Striations in molecular & atomic RF plasmas
Abstract
Some optical spectroscopy and imaging investigations were carried out in low-pressure inductively coupled plasma in order to study the striation phenomenon according to the pressure, the mass flow rate, the injected power and the gas used. Still and moving paired luminous rings were observed in pure CO2 at high injected power and in CO2-CO mixtures at moderate injected power. Emission spectroscopy showed the contribution of the triplet systems of CO and chemiluminescence to the visible emission. High-speed camera acquisitions revealed the way ascending and descending are formed from a still ring. In argon and krypton, plasma structures were observed evolving with pressure from a linear string of big slow balls to a thin plasma continuous column creeping on the reactor wall with very fast curved strings of small beads at intermediate pressure. Appearing striations is shown to result from a break-up phenomenon or to arise between two existing plasma beads.
Experimental set-up
Striations in carbon dioxide
Striations in argon
Striations in krypton
Conclusion
Short bibliography
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Experimental set-up
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Frequency: 13.56 MHz
Inner diameter: 90 mm
Pressure range: 100-1000 Pa
Mass flow rate range: 1-100 mg/s
Power range: 50-500 W
High speed camera: Kodak Ektapro 4540
Intensified CCD: Princeton PI-Max
Digital camera: Minolta Dimage Z3
Spectrometer : SpectraPro-300i
Grating : 1800 gr/mm
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Striations in carbon dioxide
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Temporal evolution of moving (descending) striations in CO2
PW=325 W, p=120 Pa, Qm=4.5 mg/s
High speed camera acquisitions (4500 i/s – in false colors throughout this webpage) show that the moving striations arise from a slowly vibrating structure at the top and move downstream with a non constant velocity. An acceleration is noticed when the nascent striation clearly breaks up. The striations can be ascending or descending. The velocities of the flow and the ionization waves are both close to 1 m/s.
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Still paired rings in the CO2 plasma
PW=280 W, p=200 Pa, Qm=1.5 mg/s
For the given injected power and mass flow rates, the rings stabilize for 3 reproducible pressures: 120, 200 and 500 Pa. Their diameter decreases with pressure.
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Spectrum of the striation emission in CO2 & CO2/CO
The spectrum is mainly composed by CO systems, a continuous chemi-luminescence radiation of CO2 and atomic oxygen lines. The vibrational temperature clearly decreases from the centre of the rings to the dark spaces. Attempts to study the striations in pure CO failed because the tube wall was cover by soot from CO dissociation. The continuous radiation was also observed in this case. Rings appear easily in a CO2+CO mixture.
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Striations in argon
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Evolution of the striated argon plasma according to pressure
According to the mass flow rate, various structures appear when the pressure increases. At high pressure, no visible plasma exists downstream to the coils where a continuous column may persist.
The injected power is constant equal to 180 W. The exposure time for each image is 40 ms.
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Qm = 10 mg/s - Plasma beads are diffuse and slowly moving. As the pressure increases, the striations disappear.
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Qm = 20 mg/s - Some pressures are able to stabilize a string of plasma beads. Those beads disappear for higher pressure.
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Qm = 50 mg/s - At low pressure, the plasma looks like a DC discharge with a violet negative glow, a Faraday dark space and a pink positive column. At high pressure, the continuous column creeps on the quartz wall.
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Qm = 125 mg/s - The strings and column are much more instable but without any reproducibility according to pressure. Some hysteresis effects are observed. Striations appears again when the plasma dies.
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Temporal evolution of an unstable string of plasma beads in argon
PW=40 W, p=266 Pa, Qm=95 mg/s
Fast instabilities may occur on strings of plasma beads as well as on continuous plasma columns but only for high mass flow rates and pressures.
Such a behaviour can be recorded with still as well as with ascending or descending striations. For some conditions, loops were observed.
Aerodynamic conditions do not allow to justify a global turbulent regime. The flow is then not homogeneous and diffusion and the wall should play a major role for higher gas velocities.
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Temporal evolution of descending and still striations in argon
PW=35 W, p=133 Pa, Qm=32 mg/s
High speed camera (500 i/s) acquisitions shows that the striations moves downstream from the coils to the still striations where they disappear. There is no coalescence or break-up phenomenon as for CO2. Another difference is that the plasma bead velocity is quite constant close to 0.9 m/s. The characteristic time of disappearance is about 4 ms.
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Other behaviours at low injected power
Moving striations
PW=31 W, p=133 Pa, Qm=30 mg/s
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Curved column
PW=41 W, p=266 Pa, Qm=30 mg/s
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Striations in krypton
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Low pressure - high mass flow rate
PW=125 W, p=93 Pa, Qm=60 mg/s, 500 i/s
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High pressure - low mass flow rate
PW=170 W, p=220 Pa, Qm=18 mg/s, 500 i/s
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Temporal evolution of descending and still striations in krypton
Click to enlarge
PW=125 W, p=93 Pa, Qm=60 mg/s, 500 i/s
The striations are still at the bottom of the reactor and ascending at the top with a velocity close to 2.5m/s. Most of striations disappear at the top (pale yellow circles) but not all (vivid yellow circles). The lifetime of a striation inside the reactor is estimated to 360 ms. Most of striations appear half-height of the reactor when the distance between two existing striations increases. The appearing striations and their trajectories are underlined in pink.
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Conclusion
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Striations in CO2 speak for a relevant role of metastable states while the behaviour of rare gas plasmas appeals for investigations without flow and with more spectroscopic measurements and electric tests such as using a moving ground and recording the time evolution of the potential. Further precious data would be electron density and electric field within the plasma.
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Short bibliography
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N. Čutić, N. Glavan, Z. Kregar, N. Krstulović, S. Milošević – "Transition phenomena and striations in inductively coupled radio-frequency plasma studied by optical emission spectroscopy." 29th International Conference on Phenomena in Ionized Gases (2007), Prague, Czech Republic
Y. Sakawa, T. Shoji – "Gas dependence of paired luminous rings in capacitive radio-frequency discharges." Physics of Plasma (2001) Vol. 8, No. 6, 2998-3007.
Y. Sakawa, M. Hori, T. Shoji, T Sato – "Optical measurements of paired luminous rings in capacitive radio-frequency hydrogen discharges." Physical Review E (1999) Vol. 60, No. 5, 6007-6015.
V.I. Kolobov – "Striations in rare gas plasmas." Journal of Physics D: Applied Physics (2006), Vol. 39, R487-R506.
A.V. Fedoseev, G.I. Sukhinin – "Self consistent hybrid model for a stratified positive column of a low pressure glow discharge." Journal of Engineering Thermophysics (2008), Vol. 17, No. 5, 74-79.
R. Kumar, S.V. Kulkarni, D. Bora – "Cylindrical stationary striations in surface wave produced plasma columns of argon." Physics of Plasmas (2007), Vol. 14, 122101.
D. Van den Akker, R. Norman – "Plasma striations in krypton gas." (2006) http://www.calvin.edu/~mwalhout/plasmastriations.htm
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