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Reflectivity measurements from a nanosecond CO2 laser produced plasma
Volume 15, number 2
OPTICS COMMUNICATIONS
October 1975
REFLECTIVITY MEASUREMENTS FROM A NANOSECOND CO 2 LASER PRODUCED PLASMA H.A. BALDIS, H. PIPPIN and T.W. JOHNSTON INRS-l£nergie, Universit~du Qubbec, C.P. 1020, Varennes,Quebec, Canada Received 9 June 1975
The back-scattered light from a laser produced plasma has been studied using a nanosecond gigawatt CO2 laser. The spectral shift and width of the reflected light have been determined as a function of the incident laser energy.
The reflection of laser light by a plasma is of fundamental importance in the heating of laser produced plasmas. In recent experiments performed with Nd lasers [ 1 - 4 ] the back-reflected light presented a shift towards longer wavelengths, indicating the presence of stimulated processes (such as Brillouin scattering) in the region of interaction between the laser and the plasma. In this paper we present the experimental resuits obtained with a nanosecond gigawatt CO 2 laser, with a wavelength ten times longer than for Nd lasers. Instead of the red shift reported with the Nd lasers, we observed a predominant blue shift which we interpreted as a Doppler shift of the laser light due to the upstream motion of the scattering irregularities. The laser used in the experiment consisted of a mode-locked oscillator, an optical gate to isolate one single pulse, and four amplifier modules. It produced at the output a single pulse with a maximum energy of 4.5 joules and a duration of 2 nsec (fwhm). The beam has a diameter of 5 cm and 95% of its energy within a divergence of 2.2 mrad. The laser was developped by a group [5] at the Canadian Centro de Recherche pour la D6fense ~ Valcartier (C.R.D.V.) and the experiment itself was a collaboration between INRS-Energie and CRDV. A schematic of the experimental setup is shown in fig. 1. The laser beam was focussed on a 150/am thick polyethylene film with a 10 cm focal length, off-axis parabolic mirror. The damage diameter of the focal spot was 250/am, in agreement with the diffraction limit and at 4.5 joules gave a power density of approximately 3 X 1012 W cm - 2 . The incident laser pulse was monitored with a
photon drag detector. A NaC1 wedge located in front of the photon drag detector reflected a known fraction of the incident beam into the spectrometer with the purpose of calibration, both in intensity and wavelength. The reflected light from the plasma was collected using the same parabolic mirror and diverted with the NaC1 beam splitter into either a one meter grating spectrometer or an energy meter. A HgCdTe detector was used and its output recorded with a Tektronix 7904 oscilloscope, giving a combined rise time of 1.5 nsec. Reflection measurements were also performed at 45 ° from incidence using all2 lens. Other diagnostics included charge collection (Faraday cups) and a two-channel X-ray detector, to mesure the ion velocity and electron temperature, respectively. The electron temperature was determined using the two-channel
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Fig. 1. Diagram of experimental apparatus. 311
Volume 15, number 2
OPTICS COMMUNICATIONS
X-ray detector by the absorber method which is fully valid as long as the electron energy has a maxwellian distribution. In our experiment various combinations of absorbers indicate a strong deviation from a maxwellian distribution. In that respect, the thin foil combination o f 12/am Be for one channel and 2.5/am AI + 12/am (CH2) n for the second channel (which measures the temperature in the spectral interval around 1 keV) enables us to get a good indication of the minimum spectral temperature. Our results indicated temperatures of 180 -+ 50, 250 -+ 50 and 300 -+ I00 eV for 1, 2 and 4 j o u l e s of incident laser energy respectively. The reflectivity of the plasma was measured as a function of the incident laser energy. Fig. 2 shows the results for two orientations of the target. With the target perpendicular to the incident laser beam (fig. 2a), the back-reflected energy is approximately 10% of the incident energy, decreasing for increasing incident energy. The reflection at 45 ° with respect to the incident laser beam is considerable lower, and
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October 1975
