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Chapter 5

Striations in Plasma Column

The studies carried out so far on surface wave produced plasma column, which

acts as a monopole plasma antenna is demonstrated in the previous chapters. We

now turn towards possible application of plasma properties which enable us to pro

vide different structures of plasma. There is however plenty of scope in imposing

the performance of different plasma structures as different plasma antennas. For

this purpose, we begin our studies on plasma structures or striations. Hence, the

objective in this chapter is to investigate the formation of striations and their phys

ical properties in the surface wave produced plasma column. It is well-known that

the glow discharge manifests into dark and bright luminous layers from cathode to

anode, namely, Aston dark space, cathode glow, cathode dark space, negative glow,

Faraday dark space, positive column, anode dark space and anode glow. Sometimes

a positive column in glow discharge has a periodic layered structure composed of

striations. Although glow discharges at low pressures using DC, RF, laser beam

etc have been studied for long time, even then it is of academic interest due to its

several manifestations which are challenging to scientists. Production of stationary

and moving striations is a good example of manifestation of a glow discharge. In

fact, striatio11:s are the periodic changes in electron density and are caused not by

redistribution of a fixed number of electrons, but by alternate regions of predom

inant production and removal of electrons which can survive in a limited range of

current values (lo-4 A to 10 A), pressure (lo-3 Torr to 102 Torr), gas species (rare

and molecular gases) and tube radius (1 em to 10 em). It has been established

87

Chapter 5: Striations in Plasma Column 88

that the striations are due to ionization instability or manifestations of ionization

oscillations and waves [1-7], which may be caused by step-wise ionization, the max

imization of the electron distribution function, and by any agent causing enhance

ment in inhomogeneities in plasma. On the basis of visual observations, two types of

striations have been classified: standing (stationary) striations, which have a static

appearance, and moving striations, which have certain velocity [8, 9]. Striations

have been observed in different plasmas in different discharge mechanism such as

DC-discharge [10], RF plasma [11-14], laser plasmas [15], ionosphere plasmas [16],

plasma display cells [17] etc. Analysis of three-dimensional striations was being

conducted using computerized tomography [18] with continuing research from the

view point of ionization [19]. Three-dimensional spherical stationary striations have

been also studied [20-23]. Robertson and Hawkins [13] found striations in a neutral

collision dominated, RF -exited Argon plasma in a uniform axial magnetic field. The

authors [13] found plasma to display a symmetric brightly glowing stationary stria

tions or balls. Number of balls and their radial and axial dimensions were pressure

dependent and not related to the applied frequency or wavelength. Robertson and

Herring [14] tried to explain this phenomenon by violation of ambipolar diffusion

due to the absence of the magnetic field.

R. A. Goldstein [24] interpreted the phenomena of balls or stationary striations

using the theory of bifurcation in which the system bifurcates after the onset of a

weak instability. The theory treated a system governed by a set of nonlinear equa

tions that has a stationary state which becomes unstable when a parameter, which

is called bifurcation parameter, a function of experimental parameters, exceeds a

critical value and small perturbations in the system grow nonlinearly. The plasma

settles down to a final state in which the density varies sinusoidally along the axis

of the cylinder (glass tube). The number of balls (bright regions) is an indication

of the number of modes to which the system has settled. The modes can be ex

cited with positive or negative amplitude depending on plasma parameters. The

theory also indicated that a gas with metastable state should be used to obtain the

phenomenon.

Although a vast literature exists on striations in the different discharge mecha

nism, stationary striations in surface wave produced plasma columns in which both


the electrodes are at one end of the glass tube, has not been still reported in the

available literature. Hence, our first objective is to study the stationary striations

and their physical properties in such plasma column.

In this chapter, we devote our attention to the study of experimental conditions

to form stationary striations and variation in the physical properties of such stria

tions with operating parameters such as input power, working pressure etc. Electric

field profile of striations is also studied. To provide a more qualitative understanding

of such striations, a discussion as a model for explaining the experimental results of

stratified plasma column is presented with the help of concept of bifurcation the

ory of Ronald A. Goldstein (24J considering the effect of two-step ionization in the

surface wave produced plasma column.

