon emission line intensities as a function of related particle densities and the ratios of the appropriate temperature dependant rate functions can then be written:
neutral densities would be given by
IiHEL IiICP
InHEL InICP
!2 n2eHEL Ri TeHEL neHEL Ri TeHEL = = neICP n2eICP Ri TeICP Ri TeICP !2 neHEL Ki TeHEL ; TeICP = neICP ! ! neHEL nnHEL Rn TeHEL neHEL nnHEL Rn TeHEL = = neICP nnICP R T neICP nnICP Rn TeICP n eICP ! ! neHEL nnHEL = Kn TeHEL ; TeICP neICP nnICP
Ri Te
where Ki =
HEL
Ri Te
and Kn =
ICP
Rn Te
HEL
Rn Te
ð4Þ
are the ratios of the rate
ICP
functions for the ion and neutral lines for electron temperatures at Helicon and ICP modes of operation. From the spectral measurements IiHEL IiICP
=
neHEL
!2 Ki ≈105:4 9:2
neICP
and using the relationship nn =
InHEL InICP
ð5Þ
1−η η
ne
0 1 1−ηHEL ! neHEL C neHEL B ηHEL B CKn = Kn = A neICP nnICP neICP @ 1−ηICP neICP ηCP !2 neHEL 1−ηHEL ηICP 1 = K ≈ neICP 1−ηICP ηHEL n 3:7 0:6 neHEL
!
nnHEL
!
ð6Þ
comparing these two ratios and rearranging the terms give
1−ηICP 1−ηHEL
ηHEL Ki ≈390 75: ηICP Kn
ð7Þ
Drawin [20] proposed analytical formulas for the cross section for electron induced excitation of an atom. These analytical formulas have been adapted by many researchers for collisional–radiative models of argon [21–25]. For optically allowed transitions, the Drawin's formula for the electron induced excitation cross-section is [26,27]: H 2 E σij Uij = 4πao ion Eij
!2 αij fij
Uij −1 Uij2
ln 1:25βij Uij
ð8Þ
where Eij is the energy difference between the higher level j and the lower level i such that Eij = Ej − Ei, Uij is the normalized electron energy, Uij = Ee/Eij, EH ion is the ionization energy of hydrogen, ao is the Bohr radius, fij is the oscillator strength, and αij and βij are collision cross section parameters which are of the order of unity [26]. Assuming a normalized Maxwellian energy distribution for the electrons: sffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Ee −kTEe fe ðEe Þ = pffiffiffi e e π ðkTe Þ3
the excitation collision rate function is calculated by the integral: D E Rij = ve σij = ∫∞0 fe ðEe Þσij
= ð3Þ
ð9Þ
153
∫∞0 2Ee
sffiffiffiffiffiffiffiffi 2Ee dE me e ð10Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 2 − e e kTe σij dEe πðkTe Þ3 me
where sffiffiffiffiffiffiffiffi me and Ee are the electron mass and energy respectively, and 2Ee is the electron velocity. From the expression it can be seen that me the excitation collision rate is a function of electron temperature, Te, only. According to Eq. (8), in addition to being a function of the electron energy and the fitting parameters, αij and βij, that are determined experimentally, the excitation cross-section is a function of the energy of the levels that the electron transition occurs. Thus, the cross section values are line-specific and will vary between different emission lines. Similarly, the value of the excitation rate function, obtained by evaluating the integral in Eq. (10), would differ depending on the plasma temperature and the assumed values of the fitting parameters, α and β. In order to obtain an order of magnitude approximation for the change in the ionization fraction, η, of the plasma between the two experimentally observed modes of operation a few major assumptions are made: It is assumed that Drawin's proposed cross-sections can be used for both the neutral and the ionic excitation cross-sections (as suggested in [28]). In addition, it is assumed that the same Drawin's parameters (α and β) can be used for all the ion and the neutral lines. This assumption, for the neutral lines in consideration, is in line with reference [27] as these lines are due to transitions between higher energy levels. Similarly, the ion lines are also due to electronic transitions between higher energy levels of argon ion. With these assumptions, the ratios Ki and Kn would be equal even though the electron temperatures for the Helicon and the ICP modes (TeHEL and TeICP) are different as reported by Sinenian [13]. Even if the used Drawin's parameters are quite different for the neutral and the ion lines this equality would still hold very well. Then, Eq. (7) can be solved for the Helicon mode ionization fraction, ηHEL, in terms of the observed line intensity ratios and the ICP mode ionization fraction, ηICP. ηHEL ≈
390 × ηICP : 390 × ηICP + ð1−ηICP Þ
ð11Þ
Thus, if the ionization fraction for the ICP mode of operation is known, then the Helicon mode ionization fraction can be approximated using the measured spectral line intensity ratios. From the current measurements, for ηICP = 0.1% ⇒ ηHEL ≈ 28.1%, for ηICP = 0.3% ⇒ ηHEL ≈ 54.0%, for ηICP = 1% ⇒ ηHEL ≈ 79.8%, etc. Thus, the presented quantitative analysis indicates a significant increase in the plasma ionization fraction for the Helicon mode of operation. Such significant increase in the plasma ionization fraction is expected based on other studies on density jump in helicon discharges such as [29–31] where invasive probes are used. Frank et al. [30] reports a sudden plasma density jump of more than a factor of 20 when the transition to Helicon mode of operation occurs, indicating a significant Table 1 Plasma source operational parameter scans.
