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2 harmonic emission from 526 nm laser produced plasmas
SIMULTANEOUS
15 May 1984
OPTICS COMMUNICATIONS
Volume 50, number 2
OBSERVATIONS
OF wo/2 AND 3w,/2
HARMONIC EMISSION
FROM 526 nm LASER PRODUCED PLASMAS
E. McGOLDRICK Department
and S.M.L. SIM
of Physics, University of Essex, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK
R.E. TURNER University of California, Lawrence Livermore National Laboratory,
Livermore,
CA 94550,
US,4
and 0. WILL1 Clarendon Laboratoty,
University of Oxford, Oxford, UK
Received 29 November 1982 Revised manuscript received 27 January 1984
Double peaked wo/2 and 3wo/2 harmonic spectra have been simultaneously observed in the light backscattered from nylon targets irradiated by 526 nm, 800 ps laser pulses. The separations between the peaks indicate the production of the q/2 and 3wo/2 harmonic emissions is consistent with a Stokes and anti-Stokes Raman scattering process of wo photons from 42 plasmons generated by the two plasmon decay process.
Near the quarter critical density surface, two mechanisms exist which generate plasmons with frequency near wo/2: the stimulated Raman scattering (SRS) instability [ 11, where the incident photon decays to give a scattered photon and a plasmon, and the twoplasmon decay instability [2], where the incident photon decays to give two daughter plasmons both with frequency wo/2. Scattered light with frequency near wo/2 is produced directly by the SRS instability but may also be generated from plasmons created by the two-plasmon decay process, either by Stokes Raman scattering of the incident pump photon off these plasmons, or by linear mode conversion [3]. Similarly, scattered light with frequency near 3w,/2 can arise either as a result of the interaction of a photon with a plasmon, or the coalescence of three plasmons. Recent observations [4,5] of wo/2 harmonic emission spectra have shown that it possesses a double peaked structure similar to that previously reported in 3wo/2 harmonic spectra [6,7]. Hence, it was sug
0 030-4018/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
gested [4,5] that the generation mechanism for the wo/2 harmonic light is connected with processes involving the two-plasmon instability. Scattered light at wo/2 due to a two-plasmon instability re-emission process has been separately identified [8] in uv-laser produced plasmas. However, the mechanism by which the wo/2 plasmons are converted to wo/2 photons is still unclear. The former of the two processes suggested earlier would explain the similarity between the wo/2 and 3 wo/2 harmonic spectra, since they then represent the Stokes and anti-Stokes lines, respectively, of Raman scattering of photons off two-plasmon instability-generated plasmons, where the separation between the peaks for both harmonics is given by the frequency spectrum of the wo/2 plasma waves. Hence, from conservation of energy considerations: Aw(l/2)
= Aw(3/2)
giving Ah(,,2j = oA+3,2). We present here new simultaneous observations of the backscattered wo/2 and 3wo/2 harmonic emis107
Volume
50, number
2
15 May 1984
OPTICS COMMUNICATIONS
BEAM
MIRRORS a. HR AT 0 53um
I
600 Imm-’ SPECTROGRAPH
Fig. 1. Experimental
sions from nylon tube targets irradiated by 526 nm laser light to investigate the relationship between these two harmonic spectra. The experiment described here was performed on the Nd/glass laser at the SERC Laser Facility, Rutherford Laboratory, using the layout shown in fig. 1. The 1.05 m light was frequency doubled by a KDP crystal and focussed onto the target by an f/l aspheric doublet lens. The residual fundamental light was attenuated by an HR 1 pm mirror in the incident beam path. Incident energies of 216 J on target were obtained for pulse lengths -800 ps. Time resolved spectra of the wo/2 harmonic light backscattered through the focussing lens were obtained by imaging the emission onto the slit of a grating spectrograph (f = 0.5 m, 600 lines mm-l grating), optically coupled by an f/l .4 lens to an S-l streak camera. High temporal resolution was not essential for this experiment and the streak camera was chiefly used for its high gain in this spectral region (due to its image intensifier). Temporal and spectral resolutions were -160 ps and -50 A respectively. Two red glass filters (transmitting for X > 7000 A) and an IR trans108
arrangement.
