Experimental investigation of different structures of a radio frequency producedplasma columnRajneesh Kumar and Dhiraj Bora

Citation: Physics of Plasmas (1994-present) 17, 043503 (2010); doi: 10.1063/1.3365575 View online: http://dx.doi.org/10.1063/1.3365575 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/17/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Radio frequency emission from high-pressure xenon arcs: A systematic experimental analysis of the underlyingnear-anode plasma instability J. Appl. Phys. 110, 073309 (2011); 10.1063/1.3646459 Dynamics and particle fluxes in atmospheric-pressure electronegative radio frequency microplasmas Appl. Phys. Lett. 99, 091501 (2011); 10.1063/1.3631758 Investigation of effect of excitation frequency on electron energy distribution functions in low pressure radiofrequency bounded plasmas Phys. Plasmas 18, 072102 (2011); 10.1063/1.3605021 Two-dimensional radio-frequency sheath dynamics over a nonflat electrode with perpendicular magnetic field Phys. Plasmas 11, 4456 (2004); 10.1063/1.1781164 Frequency self-upshifting of intense microwave radiation producing ionization in a thin gaseous layer Phys. Plasmas 9, 2803 (2002); 10.1063/1.1478556

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Experimental investigation of different structures of a radio frequencyproduced plasma column

Rajneesh Kumar1,a� and Dhiraj Bora2

1Institute for Plasma Research, Bhat, Gandhinagar 382428, India2ITER Organisation, Cadarache 13108, France

�Received 18 January 2010; accepted 25 February 2010; published online 8 April 2010�

Experimental results on the observation of spatial structures appearing in a radio frequency �rf�produced plasma column are presented in this paper. A plasma column of length 30 cm is formedin a glass tube by applying rf power from a cw generator. It is observed that by changing theoperating parameters, e.g., input power, working pressure, drive frequency, diameter of glass tube,background gases, etc., single plasma column transforms into stationary/moving striations andhelical and spiral structures. For a better understanding of these observations, experiments are alsocarried out to study the electric field profile of these structures in the plasma. Findings of this studyreveal that surface wave produced plasma column transversally bifurcates and transforms intostationary striations. With further increase in working pressure, stationary striations become movingstriations with linear and rotational motions and finally rotating plasma column bifurcateslongitudinally and a helical structure is formed, which can also be transformed into a spiralstructure. Electric field profile of each structure is entirely different from that of the otherstructure. © 2010 American Institute of Physics. �doi:10.1063/1.3365575�

I. INTRODUCTION

In the mid-18th century, striations were first observed byMichael Faraday while Abria1 first published the descriptionof the stratification of dc glow discharge into alternatingbright and dark area. Although striations in the sense of in-stabilities are common occurrences in weekly ionized plas-mas and have been most often studied in the context of directcurrent discharge in cylindrical geometries,2 in the pastyears, many experimental and theoretical efforts have alsobeen made to study the different nonlinear patterns includingvariety of investigations addressing different topics of non-linear behavior in the plasmas.3–8 In the context of nonlinearphenomenon, striations have been studied in different plas-mas with different discharge mechanisms such as dc dis-charges, rf plasmas,9–11 laser plasmas,12 ionosphericplasmas,13 plasma display cells,14 instabilities, and patternformations in low temperature plasmas15,16 for academic in-terest due to its several manifestations which are challengingto the scientists. In general, striations are periodic changes inelectron density and are caused not by the redistribution of afixed number of electrons but by alternate regions of pre-dominant production and removal of electrons, which cansurvive in a limited range of current values, pressure, gasspecies, and tube radius.2 It has been established that thestriations are caused by ionization instability or manifesta-tions of ionization oscillations and waves,17–22 which may becaused by stepwise ionization, the maximization of the elec-tron distribution function, and by any agent-causing en-hancement in inhomogeneities in plasma. Generally, on thebasis of visual observations, two types of striations havebeen classified: standing �stationary� striations having staticappearance and moving striations at a certain velocity.23,24

Observed patterns are standing or moving striations �ioniza-tion waves� which occur over a wide range of plasma param-eters and can be formed in rare as well as molecular gases.21

Moreover, complex behavior is observed ranging from regu-lar to chaotic states.25–27 However, several types of striationshave been identified and various instabilities responsible forthe striations based on the results of numerous studies21,22

have been pointed out. In spite of long research history, stria-tions are not yet fully understood or classified because oftheir diverse phenomena.28

