Effect of Ambient Pressure on the Axial Behavior of Transferred Thermal Arc-Plasma Column

A comprehensive study of different gases in inductively coupled plasmatorch operating at one atmosphereSangeeta B. Punjabi, N. K. Joshi, H. A. Mangalvedekar, B. K. Lande, A. K. Das et al. Citation: Phys. Plasmas 19, 012108 (2012); doi: 10.1063/1.3676598 View online: http://dx.doi.org/10.1063/1.3676598 View Table of Contents: http://pop.aip.org/resource/1/PHPAEN/v19/i1 Published by the American Institute of Physics. Related ArticlesOn the dynamics of the space-charge layer inside the nozzle of a cutting torch and its relation with the “non-destructive” double-arcing phenomenon J. Appl. Phys. 110, 083302 (2011) Plasma-enhanced gasification of low-grade coals for compact power plants Phys. Plasmas 18, 104505 (2011) Reactive hydroxyl radical-driven oral bacterial inactivation by radio frequency atmospheric plasma Appl. Phys. Lett. 98, 143702 (2011) Microwave N2–Ar plasma torch. I. Modeling J. Appl. Phys. 109, 023301 (2011) Pressure and arc voltage coupling in dc plasma torches: Identification and extraction of oscillation modes J. Appl. Phys. 108, 043304 (2010) Additional information on Phys. PlasmasJournal Homepage: http://pop.aip.org/ Journal Information: http://pop.aip.org/about/about_the_journal Top downloads: http://pop.aip.org/features/most_downloaded Information for Authors: http://pop.aip.org/authors

A comprehensive study of different gases in inductively coupled plasmatorch operating at one atmosphere

Sangeeta B. Punjabi,1,2,a) N. K. Joshi,3 H. A. Mangalvedekar,1 B. K. Lande,1 A. K. Das,4

and D. C. Kothari21Electrical Engineering Department, V. J.T.I, Matunga, Mumbai 400019, India2Department of Physics, University of Mumbai, Kalina, Santacruz(E) 400098, India3Faculty of Engineering and technology, MITS, lakshmangarh, (Sikar), Rajasthan 332311, India4Laser and Plasma Technology Division, BARC, Mumbai 400085, India

(Received 17 May 2011; accepted 17 November 2011; published online 19 January 2012)

A numerical study is done to understand the possible operating regimes of RF-ICP torch (3 MHz,

50 kW) using different gases for plasma formation at atmospheric pressure. A two dimensional

numerical simulation of RF-ICP torch using argon, nitrogen, oxygen, and air as plasma gas has been

investigated using computational fluid dynamic (CFD) software FLUENTVC

. The operating parameters

varied here are central gas flow, sheath gas flow, RF-power dissipated in plasma, and plasma gas.

The temperature contours, flow field, axial, and radial velocity profiles were investigated under

different operating conditions. The plasma resistance, inductance of the torch, and the heat

distribution for various plasma gases have also been investigated. The plasma impedance of ICP

torch varies with different operating parameters and plays an important role for RF oscillator design

and power coupling. These studies will be useful to decide the design criteria for ICP torches required

for different material processing applications. VC 2012 American Institute of Physics.

[doi:10.1063/1.3676598]

I. INTRODUCTION

RF-ICP (radio frequency-inductively coupled plasma)

has been subject of interest from past four decades.1–3 The

distinguishable advantage of ICP over other plasma genera-

tion techniques include pure plasma formation even with re-

active plasma gases, as it is an electrode less plasma and

discharge is free from contamination. It can be operated in

broad range of pressures, powers, and sizes. Because of its

wide range of operating parameters, RF-ICP has attracted

considerable attention from researchers for numerous indus-

trial applications, such as powder spheroidization, spectro-

chemical analysis, sintering, spray coating of ceramic and

metallic powders, synthesis of ultra fine powders of metals,

alloys and ceramics, and nano particle synthesis.4–8

In ICP torches, mostly argon is used as plasma gas, but it

may be desirable to use gases such as nitrogen, air, oxygen,

or hydrogen depending on the industrial application. The

temperature and flow fields in ICP torches operating with ar-

gon gas have been studied extensively using both 2-D and

3-D numerical simulations.9–12 However, there is paucity of

simulated data for torch operating with other plasma gases

like nitrogen, air, and oxygen. Kim13 discusses a methodol-

ogy considering the RF generator simulation to predict sus-

tainable minimum power for argon and nitrogen plasma.

