Electric-field induced fluctuations in laser generated plasma plume

National Centre for Physics,Quaid-i-Azam University Campus,45320 Islamabad,Pakistan

Abstract The effect of an external electric field on laser-generated plasma has been studied.It is observed that the laser-generated plasma can be used for the ignition of a spark in the presence of a low voltage external electric field.An eight-fold emission intensity enhancement in Cu I spectral lines are measured as compared to the signal intensity in the absence of an external electric field.The plasma parameters remain the same initially,up to a few microseconds after the generation of plasma,and this feature makes it more interesting for the quantitative analysis of any sample using laser induced breakdown spectroscopy(LIBS).In the presence of an external electric field,fluctuations(contraction and expansion)in the laser-generated plasma are observed which increase the plasma decay time and consequently result in enhanced signal intensity.

Keywords:LIBS,laser produced plasma,effect of external electric,plasma parameters

1.Introduction

Laser-induced breakdown spectroscopy(LIBS),or laserinduced plasma spectroscopy(LIPS),is an emerging technique for the elemental analysis of materials including solid,gases,and liquids[1-3].In this technique,the real-time elemental composition of any sample can be determined in situ,without any sample preparation.Potential applications of LIBS are in the field of space exploration[4,5],environmental monitoring[6,7],biomedical detection[8],industry[9,10],and geotechnical studies[11,12].In this technique,the plasma is generated by focusing a high-power laser beam on the surface of any target.The laser-generated plasma plume expands perpendicularly to the surface of the target,however,it cools down in a few microseconds.The emission spectrum of the plume is registered after a few microseconds of the plasma generation using a spectrograph equipped with a time-integrated or time-resolved setup.The transient nature of the plasma plume,and a very small amount of the ablated material,constrict the limit of detection(LOD)of LIBS as compared to other analytical techniques[13].Several attempts have been made to increase the limit of detection of LIBS for the measurement of trace elements[14].The signal intensity enhancement has been measured by applying radiofrequency and microwave radiation to the laser-generated plasma plume[15,16].A dual pulse LIBS setup has also been used to achieve the signal intensity enhancement[17-19].Ahmed and Baig[20]reported that the optimized value of the inter-pulse delay between the two laser pulses can result in 10-50 times the signal intensity enhancement in the dual pulse configuration.Further studies of dual pulse LIBS revealed that the optimized value of the laser pulse energies ratio can improve the signal intensity enhancement factor by up to 300 times as compared to single pulse LIBS[21].The signal intensity enhancement has also been measured by multiple laser pulse excitation of laser-generated plasma[22].Li et al[23]used the resonant frequency excitation technique to increase the LOD of the trace elements in laser-generated plasmas.

For the commercialization of LIBS as an analytical technique,the LOD needs to be improved.The effects of electric and magnetic fields in laser-produced plasmas have also been reported[24,25].It has been inferred that reexcitation of the laser-generated plasma by a high voltage discharge can increase the signal intensity which results in increasing the signal-to-noise ratio as well as the LOD of LIBS[26].In spark-discharge-assisted LIBS,Nassef and Ali[27]observed that the signal intensity enhancement factor increases as the spark discharge voltage increases.Further,the plasma temperature remains the same in the spark-dischargeassisted LIBS and in the conventional LIBS.The effect of the static electric field on the laser-generated plasma[28]showed signal intensity enhancement along with a small decrease in the electron number density,while the plasma temperature remains the same.In a recent article[29],a low voltage and high current arc discharge technique,coupled with LIBS,yields an increased persistence time of the plasma plume,along with the signal intensity enhancement.

In this work,the effect of external electric field in the laser-generated plasma has been studied.The fluctuations in the laser-generated plasma plume are observed in the presence of external electric field(E).The signal intensity enhancement in the emission spectrum of Cu and a slight increase in the plasma temperature and electron number density have been observed.The experimental detail has been presented in section 2,while all the experimental results are discussed in section 3.

