Physics Letters A 374 (2010) 2555–2560

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Physics Letters A

www.elsevier.com/locate/pla

Effect of magnetic field on laser-blow-off plasma plume:Structured temporal emission profile

Ajai Kumar ∗, H.C. Joshi, V. Prahlad, R.K. Singh

Institute for Plasma Research, Bhat, Gandhinagar-382 428, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 December 2009Received in revised form 19 March 2010Accepted 10 April 2010Available online 24 April 2010Communicated by F. Porcelli

Keywords:Laser-blow-offEffect of magnetic fieldAtomic processes

The effect of varying magnetic field on the lithium plasma produced by laser-blow-off technique has beenstudied. Enhancement in the intensity accompanied with structures was observed for the spectral linesfrom neutrals, which varied with the intensity of the magnetic field. In the present report we explore theorigin of these structures by invoking the role of various atomic processes.

© 2010 Elsevier B.V. All rights reserved.

The presence of magnetic field during the expansion of laser-produced plasma leads to several interesting physical phenom-ena e.g. conversion of kinetic energy into plasma thermal en-ergy, plume confinement, ion acceleration/deceleration, emissionenhancement, plasma instabilities, increase in the detection sensi-tivity of laser-induced breakdown spectroscopy (LIBS), etc. [1–4].The study of plasma flow across magnetic field lines is impor-tant in many laboratory, tokamak and space plasmas [5–7]. Super-thermal neutral atomic beams (lithium and carbon) produced bylaser induced forward transfer technique, better known as laser-blow-off (LBO) technique [8–10], are extensively used to measurethe edge parameters of high temperature tokamak plasma. Pres-ence of tokamak’s magnetic field is expected to considerably affectthe neutral and atomic ion beams. Moreover, from the applicationpoint of view, the effect of magnetic field is important in thin filmdeposition, control of laser-ablated debris, etc. [1]. Hence, the studyof the effect of magnetic field on the plume expansion is a startingpoint for its applications.

Plasma changes its physical properties during expansion acrossa magnetic field, and this ultimately affects its emission character-istics. An enhancement in optical emission under the influence ofa pulsed magnetic field has been reported in some earlier works[1–4,11–13]. Moreover, emission profiles for neutrals as well asions exhibited distinct features (structures). In some of the ear-lier works, the appearance of these features has been attributed toinstabilities [11,12]. Peyser et al. [14] also observed structured im-

* Corresponding author.E-mail address: ajai@ipr.res.in (A. Kumar).

0375-9601/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.physleta.2010.04.029

ages in the fast-framed photographs in the presence of magneticfield, which they attributed to ion–electron hybrid instabilities. Ina recent work the presence of twin structure in the neutral profilehas been attributed to the backflow of particles towards the targeton the collapse of the magnetic bubble [2]. Different types of be-haviors in the plume expansion in case of Zn and Zn+ have beenreported by Kim et al. [15], which they have attributed to accel-eration/deceleration due to J × B force. Despite these studies, theexploration of the origin of these structures still remains elusive.For any application point of view, the plume dynamics in the pres-ence of magnetic field needs a better insight. In the present work,we report the evolution of the structures in the temporal emis-sion from neutral lithium (Li I) with varying magnetic field. Wedemonstrate that the appearance of these structures is correlatedwith atomic processes rather than the growth of instabilities in theplume.

The experimental technique of time of flight emission spec-troscopy used in the present work has been described elsewhere[16,17]. Briefly, the plasma plume is generated inside a multi-portstainless steel chamber, which is evacuated to a base pressure bet-ter than 5 × 10−6 mbar. The target is composed of uniform layersof 0.5 μm thick carbon film and 0.05 μm of LiF film on a thickquartz substrate. Target is mounted on a motorized x− y translatorstage so as to expose a fresh region of the target for every succes-sive shot. An Nd: YAG laser (λ = 1.064 μm) having a pulse width of8 ns was used to generate the plasma plume. The laser beam wasfocused on the target surface (angle of incidence ∼ 6◦). The aver-age pulse energy in the experiment was ∼ 200 mJ. The spot sizeof the laser beam was set to about 1 mm diameter at the target toachieve an average power density ∼ 109 W/cm2. A pulsed power

