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2 harmonic emission from 0.35μm laser-irradiated plasmas
Volume 40, number 6
OPTICS COMMUNICATIONS
15 February 1982
OBSERVATIONS OF ¢a0/2 HARMONIC EMISSION FROM 0.35 pm LASER-IRRADIATED PLASMAS E. McGOLDRICK and S.M.L. SIM Department of Physics, University of Essex, WivenhoePark, Colchester, Essex CO4 3SQ, England Received 28 October 1981
Observations of too/2 harmonic emission from both spherical and plane targets irradiated by 0.35 pro, 800 ps laser pulses have been obtained with simultaneous high spectral and temporal resolutions of 16 A and 20 ps respectively.The too/2 harmonic emission spectrum is interpreted as providinga direct measurement of the frequency of the too/2 plasma waves and hence can be used to estimate the electron temperature.
Recent simulations [1] have shown that significant levels of scattered light with frequency near <~0/2 can be expected in longer pu~se length experiments (z > 1 ns). The underdense plasma conditions in this case favour the stimulated Raman scattering instability [2] which produces a scattered electromagnetic wave and an electron plasma wave. Light with frequency near 6o0/2 can also be generated from plasmons created by the two plasmon decay instability either by Stokes Raman scattering of the incident pump wave off these plasma waves, or possibly by linear mode conversion [3]. All of these processes occur near the quarter critical density region. The earliest report of the experimental observation of light at co0/2 was by Bobin et al. [4]. A line spectrum centred at 2X0 was seen, with 60 A fwhm compared with 30 A for the incident light. More recently, signals at co0/2 have been detected by Elazar et al. [5] and Phillion [6], whose signal extended below co0[2 as well. Electron plasma waves at ~0/2 have been directly observed by Baldis et al. [7] in a CO2-irradiated plasma using Thomson scattering techniques. We present here new observations of the ~0/2 harmonic emission from spherical and plane targets irradiated by the third harmonic (0.35/am) of the Nd: glass laser (Ela s < 10 J; fwhm "~ 800 ps) at the SERC Laser Facility, Rutherford Laboratory. The present work was obtained with better spectral resolution (~'16 A) than in the previous reports [4-6] and with simultaneous temporal resolution of '~20 ps. 0 030-4018/82/0000-0000/$ 02.75 © 1982 North-Holland
The experimental layout is shown in fig. 1. The 1.05 gm light was converted [8] to 0.35 pm (fwhm = 800 ps) by two KDP crystals and focussed onto the target by an f/2.5 doublet lens. The co0/2 harmonic f=12cm f =2Ocm i'---i
520 STREAK CAMERA
'% FILTER R;dE CEI
PRISM DOVE
~
SPECTROGRAPH
¢m
R >80% \ at 7000~# it L V ar f=2.0 m m
INCIDENT BLUE Wo BEAM
BACKSCATTERED Wo~2
DOUBLET LENS
Fig. 1. Experimental layout. 433
Volume 40, number 6
OPTICS COMMUNICATIONS
emission backscattered through the lens was imaged onto the slit of a grating spectrograph (f--- 0.5 m, 1200 lines per mm grating). The dispersed output was rotated by the dove prism so that wavelength lay along the slit of the $20 streak camera. A broadband transmission interference filter (~200 A centred at 7000 A) was placed in front of the streak camera slit and the input optics arranged so that the spectral window at the streak camera was "~200 A. Calibration marks were obtained after every shot by streaking part of the second harmonic incident beam produced by a subsequent low energy shot, with the wavelength setting of the spectrograph at 5260 A and appropriate adjustment to the camera trigger delay cable. Although this method relied on the linearity and reproducibility Of the grating drive mechanism, it was sufficiently accurate for our purposes since the instrumental error ('~2 A) was considerably smaller than our spectral resolution of 16 A. Recordings of the time resolved co0/2 spectra were made on KODAK 2485 film. A range of irradiances on target between " 5 X 1013 W cm - 2 and "1015 W cm - 2 was obtained by varying the focal spot size. Fig. 2 shows the distribution of intensity illumination on target from X-ray pinhole camera photographs for focal spot sizes 5 0 - 2 0 0 /am. No 600/2 harmonic emission was observed for irradiances below "1014 cm -2 but this may be due to the uneven distribution of intensity illumination for focal spot sizes > 50/am, as only a small spatial region " 5 0 #m wide in the centre of the target was imaged onto the spectrograph slit. Fig. 3 shows the result of a moderate irradiance (~ "~ 7 × 1014 W cm - 2 ) shot on a gold coated microballoon which was typical of our data. The ~o0/2 harmonic emission spectrum consisted of two wings symmetrically shifted to the blue and the red of the nominal ~o0/2 position. Both peaks appeared simultaneously, but with their separation decreasing in time. The emission also had a pulsed nature, where the duration of each pulse was no greater than the time resolution ( " 2 0 ps) of the system, and the pulse separation varied between 5 0 - 9 0 ps. It thus bore a striking resemblance to our previous observations [9] of 3/2 co0 harmonic emission obtained with 1.06/am incident light. Although the stimulated Raman scattering instability produces 600/2 harmonic light directly, we do not believe that this mechanism generated the radiation 434
15 February 1982
Fig. 2. X-ray pinhole camera photographs showing distribution of intensity illumination on target for three focal spot sizes. observed in our experiment, since the laser irradiance did not exceed the theoretical threshold for this instability, and the double-peaked structure is not consistent with the Raman instability. Instead we attribute the mechanism for the 6o0/2 harmonic emission in our observations to processes involving the plasmons generated by the two plasmon decay instability, which has a lower threshold. This may be followed by Stokes Raman scattering of the pump wave, or possibly by
Volume 40, number 6
OPTICS COMMUNICATIONS
15 February 1982
(O) I
7o13A (c)
TIME
,6.