5.1 Formation of Striations

By changing the external operating parameters such as working pressure (0.03

mbar to 0.30 mbar), driven frequency (3 MHz to 10 MHz), input power (40 watts to

60 watts), radius of glass tube (1.5 em to 2.5 em), length of plasma column (5cm to 30

em) and Argon gas at certain critical conditions the plasma column is transformed in

to finite number of cylindrical stationary striations. Fig.5.1 shows the few of critical

values, which are the combinations of input power and working pressure where the

stationary cylindrical striations are formed at constant driven frequency (5 MHz)

and length of plasma column (30 em). In fact, these structures in plasma column

are transformed from a stable visible inhomogeneous plasma column to unstable

inhomogeneous state, which again diffuses to stable visible inhomogeneous steady

state with non-uniform plasma structures like striations. A photograph of striations

is s~own in Fig.5.2 in which five striations can be visualized in 25 em long plasma

column. It can be seen that length of first striation is maximum and subsequently

the length of next striation decreases. Now, in order to investigate, we will study

about variation in the physical properties of striations with operating parameters in

the next section.


52 I I I

f f

.

. .

. .

.

t t t .

. f f + . I I I I

0104 0106 0.08 0.1 Working pressure (mbar)

Figure 5.1: Few combination of input power and working pressure where plasma column transformed in to striations in 3 em diameter of glass tube.

5.2 Properties of Stationary Striations

The physical properties of such striations, namely, the number and length of

individual striation which can be controlled by changing the operating parameters,

are described in this section.

5.2.1 Variation of the Length of Plasma Column

The number of striations increases with the increase in the total length of plasma

column. Fig.5.3 shows that the number of striations increases from one to six when

the total length of plasma column varies from 5 em to 30 em. However, the length of

individual cylindrical striation does not depend on the length of plasma column. The

total length of plasma column is varied by varying the input power at a particular

working pressure as shown in Fig.3.1 of Chapter 3. Hence, the different number of

striations of same dimension can be formed by changing the input power.

It is quite interesting to know that in case of surface wave produced plasma in


Figure 5.2: Five cylindrical stationary striations in 25 em long plasma column at constant operating parameters.

which the density decreases axially, the length of the first striation near the RF

exciter is maximum and length of the subsequent striations keep decreasing. This

is different as compared to the striations produced with two electrodes at the two

ends of the plasma column. Fig.5.4 shows the variations in the number of striations

with the length of striations at working pressure of 0.05 mbar, input power of 40

watts, driven frequency of 5MHz and length of plasma column of 30 em.

5.2.2 Variation with the Working Pressure

It is observed that the length of individual striation and number oi striations vary

with working pressure while keeping all other operating parameters constant. The

number of striations increases from 6 to 12 when working pressure varies from 0.03

mbar to 0.10 mbar at constant input power of 50 watts and 30 em long plasma

column, as shown in Fig.5.5. The length of first striation from RF exciter is 4 em at

0.03 mbar working pressure. When working pressure increases to 0.10 mbar, length

of first striation is reduced from 4 em to 2 em, which is shown in Fig.5.6.


. . . 6 •

5 • fl.l

= = ~ = 4· • ·c .... fl.l

c.. = 3 • .. ~ .c

~ 2 • z

1· • . I . . . 5 10 15 20 25

Length of plasma column (em) 30

Figure 5.3: Variation in number of striations with the length of plasma column at constant pressure (0.05 mbar) and driven frequency (5MHz).

5.2.3 Variation of Input Power

As mentioned before in the section 3.1 of Chapter 3, the length of the plasma

column increases with in increase input power. However, it is observed that when

the plasma column is produced in a full length of 30 em of the tube and if the power

is increased further, then the length of striation decreases keeping all other operating

parameters including number of striations constant. Fig.5. 7 shows that the length

of first striation from RF exciter is reduced from 4.1 em to 2 em by changing the

input power from 40 watts to 50 watts while the number of striations remains same

at constant working pressure of 0.03 mbar and consequently the gap between two

striations increases from 2 em to 4 em.

5.2.4 Variation of the RF Frequency

The length and number of striations are not affected by changing the driven

frequency from 3 MHz to 10 MHz when all other operating parameters are kept


- I I I I I --. a 5.5· • . u ; 5.0· . ~ 4.5· + . ~ :s 4.0· . fl.l -= 3.5· . u a1 3.0· • . c.. Q 2.5· + +: -= !., 2.0· = ~ 1.5· .