2556 A. Kumar et al. / Physics Letters A 374 (2010) 2555–2560

Fig. 1. Temporal profiles of Li (I) 670.8 nm line (a) and 610.3 nm (b) for various magnetic fields at z = 6 mm. (Color online.)

system consisting of a capacitor bank and a wire wound solenoidwas used to produce the magnetic field of 0.04–0.2 T (flat-top du-ration of the magnetic field profile is 40 μs; which is much largerthan the plume duration ∼ 5 μs) [16]. For time and space resolvedspectroscopy, the plasma plume is viewed normal to the directionof expansion and imaged at the entrance slit of a monochroma-tor (�λ = 12.5 Å for slit width of 250 μm). The monochromatorwith photomultiplier tube was mounted on a single-stage transla-tor system, which enabled space-resolved scan of the plume alongits expansion axis. The time resolution in the present experimentwas about 4 ns. Overall maximum uncertainty in our measurementis less than 8%.

In the present study two spectral lines from neutral lithium Li I;670.8 nm (2s2S1/2 ← 2p2P3/2,1/2) and 610.3 nm (2p2P1/2,3/2 ←3d2P3/2,5/2) and two from ionic Li II 548.4 nm (2s3S1 ← 2p3P2,1,0)

and 478.8 nm (3p1P1 ← 4d1D2) were chosen for investigating theeffect of the magnetic field. Fig. 1 shows the variation in the neu-tral emissions viz 670.8 nm and 610.3 nm for different magneticfields in the range 0–0.2 T observed at a distance of z = 6 mmfrom the target. There is a remarkable difference in the shapes oftemporal profiles of neutrals obtained with and without magneticfield. Enhancement in overall intensity was observed in the pres-ence of magnetic field (Fig. 2), which is 7.3 times for 670.8 nmand 1.3 for 610.3 nm for B = 0.2 T at z = 6 mm. Moreover, thetemporal profiles show multi-peak structures. Another interestingobservation is in the enhancement that starts taking place for theslowest component when the field is introduced. With further in-crease in the field, the intensities of the relatively faster compo-nents start increasing. However, it can be noted that the fastestcomponent (appearing at the initial stage) does not exhibit anysignificant change. Li (II) 548.4 nm line, on the other hand, showsa small enhancement with field, however, without any structures(Fig. 3). Similar behaviour is observed with Li II 478.8 nm line.

It is well known that in laser produced plasma, the distributionof the particles can be best described by a one-dimensional shiftedMaxwell–Boltzmann distribution function (SMB) [17,18]

f (t) = At−4 exp

[−m(z/t − v0)2 ]

2kB T

Fig. 2. Variation of the total integrated intensities of Li (I) 670.8 nm and 610.3 nmspectral lines with magnetic field at z = 6 mm. (Color online.)

where z is the distance from the target, t is the time elapsed,A is a normalization constant (corresponding to the peak inten-sity), T is the kinetic temperature, ν0 is the center-of-mass velocityof the plume and m is the atomic mass of lithium.

However, a single SMB distribution cannot be used to fit themulti-peak temporal profile observed in the present experiment.Therefore, a multi-component SMB distribution was attempted.As is evident from Fig. 4, it is found that a four componentSMB distribution fits well to the temporal profile. These struc-tures/components may hereafter be referred as P1, P2, P3, and P4respectively in increasing order of the time delay.

The percentage contribution from each of these structures couldbe obtained by normalizing their areas with the total area underthe temporal profile. These contributions were estimated for thetemporal profiles of both the neutral emission lines (670.8 nm and

A. Kumar et al. / Physics Letters A 374 (2010) 2555–2560 2557

Fig. 3. Temporal profiles of Li (II) 548.4 nm line for various magnetic fields at z =6 mm. (Color online.)