mitting black glass filter were placed in front of the streak camera slit. The input optics were arranged to give a spectral window of -470 A at the streak camera slit, centred at the nominal wo/2 position. Recordings of the time resolved spectra were made on Ilford HP5 film. The spectral dispersion along the streak camera slit was calibrated by recording low energy 1.05 pm laser shots with the spectrograph at two different wavelength counter settings. Time integrated spectra of the backscattered 3wo/2 harmonic light were resolved on an f = 0.5 m, 1200 lines mm-l grating spectrograph and recorded on Ilford HP5 film. An uv-transmitting black glass filter was used in front of the spectrograph slit. The spectrograph was used in first, second and third orders to obtain suitable recording levels of light over the range investigated. Corresponding spectral resolutions were -16 A, -8 A and -4 A. A mercury lamp was used to provide calibration lines for each shot. A range of irradiances on target between 1014 and 2 X 10” W cmm2 was obtained by varying the focal spot size between 30-100 nm diameter. Fig. 2 shows the simultaneously observed wo/2 and 3wo/2 har-
15 May 1984
OPTICS COMMUNICATIONS
Volume 50, number 2
t
zoops
I
1.05pm
A
1.05vm
I
1
1ooA
b
A
Fig. 2. Experimental data showing the simultaneous wo/2 and 3w0/2 spectra from a Nd/glass laser irradiated nylon target 9 - 9 X 1014 W cme2. (i) Time resolved w0/2 harmonic spectrum. (ii) Time integrated 3wo/2 harmonic spectrum.
manic spectra for a typical shot. The pulsed nature of the oo/2 harmonic emission is due to mode beating in the main laser pulse. The 30,/2 spectrum is time-integrated and therefore represents an “average” peak separation during the laser pulse, whereas the wo/2 spectrum, despite the poor temporal resolution, shows a variation in peak separation with time. Since the peak separation is proportional to the electron temperature [6,7] the wo/2 spectrum reflects the temporal variation in the temperature. Over the
range investigated the separation between the blueshifted and red-shifted peaks for the wo/2 harmonic spectra was observed to be A+) - (206-330 A) f 25 8, and for the 3wo/2 harmonic emission to be Ahg,;?) N (24-3 1 A) + 5 .&. These measurements show good agreement with the relationship AXCy2) = 9A1(3p, predicted by a Stokes and anti-Stokes Raman scattering generation mechanism. However, in the absence of any other process, the observed light should be split symmetrically 109
Volume
50, number
2
OPTICS COMMUNICATIONS
[7] about 0012 or 3~012 and the experimental observations show the wo/2 mean is slightly blue-shifted, while the 3w0/2 is slightly red-shifted. Although this is consistent with the idea of Raman generation it suggests the plasmons from the two-plasmon instability have lost some energy, perhaps due to ion acoustic turbulence or to some other parametric decay instability. Alternatively, the asymmetric splitting of the spectra may be explained by k-matching considerations. For the initial two plasmon decay process the usual conservation of momentum and energy matching conditions give w. = w1 + w2 and k, = k, t k,, where the subscripts 0, 1 and 2 denote the pump and decay waves respectively with w2 > wl. Raman up conversion to produce 3w,/2 scattered light requires ~312 = w. t w1,2 and k312 = k, t kL2. Similarly, Raman down conversion to produce scattered light at the frequency wo/2 requires w1/2 = w. - WQ and kl,2 = k, - k1,2 which can only be satisfied in the case of k, if the pump wave is reflected, i.e. for -k,, when propagation of the plasma wave is not allowed [9]. Also, for both wo/2 and 3wo/2 harmonic generation, the matching conditions cannot be satisfied simultaneously for both k, and k2 [lo], hence different two plasmon decay waves contribute to each wing. Linear mode conversion of the wo/2 plasma waves to give wo/2 scattered light can only occur for plasma waves that propagate up the density gradient and so would only contribute to the red wing. The shifts to the red for the centroid of the 3wo/2 harmonic spectra and to the blue for the centroid of the wo/2 harmonic spectra do not support the generation of 3wo/2 harmonic emission by the coalescence of three plasmons. In the presence of a magnetic field the SRS instability can produce w0/2 scattered light with a double peaked spectral structure but calculations by Barr [ 1 l] show that very large fields of -6 MG would be required to explain the present observations. Also, for our experimental parameters the two plasmon decay instability is expected to be the dominant mechanism at the quarter critical density layer since it has the lower threshold. Scattered light due to a convective SRS instability has been identified at plasma densities of 0.05 n, [8] and 0.1 n, [5], but
110
15 May 1984
similar observations were not obtained here. In conclusion, the experimental observations have shown that Aw(~,~) = Awc3/2), as predicted by the Stokes and anti-Stokes Raman generation mechanism. However, the shifts in the mean wavelengths of the wo/2 and 3 wo/2 spectra have yet to be fully explained. We gratefully acknowledge the assistance of the SERC Laser Facility at the Rutherford Laboratory in carrying out these observations, especially W. Toner, C. Hooker and S.J. Knight. We are also grateful to T.P. Hughes (Univesity of Essex) and R.G. Evans (Rutherford Laboratory) for helpful discussions. We are indebted to the U.K. Science and Engineering Research Council for financial support.
