Rotating copper plasmoid in external magnetic fieldPramod K. Pandey and Raj K. Thareja Citation: Phys. Plasmas 20, 022117 (2013); doi: 10.1063/1.4793729 View online: http://dx.doi.org/10.1063/1.4793729 View Table of Contents: http://pop.aip.org/resource/1/PHPAEN/v20/i2 Published by the American Institute of Physics. Related ArticlesMagnetic field advection in two interpenetrating plasma streams Phys. Plasmas 20, 032703 (2013) Magnetorotational instability in plasmas with mobile dust grains Phys. Plasmas 20, 032102 (2013) On the formation of m=1, n=1 density snakes Phys. Plasmas 20, 032504 (2013) The effect of emissive biased limiter on the magnetohydrodynamic modes in the IR-T1 tokamak Phys. Plasmas 20, 032503 (2013) Local thermodynamics of a magnetized, anisotropic plasma Phys. Plasmas 20, 022506 (2013) 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

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Rotating copper plasmoid in external magnetic field

Pramod K. Pandeya) and Raj K. Tharejab)

Department of Physics, Indian Institute of Technology Kanpur, Uttar Pradesh 208 016, India

(Received 19 October 2012; accepted 5 February 2013; published online 28 February 2013)

Effect of nonuniform magnetic field on the expanding copper plasmoid in helium and argon gases

using optical emission spectroscopy and fast imaging is presented. We report a peculiar oscillatory

rotation of plasmoid in magnetic field and argon ambient. The temporal variation and appearance

of the dip in the electron temperature show a direct evidence of the threading and expulsion of the

magnetic field lines from the plasmoid. Rayleigh Taylor instability produced at the interface

separating magnetic field and plasma is discussed. VC 2013 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4793729]

I. INTRODUCTION

Laser produced plasmas (LPP) in ambient of magnetic

field is embedded in several areas of plasmas and astrophy-

sics, i.e., supernova explosion, coronal mass ejection,1 high

altitude nuclear explosions,2 inertial confinement fusion

plasmas, etc.3–5 LPP flowing in external magnetic field can

be used to study the astrophysical processes of stellar interi-

ors in the laboratory.6 The collimation and stability proper-

ties of plasma flow across magnetic field are of importance

to understand the propagation of charged particle beams, so-

lar wind evolution, and astrophysical jets.7 These jets are

similar to that observed in laboratory plasmas with magnetic

field via ~E �~B drift.8 The propagation of laser produced

plasma in a curved magnetic field resembles the flow of

charged particles in curved magnetic field of earth in the

earth magnetosphere which gives rise to ring current flowing

westward.9,10 X-ray backlighting technique in LPP has been

used to measure opacity11 and turbulence12 which are of con-

siderable interest to the astrophysicist of dense plasmas. The

plasma expanding in magnetic field undergoes instabilities

like Rayleigh Taylor (RT) instability at the boundary of

plasma front.13,14 The external magnetic field controls the

local plasma thermofluid characteristics both macroscopic

and microscopic,15,16 and hence is important from industrial

applications point of view. The magnetic field helps to con-

trol the dynamic properties of transient and energetic plasma

species for understanding the beam heating of magnetically

confined thermonuclear plasma.17 Magnetic field has also

been shown to change the material properties.18,19 Neogi

et al.20 has reported optical emission from the laser ablated

carbon plasma in different regions of a curved magnetic field

and found that emission characteristics depend on the curva-

ture and gradient of the magnetic field.

In the present work, the optical emission characteristics

and expansion dynamics using 2-dimensional imaging of laser

ablated copper plasma plume expanding in a non-uniform

magnetic field and an ambient gas are investigated. Fourth

harmonic of Nd:YAG laser (wavelength 266 nm) was used

for creating the copper plasma plume in a vacuum chamber.

The ambient magnetic field was applied through the assembly

of two permanent magnets. The field is maximum (3.0 kG) at

the middle of the poles and decreases on either side. Attempt

is made to understand the peculiar expansion, stagnation, rota-

tion of the copper plasma plume in argon ambient, and mag-

netic field. The instability arising due to motion of the plasma

plume in a magnetic field is discussed. The present paper is

organized as follows: Sec. II gives the details of the experi-

mental setup, results and discussion are given in Sec. III, and

conclusions are presented in Sec. IV.

II. EXPERIMENTAL

The layout of the experimental setup is shown in Fig. 1.