Mostaghimi, Proulx, and Boulos14 using 1D electromag-

netic equations reported that the flow and temperature fields of

nitrogen and argon differs substantially with change in the

operating gas. Barnes and his collaborators15,16 modified the

mathematical model reported by Miller and Ayen17 to predict

that the increase in central flow rate reduced the temperature

in coil region. Barnes and Nikdel18 used the same model

developed by Miller and Ayen to obtain nitrogen ICP dis-

charge in spectrochemical applications. They investigated tem-

perature and velocity profile and energy distribution with input

power and central flow rates. Later on, Barnes and Nikdel19

did a comparative study of argon and nitrogen for spectro-

chemical analysis working at one atmosphere. In this paper, it

is mentioned that nitrogen discharges offered better conditions

for spectrochemical analysis than argon discharge. Nishiyama

et al.20 used 2-D axisymmetric turbulent RF-inductively

coupled thermal argon plasma and reported that how injection

position of cold helium gas can control temperature and axial

velocity. These papers used mostly argon gas or nitrogen for

predicting or comparing the flow, temperature fields in

RF-ICP. Tsuenwani et al.21 used low-Reynolds number k � emodel to demonstrate that oxygen plasma is confined to axis

and axial velocity decreases with increase in RF-power. Con-

sideration of gas economy is also a strong motivating factor to

study the temperature and flow fields of nitrogen, oxygen, and

air in these industrially important plasmas.

Fouladgar and Chentouf22 developed a boundary element-

finite difference method to predict the resistance and inductance

as a function of temperature in RF-ICP torch working at

3–5 MHz frequency with argon gas. Kim et al.23 used the same

method to predict the resistance and inductance as a function of

plasma power and frequency for 2-D-model of RF ICP torch

using argon as plasma gas. An integrated model was proposed24

to predict the overall electrical characteristic of 2-D RF-ICP

turbulent plasma. In this model, induction plasma is considered

as part of RF network, in actual practice also, oscillator fre-

quency and inductive power does not remain constant buta)Electronic mail: [email protected].

1070-664X/2012/19(1)/012108/12/$30.00 VC 2012 American Institute of Physics19, 012108-1

PHYSICS OF PLASMAS 19, 012108 (2012)

depends on the plasma properties and characteristics of genera-

tor circuit. This model was later experimentally validated,25

which showed 2–4% variation in impedance predicted by the

model. For designing the RF generator for ICP torch, the

knowledge of plasma impedance and its variation with different

operating conditions is of great importance.

In this paper, for practical applications of ICP torches

like material processing, the role of different plasma gases has

been studied in details. In determining the useful range of operat-

ing parameters of the ICP torch for material processing, the fol-

lowing requirements for the ICP torch are taken in to account:

(a) Hot enough temperature in the coil region, so that elec-

tron density (more than 1021/m3) should be high enough

to satisfy LTE assumption.

(b) Large volume of the hot plasma for industrial application

like material processing, the requisites are high tempera-

ture at the coil plasma region.

(c) No recirculating flow inside the plasma especially near

the inlet.

(d) Sufficient cooling of the tube wall by the sheath gas, so

that wall temperature should be below the melting point

of quartz.

The objective of present study is to provide a compara-

tive study of influential operating parameters on the tempera-

ture contours, impedance, and heat distribution. The gases

used for study are argon, nitrogen, oxygen, and air working

at atmospheric pressure with oscillator frequency of 3 MHz.

The investigated parameters can be used to design a RF-ICP

system depending on the industrial application.