References [l] C.S. Liu, M.N. Rosenbluth [2]
[ 31 [4] [S
[6] [7]
[8]
[9]
[lo] [ 1 l]
]
and R.B. Shite, Phys. Fluids 17 (1974) 1211. H.H. Chen and C.S. Liu, Phys. Rev. Lett. 39 (1974) 881; C.S. Liu and M.N. Rosenbluth, Phys. Fluids 19 (1976) 967. V.L. Ginsburg, The propagation of electromagnetic waves in plasmas (Pergamon, Oxford, 1970) p. 267. E. McGoldrick and S.M.L. Sim, Optics Comm. 40 (1982) 433. R.E. Turner, E.M. Campbell, W.C. Mead, F. Ze, C. Max, D.W. Phillion, P. Lee, B. Pruett, G. Time11 and B.F. Lasinski, Lawrence Livermore Laboratory University of California Report No. UCRL 86911. P.D. Carter, S.M.L. Sim, H.C. Barr and R.G. Evans, Phys. Rev. Lett. 44 (1980) 1407. A.I. Avrov, V.Yu. Bychenkov, O.N. Krokhin, V.V. Pustavalov, S.S. Rupasov, V.P. Silin, G.V. Skhzkov, V.T. Tikhonchuk and A.S. Shikanov, Sov. Phys. JETP 45 (1977) 507. K. Tanaka, L.M. Goldman, W. Seka, M.C. Richardson, J.M. Soures and E.A. Williams, Phys. Rev. Lett. 48 (1982) 1179. S.J. Karttunen, private communication. H.C. Barr, Annual Report to the Laser Facility Committee, Rutherford Lab. Report RL-79-036 (1979) Section 8.3.3. H.C. Barr, Annual Report to the Laser Facility Committee, Rutherford Lab. Report RL-83-043 (1983) Section 8.3.3.
15 May 1984
OPTICS COMMUNICATIONS
Volume 50, number 2
OBSERVATIONS
OF wo/2 AND 3w,/2
HARMONIC EMISSION
FROM 526 nm LASER PRODUCED PLASMAS
E. McGOLDRICK Department
and S.M.L. SIM
of Physics, University of Essex, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK
R.E. TURNER University of California, Lawrence Livermore National Laboratory,
Livermore,
CA 94550,
US,4
and 0. WILL1 Clarendon Laboratoty,
University of Oxford, Oxford, UK
Received 29 November 1982 Revised manuscript received 27 January 1984
Double peaked wo/2 and 3wo/2 harmonic spectra have been simultaneously observed in the light backscattered from nylon targets irradiated by 526 nm, 800 ps laser pulses. The separations between the peaks indicate the production of the q/2 and 3wo/2 harmonic emissions is consistent with a Stokes and anti-Stokes Raman scattering process of wo photons from 42 plasmons generated by the two plasmon decay process.