I I I I I I

1 2 3 4 5 6 Striation number

Figure 5.4: Variations in the length of striations with the number of striations at working pressure of 0.05 mbar, input power of 40 watts, drive frequency of 5 MHz and length of plasma column of 30 em.

constant.

The results of experiments of this section suggest that the length and number of

striations can be controlled by operating parameters.

5.3 Electric Field Profile

To provide a more qualitative understanding of formation of striations (bright and

dark regions) due to electric field and study of plasma density gradient, experiments

are conducted, which are described in this section. Electric field on the surface of

plasma column with striations along the axis of glass tube is measured with the

dipole probe. Profile is shown in Fig.5.8, in which solid line with squares shows the

axial electric field profile with striations. Electric field is negative, positive and zero

for striations. Electric field is due to the electron density gradient can be written as

p 'VE= --

co (5.1)


12· I .

i . fljll· =

. = ~10· • . •• .. 9· . .... flj c.

8· = • . .. ~ .c 7· . m 6 • . z

5 I I

0.02 0.04 0.06 0.08 0.10 Working pressure (mbar)

Figure 5.5: Variation in the number of striations with working pressure in the 30 em long plasma column at constant input power of 50 watts.

where p = ne- ni = b.ne is the charge density which is zero at charge neutrality

condition. Hence, electric field is due to the density gradient.

This study shows that the density gradient changes at the striations and the

position of striation can be determined by electric field profile also. In the region of

negative electric field where striations are formed, electrons do not have sufficient

energy for ionization and hence during collision, excitation of atoms takes place

by which region of striations becomes bright but after the region of the striation,

electrons experience positive electric field by which it gains more energy to give rise

to ionization during collisions and the process continue by which we get the region

of striations.

5.4 Experiments with Different Gases

In previous sections, we have investigated the variation in physical properties of

striations in Argon gas plasma, which provides two-step ionization at certain ex-


,-.. • • • • e T ~4.0· .

= Q : = 3.5· . ... a. .... Ill

+ ~ 3.0· ' ·. ct)

= ... Ill

'Q 2.5· . -= .... ct)

iii ' = 2.0· . ~

~ • • • . 0.02 0.04 0.06 0.08 0.10 0.12

working pressure (mbar)

Figure 5.6: Variation in the length of single striation with working pressure in 30 em long plasma column at constant input power of 50 watts.

perimental conditions. Here, it is interesting to investigate that the production of

striations is caused by two-step ionization. So we are interested in the study of

plasma column of different gases. Experiments are performed with Argon, Air, Ni

trogen and Oxygen. Striations are observed in the Argon gas only at the similar

experimental conditions, which are not observed in Air, Nitrogen and Oxygen. This

study came to the conclusion that striations are formed due to two-step ioniza

tion, which is effective only in Argon gas in comparison to other used gases in our

experimental limitations and conditions.

5.5 Experiments with Glass Tubes of Different

Diameters

In the previous sections, It has been demonstrated that formation of striations

and their physical properties depend on the input power, working pressure, driven


~415 I I I I

e ! ~ = 410 I

= ~

= ~c 3~5 I .... fll

~ I "Sb3101 . = I ...

fll lot-I = 215 . .c .... 1:)!)

! ~ 2~0· ! ! . ~

I I I I I

40 45 50 55 60 Input power (Watts)

Figure 5. 7: Variation in the length of striation with input power in a 30 em long plasma column at constant working pressure of 0.03 mbar and 5 MHz drive frequency.

frequency and filled gas. As we have mentioned earlier that tube radius is also

an important parameter to form the striations. Hence, experiments are performed

with similar glass tubes of different diameters keeping constant all other operating

parameters including length of glass tube of 30 em. Striations are formed in glass

tube of diameter of up to 5 em in our experimental regime.

5.6 Model for Explaining the Experimental Re-

suits

In the previous sections, formation of striations and their physical properties have

been described. On the basis of these experimental results and an available theory of

bifurcation, one can understand the phenomena of formation of striations in terms

of their number and the length of individual striations which are formed in a surface

wave produced RF plasma. The theory for striations in two electrodes at the two


5 10 15 20 25 30 Axial distance (em)

Figure 5.8: Axial electric field profile of striations on the surface of glass tube at constant length of plasma column of 30 em, working pressure of 0.03 mbar and input power of 50 watts.

end of glass tube is well established. However, in our case, both the electrodes are

fixed at the one end of the glass tube and the discharge ignites along the axis of

glass tube due to surface wave.