II. ICP SIMULATION MODEL

Using 2-D Model,26 continuity, momentum, energy, and

vector potential equations are solved for optically thin

plasma. The plasma is assumed to be in LTE under atmos-

pheric condition. The flow is laminar, steady with negligible

viscous dissipation. The tangential component of velocity is

not taken into account. Displacement current associated with

oscillatory magnetic field is assumed to be negligible. A

coupled set of governing equations of RF-ICP is simulated

within FLUENTVC

environment. The vector potential equa-

tions are solved using User Defined Scalar.27 The governing

equations used are as follows:

A. Governing equations

Continuity equation:

@ðqtzÞ@z

þ 1

r

@ðqrtrÞ@r

¼ 0: (1)

Momentum equation:

q@tz

@z� tz þ

@tz

@r� tr

� �¼� @p

@zþ @

@zl � 2

@tz

@z

� �� �

þ 1

r

@

@rlr

@tr

@zþ @tz

@r

� �� �þ Fz;

(2)

q@tr

@r� tr þ

@tr

@z� tz

� �¼� @p

@rþ @

@rl � 2

@tr

@r

� �� �

þ @

@zl@tr

@zþ @tz

@r

� �� �

þ 2lr

@tr

@r

� �� tr

r

� �þ Fr: (3)

Energy equation:

q@h

@rtr þ

@h

@ztz

� �¼ @

@z

kcp

@h

@z

� �þ 1

r

@

@r

kcp

r@h

@r

� �þ UP � UR: (4)

Vector potential equation:

@2AR

@z2þ 1

r

@

@rr@AR

@r

� �� AR

r2þ l0xrAI ¼ 0; (5)

@2AI

@z2þ 1

r

@

@rr@AI

@r

� �� AI

r2� l0xrAR ¼ 0; (6)

Ah ¼ AR þ iAI; (7)

where r is the distance in radial direction and z is the distance

in axial direction, tz is the axial velocity and tr is the radial

component of velocity; q; l; k, r, and cp are the density, vis-

cosity, thermal conductivity, electrical conductivity, and spe-

cific heat at constant pressure, respectively; h is the enthalpy,

p is the pressure, l0 is the permeability of free space,

x ¼ 2pf , and f is the oscillator frequency, Up and UR are the

local energy dissipation rate and volumetric radiation heat

losses, respectively. AR and AI are real and imaginary compo-

nents of vector potential Ah. Fr;Fz are the radial and axial

body force acting on plasma gas in the discharge region.

Fr ¼1

2l0rReal EhH�z

� �; (8)

Fz ¼ �1

2l0rReal EhH�r

� �; (9)

Up ¼1

2r EhE�h� �

: (10)

The corresponding electrical field intensity in azimuthal

direction, Eh, the axial and radial components of the mag-

netic field, Hz and Hr, can be calculated as follows:

Eh ¼ �ixAh; (11)

l0Hz ¼1

r

@

@rrAhð Þ; (12)

l0Hr ¼ �@

@zAhð Þ: (13)

The total RF discharge power dissipated into plasma is

denoted by P

P ¼ð

v0Updv0 (14)

012108-2 Punjabi et al. Phys. Plasmas 19, 012108 (2012)

B. Boundary condition

The boundary conditions for the conservation equations

of inductively coupled plasma are as follows:

� Inlet conditions (z¼ 0):

tz ¼

Q1=pr21 r < r1

0 r1 � r � r2

Q2=p r23 � r2

2

� r2 � r � r3

Q3=p R20 � r2

3

� r3 � r � R0

8>>>><>>>>:

(15)

tr ¼ 0; (16)

T ¼ 300K; (17)

@AR

@z¼ @AI

@z¼ 0: (18)

� Centreline (r¼ 0):

@tz

@r¼ tr ¼

@h

@r¼ AR ¼ AI ¼ 0: (19)

� Wall (r ¼ R0)

tz ¼ tr ¼ 0; (20)

k@T

@r¼ kw

dwTs � Twð Þ; (21)

AR ¼l0I

2p

ffiffiffiffiffiRc

R0

r Xcoil

i¼1

GðkiÞ þl0x2p

XC:V:p¼1

ffiffiffiffiffirp

R0

rrpAI;pSpGðkpÞ;

(22)