2.Experimental details

The details of the experimental apparatus used in the present study have already been described in our recent articles[30,31].A high-power pulsed Nd:YAG laser(Quantel,Brilliant B)emitting a full energy of 400 mJ at 532 nm,with a repetition rate of 10 Hz with the pulse duration of 5 ns,was used to generate the plasma.The flash lamp to Q-switch delay has been used for the variation of the laser pulse energy and an energy meter(NOVA-QTL)was used to measure the output pulse energy of the Nd:YAG laser.The second harmonic of the Nd:YAG laser@532 nm,with a pulse energy of 100 mJ,is used and focused on the pure Cu target surface with a quartz lens of focal length 10 cm.To avoid the air breakdown in front of the target surface,the distance between the sample and lens remained less than 10 cm.An optical fiber with a collimating lens has been used for collecting the light from the laser-generated plasma.The collected light from the plasma plume was sent to the spectrometer via optical fiber.The optical fiber was coupled with the high resolution spectrometer(Avantes)which is equipped with a 10 μm slit width and covers the wavelength range from 250 nm to 875 nm.This spectrometer has a 3648-element linear CCD array with the optical resolution of≈0.06 nm at 400 nm.To apply the external electric field,two Al electrodes separated by about 3 mm were placed just above the surface of the sample and electric field was applied across the lasergenerated plasma plume by a DC regulated power supply(Kenwood; PS 350).Both the width and the length of the electrodes are 10 mm.The insulation between the Cu target and electrode was also ensured in order to apply the appropriate voltage.The separation between the closest edge of the electrodes and the target surface is≈250 μm.To measure the current,a resistor has been added in the circuit and the potential drop across the resistor was monitored by oscilloscope.To register the emission spectrum of the laser-generated plasma plume,the Q-switched Nd:YAG laser has been synchronized with the spectrometer through a delay generator(SRS DG 535).The delay between the data acquisition system and the laser pulse was also controlled through the delay generator.The minimum integration time of the spectrometer is 10 μs.To check the reproducibility of the data,the experiment has been repeated at least three times in all cases.The registered emission spectrum has been corrected by subtracting the background signal of the detector using software(AvaSoft).

Figure 1.A portion of the emission spectrum measured without external electric field at different time delays between the laser pulse and the data acquisition time.

3.Results and discussion

To register the emission spectrum of Cu,the laser beam was focused on the surface of the sample and the emission spectrum was registered by the spectrometer.The emission spectra include all the major lines of Cu I at 324 nm,327 nm,510.55 nm,515.32 nm,521.82 nm,570.02 nm,578.21 nm.A small portion of the copper emission spectrum is presented in figure 1.A series of emission spectra were registered at different values of time delay between the data acquisition and laser pulse.In figure 2 the intensities of the Cu I lines at 510.55 nm,515.32 nm,521.82 nm,570.02 nm,578.21 nm are plotted as a function of the delay between the laser pulse and the data acquisition system.It is evident from the figure that the line intensities decrease as the plasma plume expands/cools down.

To study the effect of external electric field on the lasergenerated plasma,two aluminum electrodes separated by 3 mm were placed perpendicular to the direction of plasma plume expansion.A 20 V mm?1electric field was applied across the laser-generated plasma plume and the emission spectra have been registered.The electric field across the laser-generated plasma plume was increased,step by step,by increasing the applied voltage and keeping the separation of the electrodes(3 mm)constant.A series of spectra were registered at different electric fields.As the external electric filed reaches 49 V mm?1,an abrupt increase in the line intensities was observed; the line profiles of the strong Cu lines show saturation.In figure 3,the intensities of the Cu I lines at 510.55 nm,515.32 nm,521.82 nm,570.02 nm,578.21 nm are plotted as a function of the external electric field.It is quite evident from figure 3 that the threshold for the laser-ignited spark is 49 V mm?1.As the electric field was increased beyond the threshold value,no major change in the intensities of the spectral lines was observed.

