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Compression of high power KrF laser pulse by backward Raman amplification
Fusion Engineering and Design 44 Ž1999. 133]136
Compression of high power KrF laser pulse by backward Raman amplification E. Takahashi a,U , Y. Matsumoto a , I. Matsushimaa , I. Okudaa , Y. Owadano a , K. Kuwaharab a
High-Density Energy Section, Electrotechnical Laboratory AIST MITI, Umezono, 1-1-4 Tsukuba, Ibaraki 305, Japan b Science Uni¨ ersity of Tokyo, Noda, Chiba, Japan
Abstract Backward Raman pulse compression of high-power KrF laser using focused geometry was investigated. This simple configuration has various advantages suitable for creating not only high-power but also high-energy Stokes pulses. An energy conversion efficiency of 27% and power gain of 30 have been achieved. Q 1999 Elsevier Science S.A. All rights reserved. Keywords: Backward Raman amplification; KrF laser; Stokes light
1. Introduction KrF lasers have various advantages suitable for the fast ignitor concept w1x. Firstly, laser pulses of shorter wavelength Ž249 nm. propagate to higher electron density and secondly, the range of suprathermal electrons produced by UV laser light is comparable to the alpha particle range for laser intensities of interest Ž; 10 20 Wrcm2 .. However, the development of the fast ignitor laser driver requires not only the correct laser intensity but also sufficient energy in the drive pulse. Backward stimulated Raman amplification of
U
Corresponding author.
Stokes light by an intense KrF laser is one of the methods which may be used to generate such a high-power and high-energy laser pulse. It has been considered that the backward first Stokes intensity is critically limited by Ž1. the generation of backward second Stokes radiation and Ž2. the growth of self-generated forward first Stokes light which depletes the pump pulse before interaction with the backward Stokes light. Problem Ž2. was thought to be difficult to overcome due to the forward-to-backward gain asymmetry. However, amplification under transient conditions obtained by keeping the duration of first Stokes pulse shorter than the dephasing time of the Raman medium, makes the growth of second Stokes radiation small w2,3x. Furthermore, conical forward-to-backward asymmetric configuration for
0920-3796r99r$ - see front matter Q 1999 Elsevier Science S.A. All rights reserved. P I I: S 0 9 2 0 - 3 7 9 6 Ž 9 8 . 0 0 3 6 4 - 0
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E. Takahashi et al. r Fusion Engineering and Design 44 (1999) 133]136
the amplifier is effective in suppressing self-generated forward scattering. We adopt a simple focused geometry for our backward Raman generator-amplifier. Previously, focused configurations have been used as Stokes generators by the injection of weak pump laser light of less than 1 J with a large F number Žmore than 100 typically.. In this paper, we report the first application of this configuration to the injection of 11 J of KrF laser light from the ASHURA system w4x with small F number Ž; 30.. The purpose of the experiment was the verification of single Stokes pulse generation and self-steeping effect under high-density energy concentration in a Raman gas. The backward Stokes pulse grows from Raman scattered light generated in the focal region. The local high gain region around the focus produces self-shortening of the Stokes pulse. The pulse can be as short as the dephasing time of the Raman medium. The diverging Stokes cross-sectional areas in the amplifier also reduce the growth of second Stokes radiation. Thus, the property of this Stokes pulse is different from the output from conventional Stokes generators.
Fig. 1. Experimental set-up. A, KrF laser oscillator; B, KrF discharge amplifier; C, ASHURA KrF electron beam amplifier; D, streak camera; E, Raman cell; F, entrance window; G, exiting window; L1, focusing lens. Fast phototubes, a, incident KrF pulse; b, backward Stokes pulse; c, depleted KrF pulse, and d, forward Stokes pulse.
to obtain the waveform with precision. The timeevolution of incident KrF pump pulse, forward Stokes light and depleted KrF pulse were also monitored by fast-phototube ŽFig. 1.. An appropriate band-pass filter was placed in front of each fast-phototube. The pulse energy was measured by a pyro-electric energy meter ŽGentec ED-500. which was calibrated by a volume absorbing calorimeter.