In the beginning of this section, a power balance equation is shown and length of

plasma column is decided by working pressure and input power. Further, bifurcation

theory is used to understand the bifurcation of plasma column into striations. During

the formation of striations, power balance equation is modified in which an extra

term of energy due to two-step ionization in surface wave discharge is added. After

that, a model of final density profile of striations in which the length and density

of striations decreases along the length of plasma column, is proposed. Therefore,

the particular lengths of striations are decided by wavelength and a scale factor.

Finally, it is shown that bifurcation parameter is the function of working pressure

and input power.

Discharge development and sustaining due to traveling surface wave is a specific

mechanism, in which wave energy is transformed from the electromagnetic field to


the plasma. The wave is launched at axial position z = 0 and propagates in the

z-direction and column ends at z = l, where the wave power drops below the level

required to sustain the plasma. According to power balance equation, the power

absorbed per unit length by the plasma from the surface wave at a position z along

the plasma column is balanced by the power per unit length lost to the walls from

the plasma (L(z)) by the migration of electron-ion pair at Bhom velocity (us). For

a given tube radius a, the power per unit length lost to the wall from the plasma,

L(z) is (25, 26]

(5.2)

where us(Te) is migration of electron-ion pair at the Bohm velocity, Ae!f(p) is

effective surface area per unit length of the column, n(z,p) is the electron number

density which is a function of position and filling pressure and ~L(Te) is the energy

loss per electron ion pair which is made up by several contributions of collisional loss

per electron-ion pair due to ionization, excitation and elastic scattering from neutral

atom (~c), mean kinetic energy (2Te) and loss per electron and the energy loss per

ion due to acceleration across the sheath(~i)· The total energy loss per electron-ion

pair can be expressed as (27]

(5.3)

Therefore, to sustain a surface wave discharge, the power flux must exceed some

threshold value and length of plasma column can be controlled by power as (28-30]

1 L = f(Pn a)P2 (5.4)

where

(5.5)

where a is tube radius, (} represents energy which is consumed in various collisions

and Vm is collision frequency which is the function of working pressure. f(Pr, a) is

a constant at constant working pressure and tube radius. Hence, the corresponding

plasma lengths (in em) can be decided by (P)~, as can be seen in Fig.3.1 of Chapter

3. The experiments are performed to measure the length of plasma column with

power in three different glass tubes of radii (1.5 em, 2.5 em and 3.5 em).


After formation of 30 em long plasma column by surface wave discharge, op

erating parameters are varied and plasma column is transformed in to stationary

striations. Power balance equation will change during the formation of striations in

surface wave produced plasma column which is elaborated in following.

Striations originate due to growing instability during the ionization process which

may be caused by step-wise ionization when metastable atoms produced by electron

impact decay as a result of diffusion to the wall [10]. Such conditions can be achieved

in inert gas (Argon) which is filled at 0.03 mbar to 0.30 mbar in 30 em long and 3 em

to 5 em diameter of glass tube in which coupled power is varied from 40 watts to 60

watts keeping constant driven frequency of 5 MHz. Although the experiments are

performed with different gases such as Nitrogen, Oxygen, Air and Argon, striations

are formed only in the Argon gas at above given conditions. Striations in any other

gas can be formed in other experimental conditions which we could not achieve due

to our experimental limitations.

R. A. Goldstine [24] developed the theory of striations using the theory of bi

furcation for plasma produced between two electrodes. In order to apply his theory

for our experiment, it is necessary to modify existing theory to explain decreasing

length of striations in surface wave produced plasma and compare the theoretical

results with our experimental results.

According to bifurcation theory, one needs to develop a set of non-linear equa

tions that has stationary state, which becomes unstable when a certain bifurcation

parameter, p,, exceeds a certain value f.Lc, a critical value of bifurcation parameter.