Figure 2.The variation of signal intensities of Cu I spectral lines with the time delay between the laser pulse and data acquisition time.

Figure 3.Effect of external electric field on the signal intensity of Cu I lines.

Figure 4.In the presence of external electric field,the variation of the signal intensities of Cu I spectral lines with the time delay between the laser pulse and data acquisition time.

Figure 5.Comparison of the signal intensity with and without external electric field.

The temporal evolution of the laser-generated plasma in the presence of an external electrical field was also studied.The external electric field(49 V mm?1)was applied and the emission spectrum was registered at different time delays between the laser pulse and the data acquisition system.Figure 4 shows the temporal dependence of the spectral line intensities of Cu I at 510.55 nm,515.32 nm,521.82 nm,570.02 nm,578.21 nm in the presence of the external electric field.At the initial values of the delay times between the laser pulse and the data acquisition system,the intensities of the Cu I lines at 510.55 nm,515.32 nm,521.82 nm are very high and the line shapes are fully saturated due to the signal intensity enhancement induced by the laser-generated spark in the presence of the electric field.Therefore,it is not feasible to use these lines(Cu I 510.55 nm,515.32 nm,521.82 nm)to extract any useful information.A comparison of the spectral line intensities of Cu I 570.02 nm,578.21 nm,measured with the external electric field(49 V mm?1)and without the external electric field,is shown in figure 5,which reveals an eight-fold signal intensity enhancement in the Cu I line at 570.02 nm in the presence of the external electric field.The signal intensity enhancement measured in the presence of the external electric field is in good agreement with earlier reports[28,29].As the distance between the electrodes is only 3 mm,it will be hard to ignore the contribution of cavity confinement effects[32].But the emission intensity has been measured in the presence of electrodes in both cases(with and without E-field),therefore the spatial confinement of the plasma is not contributing to the calculation of intensity enhancement factor.The variation in the signal intensity of the Cu I line at 578.21 nm,as a function of the delay between the laser pulse and the data acquisition,is shown in figure 6.An exponential decay function was fitted to the experimental data points.In both cases,with and without the external electric field,the signal intensity of the Cu I 578.21 nm line decays exponentially with delay time,but with different decay rates(t).In the presence of the external electric field,the signal intensity of the Cu I 578.21 nm line decays slowly as compared to the signal intensity measured without the external electric field.

Figure 6.Variation in the signal intensity of Cu I line at 578.21 nm with the time delay between the laser pulse and data acquisition time,with and without external electric field.

Figure 7.Variations in electron temperature with the time delay between the laser pulse and data acquisition time.

The plasma parameters are essentially required for the quantitative analysis of any sample by LIBS.The plasma temperature has been estimated using the well-known Boltzmann plot method[33].The Cu I lines at 510.55 nm,515.32 nm,521.82 nm,570.02 nm,578.21 nm were used in the Boltzmann plot.The variation in the plasma temperature as a function of delay time was estimated using the Boltzmann plot method.The dependence of the plasma temperature on the delay time after the laser pulse,in the presence and absence of the external electric field,is presented in figure 7.In the presence of the external electric field,the plasma temperature was not calculated for the first 20 μs because the signal intensities of the Cu I lines at 510.55 nm,515.32 nm,521.82 nm are saturated(see figure 4),and as the time delay increases,the saturation effect in the line intensities decreases,and the plasma temperature attains the actual value.After 20 μs,the plasma temperature decreases exponentially which is attributed to the plasma expansion.However,we were unable to find the correct plasma temperature in the initial delay time of 20 μs due to limitations in the experimental setup or the saturation of major spectral lines.

Figure 8.Variations in electron number density with time delay between the laser pulse and data acquisition time.

