1. Discharge-assisted LIBS and ablation-induced current pulses.
P. Paris, M. Laan, M. Aints, A. Lissovski,
Institute of Physics, University of Tartu,
Set up
Summary, conclusions.
Significant enhancement up to 60 times of measured line intensities is observed in combined GD-LIBS method for similar laser conditions. Thus, GD-LIBS method allows using laser pulses with lower energy to ablate the studied material, causing less damage to its surface.
The gas discharge assisted LIBS is a promising technique having potential to improve the depth resolutions in characterization of surface layers.
The GD-LIBS allows at least 2-3 times reduction of required laser pulse energy.
Acknowledgements
This work has been supported by the Estonian Science Foundation grants no and 8237.
References
[1] P. Paris, M. Aints, A. Hakola, M. Kiisk, J. Kolehmainen, M. Laan, J. Likonen, C. Ruset, K. Sugiyama, S. Tervakangas, Fusion Engineering and Design (2011) in press.
[2] K. A. Tereszchuk, J. M. Vadillo, J.J. Laserna, Spectrochimica Acta Part B 64, 378–83, (2009).
Introduction
In principal, the beforehand knowledge of ablation rate (mm per laser pulse) of target material and recording of LIBS spectra as a function of laser shot number is a way for determination of elemental depth profiles of superficial layers [1].
However, the ablation rate, of about 1 mm/shot appeared to be too high for studying thinner layers. Besides, high fluencies up F 10 J/cm2 needed for recording spectra materials relevant to fusion reactor walls, can cause the delamination of coating. Morerover, high laser fluences cause arise of a strong continuum emission, which reduce the signal-to-background ratio.
These drawbacks forced us to look for alternative ways, which allow get reliable LIBS spectra at lower fluences. A considerable lowering of the fluence needed for LIBS has been achieved combining laser ablation with the extra applied electric field [2].
The aim of the present study was the comparison results of low pressure conventional LIBS with gas discharge assisted LIBS (GD-LIBS).
Figure 1. Set-up
Distance between anode and target (cathode) = 6 mm.
Capacitor C = 470 nF; Charging resistor R1 =1 M ; Current limiting resistor R2
Cathode is grounded via low ohmic resistor R3 for current measurements.
LIBS signals are recorded collinearly to laser beam with the help of spectrometer Mechelle5000 equipped with a gated ICCD camera; The focusing lens f = 1m.
Target cathode made from brass; opposite electrode, anode, has an opening in its centre.
The working pressure was chosen 1 mBar considering Pashen minimum conditions for 6 mm gap.
Anode voltage was set at 260 V, which value is just below the ignition of discharge.
In these conditions laser ablation fired the discharge between electrodes.
Figure 3. Typical emission spectra of brass target averaged over 10 laser shot for two cases: conventional LIBS and GD-LIBS. Laser fluence F 3J/cm2, R2 = 0 , R3 = 50  . The delay time between laser and emission recording was 150 ns.
During discharge the gap voltage remains constant
The current strength is inversely proportional to the resistance of external circuit.
The discharge current drops nearly exponentially accordingly to the circuit time constant RC
The discharge current and voltage are independent of laser energy (fluence) for our experimental conditions.
Figure 2. Discharge current/voltage – time curves
Figure 6. Temporal changes of the plasma emission in case of different circuit resistances
Target spectral lines and lines of background Ar decay similarly following the change of discharge current.
In the case of conventional LIBS the Ar lines mostly are not observable
Figure 5. The dependence of enhancement of spectral lines in GD_LIBS mode on the circuit time constant RC.
Integration time for both is 60 microseconds.
Figure 4. The narrowing of spectral line in GD-LIBS mode.
Temporal evolution of spectral emission lines
The intensity-time curve of emission spectra was recorded with both time gate and time steps of 1 ms (Fig.6 and 7).
Compared top conventional LIBS, plasma created by the GD-LIBS lasts for a longer time -tens of microseconds- and has a larger volume.
The intensity of spectra is proportional to the discharge current determined by the current limiting resistor and does not depend on pressure at these conditions.
At higher pressures – 5 and 10 mBar, laser fluences 3-4 J/cm2 will not fire the discharge between electrodes. Recorded spectra do not differ from those recorded by conventional LIBS.
The intensity of GD-LIBS spectra equal to that of LIBS spectra recorded with the same delay time td = 150 ns was achieved at two-fold lower laser fluence, whereas the spectral lines were considerably narrower (Fig. 4).
The dependence of enhancement of GD-LIBS on the circuit time constant has a maximum near 5ms (Fig.5).
At lower limit of laser fluence 3 J/cm2 used in our study the analytical lines were already reliably detected in conventional LIBS spectra.
The electrical discharge energy deposited in plasma forms less than 10% of laser pulse energy.
Figure 7. Intensity-time curve for different spectral lines
Discharge parameters
Electric field was created between target and parallel to it plate electrode with an opening in its center for the laser beam (Fig 1).. The current limiting resistor in series with discharge gap was varied.
At applied voltage no discharge without laser.
Laser pulse fires the discharge, voltage drops within 8 ns from 260V to about 20 V and remains almost constant until discharge extinction.
Experimental
Our experiments were concentrated on the collinear recording of LIBS spectra under reduced pressure in Ar atmosphere.
Preassured range p = 1-10 mBar.
LIBS plasma was generated with Q-switched Nd:YAG pulsed laser, 7 ns FWHM, l=1064 nm at repetition rate 0.5 s-1 .
Laser energy was monitored with the Ophir Nova energy meter.
Laser fluence F was varied within range 3-8 J/cm2
Results
Recorded GD-LIBS spectra are compared with conventional LIBS spectra with no applied voltage.
In created plasma Cu, Zn, Ar lines are observed (Fig.3). The GD-LIBS spectrum is enhanced up to 60 times and shows pronounced enhancement of signal noise ratio of different Cu and Zn lines