most of the energy is reflected in the backwards direction. F r o m these measurements the total r e f e c t e d energy is estimated to be approximately 30% of the incident. Rotation of the target (fig. 2b) increases the reflectivity towards the direction of specular reflection. Spectral measurements of the reflected light are shown in fig. 3 for three incident laser energies. Measurements across the spectral line were obtained in a shot-to-shot basis; this technique gave rise to the relatively large error bars indicated in fig. 3. Calibration of the spectrometer was made with the unshifted laser radiation reflected on the wedge (fig. 1). The instrumental width of the spectrometer was 18 A, at 10.6 /am. The shift and width of the reflected light are clearly resolved as shown in fig. 3, where the spectrum of the incident laser light has been superimposed. Due to the limited time resolution of the IR detector, only time integrated measurements have been obtained. An important feature of these measurements is the observed blue shift, which increases with the incident laser power. The Doppler velocities v = AX C/X calculated from the observed shift are shown in fig. 4. The magnitude and energy dependence of these velocilies are similar to ion velocities measured with the Faraday cup charge collector which are also indicated in fig. 4. This blue shift and the large reflectivity observed
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Fig. 2. Measured reflectivities for two directions of observation (0° and 45°). 312
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Fig. 3. Spectrum of reflected light for three different incident laser energies: (o) 4.6 J, (~) 1.4 J, and (~,) 0.35 J.
Volume 15, number 2 t0 8
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OPTICS COMMUNICATIONS I
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Fig. 4. Doppler velocities (o) calculated from the observed line shifts, and ion velocities (a) obtained with the Faraday cup.
(fig. 2a), seems to indicate that scattering irregularities exist near the critical layer (with an electron density near 1019 c m - 3 ) , which are moving away from the target with a velocity determined by the motion of the ions. In reflection measurements performed with Nd lasers [6], the reflection time obtained from the reciprocal of the linewidth (under the assumption of a reflectivity of 1) predicted a reflected energy that agreed well with the measured value. In our experiment, the reciprocal of the linewidth of the reflected light gives a minimum reflection time of 0.05 nsec for 4.5 joules of incident laser energy, ten times smaller than the minimum reflection time required to reflect 30% o f the incident light under the assumption of a reflectivity of 1. Therefore, a different broadening mechanism is responsable for the observed linewidths. A possible mechanism could be Doppler broadening, if we assume a characteristic fluctuation energy within the reflecting layer. A characteristic temperature can then be calculated for these fluctua-
October 1975
tions from the observed linewidths, and compared with temperatures obtained from X-ray emission. If we use an average ion mass of 4.67 for polyethylene, we obtain characteristic temperatures which are three times larger than the lowest electron spectral temperature obtained with X-rays, for all incident laser power. If, on the other hand, we employ a typical ion mass of 1.4 we obtain characteristic temperatures in agreement with electron spectral temperatures. In spite of the uncertainty in the choice of average ion mass, there is a reasonable agreement between these two temperatures to explain the broadening in terms of Doppler effect. This work has been possible thanks to a collaboration between INRS-Energie and the Defense Research Establishment at Valcartier. The authors wish to acknowledge valuable discussions with T.W. Johnston and K.J. Parbhakar, and the technical assistance of J. Gauthier, A. Thibaudeau, C. Tr@anier and J.G. Vall6e.
References [ l J K. Buchl, K. Eidman, H. Salzman and R. Sigel, Appl. Phys, Lett. 20 (1972) 3. [2] L.M. Goldman, J. Soures and M.J. Lubin, Phys. Rev. Lett. 31 (1973) 1184. [3] P. Lee, D.V. Giovanielli, R.P. Godwin and G.H. McCall, Appl. Phys. Lett. 24 (1974) 406. [4 ] B.H. Ripin, J.M. McMahou, E.A. McLean, W.M. Manheimer and J.A. Stamper, Phys. Rev. Lett. 33 (1974) 634. [5 ] F. Rheault, J.L. Lachambre, P. Lavigne, H. P6pin and H.A. Baldis, Rev. Sci. Instr., to be published Sept. 1975. [6] K. Boyer, Los Alamos Sc. Lab. Prog. Rep. LA-5251-PR, June 1973.