The electron number density has been estimated using the full width at half maximum(FWHM)of the Stark broadened line profile at 570.02 nm Cu I line.The time resolved spectra were calculated from the time integrated spectra,according to the method reported by Grifoni et al[34].A series of spectra were registered at different values of time delay between laser shots and spectrometer.The time resolved spectra have been used for estimating the electron number density.The variation in the electron density has been calculated using the time resolved emission spectrum registered after different values of delay after laser pulse.The variation in the electron density with the delay time after laser pulse has been presented in figure 8.In the initial delay time of 20 μs,in the presence of the external electric field,the electron density remains almost the same but decreases exponentially in the absence of electric field.The behavior of an identical electron density for the initial delay of 20 μs may lead towards the plasma temperature in the same time domain(see figure 7).During the expansion of the laser-generated plasma,the decay of electron density and plasma temperature may follow the same trend.Therefore,it is expected that the electron density and plasma temperature may follow the same trend of decay during the expansion of laser-generated plasma plume.In the presence of electric field,it can be assumed that the electron temperature may also remain the same in the initial delay of 20 μs,but it was not estimated due to the saturation of signal intensities.

The laser-generated plasma can be considered as a hot ionized gas consisting of negatively charged particles(electrons)and positively charged particles(ions).The Lorentz force ‘F’ on the charged particles can be written as

In the present case,there will be two types of electric fields,one is the internal electric field of the plasma which is responsible for the Stark broadening of the spectral lines,and the other is the external electric field applied by the DC regulated power supply.Equation(1)can be modified as:

For the sake of simplicity,equation(2)can be written in three components:

Here,F1=eEint,F2=eEextandF3=e(V′B).

In the presence of an electric field,it exerts force on the charged particles produced by the laser-generated plasma.The direction of the force on the positively and negatively charged particles will be opposite,which causes plasma expansion along the axis of the applied electric field.As the plasma plume expands and reaches the electrodes,conduction starts,a high current begins to flow and the externally-applied voltage drops,which reduces the magnitude of the external electric field.As the external electric field reduces,the internal electric field exerts a force ‘F1’ on the charged particles of the laser-generated plasma.The direction of F1is opposite to the direction of F2which causes contraction of the laser-generated plasma plume.There is no external magnetic field applied,but due to the flow of the current between the electrodes,a magnetic field will be generated.The force exerted by the magnetic field is F3and it will be perpendicular to the direction of current flowing between the electrodes.Therefore,F3is an instantaneous force and it can influence the charges in the plasma plume as long as current is flowing between the electrodes.In the presence of F3,we may think that charges may follow a spiral path around the line of force of the B field.But due to atmospheric pressure,there will be many collisions of charges before completing the spiral or circular path.At the same time,the electric field(internal/external)will force the charges to move straight along the direction of the E field.Therefore,in the presence of the electric and magnetic forces,the resultant path of the charges between the electrodes can be considered along/opposite of the electric field with some minor effect caused by instantaneous magnetic field.The magnitude of the magnetic force(F3)will be less than the magnitude of the F1or F2; it means that F3will not play a major role in the contraction and expansion(fluctuations)of laser-generated plasma plume but may slightly affect the path trajectories of the charges between the electrodes.The F3force will seize as the contraction of plasma caused by F1and conduction will stop between the electrodes through the plasma plume due to the creation of resistance between the electrodes.As the current stops flowing,the external electric field exerts a force F2again on the charged particle and expansion of the plasma may occur again.In this way a complete cycle of all the forces will be completed again.A schematic expansion of plasma in the presence of external electric field is presented in figure 9.In the meantime,the laser-generated plasma decays also,which decreases the plasma volume between the electrodes.Therefore,a cycle of these forces(F1,F2,F3)will be repeated until the plasma volume decreases to such an extent that it cannot reach or expand to the electrodes due to the force F2.In the other way around,the cycle of these forces(F1,F2,F3)can be started only if the external electric field exerts the F2force in such a way that the plasma expands and touches the electrodes.This means that there will be a threshold force required for the start of these cycles.It is quite obvious from figure 3 that no signal intensity enhancement is observed below the threshold value(49 V mm?1)of the external electric field.To confirm this assertion,the separation between the electrodes was increased to 4 mm and the same strength of electric field(49 V mm?1)was applied between the electrodes.Fortunately,with the separation of electrodes(4 mm),no signal intensity enhancement was observed while the electric field strength remained the same.This means that the threshold electric field required for the signal intensity enhancement has some correlation with the separation of electrodes.In other words,49 V mm?1electric field is not enough to expand the plasma to 4 mm so that it can touch the electrodes and start the current flowing between the electrodes.