Fig. 4. A typical SMB fitted profile for z = 6 mm and for a magnetic field of 0.20 Tfor 670.8 nm emission. The resolved components P1, P2, P3 and P4 are shown.(Color online.)

610.3 nm). We have plotted the variation of these contributionsas a function of magnetic field for a distance z = 6 mm (Fig. 5aand b). The relative contribution from P2, P3 and P4 were affectedby changes in magnetic field but P1 remains relatively unaffected(Fig. 5). P2 shows enhancement and P3 shows an initial enhance-ment followed by a slight decrease but P4 shows a continuousdecrease with increase in field after its appearance at 0.04 T. Itis interesting to note that for 0.04 T, P4 is more intense than P2and P3.

The fit parameters namely, normalizing constant A, kB T andν0 for these components, are given in Table 1. It is interesting tosee that the center-of-mass velocities (ν0) and the widths (kB T ) ofthese components are unaffected by the variation of magnetic field.Only the peak intensities are found to be affected by the variationof the magnetic field. Had there been a change in the productionof neutrals due to variation in magnetic field, a change in velocitydistribution should have also been observed apart from a changein the peak amplitude. As no changes in velocity distribution areobserved, it can be inferred that magnetic field has no significantcontribution in the production of neutrals and in their distribution.

Probably they are ejected in different groups with varying delaysfrom the target itself. In the forthcoming discussion we describethe relative change in the peak amplitudes with distance.

Fig. 6 shows the variation in the neutral emissions 670.8 nmand 610.3 nm as a function of distance from target (z = 2–10 mm)at a fixed magnetic field of 0.2 T as well as in the absence ofmagnetic field. With increase in distance, the intensity of the fastcomponent (P1) decreases irrespective of changes in field. On theother hand, intensity of slower components increases in the pres-ence of magnetic field for all the distances taken in the study.Another noteworthy observation is a considerably larger increasein the intensity of slower (P2, P3 and P4) components for 670.8nm as compared to 610.3 nm line.

As these structures start appearing even from very short dis-tances from the target, these cannot be ascribed to edge insta-bilities/fluctuations [11,12] because edge instabilities should showsystematic variation in the frequency, amplitude and position inthe observed peaks with increase in the field. Instabilities associ-ated with ions can also be ruled out, as we do not see structuredprofile in case of ions (Fig. 3). Moreover, appearance of multiplestructures and the appearance of P4 at 0.04 T and then decreasein its intensity with further increase in field and simultaneous in-crease in the amplitudes of P2 and P3 indicate that they shouldnot be not caused by back flow of the particles [2]. The differencein the temporal profiles for 610.3 nm and 670.8 nm (Fig. 1) indi-cates that the appearance of these structures should be linked tothe differences in their excitation/emission processes. It is, there-fore, essential to examine the various atomic processes that couldcontribute to such excitation and emission processes.

As can be seen from Fig. 1a and b, and Fig. 5, P1 is relativelyunaffected by the magnetic field. Since these excited neutrals havelarge kinetic velocities (Table 1) they are also known as ‘fast neu-trals’ [19], which appear even in the absence of the magnetic field.In our earlier report we identified that the formation of fast neu-trals is due to direct neutralization of fast ions by the resonantcharge exchange reaction [19]. As P1 is almost unaffected by field,we attribute this behavior to the effect of longer magnetic diffu-sion time as described below.

We calculated magnetic diffusion time (essentially the time re-quired for Joule heating) defined by

td = 4πσ R2b

where σ is the Spitzer conductivity and Rb is the bubble radiusdefined by [2,20]

Rb =(

3μ0 El

2π B2

)1/3

where El is laser energy and B is the magnetic field strength.A typical value of td for our parameters varies from ∼ 4 μs to∼ 1 μs for Z = 1, when the field is increased from 0.04 T to 0.2 T.As this fast component P1 appears at very short time (sub mi-crosecond range), it is likely to be least affected by the field.