313
OPTICS COMMUNICATIONS
October 1975
REFLECTIVITY MEASUREMENTS FROM A NANOSECOND CO 2 LASER PRODUCED PLASMA H.A. BALDIS, H. PIPPIN and T.W. JOHNSTON INRS-l£nergie, Universit~du Qubbec, C.P. 1020, Varennes,Quebec, Canada Received 9 June 1975
The back-scattered light from a laser produced plasma has been studied using a nanosecond gigawatt CO2 laser. The spectral shift and width of the reflected light have been determined as a function of the incident laser energy.
The reflection of laser light by a plasma is of fundamental importance in the heating of laser produced plasmas. In recent experiments performed with Nd lasers [ 1 - 4 ] the back-reflected light presented a shift towards longer wavelengths, indicating the presence of stimulated processes (such as Brillouin scattering) in the region of interaction between the laser and the plasma. In this paper we present the experimental resuits obtained with a nanosecond gigawatt CO 2 laser, with a wavelength ten times longer than for Nd lasers. Instead of the red shift reported with the Nd lasers, we observed a predominant blue shift which we interpreted as a Doppler shift of the laser light due to the upstream motion of the scattering irregularities. The laser used in the experiment consisted of a mode-locked oscillator, an optical gate to isolate one single pulse, and four amplifier modules. It produced at the output a single pulse with a maximum energy of 4.5 joules and a duration of 2 nsec (fwhm). The beam has a diameter of 5 cm and 95% of its energy within a divergence of 2.2 mrad. The laser was developped by a group [5] at the Canadian Centro de Recherche pour la D6fense ~ Valcartier (C.R.D.V.) and the experiment itself was a collaboration between INRS-Energie and CRDV. A schematic of the experimental setup is shown in fig. 1. The laser beam was focussed on a 150/am thick polyethylene film with a 10 cm focal length, off-axis parabolic mirror. The damage diameter of the focal spot was 250/am, in agreement with the diffraction limit and at 4.5 joules gave a power density of approximately 3 X 1012 W cm - 2 . The incident laser pulse was monitored with a
photon drag detector. A NaC1 wedge located in front of the photon drag detector reflected a known fraction of the incident beam into the spectrometer with the purpose of calibration, both in intensity and wavelength. The reflected light from the plasma was collected using the same parabolic mirror and diverted with the NaC1 beam splitter into either a one meter grating spectrometer or an energy meter. A HgCdTe detector was used and its output recorded with a Tektronix 7904 oscilloscope, giving a combined rise time of 1.5 nsec. Reflection measurements were also performed at 45 ° from incidence using all2 lens. Other diagnostics included charge collection (Faraday cups) and a two-channel X-ray detector, to mesure the ion velocity and electron temperature, respectively. The electron temperature was determined using the two-channel
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/
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'
Photon drag detector
~
A
spliTtters
Energy
~/x>meter COe2r
Grating _' v ~spG~trograph
Fig. 1. Diagram of experimental apparatus. 311
Volume 15, number 2
OPTICS COMMUNICATIONS
X-ray detector by the absorber method which is fully valid as long as the electron energy has a maxwellian distribution. In our experiment various combinations of absorbers indicate a strong deviation from a maxwellian distribution. In that respect, the thin foil combination o f 12/am Be for one channel and 2.5/am AI + 12/am (CH2) n for the second channel (which measures the temperature in the spectral interval around 1 keV) enables us to get a good indication of the minimum spectral temperature. Our results indicated temperatures of 180 -+ 50, 250 -+ 50 and 300 -+ I00 eV for 1, 2 and 4 j o u l e s of incident laser energy respectively. The reflectivity of the plasma was measured as a function of the incident laser energy. Fig. 2 shows the results for two orientations of the target. With the target perpendicular to the incident laser beam (fig. 2a), the back-reflected energy is approximately 10% of the incident energy, decreasing for increasing incident energy. The reflection at 45 ° with respect to the incident laser beam is considerable lower, and
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October 1975