Power scan Flow rate scan Magnet current scan
Power
Flow rate
Magnet current
400–1200 W 1000 W 1000 W
20 sccm 10–100 sccm 20 sccm
30 A 30 A 10–35 A
154
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
jump in the plasma ionization fraction. Similarly, Sudit et al. [31] report plasma density increase of up to 80 times between the cases when no magnetic field is applied (ICP mode) and an axial magnetic field of 80 mT is applied (Helicon mode).
[13] at the exit plane of a similar helicon source show that that the electron temperature remains roughly constant around ∼ 3.3 ± 0.3 eV for varying RF power. Thus, if the effect of temperature variations is neglected, the observations indicate that the electron density has a square root dependence to the variation in the delivered RF power.
3.2. Observations of operational parameter scans As stated earlier, several operational parameters were varied and the emission spectra of the resulting plasma were measured. The scanned parameters are tabulated in Table 1. For all measurements discussed in this section, the Helicon mode of operation, characterized by the intense “blue core” plasma, is achieved by adjusting the capacitance of the impedance-matching network circuit. In order to analyze the relative strength of the obtained plasma, a prominent argon single ion emission line at 434.8 nm was identified and studied. For the given emission line, the following relationship provides the dependence of the emission intensity on density and electron temperature 2
I434:8nm ∝ ne ni hσ434:8nm ve i ∝ ne R434:8nm ðTe Þ
ð12Þ
where ne and ni are electron and ion densities respectively, Te is the electron temperature, σ434.8nm is the total excitation collision cross section for argon ion 434.8 nm line and ve is the electron velocity. The excitation collision rate function R434.8nm(Te) = 〈σ434.8nmve〉 is only a function of electron temperature. Thus, the emission intensity of this line has quadratic dependence on the plasma electron density. This is shown to be valid when invasive probe measurements for plasma density variation is compared with similar argon ion line emission intensity variation (i.e., variation of Ar ion 488 nm line by [31] and Ar ion 443 nm line by [32]). 3.2.1. Power scan The plasma emission spectra were measured for the Helicon mode of operation as the RF power delivered to the plasma by the helical antenna was varied from 400 W to 1200 W in 100 W increments. The emission spectra were measured for the antenna region where the discharge occurs. For these measurements the Argon flow rate was kept at 20 sccm and the magnet current was set at 30 A corresponding to an axial magnetic field intensity of ∼180 mT in the antenna region. Fig. 6 shows the variation in the Argon ion 434.8 nm line intensity as the delivered RF power is varied. A linear fit to the data is also included. As seen in the graph, the intensity of the line increases linearly with the power delivered to the antenna. Since the flow rate is kept constant, the total particle density, no, can be assumed to remain roughly constant. Thus, the linear increase in the ion emission intensity is attributed to the increase in the ionization fraction and the electron temperature. Langmuir probe measurements by Sinenian
3.2.2. Flow rate scan The argon propellant flow rate was varied between 10 sccm and 100 sccm and the plasma emission spectra from the antenna region were measured for the Helicon mode of operation. In these measurements, the RF power delivered was kept constant at 1000 W and the magnet current was set at 30 A corresponding to a maximum axial magnetic field intensity of 180 mT in the antenna region. The measured emission intensity values of the argon ion 434.8 nm line are presented in Fig. 7. As seen from the figure, when the mass flow rate is varied, the ion emission intensity first increases up to a flow rate of 25 sccm and then decreases for higher flow rates. So, for the given power level and magnetic field configuration, there is an optimum propellant flow rate that maximizes ion line emission intensity. Thus, the ionization rate is limited at high flow rates, as very high background neutral gas density increases the electron energy loss. This leads to reduced electron temperature and increases the energy cost of electron-ion production. This observation is again consistent with the Langmuir probe measurements of Sinenian [13], where the author reports a drop in electron temperature from 4.34 eV to 2.39 eV when the argon propellant flow rate is increased from 10 sccm to 50 sccm. 3.2.3. Magnetic field intensity scan The magnetic field intensity in the antenna region is varied by changing the current supplied to the electromagnets. For a delivered RF power of 1000 W and argon flow rate of 20 sccm, the current is varied from 10 A to 35 A in 5 A increments, corresponding to changing the maximum axial magnetic field intensity from 60 mT to 210 mT with 30 mT increments, and the plasma emission spectra from the antenna region were recorded. Fig. 8 shows the measured emission intensity of argon ion line at 434.8 nm for varying currents to the electromagnet coils. As seen in the figure, when the applied axial magnetic field intensity is increased, the argon ion emission line first increases and then levels off. So, for the given flow rate and power level, it appears that above a certain level of magnetic field intensity the plasma density variation with magnetic field intensity variation is limited and even shows an apparent slight decrease after 150 mT of axial magnetic field intensity. This observation agrees with the RF compensated Langmuir probe measurements of the plasma density by [33] for a similar plasma source where it is observed that the plasma electron density levels off above a certain minimum magnetic 1.6
1.2 1.4 1.2
Intensity [a.u.]