According to our experimental results, p, and f.Lc are the functions of input power

and working pressure. p, can not exceed the f.Lc if working pressure is less than 0.03

mbar for any value of input power or if input power is less than 40 watts for any

value of working pressure. The few values of f.Lc (certain combination of input power

and pressure) have been shown in Fig.5.1. These are many cases in which p,- f.Lc is

small because as soon as p, exceeds f.Lc, the system actually goes unstable. When 1-"

is greater than f..Lc, small perturbation due to two-step ionization, will send the sys

tem in to a non-linear time evolution in which there are growing modes, which has

attained a certain amplitude and plasma will settle down to a final state in which

the density varies sinusoidally along the axis of cylinder. Number of striations is a


presentation of number of modes in the plasma.

Robertson [31] demonstrated that the two-step ionization of metastables states

leads to instability. In the present experiment, Argon gas is used and striations

are formed above certain value of J.Lc which is related to two-step ionization process

which becomes active above certain input power and working pressure. Input power

is related to the electric field and working pressure is related to the electron density

of the gas. Thus E IN is the governing factor for the reaction cross section of

the two-step ionization process and the experimentally measured value of J.Lc are

related to E IN. In the total ionization in Argon, ground state ionization, multi

step ionization and metastabel collisions contribute with 64%, 11% and 25% of total

ionization respectively. In Nitrogen, Air and Oxygen two step ionization does not

effective in existing parametric regime hence J.L does not exceed the J.Lc so stationary

striations are not observed in our system.

In the two-step ionization, a ground state atom of Argon gas is excited up to its

metastable state by an electron collision (11.55 eV). The excited atoms are ionized

by another electron collisions (the extra energy to ionize is 4.20 e V in Argon gas) and

collisions between metastabels. A term of energy, which contains the consumed input

power during these collisions, should be added in the total energy loss. Therefore,

the Eq.5.3 gets modified as

(5.6)

where ~M is energy lost by per electron collision with metastable atom and collisions

between metastable atoms at metastable state.

According to available literature, striations in surface wave produced plasmas are

still not experimentally studied. Hence, our experiment shows the effect of two-step

ionization in such discharge mechanism. Equation 5.6 reveals a interesting study on

the two-step ionization in surface wave discharge mechanism which can help us to

make better understanding on discharge mechanism. We now interested to under

stand the variation in physical properties of striations with operating parameters.

Let us first concentrate on the final density profile of striations. Plasma density

profile of striations can be qualitatively modeled in our experiment by,

(5.7)


where N0 is measured localized plasma density of plasma column, 'Y is decay co

efficient and kz is the wave vector, which is the function of axial position z. The

density profile on the basis of Eq.5. 7 is given in Fig.5.9 in which the profile of elec

tron density with axial distance from RF exciter is shown. This profile indicates

that each peak (amplitude) corresponds to the appearance of each striation. There

are six striations with decreasing amplitudes as well as decreasing lengths along the

z- direction which quite match with our experimental results.

The wave vector kz can be defined as

k _ 21r _ n1r z-)..- L (5.8)

where ).. is wavelength, n is the number of modes and L is length of plasma col

umn. The modes are the appearance of striations, which are excited with positive

amplitude. Substituting the values of n from 1 to 6 with corresponding plasma

column length from 5 em to 30 em in Eq.5.8, the calculated value of ).. is 10 em.

The half wavelength(>../2) is corresponding to the length of the striation close to

the RF exciter or first striation, out of the different striations in a certain length of

plasma column. The calculated length of 5 em matches very well with the measured

length of the first striation (5 em). The length of striations decreases along the

z-axis with the scale factor (! = 1.28). Scale factor is the ratio of the length of two

successive striations. Now the length of second, third and subsequent striations can

be expressed as

(5.9)

where nz is 1, 2, 3 .... for second, third, fourth, .... striation and ).. is 10 em. A com

parison between calculated and measured lengths of different striations are shown

in Fig.5.10, in which different symbols are measured values and lines are the calcu

lated values at different pressures from 0.03 mbar to 0.75 mbar at constant operating

parameters.

The wave vector is a function of z in Eq.5.7 so that kz(z) for each striation can

be expressed as 7r

kz(z) = - 1 f nz z (5.10)

The scale factor f is also equal to the damping factor of axial electric field of plasma

column. Therefore, the lengths of striations are decreased due to the exponential

Chapter 5: Striations in Plasma Column

0 5 10 15 20 25 30 Axial distance (em)

102

Figure 5.9: Plasma density profile of six striations along the axis of plasma column of length 30 em.

decay of axial electric field of surface wave in plasma column. This result emerges

our interest to study the electric field profiles with and without striations, which

can be compared with the plasma density profile.