most of the energy is reflected in the backwards direction. F r o m these measurements the total r e f e c t e d energy is estimated to be approximately 30% of the incident. Rotation of the target (fig. 2b) increases the reflectivity towards the direction of specular reflection. Spectral measurements of the reflected light are shown in fig. 3 for three incident laser energies. Measurements across the spectral line were obtained in a shot-to-shot basis; this technique gave rise to the relatively large error bars indicated in fig. 3. Calibration of the spectrometer was made with the unshifted laser radiation reflected on the wedge (fig. 1). The instrumental width of the spectrometer was 18 A, at 10.6 /am. The shift and width of the reflected light are clearly resolved as shown in fig. 3, where the spectrum of the incident laser light has been superimposed. Due to the limited time resolution of the IR detector, only time integrated measurements have been obtained. An important feature of these measurements is the observed blue shift, which increases with the incident laser power. The Doppler velocities v = AX C/X calculated from the observed shift are shown in fig. 4. The magnitude and energy dependence of these velocilies are similar to ion velocities measured with the Faraday cup charge collector which are also indicated in fig. 4. This blue shift and the large reflectivity observed
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Fig. 2. Measured reflectivities for two directions of observation (0° and 45°). 312
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Fig. 3. Spectrum of reflected light for three different incident laser energies: (o) 4.6 J, (~) 1.4 J, and (~,) 0.35 J.
Volume 15, number 2 t0 8
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OPTICS COMMUNICATIONS I
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Fig. 4. Doppler velocities (o) calculated from the observed line shifts, and ion velocities (a) obtained with the Faraday cup.
(fig. 2a), seems to indicate that scattering irregularities exist near the critical layer (with an electron density near 1019 c m - 3 ) , which are moving away from the target with a velocity determined by the motion of the ions. In reflection measurements performed with Nd lasers [6], the reflection time obtained from the reciprocal of the linewidth (under the assumption of a reflectivity of 1) predicted a reflected energy that agreed well with the measured value. In our experiment, the reciprocal of the linewidth of the reflected light gives a minimum reflection time of 0.05 nsec for 4.5 joules of incident laser energy, ten times smaller than the minimum reflection time required to reflect 30% o f the incident light under the assumption of a reflectivity of 1. Therefore, a different broadening mechanism is responsable for the observed linewidths. A possible mechanism could be Doppler broadening, if we assume a characteristic fluctuation energy within the reflecting layer. A characteristic temperature can then be calculated for these fluctua-
October 1975
tions from the observed linewidths, and compared with temperatures obtained from X-ray emission. If we use an average ion mass of 4.67 for polyethylene, we obtain characteristic temperatures which are three times larger than the lowest electron spectral temperature obtained with X-rays, for all incident laser power. If, on the other hand, we employ a typical ion mass of 1.4 we obtain characteristic temperatures in agreement with electron spectral temperatures. In spite of the uncertainty in the choice of average ion mass, there is a reasonable agreement between these two temperatures to explain the broadening in terms of Doppler effect. This work has been possible thanks to a collaboration between INRS-Energie and the Defense Research Establishment at Valcartier. The authors wish to acknowledge valuable discussions with T.W. Johnston and K.J. Parbhakar, and the technical assistance of J. Gauthier, A. Thibaudeau, C. Tr@anier and J.G. Vall6e.
References [ l J K. Buchl, K. Eidman, H. Salzman and R. Sigel, Appl. Phys, Lett. 20 (1972) 3. [2] L.M. Goldman, J. Soures and M.J. Lubin, Phys. Rev. Lett. 31 (1973) 1184. [3] P. Lee, D.V. Giovanielli, R.P. Godwin and G.H. McCall, Appl. Phys. Lett. 24 (1974) 406. [4 ] B.H. Ripin, J.M. McMahou, E.A. McLean, W.M. Manheimer and J.A. Stamper, Phys. Rev. Lett. 33 (1974) 634. [5 ] F. Rheault, J.L. Lachambre, P. Lavigne, H. P6pin and H.A. Baldis, Rev. Sci. Instr., to be published Sept. 1975. [6] K. Boyer, Los Alamos Sc. Lab. Prog. Rep. LA-5251-PR, June 1973.
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