70ps "-~
~
105]~
I
i, TIME
p
7013 A
Fig. 3 (a) Time-resolvedspectrum of too/2 harmonic emission from 0.35 um laser irradiated gold coated microballoon,~ ~ 7 X 1014 W cm-2. CO)Isodensity contour map of (a). (c) Mierodensitometertrace along the time axis. (d) Microdensitometertrace along the wavelengthaxis.
linear mode conversion of the plasma waves. The former process would explain the similarity between the co0/2 and 3/2 coo harmonic spectra since they would then represent the Stokes and anti-Stokes line respectively of the Raman scattering of the pump wave, but simultaneous observations of the two harmonic emission spectra is necessary to determine this. In either case, the co0/2 harmonic emission spectrum provides a direct determination of the frequency spectrum of the 6o0/2 electron plasma waves. The decreasing separation of the two peaks is then a reflection of the decreasing electron temperature in the falling part of the laser pulse, since in the case of no prepulse, the density scale length at quarter critical density during the rising part of the laser pulse is very small. The threshold for the two plasmon decay instability [9] will only be exceeded after the peak of the laser pulse as the density scale length increases.
If the frequency of the co0/2 plamaa waves is determined by the two plasrnon decay matching conditions and by the Bohm-Gross dispersion relation, then the temperature of the quarter critical density region can be estimated at about 800 eV. The pulsed nature of the co0/2 harmonic emission is similar to that observed in the 2 coo [10,11] and 3/2 coO [9] harmonic emissions with 1.06 tam laser light and is consistent with the explanation in refs. [9-11] of the existence of a hydrodynamic instability, though other interpretations are possible. We gratefully acknowledge the assistance of the members of the SERC Laser Facility at the Rutherford Laboratory in carrying out these observations, especially W. Toner, R. Eason, D. Pepler and J. Szechi. We are also grateful to T.P. Hughes (University of Essex) and R.G. Evans (Rutherford Laboratory) 435
Volume 40, numb.er 6
OPTICS COMMUNICATIONS
for helpful discussions. We are indebted to the U.K. Science Research Council for financial support.
References [1 ] W.L. Kruer, K. Estabrook and A.B. Langdon, Phys. Fluids 23 (1980) 1232 and references therein. [2] C.S. Liu, M.N. Rosenbluth and R.B. White, Phys. Fluids 17 (1974) 1211. [3] V.L. Ginzburg, The propagation of electromagnetic waves in plasmas (Pergamon, Oxford 1970) p. 267. [4] J.L. Bobin, M. Decroisette, B. Meyer and Y. Vitel, Phys. Rev. Lett. 30 (1973) 594.
436
15 February 1982
[5] J. Elazar, W. Toner and E. Wooding, Rutherford Laboratory report No. 80-026, Chapter 3. [6] D. Phillion, Lawrence Livermore Laboratory, University of California report No UCRL 84148 (1980). [7] H.A. Baldis, J.C. Samson and P.B. Corkum, Phys. Rev. Lett. 41 (1978) 1719. [8] W. Toner, Rutherford Laboratory report No. 81-040, Chapter 1. [9] P.D. Carter, S.M.L. Sim, H.C. Barr and R.G. Evans, Phys. Rev. Lett. 44 (1980) 1407. [10] D.R. Gray, J.D. Murdoeh, S.M.L. Sire, A.J. Cole, R.G. Evans and W.T. Toner, Plasma Phys. 22 (1980) 967. [11] P.D. Carter, S.M.L. Sire and T.P. Hughes, Optics Comm. 27 (1978) 423.