Intensity [a.u.]
1 0.8 0.6 0.4
1 0.8 0.6 0.4
0.2 0 300
0.2 0 400
500
600
700
800
900
1000 1100 1200 1300
RF Power [W] Fig. 6. Argon ion 434.8 nm line emission intensity as delivered RF power is varied.
10
30
50
70
90
110
Mass Flow Rate [sccm] Fig. 7. Argon ion 434.8 nm line emission intensity as delivered Argon flow rate is varied.
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
1.2
References
1
Intensity [a.u.]
155
0.8 0.6 0.4 0.2 0 5
10
15
20
25
30
35
40
Magnet Current [A] Fig. 8. Argon ion 434.8 nm line emission intensity as the current to the magnet is varied.
field strength. It is suggested that, this observation indicates a critical magnetic field strength to “fit” the helicon wave into the chamber [33]. In those measurements, for the case of 13 MHz antenna frequency, above the minimum required magnetic field strength, the plasma electron density shows a slight decrease for increased magnetic field intensity (see ref [33] Fig. 9) similar to our observation. However, the reasons for this slight decrease are not understood. 4. Conclusion The emission spectra of a laboratory helicon plasma source were measured for various operational parameters. Spectral measurements provided a good qualitative understanding of the plasma ionization strength. According to the spectral measurements, the intensity of the ion emission line increases as the power delivered to the RF antenna, thus to the plasma, is increased. For a set RF power, and fixed magnetic field intensity, there is an optimum mass flow rate for the highest plasma electron density. For a set RF power, and constant argon propellant flow rate, it was observed that above a certain minimum magnetic field strength, the plasma electron density shows little change with variation in the magnetic field intensity. The comparison and analysis of spectra obtained for the ICP and Helicon modes of operation indicate a significant increase in the ionization fraction for the Helicon mode where the presence of the axial magnetic field allows the propagation of the electromagnetic waves into the plasma and the efficient transfer of the wave energy to the electrons. Acknowledgments The author would like to thank Prof. Manuel Martinez-Sanchez and Dr. Oleg Batishchev of MIT Space Propulsion Laboratory for providing guidance and support, Dr. Yu-Hui Chiu and Dr. Rainer Dressler of Air Force Research Laboratory at Hanscom for lending the spectral instruments. This research is partially supported by Air Force Research Laboratory/PRSA, Edwards AFB through ERC Inc. subcontract RS060213.
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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Spectral measurements of inductively coupled and helicon discharge modes of a laboratory argon plasma source Murat Celik ⁎ Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
a r t i c l e
i n f o
Article history: Received 26 August 2010 Accepted 12 January 2011 Available online 19 January 2011 Keywords: Helicon plasma Inductively-coupled plasma Spectroscopic measurement Electric propulsion
a b s t r a c t An experimental study was conducted to investigate the effects of several operational parameters in the emission spectra, in the 400–850 nm wavelength region, of a laboratory Argon plasma source. In particular, the emission spectra of the inductively coupled plasma and the Helicon plasma modes of operation were compared. Comparisons of spectra point to a significant increase in the ionization fraction of the plasma for the Helicon mode of operation. The spectral measurements allow one to determine the major trends in the plasma electron density for various parameters such as power delivered to the helical antenna, propellant mass flow rate, and applied external magnetic field intensity. Analysis of a prominent Argon single ion line, at 434.8 nm, points out that the plasma electron density increases linearly with the power delivered to the helical antenna, and that there is an optimum propellant mass flow rate for maximum ionization fraction. Additional analysis of the same line shows that above a minimum applied axial magnetic field intensity, the variation in the magnetic field strength has little effect on the plasma electron density. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The use of helical shaped Radio Frequency (RF) antennas to create high density plasmas (n N 1018m− 3) has been widely studied. In the inductively coupled plasma (ICP) sources, typically, the region of plasma generation is surrounded by a helical shaped coil that creates a time varying magnetic field around it when supplied with RF currents. The time varying magnetic field induces a solenoidal RF electric field which accelerates the free electrons and creates the plasma [1,2]. In the helicon plasma sources, similar to the ICP sources, a radio frequency driven helical antenna is placed around a dielectric cylinder but with a direct current (DC) axial magnetic field applied in the region of the plasma generation allowing the excitation of a helicon wave within the source of the plasma [1]. Because of their efficient high density plasma production, the helicon plasma sources are getting increased attention over the past few decades [3–6]. However, the detailed mechanism of the helicon mode plasma generation is still an ongoing scientific debate [7–10]. The mini Helicon Thruster Experiment, mHTX, has been built to characterize the helicon plasma source in order to gain a better understanding of the plasma generation by helical shaped RF antenna, and identify methods by which plasma parameters can be tuned to
⁎ MIT Space Propulsion Laboratory, currently Assistant Professor at Bogazici University, Istanbul, Turkey. Tel.: + 90 212 359 7372; fax: + 90 212 287 2456. E-mail address: [email protected]. 0584-8547/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2011.01.003
accelerate the obtained high density plasma in order to achieve an efficient propulsive system [11–13]. In electric thrusters external energy is used to ionize gas and then accelerate the resulting plasma using electric and magnetic field forces. In thruster concepts such as the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), a helicon source is used to produce high density plasma, while a secondary stage is used to heat the ions by ion cyclotron resonance heating using radio frequency waves and a magnetic nozzle is used to convert azimuthal momentum into axial momentum to accelerate the gas particles [14–16]. For the mHTX concept, the goal is to obtain high density plasma using a helicon discharge and then accelerate it through thermal pressure which creates ambipolar potential gradients. The power is delivered to the particles through wave–particle coupling using the helicon waves. In the current study, emission spectroscopy is used as a means to deduce information about the plasma through the measurement of line radiation emitted from the plasma particles [17]. It is shown that change in the operational parameters significantly affects the ionization fraction of the plasma. 2. Experimental setup and procedures All spectral measurements were conducted at the MIT Space Propulsion Laboratory. The plasma source was placed inside the 1.5 m diameter 1.6 m long vacuum chamber that is equipped with a mechanical roughing pump and two cryogenic pumps with a total pumping capacity of 7000 L/s for Xenon [18].