Another interesting observation is the higher intensity for P1for 610.3 nm line as compared to 670.8 nm line. Under our ex-perimental conditions it is likely that the ground state of the610.3 transition gets more populated due to higher temperature(Te ∼ 5 eV). However, for P2 and P3 and P4, the presence of fieldshows dramatic change in intensity. Although the origin of theseneutrals is not clear so far, it appears that they should representdifferent groups of neutrals.

It is worthwhile to investigate those processes which are re-sponsible for the excitation of neutrals. To identify this, we ex-tracted photon emissivity coefficients (PEC) for electron impact ex-citation as well as recombination (it takes into account two body,three body and dielectronic recombination in directly producing

2558 A. Kumar et al. / Physics Letters A 374 (2010) 2555–2560

Fig. 5. Variation of the relative percentage contribution of P1, P2, P3 and P4 with magnetic field at z = 6 mm for (a) 670.8 nm and (b) 610.3 nm. (Color online.)

Table 1Fitted SMB parameters for 670.8 nm temporal profile for various magnetic field (B) values (z = 6 mm).

B (T)P1 P2

kb T(eV)

A(Arb. units)

ν0

(103 m/s)kb T(eV)

A(Arb. units)

ν0

(103 m/s)

0.04 0.72 0.20 12.5 0.41 0.11 8.330.08 0.72 0.21 12.5 0.41 0.18 6.940.12 0.72 0.21 12.5 0.41 0.25 6.460.16 0.72 0.24 12.5 0.41 0.40 6.460.20 0.72 0.21 12.5 0.41 0.47 6.46

B (T)P3 P4

kb T(eV)

A(Arb. units)

ν0

(103 m/s)kb T(eV)

A(Arb. units)

ν0

(103 m/s)

0.04 0.21 0.12 3.38 0.017 0.18 1.890.08 0.21 0.23 3.55 0.017 0.16 2.000.12 0.21 0.30 3.57 0.017 0.13 2.000.16 0.21 0.29 3.57 0.017 0.12 2.000.20 0.21 0.28 3.57 0.017 0.10 2.00

excited neutrals from ions) for both 610.3 nm and 670.8 nm lines(Fig. 4a and b) using Atomic Data and Analysis Structure (ADAS)[21]. Moreover, the contribution from the metastables is also takeninto account. For obtaining PEC, the estimate of Ne and Te is doneby the following method (it was not possible to measure from in-tensity measurements particularly at longer times because of lowintensity). For this, extrapolated values for Ne and Te are obtainedfrom triple Langmuir probe data, having temporal resolution of40 ns [22] and from the relations reported in the literature [23].The values of Ne (varying as t−2) and Te (varying as t−1) for thetimes corresponding to P2, P3 and P4 are calculated assuming thatthe peak value corresponds to P1. The corresponding estimated Te

and Ne values for P2, P3 and P4 are 1.8 eV, 1.1 eV and 0.6 eV and1 × 1014 cm−3, 7 × 1013 cm−3 and 1 × 101 cm−3 respectively forz = 6 mm. Therefore, PEC are estimated for the temperature rang-ing from 0.5 eV to 10 eV. Typical plots of PEC vs Te for these Ne

values are shown in Fig. 7. From this figure it can be seen that forelectron impact excitation, the PEC are much higher for 670.8 nmas compared to 610.3 nm (Fig. 6a) but there is no significant dif-ference in PEC for recombination (Fig. 6b).

In ADAS it is assumed that the collisional and radiative pro-cesses between all excited levels redistribute the populations and

the excited levels are in quasi-static equilibrium with the metasta-bles. The emissivity of an individual line between states j and kfor electron impact excitation is given by

ε j→k(exc) = A j→k

M∑σ=1

F (exc)jσ Ne Nσ (1)

and emissivity of an individual line due to recombination is givenby

ε j→k(rec) = A j→k

M∑υ=1

F (rec)jυ Ne Nυ (2)

where A j→k is the transition probability for transition between jand k levels, F exc

σ and F recυ are the effective contributions to the

populations of the excited state from metastable σ of the atomand ν of the ion for electron impact excitation and recombinationrespectively (electron density and temperature dependent) and Ne ,Nν and Nσ are electron density, density of ions in metastable νand density of atoms in metastable σ respectively. The respectiveemissivities for electron impact excitation and recombination canbe expressed as

A. Kumar et al. / Physics Letters A 374 (2010) 2555–2560 2559

Fig. 6. Temporal profiles of Li (I) 670.8 nm (a) without field, (b) 0.2 T; Li (I) 610.3 nm (c) without field and (d) 0.20 T for various distances. (Color online.)