Near the quarter critical density surface, two mechanisms exist which generate plasmons with frequency near wo/2: the stimulated Raman scattering (SRS) instability [ 11, where the incident photon decays to give a scattered photon and a plasmon, and the twoplasmon decay instability [2], where the incident photon decays to give two daughter plasmons both with frequency wo/2. Scattered light with frequency near wo/2 is produced directly by the SRS instability but may also be generated from plasmons created by the two-plasmon decay process, either by Stokes Raman scattering of the incident pump photon off these plasmons, or by linear mode conversion [3]. Similarly, scattered light with frequency near 3w,/2 can arise either as a result of the interaction of a photon with a plasmon, or the coalescence of three plasmons. Recent observations [4,5] of wo/2 harmonic emission spectra have shown that it possesses a double peaked structure similar to that previously reported in 3wo/2 harmonic spectra [6,7]. Hence, it was sug
0 030-4018/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
gested [4,5] that the generation mechanism for the wo/2 harmonic light is connected with processes involving the two-plasmon instability. Scattered light at wo/2 due to a two-plasmon instability re-emission process has been separately identified [8] in uv-laser produced plasmas. However, the mechanism by which the wo/2 plasmons are converted to wo/2 photons is still unclear. The former of the two processes suggested earlier would explain the similarity between the wo/2 and 3 wo/2 harmonic spectra, since they then represent the Stokes and anti-Stokes lines, respectively, of Raman scattering of photons off two-plasmon instability-generated plasmons, where the separation between the peaks for both harmonics is given by the frequency spectrum of the wo/2 plasma waves. Hence, from conservation of energy considerations: Aw(l/2)
= Aw(3/2)
giving Ah(,,2j = oA+3,2). We present here new simultaneous observations of the backscattered wo/2 and 3wo/2 harmonic emis107
Volume
50, number
2
15 May 1984
OPTICS COMMUNICATIONS
BEAM
MIRRORS a. HR AT 0 53um
I
600 Imm-’ SPECTROGRAPH
Fig. 1. Experimental
sions from nylon tube targets irradiated by 526 nm laser light to investigate the relationship between these two harmonic spectra. The experiment described here was performed on the Nd/glass laser at the SERC Laser Facility, Rutherford Laboratory, using the layout shown in fig. 1. The 1.05 m light was frequency doubled by a KDP crystal and focussed onto the target by an f/l aspheric doublet lens. The residual fundamental light was attenuated by an HR 1 pm mirror in the incident beam path. Incident energies of 216 J on target were obtained for pulse lengths -800 ps. Time resolved spectra of the wo/2 harmonic light backscattered through the focussing lens were obtained by imaging the emission onto the slit of a grating spectrograph (f = 0.5 m, 600 lines mm-l grating), optically coupled by an f/l .4 lens to an S-l streak camera. High temporal resolution was not essential for this experiment and the streak camera was chiefly used for its high gain in this spectral region (due to its image intensifier). Temporal and spectral resolutions were -160 ps and -50 A respectively. Two red glass filters (transmitting for X > 7000 A) and an IR trans108
arrangement.
mitting black glass filter were placed in front of the streak camera slit. The input optics were arranged to give a spectral window of -470 A at the streak camera slit, centred at the nominal wo/2 position. Recordings of the time resolved spectra were made on Ilford HP5 film. The spectral dispersion along the streak camera slit was calibrated by recording low energy 1.05 pm laser shots with the spectrograph at two different wavelength counter settings. Time integrated spectra of the backscattered 3wo/2 harmonic light were resolved on an f = 0.5 m, 1200 lines mm-l grating spectrograph and recorded on Ilford HP5 film. An uv-transmitting black glass filter was used in front of the spectrograph slit. The spectrograph was used in first, second and third orders to obtain suitable recording levels of light over the range investigated. Corresponding spectral resolutions were -16 A, -8 A and -4 A. A mercury lamp was used to provide calibration lines for each shot. A range of irradiances on target between 1014 and 2 X 10” W cmm2 was obtained by varying the focal spot size between 30-100 nm diameter. Fig. 2 shows the simultaneously observed wo/2 and 3wo/2 har-
15 May 1984
OPTICS COMMUNICATIONS
Volume 50, number 2
t
zoops
I
1.05pm
A
1.05vm
I
1
1ooA
b
A
Fig. 2. Experimental data showing the simultaneous wo/2 and 3w0/2 spectra from a Nd/glass laser irradiated nylon target 9 - 9 X 1014 W cme2. (i) Time resolved w0/2 harmonic spectrum. (ii) Time integrated 3wo/2 harmonic spectrum.