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2.1. The plasma source The mHTX Helicon plasma source consists of a pair of cylindrical electromagnets surrounding a long 2 cm diameter cylindrical quartz tube where the propellant gas flows through. A 9.86 cm long 2 cm diameter right hand polarized half-helical antenna made of copper is placed over the quartz tube between the electromagnets [11]. The length of the antenna, 9.86 cm, was chosen to be the half wavelength of the 13.56 MHz frequency m = 1 Helicon waves in a plasma of 1020 m− 3 in density for 20 eV electron energy [11]. A schematic of the experimental Helicon plasma source is shown in Fig. 1. One end of the quartz tube is attached to a propellant gas flow line, and the other is open to the vacuum of the chamber. The gas flow is controlled by a digital flow meter located outside of the vacuum chamber. Even though in this study only the tests on Argon gas are presented, the plasma source was run on xenon, nitrogen, neon and air as well as nitrogen–argon mixture of varying ratios. The helical antenna is powered by a 1200 W RF power supply, Advanced Energy RFPP-10, operating at 13.56 MHz. The antenna is connected to the RF power supply by an in-house built coaxial transmission line located inside the chamber, a 13.56 MHz vacuum RF power feedthrough, and an impedance-matching network attached to the vacuum port on the outside of the chamber. The impedancematching network has a classic L-network circuit structure that employs two adjustable vacuum capacitors [11]. In order to produce the helicon discharge, the external magnetic field is generated through the use of a pair of electromagnets in Helmholtz configuration. The magnet system produces a maximum axial magnetic field intensity of 210 mT at the center, axial region between the electromagnets for 35 A of current to each coil. The helicon antenna is located in this high, axial magnetic field region. During the experiments the magnet current is modified for the desired magnetic field intensity. The plasma source was located in the center of the vacuum tank on a metal platform, and the antenna region was aligned with one of the side windows.
2.2. Spectral measurement setup The radiation collection, from the plasma region of interest, is accomplished by a pair of 2.54 cm diameter collimating–focusing lenses with 100 cm and 10 cm focal lengths, respectively. The whole optical setup is placed on a metal shelf attached to the vacuum tank window as shown in Fig. 2. The end of a 91.44 cm long Oriel fiber bundle is placed at the focal point of the focusing lens. In order to reduce the stray light from inside the vacuum tank, the window is covered with a black optical card board with a 2.54 cm hole cut in the middle, in line with the lens axis. A mechanical diaphragm is placed before the collimating lens in order to adjust the intensity of the
collected light. The shelf is covered with an optically opaque black cloth during the data acquisition. Mechanical translation stages were used to adjust the exact location and the proper alignment of the optical components. A fiber adapter is used to hold the fiber bundle directly at the entrance slit of the spectrometer. A Thermo Jarrel Ash Monospec-18 spectrometer was used as the dispersive instrument. This 15.6 cm focal length, f/3.8 aperture Czerny turner type spectrometer provides a resolution of ∼0.7 nm for 1200 g/mm grating. An Andor iDus DU420A CCD detector was attached to the exit port of the spectrometer for the presented spectral measurements. The intensity calibration of the measured spectra was achieved by using a tungsten lamp with known continuum emission intensity. The tungsten lamp was placed in the same location as that of the helicon plasma source inside the vacuum chamber. The emission intensity produced by the tungsten lamp was measured with the same optical path and exposure time for each spectrometer dial setting that a helicon plasma emission data was taken. 2.3. Spectral measurement procedure A vacuum pressure level of 1.2 × 10− 7 Torr was obtained before the plasma source started operating. First, the propellant flow was turned on by digitally setting the flow rate on the flow-meter located outside of the vacuum chamber. For 20 sccm of argon flow, the background pressure inside the vacuum tank stabilized around 3.2 × 10− 5 Torr. Second, the magnet power supplies were turned on and currents of up to 35 A were delivered to the electromagnet coils to create the desired magnetic field intensity in the antenna region between the electromagnets. Next, the RF power supply was turned on and was set to a desired power level up to the maximum 1200 W. As the RF power was delivered to the antenna, a plasma discharge was observed. The capacitance of the impedance-matching network circuit was adjusted by changing the dial settings on the two capacitors until the best impedance match for the plasma discharge was obtained. The best impedance-match was verified by monitoring the total RF power delivered to the plasma on the RF power supply display. The match was also confirmed by visibly observing the brightness and stability of the obtained plasma from a vacuum window port. Spectral measurements were then taken for varying operational parameters. 