2. Experimental set-up 3. Results and discussion The experimental set-up is shown in Fig. 1. The KrF pump laser pulse was generated by a discharge-type injection-locked narrow-band oscillator ŽLambda Physik COMPEX150T.. Pulse duration ŽFWHM., bandwidth and beam divergence of the oscillator output are 25 ns, 0.45 cmy1 and 0.3 mrad, respectively. The KrF laser pulse was amplified by a discharge KrF amplifier and successive electron-beam-pumped KrF laser amplifiers ŽASHURA.. The pulse had an energy of typically 11 J in front of the entrance window of the Raman gas cell Ž3 m long. and was focused into the cell by a convex lens. Several atm of methane gas were used as the Raman medium due to its short-dephasing time and high-gain. The beam-diameter at the entrance window was 7 cm. The focal length was fixed at 2.2 m to avoid damage on the exit window. The backward Stokes light was simultaneously measured by a fast-phototube and streak camera
The temporal evolution of the incident KrF pump, backward Stokes, forward Stokes and depleted pump are shown in Fig. 2. The temporal axis is adjusted to show simultaneous waveforms at the focus by calculating the delay time derived from the optical path length from the focus to each measuring point. A single high-intensity backward Stokes pulse was observed. No other post-pulse with more than 10y3 intensity of the first pulse was found. The generation of the backward Stokes pulse was located at the rising edge of the KrF pump pulse. This assured us that the backward Stokes light was generated by the incidence of the KrF pump pulse on the focus. The focal length of 2.2 m corresponds to an interaction time of 14.7 ns. The depleted KrF pulse stays low during this interaction period but increases at later times as can be seen in Fig. 2. The following energy measurement was per-
E. Takahashi et al. r Fusion Engineering and Design 44 (1999) 133]136
135
Fig. 2. Waveforms: backward Stokes, forward Stokes, input KrF, and depleted KrF pulse. Fig. 3. Streak image of backward Stokes pulse.
formed at a fixed Raman gas pressure of 2.5 atm. The depleted KrF laser energy was 3.6 J and the rest of the forward Stokes light including highercomponents was 5.4 J. The typical energy of the backward Stokes light was 2 J. Therefore, input and output energies were balanced within the measurement errors Ž; 10%.. Gas breakdown was expected to cause a bad influence, for example by absorbing light. The breakdown threshold power density for methane gas at 2.5 atm is 1.2= 10 10 Wrcm2 according to Ref. w5x. Our experimental intensity of 6.4= 10 10 Wrcm2 was much larger than the gas breakdown threshold. However, the result of the energy measurement showed the effect was negligible from an energy point of view. The focal length was not long enough for the Stokes pulse to interact with all of the pump pulse. Consequently, an energy of only 7.4 J was used to amplify the backward Stokes pulse giving an energy conversion efficiency of 27%. A streak image of the backward Stokes pulse is shown in Fig. 3. The pulse width was 150 ps. The power gain was estimated to be 30. The dependence of peak backward Stokes intensity and depleted KrF pulse Žaround 20 ns from rising-edge. intensity on methane gas pressure is shown in Fig. 4. The depleted KrF pulse decreased almost exponentially with increasing methane gas pressure. The backward Stokes has a peak at around 2 atm of methane. Despite increasing Raman gain with increasing methane gas
pressure, the intensity of the forward and backward Stokes light is low above 3 atm. The breakdown of the Raman gas medium or the generation of higher-component forward Raman scattering is considered to be the cause of this suppression of first Stokes light. Backward second Stokes was observed and its intensity was 10% of the first Stokes intensity. This is not sufficient intensity to reduce the efficiency of first Stokes light as described above. We conducted a simple numerical calculation of the wave equations with a configuration as shown in Fig. 5a. Calculation parameters are adjusted for the experimental conditions noted on the figure caption. The seed Stokes light for backward Raman scattering is introduced as a very
Fig. 4. Pressure dependence of backward Stokes and depleted KrF laser.