Fourth harmonic of the Nd:YAG pulsed laser (Lab 190-10,

Spectra Physics) with a maximum energy 110 mJ/pulse at

wavelength 266 nm with pulse width 4 ns [full width at half

maximum (FWHM)] at repetition rate 10 Hz was used to cre-

ate copper plasma. The laser radiation was focused on to the

copper target to a focused spot of 250 lm; the laser irradiance

was kept at 1.5� 1010 W/cm2 for all measurements. The tar-

get was mounted in vacuum chamber, which could be evac-

uated to pressure better than 10�6 mbar. The chamber was

purged with helium/argon prior to filling in the required am-

bient gas in a controlled manner. The experiment was done

using helium/argon at ambient pressure of 7.4 mbars. The

2-dimensional images of the expanding copper plasma plume

were recorded, which is performed using gated intensified

charge couple device (ICCD DH-720, ANDOR Technology,

USA) (ICCD) interfaced with computer at sampling rate

90 Hz [refer Fig. 1]. To observe optical emission, the ICCD

shown in Fig. 1 is replaced with a collecting lens of focal

length 10 cm and an optical fiber coupled to the entrance slit

of spectrograph (Shamrock SR 303i, ANDOR Technology,

USA) with gated ICCD and interfaced with the computer.

The gate width (exposure time) used for recording images

and spectral data was 50 ns with gate step size of 50 ns. The

overall resolution of the spectrograph is 0.65 nm and its en-

trance slit width was fixed at 180 lm. The resolution was esti-

mated using mercury spectrum.21 The spectral measurements

are done following the conventional geometry, where the

a)Present address: WCI-Center for Quantum-Beam Based Radiation

Research, Korea Atomic Energy Research Institute, Daejeon, South Korea.

Electronic mail: panditpkp@gmail.com.b)Author to whom correspondence should be addressed. Electronic mail:

thareja@iitk.ac.in.

1070-664X/2013/20(2)/022117/8/$30.00 VC 2013 American Institute of Physics20, 022117-1

PHYSICS OF PLASMAS 20, 022117 (2013)

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expanding plasma plume is imaged using a lens so as to have

1:1 image outside the interaction chamber on to an optical

fiber of diameter of 200 lm, thereby we can get spatial reso-

lution of about 0.1 mm.22 The magnetic trap used with varia-

tion of magnetic field is shown in Fig. 2. The distance

between the poles is 3.2 cm. The field is maximum (3.0 kG)

at the middle of poles and decreases on either side (along the

x-axis), whereas along the z-direction, it is minimum at the

middle and maximum (5.1 kG) at the pole surface. The target

was kept in between the pole pieces at two different positions

(x¼ 2.5 cm and x¼�1.0 cm). Figures 2(c) and 2(d) shows

the distortions in the magnetic field lines when the spherically

expanding plasmoid enters into the magnetic field; the

induced current direction due to change in the magnetic flux

as well as direction of the induced magnetic forces (in terms

of torque) acting on the plasmoid are also shown.

III. RESULTS AND DISCUSSION

A. Plume imaging

Expansion dynamics and evolution of copper plasma

plume at 7.4 mbars ambient pressure of He/Ar ambient are

extensively studied in the presence and in the absence of

magnetic field. We have used 2-dimensional imaging of the

plasma plume recorded using ICCD at different time delays

with respect to the ablating pulse to study the plasma plume

parameters for imbedded applications.

Figures 3 and 4 show the copper plasma plume expan-

sion in helium ambient with and without magnetic field and

the corresponding plume front position with respect to time

(R-t) at different time delays with respect to ablating pulse.

FIG. 2. (a) Magnetic trap used, (b) mag-

netic field variation along x and z direc-

tions, (c) compression in the magnetic

field lines due to plasma plume, and (d)

depiction of the forces and induced cur-

rent direction in the magnetic field.

FIG. 1. Schematic of the experimental setup.FIG. 3. Evolution of copper plasma plume at 7.4 mbars pressure of helium: (a)

without M, (b) with M at target position x¼ 2.5 cm, and (c) at x¼�1.0 cm.