3. Results and discussion During the spectral measurements, several operational parameters were varied and the plasma emission spectra were recorded. The RF power delivered to the plasma was varied from 400 W to 1200 W. The magnet currents were varied from 0 to 35 A. An ampere of magnet coil current corresponds to ∼ 6 mT maximum axial magnetic field intensity in the antenna region. Thus the maximum axial magnetic field strength of up to 210 mT was obtained. The argon propellant flow rate was varied from 10 to 100 sccm. Spectral data were taken for the wavelength range of 400– 850 nm as this was the high sensitivity range for the available CCD detector. A study of the prominent argon neutral and single-ion emission lines in this wavelength region shows that the spectrum is dominated by the argon single-ion lines in the 400–550 nm region, however in the 700–850 nm region all the prominent lines are those of the neutral argon atom according to NIST Atomic Spectra Database [19]. 3.1. ICP vs. helicon regimes
Fig. 1. Schematic of the Helicon Plasma Source.
From the visual observation of the plasma, the increased magnetic field makes a significant difference in the color and the intensity of the radiation emanating from the discharge. Also, for the case with no magnetic field, which can be called Inductively Coupled Plasma mode,
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
151
Fig. 2. Schematic of the experimental setup for spectroscopic measurements.
there was no visible plume coming from the plasma source. However, for the high magnetic field case, the Helicon mode, a very bright plume formed at the open end of the quartz tube. Fig. 3 shows the comparison of the high resolution emission spectra for the Helicon and the ICP modes of operation for the argon discharge in the 400–850 nm wavelength range. The compared two spectra constructed from data were taken with the same exposure time and spectrometer dial settings. For both modes the argon propellant flow rate was set at 20 sccm. For the ICP mode of operation, the current to the magnets was turned off and for the Helicon mode of operation the magnetic field intensity in the antenna region was 180 mT (with 30 A of current to each magnet coils). In the Helicon mode operation the total power delivered to the helical antenna was set at 1000 W. As seen from the inset pictures in Fig. 3, for the Helicon mode of operation a bright blue plasma is observed, whereas for the ICP mode the observed plasma is dimmer and more reddish. This observation is substantiated by the measured spectra of the two modes of the operation. As seen from the spectral graphs when the magnetic field is turned on, the intensity of the plasma ionic emission lines increases as observed from the increase in the emission intensity
of the blue portion of the spectrum where the ion emission lines are dominant. Focusing only on the 800–850 nm portion of the spectrum where there are only strong argon neutral emission lines, Fig. 4 shows the comparison of spectra for the ICP and Helicon modes of operation. As observed from this spectral graph, the neutral emission intensity drops as the mode changes from ICP to Helicon. As discussed, the magnetic field effects on the discharge are profound. Spectral measurements of the antenna region emission spectrum are presented in Fig. 5 for the blue portion of the electromagnetic visible spectrum. In Fig. 5, the ICP emission intensity is 50 times amplified to make the comparison of the spectrum with that of the helicon discharge emission more clear. A few prominent argon neutral and single ion emission lines are identified with red and blue labels, respectively. As observed from the presented spectra, prominent argon ion lines appear in the Helicon mode of operation. In the ICP mode of operation the emission due to argon neutral lines are more prominent. In the Helicon mode, the neutral emission intensity seems to drop, as the argon ion emission lines in the 405–485 nm wavelength range
Fig. 3. Comparison of the Argon Plasma Spectra for the ICP and Helicon modes of operation.
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6 ICP Helicon
811.5nm
5
Intensity [a.u.]