OPTICS COMMUNICATIONS
15 February 1982
OBSERVATIONS OF ¢a0/2 HARMONIC EMISSION FROM 0.35 pm LASER-IRRADIATED PLASMAS E. McGOLDRICK and S.M.L. SIM Department of Physics, University of Essex, WivenhoePark, Colchester, Essex CO4 3SQ, England Received 28 October 1981
Observations of too/2 harmonic emission from both spherical and plane targets irradiated by 0.35 pro, 800 ps laser pulses have been obtained with simultaneous high spectral and temporal resolutions of 16 A and 20 ps respectively.The too/2 harmonic emission spectrum is interpreted as providinga direct measurement of the frequency of the too/2 plasma waves and hence can be used to estimate the electron temperature.
Recent simulations [1] have shown that significant levels of scattered light with frequency near <~0/2 can be expected in longer pu~se length experiments (z > 1 ns). The underdense plasma conditions in this case favour the stimulated Raman scattering instability [2] which produces a scattered electromagnetic wave and an electron plasma wave. Light with frequency near 6o0/2 can also be generated from plasmons created by the two plasmon decay instability either by Stokes Raman scattering of the incident pump wave off these plasma waves, or possibly by linear mode conversion [3]. All of these processes occur near the quarter critical density region. The earliest report of the experimental observation of light at co0/2 was by Bobin et al. [4]. A line spectrum centred at 2X0 was seen, with 60 A fwhm compared with 30 A for the incident light. More recently, signals at co0/2 have been detected by Elazar et al. [5] and Phillion [6], whose signal extended below co0[2 as well. Electron plasma waves at ~0/2 have been directly observed by Baldis et al. [7] in a CO2-irradiated plasma using Thomson scattering techniques. We present here new observations of the ~0/2 harmonic emission from spherical and plane targets irradiated by the third harmonic (0.35/am) of the Nd: glass laser (Ela s < 10 J; fwhm "~ 800 ps) at the SERC Laser Facility, Rutherford Laboratory. The present work was obtained with better spectral resolution (~'16 A) than in the previous reports [4-6] and with simultaneous temporal resolution of '~20 ps. 0 030-4018/82/0000-0000/$ 02.75 © 1982 North-Holland
The experimental layout is shown in fig. 1. The 1.05 gm light was converted [8] to 0.35 pm (fwhm = 800 ps) by two KDP crystals and focussed onto the target by an f/2.5 doublet lens. The co0/2 harmonic f=12cm f =2Ocm i'---i
520 STREAK CAMERA
'% FILTER R;dE CEI
PRISM DOVE
~
SPECTROGRAPH
¢m
R >80% \ at 7000~# it L V ar f=2.0 m m
INCIDENT BLUE Wo BEAM
BACKSCATTERED Wo~2
DOUBLET LENS
Fig. 1. Experimental layout. 433
Volume 40, number 6
OPTICS COMMUNICATIONS
emission backscattered through the lens was imaged onto the slit of a grating spectrograph (f--- 0.5 m, 1200 lines per mm grating). The dispersed output was rotated by the dove prism so that wavelength lay along the slit of the $20 streak camera. A broadband transmission interference filter (~200 A centred at 7000 A) was placed in front of the streak camera slit and the input optics arranged so that the spectral window at the streak camera was "~200 A. Calibration marks were obtained after every shot by streaking part of the second harmonic incident beam produced by a subsequent low energy shot, with the wavelength setting of the spectrograph at 5260 A and appropriate adjustment to the camera trigger delay cable. Although this method relied on the linearity and reproducibility Of the grating drive mechanism, it was sufficiently accurate for our purposes since the instrumental error ('~2 A) was considerably smaller than our spectral resolution of 16 A. Recordings of the time resolved co0/2 spectra were made on KODAK 2485 film. A range of irradiances on target between " 5 X 1013 W cm - 2 and "1015 W cm - 2 was obtained by varying the focal spot size. Fig. 2 shows the distribution of intensity illumination on target from X-ray pinhole camera photographs for focal spot sizes 5 0 - 2 0 0 /am. No 600/2 harmonic emission was observed for irradiances below "1014 cm -2 but this may be due to the uneven distribution of intensity illumination for focal spot sizes > 50/am, as only a small spatial region " 5 0 #m wide in the centre of the target was imaged onto the spectrograph slit. Fig. 3 shows the result of a moderate irradiance (~ "~ 7 × 1014 W cm - 2 ) shot on a gold coated microballoon which was typical of our data. The ~o0/2 harmonic emission spectrum consisted of two wings symmetrically shifted to the blue and the red of the nominal ~o0/2 position. Both peaks appeared simultaneously, but with their separation decreasing in time. The emission also had a pulsed nature, where the duration of each pulse was no greater than the time resolution ( " 2 0 ps) of the system, and the pulse separation varied between 5 0 - 9 0 ps. It thus bore a striking resemblance to our previous observations [9] of 3/2 co0 harmonic emission obtained with 1.06/am incident light. Although the stimulated Raman scattering instability produces 600/2 harmonic light directly, we do not believe that this mechanism generated the radiation 434
15 February 1982
Fig. 2. X-ray pinhole camera photographs showing distribution of intensity illumination on target for three focal spot sizes. observed in our experiment, since the laser irradiance did not exceed the theoretical threshold for this instability, and the double-peaked structure is not consistent with the Raman instability. Instead we attribute the mechanism for the 6o0/2 harmonic emission in our observations to processes involving the plasmons generated by the two plasmon decay instability, which has a lower threshold. This may be followed by Stokes Raman scattering of the pump wave, or possibly by
Volume 40, number 6
OPTICS COMMUNICATIONS
15 February 1982
(O) I
7o13A (c)
TIME
,6.