AI ¼ �l0x2p

XC:V:p¼1

ffiffiffiffiffirp

R0

rrpAR;pSpGðkpÞ; (23)

where

GðkÞ ¼ ð2� k2ÞKðkÞ � 2EðkÞk

; (24)

k2p ¼

4R0rp

ðrp þ R0Þ2 þ ðzb � zpÞ2; k2

i ¼4RiR0

ðRi þ R0Þ2 þ ðzi � zbÞ2:

(25)

Q1, Q2, Q3 is the flow rate of central, plasma, and sheath gas.

r1 is the inner radius of injection tube, r2 is the outer radius

of injection tube, r3 is the radius of intermediate tube as

shown in Figure 1. T is the temperature, and KðkÞ and EðkÞare complete elliptic integrals.28 The vector potential at each

point depends on the current carrying region of space.

Hence, superposition of coil and plasma effect is used to

determine the vector potential. Therefore, in AR at wall

boundary, the first summation extends over the number of

coils and the second one extends over the current carrying

region of the discharge as shown in equation (22). Here R0 is

the radius of the confinement tube as shown in Figure 1; Rc

is the radius of the coil, and rp and Sp are the radius and

crossection of the pth control volume. Ri is the radius of ithcoil, zi is the height of the ith coil, zb is the height of the

boundary, and rp is the electrical conductivity at pth control

volume. kw is the thermal conductivity of the quartz confine-

ment tube (kw¼ 1.047 W/mK), dw is the tube wall thickness,

dc is the coil tube diameter, I is the coil current, Ts is inside

surface temperature of quartz tube, and Tw is the external

(wall) surface temperature of quartz tube (300 K).

� Exit

@ðqtzÞ@z

¼ @tr

@z¼ @h

@z¼ @AR

@z¼ @AI

@z¼ 0: (26)

C. Impedance calculation

Estimation of operating conditions of plasma in RF-ICP

depends on frequency, RF-power dissipated in plasma, torch

dimension, coil positioning, and so on. Evidently, to find

experimentally the controlled parameters of RF-oscillator

and plasma torch is a wearisome and complex problem. This

is due to the effect that RF-oscillator has on frequency and

power dissipated in plasma. Often for experimental measure-

ment of power dissipated in plasma and its impedance,

experiment is performed using a dummy resistive load in

place of plasma and by calorimetric measurements estimate

the power dissipated in the dummy load. Impedance calcula-

tion by this method over estimates the efficiency.29 A fixed

dummy resistance load cannot predict the dynamic charac-

teristic of plasma load.30 The numerical simulation of plasma

is an excellent tool to predict the impedance and power dissi-

pated in plasma to avoid time-consuming fabrication of the

torches31 and costly experimental process, these data are also

required for efficient design of RF-generator. The method

used for calculation of plasma impedance is same as done by

FIG. 1. Schematic diagram of inductively coupled plasma torch.

012108-3 Study of different gases in inductively coupled plasma torch Phys. Plasmas 19, 012108 (2012)

Kim, Mostaghimi, and Iravani.23 Resistance and inductance

of the torch are obtained from impedance expression and

hence the equation is as follows:

Rtorch ¼ Rcoil þ Rplasma;

Rtorch¼Xcoil

i¼1

2pRi

rcoilScoilþx2l2

0

2p�XC:V:p¼1

RirpA�R;pSp

ffiffiffiffiffirp

Ri

rG ki;p

� !;

(27)

Ltorch ¼ Lcoil � Lplasma;

Ltorch ¼Xcoil

i¼1

Xcoil

n¼1

Kn �xl2

0

2p�XC:V:p¼1

RirpA�I;pSp

ffiffiffiffiffirp

Ri

rG ki;p

� !;

(28)

where Kn ¼ Ril0

ffiffiffiffiRn

Ri

qG ki;n

� if i= n,

¼ N2RiP0 � 1� 10�9 if i ¼ n;

A� ¼ 2pA

l0I:

P0 is the shape correction factor in Grover’s self inductance

formula.32 N is the number of coil turn. In Eq. (28), the first

term on RHS represents mutual inductance of single coil

turn i with the other coil turns, and the second term repre-

sents mutual inductance between coil i and the plasma.