This paper is aimed at presenting the experimental inves-tigations of stationary and moving striations and helical andspiral structures of an rf produced plasma column in whichboth electrodes are at one end of the glass tube. It is alsoobserved that striations and helical structure are the result oftransverse and longitudinal bifurcation of a plasma columnin an experiment. For better understanding of visual obser-vations, electric field profiles of different structures of aplasma column are also studied. To the best of our knowl-edge, striations in high frequency discharges are poorly un-derstood in earlier studies and helical structures are still notobserved. We would also like to compare our study withearlier studies where it has been reported that striations in dcglow discharges can be described by plasma density, densi-ties of excited state atoms, electric fields, discharge current,neutral gas pressure and length, and diameter of the positivecolumn. Thus taking into account of the above, we have alsostudied the surface wave discharge mechanism for obtainingthe discharge or operating parameters e.g., input power,length of plasma column, density profile, electric field, com-positing of gas, diameter of glass tube or plasma column,etc., which are responsible for exciting different structures/patterns or striations. It is worthwhile to mention that surfacewave discharge is entirely different from dc discharge.a�Electronic mail: rajneeshipr@gmail.com.

PHYSICS OF PLASMAS 17, 043503 �2010�

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Therefore, in the present experimental study, a surface waveproduced plasma column is characterized with the help ofdischarge mechanism, electric field profiles, etc. Moreover,by varying the operating parameters single plasma columncan be transformed into different structures/patterns due totransverse and longitudinal bifurcations. Finally, electric fieldprofiles of different structures are also compared.

The manuscript is organized in the following sections. InSec. II, experimental setup to produce surface wave drivenplasma column is described. Formation of plasma column,electric field profiles of different structures are described inSec. III. In Sec. IV, a brief discussion of the experimentalresults is given while Sec. V outlines overall conclusions.

II. EXPERIMENTAL SETUP

A schematic drawing of the experimental setup for sur-face wave produced plasma column is shown in Fig. 1. Inthis setup, a 30 cm long glass tube of a diameter of 3 cm isevacuated by a combined system of rotary and diffusionpumps. The glass tube is filled with argon gas to variousworking pressures. A capacitive coupler of width 35 mm ismounted 2 mm above �the ground plate of diameter 120 mmand thickness 20 mm� at one end of the glass tube. Some-time, arrangement of capacitive coupler and ground plate inthe composite form is called field applicator because rf fieldis applied between them. The initial breakdown takes placeinside the tube in the gap between coupler and groundplate by a cw rf generator operating between 3–10 MHz upto 100 W power. The gap is varied from 1 to 5 mm andrequired power for initial breakdown varies from 10 to 20 Wat a working gas pressure of 0.01 mbar. An L-type matchingnetwork with variable inductors and capacitors is used tomaintain the 50 � impedance of the rf generator with andwithout plasma. The coupled power is measured with thehelp of a power meter.

III. MEASUREMENTS AND RESULTS

A. Formation of plasma column

It is well known that striations can be governed by thedischarge mechanism and parameters. Therefore, it may bequite interesting to study discharge mechanism for the for-mation of 30 cm long plasma column while rf power is fed atone end of glass tube. The initial breakdown takes place inthe discharge tube close to the gap in the field applicator. Thegap is varied from 0.5 to 5 mm and the required power forinitial breakdown varies from 10 to 20 W. The potential dif-ference between the two should be more than 300 Volts. Dueto the field gradients within the gap region in the field appli-cator electrons are driven along the tube axis by the ponder-motive force.29 These conditions for breakdown are achievedby using a matching network between the rf generator andthe field applicator. It has been established earlier that at theinterface of plasma and glass tube, surface wave gets excitedand moves along the interface of these two dielectric mediaand the ionization front moves along with the wave electricfield. The discharge build-up goes on until the plasma col-umn is fully filled in the glass tube and the resulting dis-charge mechanism is called surface wave discharge.29 In theavailable literature numerous studies have been done on dif-ferent kinds of surface wave discharges using surfatrons, sur-faguides, etc.29 Briefly, the properties of surface wave dis-charge depend on the amount of power absorbed per unitlength of the plasma and on the discharge conditions, i.e.,tube dimension, material, operational frequency, inputpower, working pressure, composition of filled gas, wall ma-terial of tube, and the mode number.29 Similarly it is highlypossible in our experiment that the surface wave is excited atthe axial position and propagates in z-direction �the powerflux of the wave decreases with increasing z as the power isgradually expended in sustaining the discharge�. The plasmacolumn ends where wave power drops below the level nec-essary to sustain the plasma. The power dissipated per unitlength of the plasma column varies along the axis of theglass tube. The axial distribution of the high frequency elec-tric field E�z� gives directly the value of the attenuation con-stant of the surface wave.30 The power is lost by electronsthrough collisions with ions and neutrals. Thus, plasma col-umn with input power at various working pressures can beobtained.31 Taking into account the above facts, we earlierstudied and reported the relation between length of plasmacolumn with input power and plasma density profiles.32