manic spectra for a typical shot. The pulsed nature of the oo/2 harmonic emission is due to mode beating in the main laser pulse. The 30,/2 spectrum is time-integrated and therefore represents an “average” peak separation during the laser pulse, whereas the wo/2 spectrum, despite the poor temporal resolution, shows a variation in peak separation with time. Since the peak separation is proportional to the electron temperature [6,7] the wo/2 spectrum reflects the temporal variation in the temperature. Over the
range investigated the separation between the blueshifted and red-shifted peaks for the wo/2 harmonic spectra was observed to be A+) - (206-330 A) f 25 8, and for the 3wo/2 harmonic emission to be Ahg,;?) N (24-3 1 A) + 5 .&. These measurements show good agreement with the relationship AXCy2) = 9A1(3p, predicted by a Stokes and anti-Stokes Raman scattering generation mechanism. However, in the absence of any other process, the observed light should be split symmetrically 109
Volume
50, number
2
OPTICS COMMUNICATIONS
[7] about 0012 or 3~012 and the experimental observations show the wo/2 mean is slightly blue-shifted, while the 3w0/2 is slightly red-shifted. Although this is consistent with the idea of Raman generation it suggests the plasmons from the two-plasmon instability have lost some energy, perhaps due to ion acoustic turbulence or to some other parametric decay instability. Alternatively, the asymmetric splitting of the spectra may be explained by k-matching considerations. For the initial two plasmon decay process the usual conservation of momentum and energy matching conditions give w. = w1 + w2 and k, = k, t k,, where the subscripts 0, 1 and 2 denote the pump and decay waves respectively with w2 > wl. Raman up conversion to produce 3w,/2 scattered light requires ~312 = w. t w1,2 and k312 = k, t kL2. Similarly, Raman down conversion to produce scattered light at the frequency wo/2 requires w1/2 = w. - WQ and kl,2 = k, - k1,2 which can only be satisfied in the case of k, if the pump wave is reflected, i.e. for -k,, when propagation of the plasma wave is not allowed [9]. Also, for both wo/2 and 3wo/2 harmonic generation, the matching conditions cannot be satisfied simultaneously for both k, and k2 [lo], hence different two plasmon decay waves contribute to each wing. Linear mode conversion of the wo/2 plasma waves to give wo/2 scattered light can only occur for plasma waves that propagate up the density gradient and so would only contribute to the red wing. The shifts to the red for the centroid of the 3wo/2 harmonic spectra and to the blue for the centroid of the wo/2 harmonic spectra do not support the generation of 3wo/2 harmonic emission by the coalescence of three plasmons. In the presence of a magnetic field the SRS instability can produce w0/2 scattered light with a double peaked spectral structure but calculations by Barr [ 1 l] show that very large fields of -6 MG would be required to explain the present observations. Also, for our experimental parameters the two plasmon decay instability is expected to be the dominant mechanism at the quarter critical density layer since it has the lower threshold. Scattered light due to a convective SRS instability has been identified at plasma densities of 0.05 n, [8] and 0.1 n, [5], but
110
15 May 1984
similar observations were not obtained here. In conclusion, the experimental observations have shown that Aw(~,~) = Awc3/2), as predicted by the Stokes and anti-Stokes Raman generation mechanism. However, the shifts in the mean wavelengths of the wo/2 and 3 wo/2 spectra have yet to be fully explained. We gratefully acknowledge the assistance of the SERC Laser Facility at the Rutherford Laboratory in carrying out these observations, especially W. Toner, C. Hooker and S.J. Knight. We are also grateful to T.P. Hughes (Univesity of Essex) and R.G. Evans (Rutherford Laboratory) for helpful discussions. We are indebted to the U.K. Science and Engineering Research Council for financial support.
References [l] C.S. Liu, M.N. Rosenbluth [2]
[ 31 [4] [S
[6] [7]
[8]
[9]
[lo] [ 1 l]
]
and R.B. Shite, Phys. Fluids 17 (1974) 1211. H.H. Chen and C.S. Liu, Phys. Rev. Lett. 39 (1974) 881; C.S. Liu and M.N. Rosenbluth, Phys. Fluids 19 (1976) 967. V.L. Ginsburg, The propagation of electromagnetic waves in plasmas (Pergamon, Oxford, 1970) p. 267. E. McGoldrick and S.M.L. Sim, Optics Comm. 40 (1982) 433. R.E. Turner, E.M. Campbell, W.C. Mead, F. Ze, C. Max, D.W. Phillion, P. Lee, B. Pruett, G. Time11 and B.F. Lasinski, Lawrence Livermore Laboratory University of California Report No. UCRL 86911. P.D. Carter, S.M.L. Sim, H.C. Barr and R.G. Evans, Phys. Rev. Lett. 44 (1980) 1407. A.I. Avrov, V.Yu. Bychenkov, O.N. Krokhin, V.V. Pustavalov, S.S. Rupasov, V.P. Silin, G.V. Skhzkov, V.T. Tikhonchuk and A.S. Shikanov, Sov. Phys. JETP 45 (1977) 507. K. Tanaka, L.M. Goldman, W. Seka, M.C. Richardson, J.M. Soures and E.A. Williams, Phys. Rev. Lett. 48 (1982) 1179. S.J. Karttunen, private communication. H.C. Barr, Annual Report to the Laser Facility Committee, Rutherford Lab. Report RL-79-036 (1979) Section 8.3.3. H.C. Barr, Annual Report to the Laser Facility Committee, Rutherford Lab. Report RL-83-043 (1983) Section 8.3.3.