022117-2 P. K. Pandey and R. K. Thareja Phys. Plasmas 20, 022117 (2013)

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Figure 5 shows the plume expansion in Ar ambient in the ab-

sence of magnetic field. Figures 6 and 7 show the plume

expansion in Ar ambient in the presence of magnetic field

and R-t plots (with and without magnetic field), respectively,

at different time delays of the ablating pulse. All images in

the Figs. 3, 5, and 6 are normalized to the maximum intensity

in that image and the plume front position has been taken at

10% intensity position of the corresponding maximum inten-

sity of that particular image. It is observed from Fig. 3 that

bifurcation of the plume occurs at earlier time (200 ns) in the

presence of magnetic field for both the target positions

x¼ 2.5 cm and x¼�1.0 cm; Fig. 3(b) and 3(c) than that

without magnetic field (300 ns). The plasma constituents in

the plume make collisions with the ablated material resulting

in ultraviolet (UV) emission of radiation that reaches the tar-

get surface which results in further vaporization. In helium

ambient, the appearance of bifurcation of the plume at latter

time may be due to faster cooling in helium that slows the

process of UV emission and hence in slower secondary

evaporation of the target material. However, in the presence

of magnetic field, bifurcation occurs earlier due to enhanced

UV emission because of magnetic heating. The temporal

evolution of the plume with respect to distance in helium

ambient is shown in Fig. 4; it follows drag model in the pres-

ence and in the absence of the magnetic field. However, in

the presence of argon ambient (Fig. 5) and without magnetic

field, the fraction of energy lost in each collision,23

DEAr ¼ 2 mCu

mAr

� �h i, where mCu is the mass of the ablated cop-

per atom and mAr is the mass of the argon atom, is much

larger than that in helium ambient that results in confinement

of the plasma plume near to the target surface. Further, due

to lower thermal conductivity of argon (17.72� 10�3 mW

m�1 K�1) than that of helium, (148.1 mW m�1 K�1)24 plume

emission is sustained for longer time and the expansion fol-

lows the drag model. At an ambient pressure of 7.4 mbars of

argon in the presence of magnetic field, the instability with

stagnation and rotation of the plume are observed at target

position of x¼ 2.5 cm [refer Fig. 6(a)]. However, no plume

rotation was observed when the target was at x¼�1.0 cm

[refer Fig. 6(b)]. Since, in an expanding laser ablated plasma,

the density of the plasma ions decreases due to expansion

and recombination processes, it facilitates the diffusion of

the ambient gas atoms into the plasma and vice versa.

Increasing the ambient pressure leads to the plasma confine-

ment and hence decreases the diffusion of the plasma ions

into the ambient gas and ambient gas atoms in the plasma. A

narrow interface formed at the plasma gas interface may be

stable or unstable depending upon whether the acceleration

is from lighter to heavier fluid or from heavier to lighter

fluid, respectively. The growth of instability occurs in the

region of maximum acceleration that can be derived from

the derivative of momentum conservation equation25

d

drmþ 4pR3q0

3

� �u

� �¼ 0; (1)

where m is the mass of the ablated material, u is the plasma

front velocity, R is the distance of the plume front from the

target surface, and q0 is the gas density. Solving Eq. (1), we

get q0 ¼ 6m28pR3. Mass of the ablated material can be calculated

using the following equation:26

_m ¼ 143I

1014

� �13

k�43 kg=cm2 s: (2)

FIG. 4. Temporal variation of plume

front position at 7.4 mbars pressure of

helium with and without magnetic field.

FIG. 5. Evolution of copper plasma plume at 7.4 mbars pressure of argon

without magnetic field.

022117-3 P. K. Pandey and R. K. Thareja Phys. Plasmas 20, 022117 (2013)

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Following Figs. 6(a) and 7(b), we observe that the instability

in the plume starts at delay of 200 ns after the ablating pulse

at 7.4 mbars Ar ambient with magnetic field at the target

position of x¼ 2.5 cm with corresponding plume front posi-

tion of R ¼ 0:30 cm from the target surface. Using Eq. (1)