Briefly, the length of plasma column varies from 5 to 30 cmwhen input power increases from 5 to 36 W �with all otheroperating parameters being constant�. Meanwhile plasmadensity and temperature are also measured by Langmuirprobe and its axial and radial profiles are studied.32 Typicalplasma electron density and temperature are 5�1010 cm−3

and 2.5 eV, respectively. Axial plasma density profile indi-cates negative density gradient �dn�z� /dz�0� of plasmawhich is also an intrinsic feature of the discharges sustainedwith surface waves in a traveling mode. In addition, radialdensity profile indicates that plasma density is maximum at

� �

� �

� �

� �

� �

� �

� �

� �

� �

� �

� �

� �

�

Capacitive coupler

Gas

Vacuum

Glass tube

Plasma

Probe shaft

Port

Probe

30cm

Field applicator

Aluminium plate

watts)

Powermeter

RF generator(3−10 MHz,100

networkMatching

Gap

FIG. 1. Schematic drawing of the experimental setup of the rf producedplasma column.

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the center of the plasma column while it falls with the radialdistance from the center toward the inner surface of the glasstube.32

B. Electric field profiles in plasma column

Considering plasma column in cylindrical geometry�z, r, and ��, axial electric field profile E�z�, radial electricfield profile E�r�, and azimuthal electric field profile E��� arestudied sequentially and discussed in the following para-graph. Electric field profiles are studied by normalizing themaximum value of electric field which is obtained by stan-dard dipole probe.33 In the course of investigations the axialelectric field component E�z� is measured by a dipole probewhich is moved along the axis of the plasma column. Theaxial electric field exponentially decreases along the axis ofplasma column as shown in Fig. 2. Variation in electric fieldwith axial distance indicates that the power is attenuatedalong the length of the plasma column. The loss of wavepower is due to the damping of the wave in the direction ofpropagation as well as in the transverse direction. For thedamping of wave in the direction of the propagation of thetraveling wave, the intensity of the wave field component isassumed to vary as

Iz � ei�kz−m�−�t�e−�z, �1�

where � is the attenuation coefficient and � is the azimuthalangel. �−1 represents the longitudinal distance �absorbinglength� over which the intensity of the field component at-tenuates by a factor e. It is also interesting to note that � isnot constant because the plasma density varies along the di-rection of propagation. Thus, local value of � is ��z�. Thereare many possible physical mechanisms that could be re-sponsible for the damping of the surface wave, e.g., electronneutral collisions, collisions with the tube walls �electron-wall collision�, dielectric losses in the glass tube, Landaudamping, as well as resonant absorption.30 At high pressures,the collisions between electron and neutral atoms play a

dominant role in the wave damping process. For our plasmaparameters where collision frequency ��108 Hz� is slightlygreater then the wave angular frequency ��107 Hz�, thewave is weakly attenuated. It is also noticed that the phasevelocity ��107 m /s� is much higher than the electron ther-mal velocity �105 m /s�. Therefore, it can be assumed as coldplasma and at low pressures the collisional damping super-seding to the Landau damping. The electron-glass collisionand dielectric losses are not dominating due to tube dimen-sion and material. Hence, surface wave propagates along theaxis of plasma column with weak collisional damping.

After the study of the axial electric field profile we arenow interested to examine the azimuthal electric field profilewhich can help to know possible wave mode in our experi-mental conditions. Hence, azimuthal electric field of plasmacolumn is measured by the moving the dipole probe in thehorizontal plane around the plasma column in a 15° incre-ment from 0° ���360° at different heights from the bottomend of the plasma column �where the source is situated�.Measurements are presented in Fig. 3 which shows the sur-face wave field intensity as a function of the azimuthal angel� at particular axial position of 15 cm from the rf exciter. Thefield is symmetric around the axis of plasma column. How-ever, it is not perfectly circular due to nonuniformity of theglass tube thickness. Results of this experiment suggest azi-muthal mode number m=0, as azimuthal electric field issymmetric around the axis.