4 800.6nm
3 801.5nm
2
842.5nm
810.4nm
840.8nm 826.5nm
1
0 800
810
820
830
840
850
Wavelength [nm] Fig. 4. Comparison of the Argon Plasma Spectra for Helicon and ICP modes of operation in 800–850 nm region.
become visible. This also explains why the helicon discharge plasma looks bright blue as opposed to reddish ICP plasma for the argon gas. The decrease in the neutral emission can be associated with the increase in the ionization fraction of the gas due to the more efficient heating of the electrons through wave–particle interactions of the Helicon mode.
ionization fraction varies between the two modes of operation. For a given ion line, assuming that the plasma is uniform, optically-thin and quasi-neutral, and that the excitation processes are dominated by electron induced collisions, the emission intensity will be given by
3.1.1. Change in ionization fraction for the ICP and Helicon modes of operation As discussed, the dramatic change in the observed emission spectra of the ICP and the Helicon modes of operation indicates a sharp increase in the ionization fraction of the plasma when the external magnetic field of 180 mT is provided. It is observed from Fig. 4 that when compared with the ICP mode of operation, the emission intensity of the argon neutral lines at 800.6 nm, 801.5 nm, 810.4 nm, 811.5 nm, 826.5 nm, 840.8 nm and 842.5 nm drops by a factor of 3.7 ± 0.6 for the Helicon mode of operation. Similarly, as observed from the spectra shown in Fig. 5, the emission intensity of the prominent argon ion lines at 434.8 nm, 473.6 nm, and 480.6 nm increases by a factor of 105.4 ± 9.2, as the magnetic field is turned on, indicating a sharp increase in the ionization fraction of the propellant gas. Using these ratios, it is possible to obtain a quantitative estimate on how significantly the
similarly for a neutral line, the emission intensity will be
2
Ii = ne ni hσi ve i = ne Ri ðTe Þ
ð1Þ
In = ne nn hσn ve i = ne nn Rn ðTe Þ
ð2Þ
where ne, ni and nn represent the electron, ion and neutral densities respectively, Te is the electron temperature, σi and σn are the total excitation collision cross section for given ion and neutral emission lines respectively and ve is the electron velocity. The respective excitation collision rate functions Ri(Te) = 〈σive〉 and Rn(Te) = 〈σnve〉 are only functions of the plasma electron temperature. For a given background density no, assuming single ionization only, the sum of ion and neutral particle densities would be a constant such that ni + nn = no. The plasma ionization fraction, η, can be defined as the ratio of the ion number density to the total particle density such n that η = i . Thus for a given plasma, the ratio of the electron and no
6 Helicon ICP*
480.6nm
Intensity [a.u.]
5
4
420.0nm
434.8nm
3
415.9nm 473.6nm
2
. 4259nm 430.0nm
442.6nm
451.1nm 461.0nm
1
0 405 410 415 420 425 430 435 440 445 450 455 460 465 470 475 480 485
Wavelength [nm] Fig. 5. Comparison of the Argon Plasma Spectra for Helicon and ICP modes of operation in 405–485 nm region. *The ICP spectrum is amplified 50 times.
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
ne η . The ratio of the ICP = 1−η nn and helicon emission line intensities as a function of related particle densities and the ratios of the appropriate temperature dependant rate functions can then be written:
neutral densities would be given by
IiHEL IiICP
InHEL InICP
!2 n2eHEL Ri TeHEL neHEL Ri TeHEL = = neICP n2eICP Ri TeICP Ri TeICP !2 neHEL Ki TeHEL ; TeICP = neICP ! ! neHEL nnHEL Rn TeHEL neHEL nnHEL Rn TeHEL = = neICP nnICP R T neICP nnICP Rn TeICP n eICP ! ! neHEL nnHEL = Kn TeHEL ; TeICP neICP nnICP
Ri Te
where Ki =
HEL
Ri Te
and Kn =
ICP
Rn Te
HEL
Rn Te
ð4Þ
are the ratios of the rate
ICP
functions for the ion and neutral lines for electron temperatures at Helicon and ICP modes of operation. From the spectral measurements IiHEL IiICP
=
neHEL
!2 Ki ≈105:4 9:2
neICP
and using the relationship nn =
InHEL InICP
ð5Þ
1−η η
ne
0 1 1−ηHEL ! neHEL C neHEL B ηHEL B CKn = Kn = A neICP nnICP neICP @ 1−ηICP neICP ηCP !2 neHEL 1−ηHEL ηICP 1 = K ≈ neICP 1−ηICP ηHEL n 3:7 0:6 neHEL
!
nnHEL
!