D. Numerical procedure

Computations are performed using a CFD software

FLUENTVC

with geometry and torch dimension as shown in Fig-

ure 1 and Table 1, respectively. The governing equations are

solved by using control volume technique. The SIMPLE algo-

rithm developed by Patankar and Spalding33,34 is chosen to

TABLE I. Torch dimension and operating conditions.

Dimensions For 3 MHz ICP

r1 1.70 mm

r2 3.70 mm

r3 18.8 mm

R0 25.0 mm

F 3.0 MHz

dw 2.0 mm

Lc 32.0 mm

Pc 10.0 mm

Rc 33.0 mm

dc 5.0 mm

LT 150. 00 mm

FIG. 2. (Color online) Temperature contour at different power for argon

plasma.

FIG. 3. (Color online) Temperature contour at different power for nitrogen

plasma.

FIG. 4. (Color online) Temperature contour at different power for oxygen

plasma.

012108-4 Punjabi et al. Phys. Plasmas 19, 012108 (2012)

solve the continuity, momentum, energy equations written in

terms of velocity, pressure, and temperature. Continuity mo-

mentum, energy and vector potential equations are discretised

using upwind scheme. The under-relaxation parameter is used

for dependent variables. UDF is used to solve the boundary

condition for vector potential, velocity, and impedance calcu-

lation27 and to update transport and thermodynamic proper-

ties. Data are taken from Refs. 35, and 36, which are required

for transport and thermodynamic properties for argon, nitro-

gen, oxygen, and air plasma. Radiation data for nitrogen, oxy-

gen is from Ref. 37 and argon from Ref. 17. For air, the

radiation data assumed to be similar to nitrogen. The compu-

tation domain consists of entire area inside the RF-ICP torch,

with 24� 60 nonuniform grid network in radial and axial

direction, respectively. Non-uniformity of the grid system

ensures gradients are steep. Grid was generated using GAMBITVC

software. The oscillator frequency is 3 MHz. The operating

parameters that have been varied are central gas flow, sheath

flow, RF-power dissipated in plasma, and type of plasma gas,

and the investigated parameters are flow fields, temperature

contours, axial veloicity, radial, axial, and wall temperature,

impedance, and heat distribution (%).

III. RESULTS AND DISCUSSION

A. Effect of variation of RF-power on differentplasma gas

In this case, the inductively coupled torch was operated

at different dissipated RF-power with constant central,

plasma, and sheath gas flow as 1, 3, 21 lpm, respectively.

Figure 2 shows the temperature contour for argon gas operat-

ing at RF-power of 8.8 kW and 15 kW. The maximum tem-

perature for 8.8 kW is 10 200 K and for 15 kW is 10 600 K.

Figures 3–5 show temperature contours of nitrogen, oxygen,

and air, respectively, at different RF-power. The maximum

core temperatures for nitrogen at 15 kW and 25 kW are

9100 K and 9700 K, respectively.

For oxygen plasma, the maximum temperature for 15

and 20 kW are 9000 K and 10 000 K, respectively. Similarly,

for air plasma, maximum temperature is 6600 K for 15 kW

and 7200 K for 29.5 kW. These figures show that as the RF-

power is increased, the plasma expands radially and axially

because plasma core temperature increases. This results in

increase in electrical conductivity of plasma, which is due to

increase in ionization. Consequently, skin depth reduces, and

so plasma core volume increases and moves towards the

wall.

For 15 kW RF-power, argon plasma has more plasma

volume, and oxygen plasma has least plasma volume in com-

parison with other gas plasma. If high temperature and maxi-

mum plasma volume is desired then argon is best. However,

if less plasma volume with high temperature is desired, then

oxygen plasma is the best.

The RF-power dissipated to an ICP discharge is dissi-

persed through radiation (Qrad), thermal conduction through

FIG. 5. (Color online) Temperature contour at different power for air

plasma.

FIG. 6. (Color online) Heat distribution profile as a function of RF-power

for argon plasma.

FIG. 7. (Color online) Heat distribution profile as a function of RF-power

for nitrogen plasma.

