formation of a double layer in electronegative ${\rm o}_{2}$ plasma
TRANSCRIPT
2798 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014
Formation of a Double Layer inElectronegative O2 Plasma
Shailesh Sharma, Chanel (Hayden) Linnane, David Gahan, Stephen Daniels, and Mike B. Hopkins
Abstract— A double layer (DL) was observed at the boundarybetween the source region and expansion region of an induc-tively coupled, radio frequency, plasma reactor when an oxygendischarge was ignited at low pressure. A DL is a narrow localizedregion with relatively large potential difference and electric field,which can be formed in electropositive as well as electronegativeplasmas. In an inductively coupled plasma reactor of this type, itseems to be formed at the interface between the smaller sourceregion and the larger expansion chamber and acts as an internalboundary separating two plasma regions of significantly differentcompositions, density, and plasma potentials.
Index Terms— Plasma chemistry, plasma density, plasmaproperties.
DOUBLE layers (DLs) are found in a diverse range ofplasmas, from discharge tubes to space plasmas (aurora,
solar corona, extragalactic jets, etc.) and can be generatedusing various types of current-driven discharges. A DL is apermeable structure in plasma, which separates two oppositelycharged parallel layers and can be formed in electropositive aswell as electronegative plasmas [1]. The difference in opticalemission allows for the observation of the DL separatinghigh density (highly emissive) and low density (less emissive)regions of the plasma.
In this letter, we show that a DL can be formed in aninductively coupled plasma reactor, which consists of a sourceregion and an expansion chamber. Oxygen gas is used togenerate an electronegative plasma with similar experimentalreactor and conditions presented in [2].
In the experiment (at a background oxygen pressure of0.2 Pa) 55 W of 13.56-MHz radio frequency power iscoupled through the copper antenna and plasma ignites inthe source region before expanding into the larger expansionchamber. Electron–neutral collisions limit plasma diffusionalong the expansion axis leading to an electron density and
Manuscript received November 1, 2013; revised February 5, 2014; acceptedMarch 9, 2014. Date of publication April 30, 2014; date of current versionOctober 21, 2014. This work was supported by the Irish Research Council.
S. Sharma is with the Department of Electronic Engineering, Dublin CityUniversity, Dublin 9, Ireland, and also with Impedans Ltd., Dublin 17, Ireland(e-mail: [email protected]).
C. Linnane, D. Gahan, and M. B. Hopkins are with Impedans Ltd.,Dublin 17, Ireland (e-mail: [email protected]; [email protected]; [email protected]).
S. Daniels is with the National Centre for Plasma Science and Technology,Dublin City University, Dublin 9, Ireland (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPS.2014.2313179
plasma potential profile that decrease away from the source.As a result of this decrease in potential, positive ions areaccelerated toward the expansion chamber and conversely,negatively charged particles toward the source. When therespective charged species reach the Bohm velocity, quasi-neutrality breaks down and an internal sheath or DL isformed [2]. The photograph in Fig. 1 shows the curved-shapedDL protruding from the source region into the expansionchamber. The DL seems to have a spherical shape that isattached to the boundary between the source and the diffusionchamber and that expands into the diffusion chamber. The DLis found to be static in our case, however, propagating DL hasbeen found to form with increasing negative ion fraction invarious electronegative gas mixtures [3]. High speed imagingof propagating DL is now becoming an important techniquefor researchers to investigate this phenomenon.
The DL acts as an internal boundary or sheath, whichseparates two plasma regions with significantly different com-positions, density and plasma potentials. A high electrondensity, high electron temperature, and low electronegativityplasma exists upstream at the source side, while a low electrondensity, low electron temperature, and high electronegativityplasma exists downstream on the lower potential side [4].The negatively charged particles entering at the low potentialside and positive ions entering at the high potential side ofthe DL are accelerated. Negatively charged particles enter-ing at the high potential side and positive ions enteringat the low potential side of the DL are decelerated andpartially or fully reflected based on the intensity of thepotential difference. The electric field across a DL maintainsa balance between accelerated electrons flowing upstreamand ions flowing in the other direction. Electrostatic probesmeasurements and laser-induced photodetachment have showndiscontinuities in all plasma parameters (electron density,electron temperature, and negative ion fraction) at the DLposition [1].
In electronegative plasmas, DL formation is the nonlin-ear evolution of large amplitude ion acoustic waves drivenunstable by oppositely streaming positive and negative ions[3], [5]. The DLs find important applications in the field ofdusty plasmas, which utilizes the high potential side of the DLto electrostatically trap the dust particles. The DLs are alsoused in the development of plasma nozzle thrusters utilizingplasma acceleration to generate high thrust. High impulse andhigh efficiency plasma thrusters for space plasma propulsionapplications are also being investigated [6].
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SHARMA et al.: FORMATION OF A DL IN ELECTRONEGATIVE O2 PLASMA 2799
Fig. 1. Photograph of a DL protruding from the source region at 0.2 Pa in an O2 inductively coupled radio frequency plasma at low power (55 W).
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