and value of the plume front position at 200 ns delay; we get

q0�1014cm�3; compared to experimentally observed plasma

density of q1� 1016 cm�3 at 7.4 mbars argon ambient in the

presence of magnetic field. Further, the RT instability arises

due to the difference in the density of two immiscible fluids

at the interface. The interface of the two fluids is stable and

unstable according to whether the acceleration is directed

from lighter to heavier fluids or vice-versa.27,28 Substituting

these values in x21 ¼ �k1ac � ðq1�q0Þ

ðq1þq0Þ; where k1 and ac are

perturbation propagation vector and acceleration of the con-

tact boundary, respectively; we get x21 < 0 justifying the

occurrence of RT instability.13,29 It has been observed that at

7.4 mbars argon ambient and target position x¼ 2.5 cm in the

magnetic field, the plume expansion stops but RT instability

prevails. The plume expansion stops when magnetic pressure

is balanced by the plasma pressure or b ¼ 8pNkBTe

B20

� �¼ 1,

where N; kB;Te; and B0 are the total number density of the

plasmoid, Boltzmann constant, electron temperature of the

plasmoid, and value of the magnetic field, respectively.30 The

expansion of the plume pushes the magnetic field ahead

because of internal diamagnetic currents shielding the field

from its interior [refer Figs. 2(c) and 2(d)].31 Thus, plasma

plume behaves like a diamagnetic cavity and opposes the

magnetic flux threading and hence the expansion of the

plume occurs until the work done by the plasma to push the

magnetic field out of its volume becomes equal to the total

plasma energy. Assuming that the plume expands spherically,

the stopping radius can be estimated by energy conservation

E0 ¼1

2Mv2 þ B2R

2l0

; (3)

where E0;M, v, and R are the total plasma energy, total mass

of the plasmoid, velocity of the plume front, and volume of

the plasmoid, respectively.

FIG. 6. (a) Evolution of copper plasma plume at 7.4 mbars pressure of argon with magnetic field at target position x¼ 2.5 cm and (b) at target position

x¼�1.0 cm.

FIG. 7. Temporal variation of plume

front position at 7.4 mbars argon ambi-

ent: (a) without magnetic field and (b)

with magnetic field.

022117-4 P. K. Pandey and R. K. Thareja Phys. Plasmas 20, 022117 (2013)

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Considering our experimental conditions and spherical

expansion of the plasmoid, as shown in Fig. 7(b), in 7.4 mbars

Ar ambient with magnetic field at target position of

x ¼ 2:5 cm; the plume front velocity is found to be

v ¼ 3:2� 104 m=s at plume front position of R ¼ 0:16 cm at

t ¼ 50 ns. Calculated volume of the plasmoid is

R¼2:14�10�9 m3, M¼3:32�10�12 kg, and B0¼0:62 kG.

Now using Eq. (3), we get total energy of the plasmoid to be

E0¼1:7�10�3 J. Since the plume front velocity, v¼0 at the

stopping radius; Eq. (3) can be written as

E0 ¼B2R2l0

; (4)

and stopping radius (bounce radius) in this case can be written as

rb ¼3l0E0

2pB20

� �13

: (5)

Substituting the value of B0 ¼ 0:62 kG and the calculated E0,

the value of stopping (bounce) radius is rb ¼ 0:63 cm. It

is greater than the experimentally measured value of

rb ¼ 0:418 cm at t ¼ 550 ns [refer Fig. 7(b) for x¼ 2.5 cm

target position]. This difference may be due to the compres-

sion of magnetic field lines upon the expansion of the plume

resulting in the enhancement in the magnetic field and hence

more confinement of the plume.24

It is observed from Fig. 7(b) that though the plume

decelerates as it approaches rb but it does not stop fully. This

may be due to incomplete exclusion of field inside the

plasma, ~E �~B motion of the plasma in the boundary layer,

and deviation from the spherical plume expansion approxi-

mation. Following Fig. 6(a) of plasma plume front, we

observe a peculiar behavior of plasmoid rotation on reaching

at the bounce radius at t ¼ 550 ns and its rotation direction

alternatively changes after a time interval t ¼ 200 ns. After

t ¼ 950 ns, the time required to change the rotation direction

increases; may be due to decrease in temperature.

In order to explain rotation of the plume, we assume the

plasma plume in magnetic field is made of number of loops

moving through the magnetic field [refer Fig. 2(d)]. Induced

emf in the loop when it passes through the magnetic field is

eemf ¼ �d/B

dt¼ i Rresistance; (6)

where i and Rresistance represent the current in the loop and re-

sistance of the loop, respectively. This induced diamagnetic

current on the surface of the plasmoid occurs when the mag-

netic flux threads through the plasmoid. It always opposes the

change in the flux through the plasmoid and an induced~j �~Bforce ðFm and F0mÞ shown in Fig. 2(d) exerts the torque on the

plasmoid that causes the rotation [refer Fig. 2(c) and 2(d)].