012108-5 Study of different gases in inductively coupled plasma torch Phys. Plasmas 19, 012108 (2012)

wall (Qwall), and convection in form of exhausted flame en-

thalpy (Qout). To study, how the RF power (heat) is distrib-

uted in the ICP torch using different plasma gases, the flow

rates of central, plasma, and sheath gas were kept at 1, 3, and

21 lpm, respectively. As the power increases, loss due to

radiation (Qrad) increases for all plasma gas considered.

Viewing heat distribution profile for argon gas as shown

in Figure 6, increase in RF power with constant conditions

results in rapid increase in heat lost due to radiation and slow

increase in thermal conduction through the wall. As power

increases, temperature also increases that results in increase

in radiation losses. The slow increase in Qwall is due to slow

increase in thermal conductivity of argon gas with increase

in temperature. The gas expands with increase in power

below 10 kW, beyond that due to finite size of quartz tube it

cannot expand; therefore, Qwall reaches a saturation after

10 kW. To balance this, there is a rapid decrease in heat lost

at the exit of the torch.

For nitrogen, oxygen, and air plasma as the power

increases, there is a slow increase in heat loss due to radia-

tion as seen in Figures 7–9. This is again due to increase in

temperature with power. To balance this thermal conduction

through wall (Qwall) and the heat lost at the exit show a

slow decrease.

The wall temperature profile with respect to wall of the

quartz tube for various RF-powers for argon plasma is shown

in Figure 10. As power increases, the wall temperature also

increases. Therefore, for designing an ICP with RF-power

beyond 15 kW, the wall temperature should be monitered (as

melting point of quartz tube is 1683 K) or water can be used

for cooling the wall. For nitrogen plasma as observed earlier

in Figure 7, that Qwall loss is around 60–70%. However, the

wall temperature of quartz tube does not go beyond 900 K as

RF-power is increased from 10.4 kW to 25 kW as seen in

Figure 11. Same situation is for oxygen plasma as shown in

Figure 12, even though the losses at the wall are 70–80%,

the wall temperature are well below melting point of quartz

FIG. 9. (Color online) Heat distribution profile as a function of RF-power

for air plasma.

FIG. 10. (Color online) Wall temperature for argon plasma at various

RF-power.

FIG. 11. (Color online) Wall temperature for nitrogen plasma at various

RF-power.

FIG. 8. (Color online) Heat distribution profile as a function of RF-power

for oxygen plasma.

012108-6 Punjabi et al. Phys. Plasmas 19, 012108 (2012)

tube. Qwall losses are more for oxygen than nitrogen but still

the maximum wall temperature at 15 kW is less for oxygen

plasma than nitrogen. This is because the axial plasma core

volume of nitrogen is more compared to oxygen plasma. The

wall temperature is around 700 K even at 30 kW for air

plasma as shown in Figure 13.

The wall temperature profile of argon gas (as shown in

Figure 10) shows a large drop within the coil region compared

to other gas profiles. This is due to the real part of vector

potential profile along the wall shows a large variation

(8.95� 10�5 Vsec/m as shown in Figure 14) in magnitude

within coil region when compared to nitrogen, oxygen, and

air plasma gases as seen in Figures 15, 16, and 17, respec-

tively (variation in magnitude of nitrogen is 5� 10�5 Vsec/m,

oxygen 4.9� 10�5 Vsec/m, and air 4.92� 10�5 Vsec/m).

Observation of wall temperature profile for all the gases

show three peaks in the profile. These peaks are at the coil

turn position of the ICP torch. This is due to the electric field

intensity in azimuthal direction.

B. Variation of impedance with RF-power

For variation of impedance with RF power, ICP was

simulated with flow rate of central, plasma, and sheath gas

as 1, 3, 21 lpm, respectively. Figures 18–21 show the varia-

tion of Ltorch and Rplasma with RF-power for argon, nitro-

gen, oxygen, and air gas, respectively. Argon, air, and

nitrogen shows as the RF-power increases the plasma resist-

ance increases. The plasma resistance depends on two fac-

tors (a) plasma volume and (b) plasma temperature. As the

power dissipated in plasma increases, plasma volume

increases and hence the plasma resistance. Increase in RF-

power results in increase in temperature that in turn

increases electrical conductivity; consequently, these results

in decrease in plasma resistance. The combined effect of

volume and temperature helps plasma resistance increase as

the power increases.