The electric field profiles on the surface of plasma column without striations and

with striations with comparison to density profile are shown in Fig.5.11, in which

solid line with circles shows axial electric field profile of plasma column without

striations, solid line with squares shows the axial electric field profile with striations

and dotted line shows the plasma density profile. From Fig.5.11, it can be shown

that electric field is negative, positive and zero for striations and if this profile is

compared to the profile of plasma density, then it is clear that, when plasma den

sity is higher, electric field is negative and axial distance is the length of particular

striation and when plasma density is minimum, electric field is positive where axial

distance is gap between two striations while electric field becomes zero at transit

region.

Till now, we have made an attempt to understand the discharge mechanism, forma-


• p = 0.03 mbar r

• p = 0.04 mbar r

• p = 0.05 mbar r

• p = 0.075 mbar r

0 1 2 3 4 5 6 7 8 9 Number of striations

103

Figure 5.10: Comparison between theoretically calculated and experimentally measured particular lengths of different striations at different pressures where all other operating parameters are constant.

tion of striations and their physical properties. As we have assumed in the begin

ning of this section the value of bifurcation parameter decides the transformation

of plasma column into striations. Hence, we now devote our attention to the study

of bifurcation parameter and its dependency on the operating parameters in our

experimental conditions.

The experimental results show that the striations can be controlled by input

power and working pressure. Hence, bifurcation parameter should depend on input

power and working pressure which finally controls the reaction cross section of two

step ionization process. Bifurcation theory [24] also treats that number of striations

can be controlled by bifurcation parameter which depends only on the L , the length

of glass tube, equation in Ref [24] can be written as

£2 J.L=

D1r (5.11)

However in our experiment, L is the length of plasma column which is not constant

and varies with input power as shown in Fig.3.1 in Chapter 3 and Eq.5.4. Dis the


diffusion coefficient of Argon gas. Number of stationary striations also vary with

length of plasma column (Fig.5.3). Length of particular striation decreases with

input power (Fig.5. 7) when 30 em long plasma column, number of striations and

working pressure are kept constant. It is because that the input power is func

tion of electric field and damping coefficient of this field is equal to the scale factor

of striations (Eq.5.9). The number of striations increases with working pressure

(Fig.5.5)and consequently length of striation decreases(Fig.5.6) because the neutral

density and plasma density are increased with working pressure. Therefore, bifur

cation parameter is a function of input power and working pressure which can be

written by putting the expression for L from Eq.5.4 in to Eq.5.11 then,

J1, = (7ra;8vm) :71" (5.12)

where a, Vm and D is constant for fixed diameter of glass tube, working pressure

and diffusion coefficient. This equation can be written as

J1, = it (Pr, a, D) P (5.13)

where

( 2 ) ! 1

it (pn a, D) = 1raB8vm D1r (5.14)

Thus the modified bifurcation theory can briefly explain the formation of striations

and their physical properties in the surface wave produced plasma column.

5. 7 Conclusions

Plasma column is formed with the help of radio frequency source operating be

tween 3 MHz to 10 MHz. By changing the operating parameters (input power from

40 watts to 60 watts and working pressure from 0.03 mbar to 0.30 mbar) plasma

column is transformed in to finite number of cylindrical stationary striations ( 4 to

12) in Argon gas due to the presence of metastable atoms for providing two-step

ionization. Number and length of striations can be controlled by changing working

pressure, input power and length of plasma column. A power balance equation is

modified for striations in surface wave produced plasma column. An attempt is

made to explain the lengths of different striations with the help of stable bifurcation


1.0

10

Electric field without striations I Electron density profile

Electric field with striatio I

15 20 25 Axial position (em)

105

30

Figure 5.11: Normalised axial electric field profiles of with and without striations on the surface of glass tube at constant length of plasma column of 30 em, working pressure of 0.03 mbar and input power of 50 watts.

theory in the literature with few modifications in the density profile. The calculated

values of lengths of striations match quite well with the measured values. It is also

interesting to see that there is a relation between the length of striations, axial den

sity and electric field profiles. Bifurcation parameter is a function of input power

and working pressure. This study reveals a lot of interesting and novel results with

proper explanation for academic interest to understand the formation of striations

and their physical properties in surface wave produced plasma column.

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