70ps "-~
~
105]~
I
i, TIME
p
7013 A
Fig. 3 (a) Time-resolvedspectrum of too/2 harmonic emission from 0.35 um laser irradiated gold coated microballoon,~ ~ 7 X 1014 W cm-2. CO)Isodensity contour map of (a). (c) Mierodensitometertrace along the time axis. (d) Microdensitometertrace along the wavelengthaxis.
linear mode conversion of the plasma waves. The former process would explain the similarity between the co0/2 and 3/2 coo harmonic spectra since they would then represent the Stokes and anti-Stokes line respectively of the Raman scattering of the pump wave, but simultaneous observations of the two harmonic emission spectra is necessary to determine this. In either case, the co0/2 harmonic emission spectrum provides a direct determination of the frequency spectrum of the 6o0/2 electron plasma waves. The decreasing separation of the two peaks is then a reflection of the decreasing electron temperature in the falling part of the laser pulse, since in the case of no prepulse, the density scale length at quarter critical density during the rising part of the laser pulse is very small. The threshold for the two plasmon decay instability [9] will only be exceeded after the peak of the laser pulse as the density scale length increases.
If the frequency of the co0/2 plamaa waves is determined by the two plasrnon decay matching conditions and by the Bohm-Gross dispersion relation, then the temperature of the quarter critical density region can be estimated at about 800 eV. The pulsed nature of the co0/2 harmonic emission is similar to that observed in the 2 coo [10,11] and 3/2 coO [9] harmonic emissions with 1.06 tam laser light and is consistent with the explanation in refs. [9-11] of the existence of a hydrodynamic instability, though other interpretations are possible. We gratefully acknowledge the assistance of the members of the SERC Laser Facility at the Rutherford Laboratory in carrying out these observations, especially W. Toner, R. Eason, D. Pepler and J. Szechi. We are also grateful to T.P. Hughes (University of Essex) and R.G. Evans (Rutherford Laboratory) 435
Volume 40, numb.er 6
OPTICS COMMUNICATIONS
for helpful discussions. We are indebted to the U.K. Science Research Council for financial support.
References [1 ] W.L. Kruer, K. Estabrook and A.B. Langdon, Phys. Fluids 23 (1980) 1232 and references therein. [2] C.S. Liu, M.N. Rosenbluth and R.B. White, Phys. Fluids 17 (1974) 1211. [3] V.L. Ginzburg, The propagation of electromagnetic waves in plasmas (Pergamon, Oxford 1970) p. 267. [4] J.L. Bobin, M. Decroisette, B. Meyer and Y. Vitel, Phys. Rev. Lett. 30 (1973) 594.
436
15 February 1982
[5] J. Elazar, W. Toner and E. Wooding, Rutherford Laboratory report No. 80-026, Chapter 3. [6] D. Phillion, Lawrence Livermore Laboratory, University of California report No UCRL 84148 (1980). [7] H.A. Baldis, J.C. Samson and P.B. Corkum, Phys. Rev. Lett. 41 (1978) 1719. [8] W. Toner, Rutherford Laboratory report No. 81-040, Chapter 1. [9] P.D. Carter, S.M.L. Sim, H.C. Barr and R.G. Evans, Phys. Rev. Lett. 44 (1980) 1407. [10] D.R. Gray, J.D. Murdoeh, S.M.L. Sire, A.J. Cole, R.G. Evans and W.T. Toner, Plasma Phys. 22 (1980) 967. [11] P.D. Carter, S.M.L. Sire and T.P. Hughes, Optics Comm. 27 (1978) 423.