However, in oxygen plasma, the effect of temperature is

more pronounced than volume as seen from Figure 4. So the

plasma resistance decreases as power increases as shown in

FIG. 12. (Color online) Wall temperature for oxygen plasma at various

RF-power.

FIG. 13. (Color online) Wall temperature for air plasma at various

RF-power.

FIG. 14. (Color online) Real part of vector potential along the wall for

argon plasma at various RF-power.

FIG. 15. (Color online) Real part of vector potential along the wall for

nitrogen plasma at various RF-power.

012108-7 Study of different gases in inductively coupled plasma torch Phys. Plasmas 19, 012108 (2012)

FIG. 16. (Color online) Real part of vector potential along the wall for

oxygen plasma at various RF-power.

FIG. 17. (Color online) Real part of vector potential along the wall for air

plasma at various RF-power.

FIG. 18. (Color online) Plasma resistance and torch inductance versus

RF-power for argon plasma.

FIG. 21. (Color online) Plasma resistance and torch inductance versus

RF-power for air plasma.

FIG. 19. (Color online) Plasma resistance and torch inductance versus

RF-power for nitrogen plasma.

FIG. 20. (Color online) Plasma resistance and torch inductance versus

RF-power for oxygen plasma.

012108-8 Punjabi et al. Phys. Plasmas 19, 012108 (2012)

Figure 20. As the power increases plasma volume increases

due to which the cross-sectional area of induced current

increases, and hence, there is less flux leakage between the

plasma and the coil. Therefore, inductance of the plasma

decreases, and therefore, inductance of the torch for argon,

air, and nitrogen decreases. For oxygen, the plasma volume

slightly decreases with power, and hence, the torch induct-

ance (Ltorch) increases from 6.02 micro H to 6.04 micro H.

C. Variation in flow rate

In this study for comparison, the dissipated RF-power is

kept at 15 kW for all gases and flow of central gas is varied

from 1 to 5 lpm and sheath gas from 21 to 17 lpm keeping

the plasma gas flow rate constant at 3 lpm. Total gas flow

rate for all the gases is constant (25 lpm). Computational

simulation show that the variation of sheath and the central

gas flow affects the temperature contour and the flow pat-

terns. Increase in central injection flow from 1 to 5 lpm indi-

cates the plasma volume shrinks radially at the axis. This is

due to the convective cooling by high flow rate of the gas.

Decrease in sheath gas flow rate from 21 to 17 lpm results in

marginally increase of radial plasma volume towards the

wall. Therefore, due to variation in flow rate, the radial

plasma volume decreases for all the gases as shown in

Figures 22–25.

Increase in central gas flow from 1 to 5 lpm and

decrease in the sheath gas flow from 21 to 17 lpm, the axial

temperature profile indicates that the plasma core region

shifts to the wall as mentioned earlier. For argon gas and

nitrogen plasma, the axial maximum temperature does not

change much with variation in sheath or central gas flow rate

as shown in Figures 26 and 27. However for air and oxygen

FIG. 22. (Color online) Temperature contour different flow rates for argon

plasma.FIG. 24. (Color online) Temperature contour at different flow rates for oxy-

gen plasma.

FIG. 23. (Color online) Temperature contour at different flow rates for

nitrogen plasma.

FIG. 25. (Color online) Temperature contour at different flow rates for air

plasma.

012108-9 Study of different gases in inductively coupled plasma torch Phys. Plasmas 19, 012108 (2012)

FIG. 26. (Color online) Axial temperature for argon plasma at different

flow rates.

FIG. 27. (Color online) Axial temperature for nitrogen plasma at different

flow rates.

FIG. 28. (Color online) Axial temperature for oxygen plasma at different

flow rates.

FIG. 29. (Color online) Axial temperature for oxygen plasma at different

flow rates.

FIG. 30. (Color online) Radial temperature for argon plasma at different

flow rates.

FIG. 31. (Color online) Radial temperature for nitrogen plasma at different

flow rates.

012108-10 Punjabi et al. Phys. Plasmas 19, 012108 (2012)

plasma, the flow rate affects the axial maximum temperature

as seen in Figures 28 and 29.