136
E. Takahashi et al. r Fusion Engineering and Design 44 (1999) 133]136
weak Stokes signal Ž10y7 times to pump intensity . from the focus in the backward direction. Forward seed Stokes light was also introduced in the procedure starting from the entrance window. A backward Stokes pulse having an intensity of 1 = 10 11 W and a pulse duration of 250 ps was obtained numerically as shown in Fig. 5b. The experimental result agreed well with the calculated intensity of first and second backward Stokes pulses. The result of the code shows the growth of forward Stokes was localized near the focus. This is caused by the weak interaction between the pump and backward Stokes which leaves undepleted pump energy available to amplify forward Stokes. The energy of these forward components
can be transferred to backward first Stokes by adjusting the beam diameter at the entrance. In this case, backward second Stokes will be the main energy drain for the first Stokes pulse. Insertion of a dichroic mirror into the amplifier would suppress backward second Stokes generation. The Super-ASHURA main amplifier has recently been demonstrated. Pump energies of several hundred Joules will be available in the near future. Stokes pulse Tera Watt with power and several tens of Joules-energy will be possible by the scale up of the concept reported here. 4. Conclusion A focused geometry for a backward Raman generator-amplifier was investigated. This configuration has the advantages of avoiding complexity, realizing self-shortening of Stokes pulse, and reducing the growth of second Stokes radiation. An amplified single, high-intensity backward pulse was obtained. A power gain of 30 was achieved and will be improved by optimizing the focusing configuration. References
Fig. 5. Ža. Calculation configuration, A, pump; B, seed forward Stokes; C, seed backward first Stokes; D, seed backward second Stokes. Žb. Numerical result. Assuming laser raise time 2 ns; incident pump laser power 7 = 10 8 W; beam diameter at entrance window 7 cm; focus radius 1.4 mm; Raman gain 1.9= 10y1 2 mrW; and focal length 2.2 m.
w1x M. Tabak, J. Hammer, M.E. Glinsky, W.L. Kruer, S.C. Wilks, R.J. Mason, Phys. Plasmas 1 Ž1994. 1626]1634. w2x H. Nishioka, K. Kimura, K. Ueda, H. Takuma, IEEE J. Quantum Electron. 29 Ž1993. 2251]2258. w3x J.R. Murray, J. Goldhar, D. Eimerl, A. Szoke, IEEE J. Quantum Electron. 15 Ž1979. 342]368. w4x Y. Owadano, I. Okuda, Y. Matsumoto, et al., Laser Part. Beams 11 Ž1993. 347]351. w5x M.C. Gower, Opt. Commun. 36 Ž1981. 43]45.
Compression of high power KrF laser pulse by backward Raman amplification E. Takahashi a,U , Y. Matsumoto a , I. Matsushimaa , I. Okudaa , Y. Owadano a , K. Kuwaharab a
High-Density Energy Section, Electrotechnical Laboratory AIST MITI, Umezono, 1-1-4 Tsukuba, Ibaraki 305, Japan b Science Uni¨ ersity of Tokyo, Noda, Chiba, Japan
Abstract Backward Raman pulse compression of high-power KrF laser using focused geometry was investigated. This simple configuration has various advantages suitable for creating not only high-power but also high-energy Stokes pulses. An energy conversion efficiency of 27% and power gain of 30 have been achieved. Q 1999 Elsevier Science S.A. All rights reserved. Keywords: Backward Raman amplification; KrF laser; Stokes light
1. Introduction KrF lasers have various advantages suitable for the fast ignitor concept w1x. Firstly, laser pulses of shorter wavelength Ž249 nm. propagate to higher electron density and secondly, the range of suprathermal electrons produced by UV laser light is comparable to the alpha particle range for laser intensities of interest Ž; 10 20 Wrcm2 .. However, the development of the fast ignitor laser driver requires not only the correct laser intensity but also sufficient energy in the drive pulse. Backward stimulated Raman amplification of
U
Corresponding author.