So far we have demonstrated axial and azimuthal electricfield profiles, we now pay our attention to the radial profileof the electric field. For this purpose, dipole probe is movedoutside and inside of the plasma column keeping constantaxial and azimuthal positions of the probe. Measurements arepresented in Fig. 4 which shows the radial variation of theelectric field of surface wave propagating over a cylindricalplasma column enclosed in a glass tube surrounded by air.

FIG. 2. �Color online� Normalized profile of axial electric field E�Z� atconstant radial position of 3 cm form the center of plasma column.

FIG. 3. �Color online� Normalized profile of azimuthal electric field E��� atconstant axial position of 15 cm form the rf exciter and radial position of3 cm form the center of plasma column.

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The profiles suggest that the field inside the plasma columndecays faster than the one that is outside of the glass tube.

Our experimental results outlined above indicate that theaxis-symmetric surface wave propagates along the interfaceof the plasma and the glass tube in traveling mode. By tuningthe operating parameters, the plasma column bifurcatestransversely and longitudinally and different plasma struc-tures are formed. These experimental findings will be de-scribed in the following section.

C. Transverse and longitudinal bifurcationof a plasma column

Surface wave driven plasma column is transformedinto finite number of small cylindrical stationary striationsor plasma balls by changing the operating parameters suchas working pressure �0.03–0.10 mbar�, drive frequency�3–10 MHz�, input power �40–50 W�, radius of glass tube�1.5–2.5 cm�, length of plasma column �5-30 cm�, and back-ground gas. It seems that the formation of stationary stria-tions is due to transverse bifurcation of plasma column ineven numbers �2, 4, 6, 8,…� of striations. It is worthwhile tomention here that certain combinations of operating param-eters can transform a plasma column into striations. All stria-tions are arranged along the axis of the glass tube. A photo ofstanding plasma balls or striations is shown in Fig. 5. Fourstationary striations can be seen in series along 30 cm longplasma column at 50 W input power and 0.03 mbar workingpressure. It is quite interesting to study the physical charac-teristics of such stationary striations. Hence, a number ofexperiments are carried out to determine the variation innumber, length, separation between two striations, etc. Find-ings of our study indicate that the number of striations in-creases from one to six when the total length of plasma col-umn is varied from 5 to 30 cm. The length of each striationdoes not depend on the length of the plasma column. Fur-thermore, it is also observed that number and length of thestriations vary with working pressure. The length of firststriation toward field applicator is 4 cm and number of stria-tions is six at working pressure of 0.03 mbar while the length

and the number are 2 cm and 12, respectively, at a pressureof 0.10 mbar. It is also found that the length of striationsdecreases while the input power is increased at constantworking pressure, drive frequency, length of plasma column,and number of striations. The length of first striation fromthe rf exciter is shortened from 4 to 2 cm by changing theinput power from 40 to 50 W, while the number of striationsremain the same and consequently separation between twostriation increases from 1 to 3 cm. In addition, it is alsoobserved that length and number of striations are not affectedby changing the drive frequency from 3 to 10 MHz �all otherparameters are kept constant�. After the study of stationarystriations, experimental attempts are made to form the helicaland spiral structures of the plasma column. When at a con-stant input power of 60 W, working pressure is finely tunedand increased up to 0.15 mbar; it is observed that stationarystriations start moving axially and rotationally. It is noticedthat as the working pressure is increased, the velocity ofstriations increases without a change in the number of stria-

FIG. 4. �Color online� Normalized profile of radial electric field inside andoutside of the plasma column at constant axial position of 15 cm form the rfexciter.

FIG. 5. �Color online� Four cylindrical stationary striations or plasma blobsin 25 cm long plasma column at constant operating parameters.

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tions. With further change in working pressure from 0.15 to0.30 mbar, it is observed that velocity of striations is veryfast and they start to merge into each other and a plasmacolumn in rotational motion appears. With further increase inthe working pressure, it is observed that at working pressureof 0.4 mbar, rotating plasma column longitudinally bifurcatesin two filaments with rotation and plasma column transformsinto helical structure. A photo of helical structure is shown inFig. 6. Surprisingly, it is observed that two filaments of ahelical structure are merged into each other and becomesingle plasma column in spiral structure or shape either byapplying external capacitive effect �any conductor is placedor wrapped on the outer surface of glass tube� or increasingworking gas pressure up to 0.50 mbar. A photo of spiralstructure is shown in Fig. 7. For presenting to the point re-lation of operating parameters with observed plasma struc-tures, Fig. 8 depicted in which it is indicated that at certaincombinations of gas working pressure and input power,plasma column transforms into stationary, moving, helical/spiral structures, or striations.