The change in the direction of the rotation of the plasmoid

may be due to alternate threading and expulsion of the mag-

netic field from the plasmoid. However, no plasmoid rotation

was observed when the target position was at x ¼ �1:0 cm

[refer Fig. 6(b)]. This may be happening because upon the

plume expansion the field lines becomes rarefied24 that results

in reduced field values and hence induced~j �~B force is not

enough to exert enough torque to rotate the plasmoid.

B. Spectroscopy

Emission spectra of copper plasma plume were recorded

at 2.5 mm from the target surface at varying time delays with

respect to ablating pulse. Figures 8 and 9 shows the intensity

variation of Cu I lines with and without magnetic field in 7.4

mbars helium and argon ambient pressures at target position

x¼ 2.5 cm and x¼�1.0 cm, respectively. It is observed that

emission line intensities of Cu I (Cu I at 510.5 nm, 515.32 nm,

and 521.82 nm) transitions at 7.4 mbars helium ambient and at

two different target positions in the magnetic field (x¼ 2.5

and x¼�1.0 cm) [refer Figs. 8(a), 8(b), and 9(a)] decrease

very rapidly compared to the corresponding argon ambient

with and without magnetic field [refer Figs. 8(c), 8(d), and

9(b)]. The observed line intensities are sustained for longer

time in argon ambient; may be the presence of argon induces

more recombination due to the confinement of the plume.

In order to understand the plasmoid oscillatory behavior,

we have calculated the temperature and density of the plasma.

The temperature and density of the plasma are determined

by using spectroscopic methods.32 Temporal variation of the

electron temperature is determined by assuming local

thermodynamic equilibrium (LTE) and using relative inten-

sities of spectral transitions.32,33 We have used Cu I transi-

tions ½4p 2P3=2 � 4s 2S1=2� at 324.75 nm, ½4p 2P1=2 � 4s 2S1=2�at 327.39 nm, ½3d10ð1sÞ5d � 3d10ð1sÞ4p� at 406.26 nm,

½4p 2P3=2 �4s2 2D5=2� at 510.5 nm, ½4d 2D3=2 � 4p 2P1=2� at

515.32 nm, ½4d 2D5=2 � 4p 2P3=2� at 521.82 nm, and ½4s2 2D

�4p 2P0� at 578.21 nm for calculating the electron

temperature34

lnInm knm

Anmgnm

� �¼ ln

nnhc

ZðTÞ

� �� Em

kBTe

� �; (7)

where Inm is the intensity of the observed transition line, Anm

is the transition probability, knm is the transition wave-length,

gnm is the statistical weight, Em is the energy of the upper

level, nn is the number density of the nth state, ZðTÞ is the

partition function, and Te is the electron temperature.

The upper and lower transitions are, respectively, labeled as

m and n. The slope 1kBTe

� �of the plot of ln Inmknm

Anmgnm

� �versus Em

gives electron temperature. The various parameters appearing

in Eq. (7) are available in the literature.35 The plasma electron

density is measured using the Stark broadened36 profile of Cu

I transition ½4p 2P3=2 � 4s2 2D5=2� at 510.5 nm. The Stark

broadening is weakly dependent on the temperature, the

electron number density (ne) can be estimated using the

FWHM Dk1=2 of the Stark broadened line given by Dk1=2

¼ 2w ne

1016 nm; where w is the stark width parameter and ne is

the plasma electron density. The Stark width parameter w is a

weak function of temperature and its value is available in the

literature.37 Since the observed line has an approximately

Lorentzian line shape, the correction for the instrumental

line broadening ðDkinstr ¼ 0:20 nmÞ is accounted for by

Dktrue ¼ Dkmeasured � Dkinstr. In order to ascertain the condi-

tion of LTE, we use the McWhirter Criterion;

022117-5 P. K. Pandey and R. K. Thareja Phys. Plasmas 20, 022117 (2013)

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ne � 1:6� 1012T12eðDEÞ3, where DEðeVÞ is the energy differ-

ence between the states and TeðKÞ is the electron temperature.

For Cu I transition at 510:5 nm DE ¼ 2:43 eV and

Te ¼ 8400 K, lowest value of ne ¼ 2:1� 1015cm�3 is less

than our measured lowest value of neð�9:2� 1015cm�3Þ,thus, implying the use of LTE approximation is justified.