Stokes light by an intense KrF laser is one of the methods which may be used to generate such a high-power and high-energy laser pulse. It has been considered that the backward first Stokes intensity is critically limited by Ž1. the generation of backward second Stokes radiation and Ž2. the growth of self-generated forward first Stokes light which depletes the pump pulse before interaction with the backward Stokes light. Problem Ž2. was thought to be difficult to overcome due to the forward-to-backward gain asymmetry. However, amplification under transient conditions obtained by keeping the duration of first Stokes pulse shorter than the dephasing time of the Raman medium, makes the growth of second Stokes radiation small w2,3x. Furthermore, conical forward-to-backward asymmetric configuration for
0920-3796r99r$ - see front matter Q 1999 Elsevier Science S.A. All rights reserved. P I I: S 0 9 2 0 - 3 7 9 6 Ž 9 8 . 0 0 3 6 4 - 0
134
E. Takahashi et al. r Fusion Engineering and Design 44 (1999) 133]136
the amplifier is effective in suppressing self-generated forward scattering. We adopt a simple focused geometry for our backward Raman generator-amplifier. Previously, focused configurations have been used as Stokes generators by the injection of weak pump laser light of less than 1 J with a large F number Žmore than 100 typically.. In this paper, we report the first application of this configuration to the injection of 11 J of KrF laser light from the ASHURA system w4x with small F number Ž; 30.. The purpose of the experiment was the verification of single Stokes pulse generation and self-steeping effect under high-density energy concentration in a Raman gas. The backward Stokes pulse grows from Raman scattered light generated in the focal region. The local high gain region around the focus produces self-shortening of the Stokes pulse. The pulse can be as short as the dephasing time of the Raman medium. The diverging Stokes cross-sectional areas in the amplifier also reduce the growth of second Stokes radiation. Thus, the property of this Stokes pulse is different from the output from conventional Stokes generators.
Fig. 1. Experimental set-up. A, KrF laser oscillator; B, KrF discharge amplifier; C, ASHURA KrF electron beam amplifier; D, streak camera; E, Raman cell; F, entrance window; G, exiting window; L1, focusing lens. Fast phototubes, a, incident KrF pulse; b, backward Stokes pulse; c, depleted KrF pulse, and d, forward Stokes pulse.
to obtain the waveform with precision. The timeevolution of incident KrF pump pulse, forward Stokes light and depleted KrF pulse were also monitored by fast-phototube ŽFig. 1.. An appropriate band-pass filter was placed in front of each fast-phototube. The pulse energy was measured by a pyro-electric energy meter ŽGentec ED-500. which was calibrated by a volume absorbing calorimeter.
2. Experimental set-up 3. Results and discussion The experimental set-up is shown in Fig. 1. The KrF pump laser pulse was generated by a discharge-type injection-locked narrow-band oscillator ŽLambda Physik COMPEX150T.. Pulse duration ŽFWHM., bandwidth and beam divergence of the oscillator output are 25 ns, 0.45 cmy1 and 0.3 mrad, respectively. The KrF laser pulse was amplified by a discharge KrF amplifier and successive electron-beam-pumped KrF laser amplifiers ŽASHURA.. The pulse had an energy of typically 11 J in front of the entrance window of the Raman gas cell Ž3 m long. and was focused into the cell by a convex lens. Several atm of methane gas were used as the Raman medium due to its short-dephasing time and high-gain. The beam-diameter at the entrance window was 7 cm. The focal length was fixed at 2.2 m to avoid damage on the exit window. The backward Stokes light was simultaneously measured by a fast-phototube and streak camera
The temporal evolution of the incident KrF pump, backward Stokes, forward Stokes and depleted pump are shown in Fig. 2. The temporal axis is adjusted to show simultaneous waveforms at the focus by calculating the delay time derived from the optical path length from the focus to each measuring point. A single high-intensity backward Stokes pulse was observed. No other post-pulse with more than 10y3 intensity of the first pulse was found. The generation of the backward Stokes pulse was located at the rising edge of the KrF pump pulse. This assured us that the backward Stokes light was generated by the incidence of the KrF pump pulse on the focus. The focal length of 2.2 m corresponds to an interaction time of 14.7 ns. The depleted KrF pulse stays low during this interaction period but increases at later times as can be seen in Fig. 2. The following energy measurement was per-
E. Takahashi et al. r Fusion Engineering and Design 44 (1999) 133]136
135
Fig. 2. Waveforms: backward Stokes, forward Stokes, input KrF, and depleted KrF pulse. Fig. 3. Streak image of backward Stokes pulse.