Briefly, the study shows that a plasma column is trans-versely and longitudinally bifurcated by operating param-eters and different plasma structures are observed. For better

understanding of this phenomenon, electric field profile ofdifferent structures is measured which is described in follow-ing section.

D. Electric field profile of bifurcated plasma column

Axial, radial, and azimuthal electric field profiles of aplasma column have been discussed in earlier Sec. III B. It ishighly possible that electric field profile may be different indifferent plasma structures. Here, axial electric field �E�z��,radial electric field �E�r��, and azimuthally electric field�E���� profiles of different structures of plasma column arestudied and described in following. Electric field on the sur-face of the plasma column with stationary striations alongthe axis of glass tube is measured with the dipole probe. Theprofile is shown in Fig. 9, in which solid line with squares

FIG. 6. �Color online� A snap shot of helical structure of plasma column.

FIG. 7. �Color online� A snap shot of spiral structure of plasma column.

FIG. 8. �Color online� A plot for representing the different plasma structureswith certain combinations of working gas pressure and input power where�T and �L are the transverse and longitudinal bifurcation parameter,respectively.

FIG. 9. Axial electric field profile of stationary striations on the surface ofglass tube at constant length of 30 cm, working pressure of 0.03 mbar, andinput power of 40 W.

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shows the axial electric field profile with striations and dot-ted line shows estimated plasma electron density profile andposition of striations, indicated for better understanding ofthe measurements. Findings of this study suggest that electricfield is negative in a length of striation while it is positive inthe gap of two successive striations. Electric field profile canbe explained with the help of earlier explanations of brightand dark regions of glow discharge.2 Hence it appears that inthe region of negative electric field where striations areformed, electrons do not have sufficient energy for ionizationand hence during collisions, excitation of atoms takes place.As a result the region of striations becomes bright but afterthe region of the striation, electrons experience positive elec-tric field by which it gains more energy to give rise to ion-ization during collisions and the process continues. It isworthwhile to mention here that radial and azimuthal electricprofiles are also measured by changing the position of dipoleprobe in radial position and angular position, respectively, inthe presence of stationary striations. Results of this studyindicate that radial and azimuthal electric field profiles aresimilar to electric field profiles of a plasma column, whichhave been discussed in Figs. 3 and 4, respectively. It is quiteinteresting to study the electric field profiles of helical struc-ture of plasma column. Findings of this study indicate thataxial and radial electric field profiles are similar as electricfield profiles of a plasma column while azimuthal electricfield profile is entirely different. An azimuthal electric fieldprofile �E���� of helical structure of plasma column is shownin Fig. 10. Results of this study suggests that electric field ismaximum at 270° and 90° and minimum at 0° and 180°while azimuthal electric field of a plasma column is symmet-ric around the axis as shown in Fig. 3.

Briefly it has been experimentally demonstrated that anrf produced plasma column of length 30 cm can be trans-versely and longitudinally bifurcated by tuning the operatingparameters and electric field profiles of different structures ofplasma column are different. To the best of our knowledge

such kind of experimental study has not been explainedwell and reported in the available literature. Therefore, anattempt is made to express experimental results in the fol-lowing discussion.

IV. DISCUSSION

Striations originate due to growing instability during theionization process, which may be caused by stepwise ioniza-tion when metastable atoms produced by electron impact de-cay as a result of diffusion to the wall.2 In earlier studies,ionization wave in dc discharge of rare gases21,22 have beenextensively studied by using numerical analysis,34 modetransitions,35,36 scaling laws,37 etc., considering that the non-linear phenomenon or striations depend on system param-eters, i.e., discharge current, the neutral gas pressure, length,and diameter of plasma column. Most analytical and numeri-cal calculations are based on a hydrodynamic model that issimplified by some reasonable assumptions.38 In the contextof the above, the transitions between different dynamic statesare described by bifurcation39,40 connected with correspond-ing pattern falling instabilities. Recently, study on hysteresisof ionization wave has suggested that the maximum spatialamplification becomes positive above a critical current�supercritical Hopf bifurcation� which means that bifurcationtakes place after a certain value of current.22 With the help ofthe above given studies for dc discharges, different structuresof surface wave produced plasma column can be explainedin which striations are achieved in inert gas �argon� which isfilled at 0.03–0.10 mbar in 30 cm long and 3–5 cm diameterof glass tube in which coupled power is varied from 40 to50 W keeping constant driven frequency of 5 MHz. Al-though, the experiments are performed with different gasessuch as nitrogen, oxygen, air, and argon, striations areformed only in argon gas. Striations in other gases can beformed in other experimental conditions which we could notachieve due to our experimental limitations. In our experi-ment different structures are a result of transverse and longi-tudinal bifurcation �in both cases we get even number ofstriations, 2, 4, 6,…� of single plasma column. Hence, bifur-cation theory can be used for better understanding of theexperimental results. According to bifurcation theory, oneneeds to develop a set of nonlinear equations that have sta-tionary state which becomes unstable when a certain bifur-cation parameter � exceeds a certain value �c �critical valueof bifurcation parameter�. It is worthwhile to mention thatplasma column bifurcates transversally and longitudinally atdifferent operating parametric regime hence we will use �T