Fig. 7. Photon emissivity coefficients (PEC) vs electron temperature for (a) electron impact excitation and (b) recombination. (Color online.)

ε j→k(exc) = PECexc Ne Na (3)

and

ε j→k(rec) = PECrecNe Ni (4)

where PECexc and PECrec are photon emissivity coefficients for elec-tron impact excitation and recombination respectively and Ne , Niand Na are electron density, density of ions and density of atomsrespectively.

2560 A. Kumar et al. / Physics Letters A 374 (2010) 2555–2560

From the knowledge of PEC, the intensity of a particular linecan be represented by [24]

I = K Ne(PECexc Na + PECrecNi

)(5)

where K is the factor that depends on the geometry of observa-tion and detector response. Evidently intensity will depend on (i)electron density, (ii) number density of atoms in the ground statefor a particular transition, (iii) PEC and (iv) geometrical factor.

As the electron density is same for Li I (670.8 nm and610.3 nm) transitions and also the number atoms in the groundstate should nearly be same for these two transitions for P2 andP3 and P4, it can be inferred that PEC values should contribute tothe difference in the intensities for these transitions (K is samebecause of same geometry and flat detector response). Regarding,the enhancement in P2, we can attribute it to enhanced electronimpact excitation because of following arguments. With increase inthe field, electron temperature will increase due to Joule heating[2] and of course, the effect will depend on the magnetic diffusiontime for heating. Moreover, ADAS analysis shows that PECex in-creases with Te whereas PECre decreases (Fig. 6). The larger changein the intensity of 670.8 nm line as compared to 610.3 nm line(Fig. 1) reflects this behaviour as the former has higher electronimpact PECex . This also rules out the possibility of recombinationas there is no significant difference in PECre for these transitions.These points indicate that excitation process is electron impact.

In case of P3 also electron impact excitation appears to con-tribute to the enhancement as in case of P2 but due to lowerelectron temperature this shows less enhancement. A slight de-crease in intensity for higher field values may be due to the shiftof distribution towards P2 because of increase in temperature.

The component P4 starts appearing at a large time delay and isprominent for 0.04 T. As the magnetic diffusion time is longer incase of 0.04 T, P4, being the slowest, appears to be affected for thisvalue of field. Again the excitation mechanism can be attributed toelectron impact. It has been reported [2] that in the presence ofmagnetic field, electron temperature decreases for longer times inthe temporal profile. Hence it is expected that P2 will show moreenhancement as compared to P3 whereas for P4 (appearing atlongest time) it should decrease with increase in field. The behav-ior of the observed results are in agreement with the fact that theincrease in the line emission is due to electron impact excitation.

Summarizing, in the present communication we report the ef-fect of magnetic field on the plasma plume in LBO study of LiFtarget. In the presence of magnetic field, the temporal profiles ofneutral lines show four distinct features with an enhancement intheir intensities. Electron impact excitation seems to be responsi-ble for the appearance of the these components. By using a vari-able magnetic field we could clearly identify the evolution of thesefeatures. Thus, in the present case we could demonstrate that theobserved enhancement in emission from neutrals is largely due toelectron impact excitation and not due to increase in radiative re-combination as suggested for some other systems [4,11,13] andatomic processes are responsible for the apparent structured pro-files. The study reveals that the atomic processes play an importantrole in understanding the shape of the plume temporal profile. Wefeel that this study will have important implications in thin filmdeposition, debris mitigation, etc. [1].

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