Figure 10(a) shows temporal variation of electron temperature

in He ambient at 7.4 mbars with and without magnetic field

(at target position of x¼ 2.5 cm and x¼�1.0 cm).

Fluctuations in the electron temperature have been observed

at longer delays. The fluctuations in the electron temperature

and density [refer Figs. 10(a) and 10(b)] occur due to the

FIG. 8. Temporal evolution of Cu I emission lines at 510.5 nm, 515.32 nm, and 521.82 nm at 7.4 mbars ambient pressure of helium and argon with and without

magnetic field at target position x¼ 2.5 cm.

FIG. 9. Temporal evolution of Cu I emission lines at 510.5 nm, 515.32 nm, and 521.82 nm at 7.4 mbars ambient pressure of helium and argon with magnetic

field at target position x¼�1.0 cm.

022117-6 P. K. Pandey and R. K. Thareja Phys. Plasmas 20, 022117 (2013)

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competition between cooling of the plasma due to the pres-

ence of helium and joule heating due to magnetic field. In the

case of argon ambient and with magnetic field (target position

at x¼ 2.5 cm) [refer Fig. 11(a)], the electron temperature

shows the oscillatory behavior with position of the dip exactly

at the same time at which the plasma plume changes its direc-

tion of rotation [refer Fig. 11(a)]. Oscillatory behavior of elec-

tron temperature corroborates the change of direction of

rotation of the plasmoid such that as the time passes circulat-

ing current in the plasmoid decreases due to finite resistivity

of the plasma resulting less joule heating and decrease in the

temperature. Now as the plasma temperature decreases, mag-

netic field starts threading through the plasmoid leading to the

change in magnetic flux. This sets in a current that flows

through the plasmoid in such a way that it opposes the change

in the flux. This current loop in the magnetic field experiences

a torque due to which plasmoid rotates, the torque changes

the direction as current changes the direction and hence the

direction of rotation of the plasmoid. This process prevails

until b � 1 is maintained and lasts for longer time because of

the decrease in electron temperature. Due to the decrease in

the temperature of plasmoid, current produced in it is not

enough to expel the magnetic field out of it and complete

threading of the field lines takes place and plume rotation

eventually stops. At target position x¼�1.0 cm, fluctuation

in electron temperature [Fig. 11(c)] shows only the joule heat-

ing due to the induced current produced in the plasmoid due

to rarefied magnetic field which corroborates the absence of

rotation and stagnation of the plume [refer Fig. 11(c)]. Fig.

11(d) represents the electron density variation in the absence

of magnetic field, whereas Fig. 11(b) shows the fluctuation in

electron density is absent at the time when the plume was

rotating, may be due to the averaging of the density due to the

current flow in the plasmiod.

FIG. 10. Temporal evolution of electron temperature (a); electron density with and without magnetic field at 7.4 mbars He ambient (b).

FIG. 11. Temporal evolution of (a) electron temperature with magnetic field at target position x¼ 2.5 cm; (b) electron density with magnetic field at target

positions x¼ 2.5 and x¼�1.0 cm; (c) electron temperature with magnetic field at target position x¼�1.0 cm and without magnetic field; and (d) electron den-

sity without magnetic field, at 7.4 mbars of Ar ambient.

022117-7 P. K. Pandey and R. K. Thareja Phys. Plasmas 20, 022117 (2013)

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IV. CONCLUSION

Dynamics of laser produced copper plasma with and

without magnetic field in He and Ar ambient atmosphere has

been investigated using fast photography and emission spec-

troscopy at 7.4 mbars pressure. The plasma parameters such

as electron temperature and electron density are calculated

using emission spectra. In the presence of magnetic field and

argon ambient of 7.4 mbars pressure, Rayleigh Taylor insta-

bility in the plume is shown to develop. In the presence of

magnetic field and at 7.4 mbars ambient of argon, stagnation

and oscillatory rotation of the plume have been found.

ACKNOWLEDGMENTS

The authors would like to thank anonymous reviewer

for his critical review. Discussion with Shyam Lal Gupta and

V. Narayanan are gratefully acknowledged. P.K.P. acknowl-

edges the support by the World Class Institute (WCI) pro-

gram of the National Research Foundation of Korea (NRF)

funded by Ministry of Education, Science and Technology

of Korea (MEST) (NRF Grant No. 2011-001).

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