as a critical transverse bifurcation parameter and �L as acritical longitudinal bifurcation parameter. According to ourexperimental results, �, �T, and �L are functions of workingpressure and input power as shown in Fig. 8. There are manycases in which �-�T is positive and small because as soon as� exceeds �T, the system actually becomes unstable. When� is greater than �T, small perturbations due to two-stepionization will send the system into a nonlinear time evolu-tion in which there are growing modes, which has attainedcertain amplitude and plasma settles down to a final state andthe plasma density varies sinusoidally along the axis of glass

FIG. 10. �Color online� Normalized azimuthal electric field profile of helicalstructure at constant operating parameters.

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tube. Number of striations is a presentation of number ofmodes in the plasma. A long time back, Robertson41 demon-strated that the two-step ionization of the metastables statesleads to instability. In the present experiment, argon gas isused and striations are formed above certain value of �T

which is related to two-step ionization process, which be-comes active above certain rf power and working pressure. rfpower is related to the electric field �E� and working pressureis related to the electron density �N� of the gas. Thus E /Nshould be the governing factor for the reaction cross sectionof the two-step ionization process and the experimentallymeasured value of �T are related to E /N. In the total ioniza-tion in argon, ground state ionization, multistep ionization,and metastable collisions contribute 64%, 11%, and 25% to-ward the total ionization, respectively. In nitrogen, air, andoxygen, two-step ionizations may not be effective in existingparametric regime hence � does not exceed �T and station-ary striations are not observed in our system. Therefore, sta-tionary striations are formed when weak instability due totwo-step ionization is excited in argon plasma column whichbifurcates at transverse axis. On the bases of experimentalresults, when working pressure is increased up to 0.15 mbar,stationary striations become moving and finally a rotatingplasma column appears. After a certain value of workingpressure and input power, rotating plasma column bifurcatesin the longitudinal axis and a helical structure is formed asshown in points in Fig. 8. This phenomenon can also beunderstood by above given bifurcation theory when bifurca-tion parameter � exceeds �L and present dynamic state ofsystem �rotating plasma column� goes to another dynamicstate �helical structure�.

We feel that a more detailed theoretical description willbe interesting to explain the experimental results where bothbifurcation parameters can be obtained by hydrodynamicmodel take into account of two-step ionization in surfacewave discharge mechanism. Nonetheless the present descrip-tion explains the observed results to a certain extent.

V. CONCLUSIONS

A plasma column is formed with the help of the radiofrequency source operating between 3–10 MHz. Axial, radialand azimuthal electric field profiles of plasma column indi-cate that 30 cm long plasma column is driven by the axis-symmetric traveling surface wave. By changing the operat-ing parameters �input power from 30 to 50 W and workingpressure from 0.03 to 0.1 mbar� plasma column is trans-formed into stationary striations in argon gas due to thepresence of metastable atoms for facilitating two-step ioniza-tion. With further increase in the working pressure up to0.15 mbar, stationary striations start moving. Further, athigher working pressures around 0.40 mbar, rotating plasmacolumn bifurcates in two filaments and helical structure isformed. In addition helical structure can be transformed intospiral structure by using external capacitive effect. Axial andazimuthal electric field profiles of stationary striations and

helical structure are entirely different form the field profilesof a plasma column. Our study reveals some interesting ex-perimental observations and measurements giving a wideverity of striations or pattern formations in plasma.

ACKNOWLEDGMENTS

Authors would like to thank the referees of the journaland Professor R. Jha for critically evaluating the manuscript,Dr. H. C. Joshi for linguistic corrections, and RF groupmembers of the Institute for their kind help at various stagesof the experiments.

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