formed at a fixed Raman gas pressure of 2.5 atm. The depleted KrF laser energy was 3.6 J and the rest of the forward Stokes light including highercomponents was 5.4 J. The typical energy of the backward Stokes light was 2 J. Therefore, input and output energies were balanced within the measurement errors Ž; 10%.. Gas breakdown was expected to cause a bad influence, for example by absorbing light. The breakdown threshold power density for methane gas at 2.5 atm is 1.2= 10 10 Wrcm2 according to Ref. w5x. Our experimental intensity of 6.4= 10 10 Wrcm2 was much larger than the gas breakdown threshold. However, the result of the energy measurement showed the effect was negligible from an energy point of view. The focal length was not long enough for the Stokes pulse to interact with all of the pump pulse. Consequently, an energy of only 7.4 J was used to amplify the backward Stokes pulse giving an energy conversion efficiency of 27%. A streak image of the backward Stokes pulse is shown in Fig. 3. The pulse width was 150 ps. The power gain was estimated to be 30. The dependence of peak backward Stokes intensity and depleted KrF pulse Žaround 20 ns from rising-edge. intensity on methane gas pressure is shown in Fig. 4. The depleted KrF pulse decreased almost exponentially with increasing methane gas pressure. The backward Stokes has a peak at around 2 atm of methane. Despite increasing Raman gain with increasing methane gas
pressure, the intensity of the forward and backward Stokes light is low above 3 atm. The breakdown of the Raman gas medium or the generation of higher-component forward Raman scattering is considered to be the cause of this suppression of first Stokes light. Backward second Stokes was observed and its intensity was 10% of the first Stokes intensity. This is not sufficient intensity to reduce the efficiency of first Stokes light as described above. We conducted a simple numerical calculation of the wave equations with a configuration as shown in Fig. 5a. Calculation parameters are adjusted for the experimental conditions noted on the figure caption. The seed Stokes light for backward Raman scattering is introduced as a very
Fig. 4. Pressure dependence of backward Stokes and depleted KrF laser.
136
E. Takahashi et al. r Fusion Engineering and Design 44 (1999) 133]136
weak Stokes signal Ž10y7 times to pump intensity . from the focus in the backward direction. Forward seed Stokes light was also introduced in the procedure starting from the entrance window. A backward Stokes pulse having an intensity of 1 = 10 11 W and a pulse duration of 250 ps was obtained numerically as shown in Fig. 5b. The experimental result agreed well with the calculated intensity of first and second backward Stokes pulses. The result of the code shows the growth of forward Stokes was localized near the focus. This is caused by the weak interaction between the pump and backward Stokes which leaves undepleted pump energy available to amplify forward Stokes. The energy of these forward components
can be transferred to backward first Stokes by adjusting the beam diameter at the entrance. In this case, backward second Stokes will be the main energy drain for the first Stokes pulse. Insertion of a dichroic mirror into the amplifier would suppress backward second Stokes generation. The Super-ASHURA main amplifier has recently been demonstrated. Pump energies of several hundred Joules will be available in the near future. Stokes pulse Tera Watt with power and several tens of Joules-energy will be possible by the scale up of the concept reported here. 4. Conclusion A focused geometry for a backward Raman generator-amplifier was investigated. This configuration has the advantages of avoiding complexity, realizing self-shortening of Stokes pulse, and reducing the growth of second Stokes radiation. An amplified single, high-intensity backward pulse was obtained. A power gain of 30 was achieved and will be improved by optimizing the focusing configuration. References
Fig. 5. Ža. Calculation configuration, A, pump; B, seed forward Stokes; C, seed backward first Stokes; D, seed backward second Stokes. Žb. Numerical result. Assuming laser raise time 2 ns; incident pump laser power 7 = 10 8 W; beam diameter at entrance window 7 cm; focus radius 1.4 mm; Raman gain 1.9= 10y1 2 mrW; and focal length 2.2 m.
w1x M. Tabak, J. Hammer, M.E. Glinsky, W.L. Kruer, S.C. Wilks, R.J. Mason, Phys. Plasmas 1 Ž1994. 1626]1634. w2x H. Nishioka, K. Kimura, K. Ueda, H. Takuma, IEEE J. Quantum Electron. 29 Ž1993. 2251]2258. w3x J.R. Murray, J. Goldhar, D. Eimerl, A. Szoke, IEEE J. Quantum Electron. 15 Ž1979. 342]368. w4x Y. Owadano, I. Okuda, Y. Matsumoto, et al., Laser Part. Beams 11 Ž1993. 347]351. w5x M.C. Gower, Opt. Commun. 36 Ž1981. 43]45.