The radial temperature profile taken at z¼ 0.042 m for

argon and nitrogen plasma is not affected with increase in

central injection flow from 1 to 3 lpm. But for 5 lpm central

injection flow rate, there is considerable fall in radial temper-

ature at the inlet as seen in Figures 30 and 31. For oxygen

and air plasma, the radial temperature profile is shown in

Figures 32 and 33, respectively. Profile indicates that if cen-

tral gas injection flow is increased from 1 to 3 lpm, then

there is major drop in temperature at the inlet region. How-

ever, for 5 lpm, there is marginal drop in temperature at the

inlet region.

IV. CONCLUSION

The present numerical simulation study has been carried

out with different gases such as argon, nitrogen, oxygen, and

air for plasma. The inductively coupled plasma torch were

simulated with atmospheric pressure with RF oscillator

working at 3 MHz frequency. The objective is to study the

change in temperature and flow fields by varying the operat-

ing parameters such as RF-power dissipated in plasma,

plasma and sheath gas flow rate for different plasma gases.

Impedance of different plasma gas has been investigated.

The summary of the results obtained are as follows:

� Increase in RF-power shows that the plasma volume

increases for all gases. However, oxygen plasma has less

plasma core volume as compared to air, argon, and nitrogen.

The heat distribution for all plasma gases has been investi-

gated. The wall temperature for different gases, operating at

different RF-powers was determined. The results indicated

that for argon plasma, maximum wall temperature was

around 950 K for 15 kW. For nitrogen plasma, it is around

800 K, and for oxygen and air plasma, it is nearly 650 K.

Therefore, if high RF-power and less wall temperature is

required, then nitrogen, oxygen, and air plasma are better.

� Plasma impedance have been simulated for various RF-

powers keeping the central, plasma, and sheath gas flow as

1, 3, and 21 lpm, respectively. It was observed that as RF-

power increases, the plasma resistance increases for argon,

air, and nitrogen plasma. This is due to two factors, which

are plasma volume and temperature. As plasma volume

increases, plasma resistance also increases. As the RF-

power increases, the temperature increases, consequently

electrical conductivity increases hence plasma resistance

decreases. Therefore, the combined effect of both the fac-

tors leads to increase in plasma resistance. For oxygen

plasma, the effect of temperature is more so plasma resist-

ance decreases as power is increased.

As the power increases, plasma volume increases due to

which the cross-sectional area of induced current

increases, and hence, there is less flux leakage between the

plasma and the coil. Therefore, inductance of the plasma

decreases, and therefore, inductance of the torch for argon,

air, and nitrogen decreases. For oxygen, the plasma vol-

ume slightly decreases with power and hence the torch in-

ductance (Ltorch) increases from 6.02 micro H to 6.04

micro H. This should be verified with experimental result.

� When central and sheath gas is varied keeping plasma gas

flow constant, it was seen that the plasma volume radially

shrinks due to convective cooling. The maximum tempera-

ture at the axis shows that for argon and nitrogen plasma, it

does not change noticeably. However, in oxygen and air

plasma, the axial temperature shows noticeable variation

with central and sheath gas variation. The radial temperature

variation shows that for 5 lpm as central gas, the argon, air,

and oxygen plasma retract from the axis of the torch. This

result could be useful for reactant injection probe located at

the centre of the torch as the temperature is less than 600 K.

Similarly, when central gas flow rate is 3 lpm, air and oxygen

plasma gas can be used for reactant injection. This could be

an important criteria for material processing.FIG. 33. (Color online) Radial temperature for air plasma at different flow

rates.

FIG. 32. (Color online) Radial temperature for oxygen plasma at different

flow rates.

012108-11 Study of different gases in inductively coupled plasma torch Phys. Plasmas 19, 012108 (2012)

ACKNOWLEDGMENT

The authors are thankful to Dr. L. M. Gantayet, Director,

Beam Technology Development Group, for his support during

the course of this work. This work was made possible through

continuing research grants from BRNS. The fellowship given

by B.R.N.S. to S.B.P. during the course of this work is grate-

fully acknowledged.

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