ð6Þ
comparing these two ratios and rearranging the terms give
1−ηICP 1−ηHEL
ηHEL Ki ≈390 75: ηICP Kn
ð7Þ
Drawin [20] proposed analytical formulas for the cross section for electron induced excitation of an atom. These analytical formulas have been adapted by many researchers for collisional–radiative models of argon [21–25]. For optically allowed transitions, the Drawin's formula for the electron induced excitation cross-section is [26,27]: H 2 E σij Uij = 4πao ion Eij
!2 αij fij
Uij −1 Uij2
ln 1:25βij Uij
ð8Þ
where Eij is the energy difference between the higher level j and the lower level i such that Eij = Ej − Ei, Uij is the normalized electron energy, Uij = Ee/Eij, EH ion is the ionization energy of hydrogen, ao is the Bohr radius, fij is the oscillator strength, and αij and βij are collision cross section parameters which are of the order of unity [26]. Assuming a normalized Maxwellian energy distribution for the electrons: sffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Ee −kTEe fe ðEe Þ = pffiffiffi e e π ðkTe Þ3
the excitation collision rate function is calculated by the integral: D E Rij = ve σij = ∫∞0 fe ðEe Þσij
= ð3Þ
ð9Þ
153
∫∞0 2Ee
sffiffiffiffiffiffiffiffi 2Ee dE me e ð10Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 2 − e e kTe σij dEe πðkTe Þ3 me
where sffiffiffiffiffiffiffiffi me and Ee are the electron mass and energy respectively, and 2Ee is the electron velocity. From the expression it can be seen that me the excitation collision rate is a function of electron temperature, Te, only. According to Eq. (8), in addition to being a function of the electron energy and the fitting parameters, αij and βij, that are determined experimentally, the excitation cross-section is a function of the energy of the levels that the electron transition occurs. Thus, the cross section values are line-specific and will vary between different emission lines. Similarly, the value of the excitation rate function, obtained by evaluating the integral in Eq. (10), would differ depending on the plasma temperature and the assumed values of the fitting parameters, α and β. In order to obtain an order of magnitude approximation for the change in the ionization fraction, η, of the plasma between the two experimentally observed modes of operation a few major assumptions are made: It is assumed that Drawin's proposed cross-sections can be used for both the neutral and the ionic excitation cross-sections (as suggested in [28]). In addition, it is assumed that the same Drawin's parameters (α and β) can be used for all the ion and the neutral lines. This assumption, for the neutral lines in consideration, is in line with reference [27] as these lines are due to transitions between higher energy levels. Similarly, the ion lines are also due to electronic transitions between higher energy levels of argon ion. With these assumptions, the ratios Ki and Kn would be equal even though the electron temperatures for the Helicon and the ICP modes (TeHEL and TeICP) are different as reported by Sinenian [13]. Even if the used Drawin's parameters are quite different for the neutral and the ion lines this equality would still hold very well. Then, Eq. (7) can be solved for the Helicon mode ionization fraction, ηHEL, in terms of the observed line intensity ratios and the ICP mode ionization fraction, ηICP. ηHEL ≈
390 × ηICP : 390 × ηICP + ð1−ηICP Þ
ð11Þ
Thus, if the ionization fraction for the ICP mode of operation is known, then the Helicon mode ionization fraction can be approximated using the measured spectral line intensity ratios. From the current measurements, for ηICP = 0.1% ⇒ ηHEL ≈ 28.1%, for ηICP = 0.3% ⇒ ηHEL ≈ 54.0%, for ηICP = 1% ⇒ ηHEL ≈ 79.8%, etc. Thus, the presented quantitative analysis indicates a significant increase in the plasma ionization fraction for the Helicon mode of operation. Such significant increase in the plasma ionization fraction is expected based on other studies on density jump in helicon discharges such as [29–31] where invasive probes are used. Frank et al. [30] reports a sudden plasma density jump of more than a factor of 20 when the transition to Helicon mode of operation occurs, indicating a significant Table 1 Plasma source operational parameter scans.
Power scan Flow rate scan Magnet current scan
Power
Flow rate
Magnet current
400–1200 W 1000 W 1000 W
20 sccm 10–100 sccm 20 sccm
30 A 30 A 10–35 A
154
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
jump in the plasma ionization fraction. Similarly, Sudit et al. [31] report plasma density increase of up to 80 times between the cases when no magnetic field is applied (ICP mode) and an axial magnetic field of 80 mT is applied (Helicon mode).
[13] at the exit plane of a similar helicon source show that that the electron temperature remains roughly constant around ∼ 3.3 ± 0.3 eV for varying RF power. Thus, if the effect of temperature variations is neglected, the observations indicate that the electron density has a square root dependence to the variation in the delivered RF power.