In short,the external electric field will be responsible for the expansion of the plasma plume and the internal electric field will be responsible for the contraction of the plasma plume,while the instantaneous magnetic field may slightly affect the trajectory of the charges between the electrodes.The contraction and expansion of the laser-generated plasma plume will be repeated again and again until the plasma volume decreases below the required value of the threshold for expansion of the plasma plume in the presence of the external electric field.In the presence of the external E-field,the additional excitation/reheating of the laser-generated plasma due to laser-generated sparks can be attributed to the observed emission intensity enhancement.

To confirm the presented model of plasma fluctuations in the presence of external electric field,the electric current between the electrodes was measured.The time dependent variations of electric current between the electrodes through the laser-generated plasma are presented in figure 10.In the first 10 μs,the electric current between the electrodes oscillates and these oscillations may be attributed to the plasma expansion and contraction.During the period of expansion of the laser-generated plasma,the electric current flows between the electrodes through the laser-generated plasma.The flow of electric current through the laser-generated plasma can reheat/re-ionize the plasma which may increase the plasma’s cooling down time.The observed fluctuations in the current can be due to parasitic capacitance/inductance of the circuit.To resolve this ambiguity,a manual switch was installed between the electrodes and current was measured in the closed/open circuit.The behavior of the current in the open and closed circuit has also been presented in figure 11.It clearly depicts that there are no oscillations in the closed circuit current,which confirms that the observed fluctuation in the current in the presence of laser-generated plasma is real but not due to parasitic capacitance/inductance.Similar oscillations in the discharge current have also been reported by Kexue et al[35].It has been observed that the electron number density remains constant and does not decay in the initial 20 μs of the plasma generation(see figure 8).This means that the natural decay of the electron number density is compensated by the electric current fluctuations through the plasma.A rapid decay of the plasma parameters is problematic for the quantitative analysis of any material by LIBS.In this study,we report that a temporal window for the quantitative analysis of any material can be selected in the initial 20 μs,because the electron number density remains the same during this period.In the presence of electric field,a small increase in the plasma parameters may be attributed to the reheating/re-ionization of the laser-generated plasma by the flow of electric current through the laser-generated plasma plume.

Figure 9.Schematic diagram of plasma expansion and contraction in the presence of external electric field.

Figure 10.Variation in the electric current between the electrodes as a function of time delay between the laser pulse and data acquisition time.

Figure 11.Electric current measured without laser-generated plasma.The circuit was closed/opened using a manual switch between electrodes.

4.Conclusion

The effect of a low voltage external electric field on lasergenerated plasma was studied.In the presence of an external electric field,an eight-fold signal intensity enhancement was observed.In addition,a small increase in the plasma parameters was measured.In the presence of an external electric field,the plasma parameters remain the same for a few microseconds after plasma generation,and this feature can be used for a better quantitative analysis of any material by LIBS.The fluctuation in the laser-generated plasma was observed in the presence of an external electric field,which increases the plasma decay time and consequently enhances the signal intensity.

Acknowledgments

We are grateful to the Pakistan Academy of Sciences for providing funds to acquire the laser system to perform the relevant experiments,and the National Centre for Physics for the infrastructure development.