3.2. Observations of operational parameter scans As stated earlier, several operational parameters were varied and the emission spectra of the resulting plasma were measured. The scanned parameters are tabulated in Table 1. For all measurements discussed in this section, the Helicon mode of operation, characterized by the intense “blue core” plasma, is achieved by adjusting the capacitance of the impedance-matching network circuit. In order to analyze the relative strength of the obtained plasma, a prominent argon single ion emission line at 434.8 nm was identified and studied. For the given emission line, the following relationship provides the dependence of the emission intensity on density and electron temperature 2
I434:8nm ∝ ne ni hσ434:8nm ve i ∝ ne R434:8nm ðTe Þ
ð12Þ
where ne and ni are electron and ion densities respectively, Te is the electron temperature, σ434.8nm is the total excitation collision cross section for argon ion 434.8 nm line and ve is the electron velocity. The excitation collision rate function R434.8nm(Te) = 〈σ434.8nmve〉 is only a function of electron temperature. Thus, the emission intensity of this line has quadratic dependence on the plasma electron density. This is shown to be valid when invasive probe measurements for plasma density variation is compared with similar argon ion line emission intensity variation (i.e., variation of Ar ion 488 nm line by [31] and Ar ion 443 nm line by [32]). 3.2.1. Power scan The plasma emission spectra were measured for the Helicon mode of operation as the RF power delivered to the plasma by the helical antenna was varied from 400 W to 1200 W in 100 W increments. The emission spectra were measured for the antenna region where the discharge occurs. For these measurements the Argon flow rate was kept at 20 sccm and the magnet current was set at 30 A corresponding to an axial magnetic field intensity of ∼180 mT in the antenna region. Fig. 6 shows the variation in the Argon ion 434.8 nm line intensity as the delivered RF power is varied. A linear fit to the data is also included. As seen in the graph, the intensity of the line increases linearly with the power delivered to the antenna. Since the flow rate is kept constant, the total particle density, no, can be assumed to remain roughly constant. Thus, the linear increase in the ion emission intensity is attributed to the increase in the ionization fraction and the electron temperature. Langmuir probe measurements by Sinenian
3.2.2. Flow rate scan The argon propellant flow rate was varied between 10 sccm and 100 sccm and the plasma emission spectra from the antenna region were measured for the Helicon mode of operation. In these measurements, the RF power delivered was kept constant at 1000 W and the magnet current was set at 30 A corresponding to a maximum axial magnetic field intensity of 180 mT in the antenna region. The measured emission intensity values of the argon ion 434.8 nm line are presented in Fig. 7. As seen from the figure, when the mass flow rate is varied, the ion emission intensity first increases up to a flow rate of 25 sccm and then decreases for higher flow rates. So, for the given power level and magnetic field configuration, there is an optimum propellant flow rate that maximizes ion line emission intensity. Thus, the ionization rate is limited at high flow rates, as very high background neutral gas density increases the electron energy loss. This leads to reduced electron temperature and increases the energy cost of electron-ion production. This observation is again consistent with the Langmuir probe measurements of Sinenian [13], where the author reports a drop in electron temperature from 4.34 eV to 2.39 eV when the argon propellant flow rate is increased from 10 sccm to 50 sccm. 3.2.3. Magnetic field intensity scan The magnetic field intensity in the antenna region is varied by changing the current supplied to the electromagnets. For a delivered RF power of 1000 W and argon flow rate of 20 sccm, the current is varied from 10 A to 35 A in 5 A increments, corresponding to changing the maximum axial magnetic field intensity from 60 mT to 210 mT with 30 mT increments, and the plasma emission spectra from the antenna region were recorded. Fig. 8 shows the measured emission intensity of argon ion line at 434.8 nm for varying currents to the electromagnet coils. As seen in the figure, when the applied axial magnetic field intensity is increased, the argon ion emission line first increases and then levels off. So, for the given flow rate and power level, it appears that above a certain level of magnetic field intensity the plasma density variation with magnetic field intensity variation is limited and even shows an apparent slight decrease after 150 mT of axial magnetic field intensity. This observation agrees with the RF compensated Langmuir probe measurements of the plasma density by [33] for a similar plasma source where it is observed that the plasma electron density levels off above a certain minimum magnetic 1.6
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RF Power [W] Fig. 6. Argon ion 434.8 nm line emission intensity as delivered RF power is varied.
10
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Mass Flow Rate [sccm] Fig. 7. Argon ion 434.8 nm line emission intensity as delivered Argon flow rate is varied.
M. Celik / Spectrochimica Acta Part B 66 (2011) 149–155
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References
1
Intensity [a.u.]
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0.8 0.6 0.4 0.2 0 5
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Magnet Current [A] Fig. 8. Argon ion 434.8 nm line emission intensity as the current to the magnet is varied.
field strength. It is suggested that, this observation indicates a critical magnetic field strength to “fit” the helicon wave into the chamber [33]. In those measurements, for the case of 13 MHz antenna frequency, above the minimum required magnetic field strength, the plasma electron density shows a slight decrease for increased magnetic field intensity (see ref [33] Fig. 9) similar to our observation. However, the reasons for this slight decrease are not understood. 4. Conclusion The emission spectra of a laboratory helicon plasma source were measured for various operational parameters. Spectral measurements provided a good qualitative understanding of the plasma ionization strength. According to the spectral measurements, the intensity of the ion emission line increases as the power delivered to the RF antenna, thus to the plasma, is increased. For a set RF power, and fixed magnetic field intensity, there is an optimum mass flow rate for the highest plasma electron density. For a set RF power, and constant argon propellant flow rate, it was observed that above a certain minimum magnetic field strength, the plasma electron density shows little change with variation in the magnetic field intensity. The comparison and analysis of spectra obtained for the ICP and Helicon modes of operation indicate a significant increase in the ionization fraction for the Helicon mode where the presence of the axial magnetic field allows the propagation of the electromagnetic waves into the plasma and the efficient transfer of the wave energy to the electrons. Acknowledgments The author would like to thank Prof. Manuel Martinez-Sanchez and Dr. Oleg Batishchev of MIT Space Propulsion Laboratory for providing guidance and support, Dr. Yu-Hui Chiu and Dr. Rainer Dressler of Air Force Research Laboratory at Hanscom for lending the spectral instruments. This research is partially supported by Air Force Research Laboratory/PRSA, Edwards AFB through ERC Inc. subcontract RS060213.
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