Journal of the Korean Physical Society, Vol. 56, No. 1, January 2010, pp. 177∼183
Current Trends in Laser Fusion Driver and Beam Combination Laser Systems Using Stimulated Brillouin Scattering Phase Conjugate Mirrors for a Fusion Driver Hong Jin Kong,∗ Jae Sung Shin, Du Hyun Beak and Sangwoo Park Department of Physics, KAIST, Daejeon 305-701
Jin Woo Yoon Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712 (Received 11 January 2009, in final form 22 June 2009) Laser facilities in the world have been developing flash-lamp-pumped ultrahigh-energy solid-state lasers for fusion research and high-repetition diode-pumped solid-state lasers to act as commercial fusion drivers. A commercial laser fusion driver requires a high-energy beam with a total energy of several megajoules per pulse in several nanoseconds with a ∼10-Hz repetition rate. However, current laser technologies have limitations in raising the beam energy when operating with a high repetition rate, which is necessary for a commercial fusion driver to function properly. The beam combination laser system, which that uses stimulated Brillouin scattering phase conjugate mirrors, is a promising candidate for a fusion driver because it can obtain both a high energy and a high repetition rate with separate amplifications. For the realization of the beam combination laser system, a self-phase control technique was proposed for the coherent beam combined output, and its principle was demonstrated experimentally. PACS numbers: 52.58.N, 42.60, 42.65.H, 42.65.E Keywords: Laser fusion driver, High-energy laser, Beam combination, Phase conjugation, Stimulated Brillouin scattering DOI: 10.3938/jkps.56.177
I. INTRODUCTION Laser Fusion Energy (LFE) is one of the most promising sources of clean energy for mankind. A commercial laser fusion driver requires a high-energy beam with a total energy of several megajoules per pulse in several nanoseconds at 0.5-0.3 µm with a ∼10-Hz repetition rate [1–4]. Although fast ignition generally reduces the required energy, an order of several hundred kilojoules is necessary for the fusion energy to occur [5, 6]. For achievement of fusion energy, many countries have been constructing their own laser facilities. Figure 1 shows the progress of high-energy laser developments in the world [4,7–42]. Flashlamp-pumped solid-state lasers for ultrahigh energy have been developed for laser fusion research. However, such high-energy lasers need a large active media in order to avoid the optical damages that can appear due to the thermal loads. Thus, it can take a long time to cool down the active media of those lasers, which forces the system to operate with only a single shot. For the commercial use of LFE, however, the lasers ∗ E-mail:
Fig. 1. Progress of high-energy laser developments in the world.
should operate with a high repetition rate around 10 Hz. Diode-pumped solid-state lasers (DPSSLs) are good candidates of high-repetition fusion drivers because of their ability to reduce the thermal loads of the laser media [1, 4]. Many researchers have developed new laser materials
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Journal of the Korean Physical Society, Vol. 56, No. 1, January 2010
with high thermal conductivities and fast cooling techniques for high-repetition, high-energy pulses. Nevertheless, DPSSLs do not solve the fundamental thermal problems and have limitations in energy scaling with such a high repetition rate. Kong et al. proposed a new concept for a laser fusion driver and a beam combination laser system using stimulated Brillouin scattering phase conjugate mirrors (SBSPCMs) [43–57]. This beam combined system can simultaneously achieve a high power, a high energy, and a high repetition rate by combining many separately amplified beams without any changes in pump sources, laser materials, and cooling techniques. This separate amplification does not need a large active media, which means that the whole system can operate with a high repetition rate by using previously developed cooling techniques. Therefore, the beam combination laser system is a promising candidate for commercial laser fusion drivers.
II. HIGH-ENERGY LASERS FOR FUSION DRIVERS 1. Flashlamp-pumped Ultrahigh-energy Solidstate Lasers
Flashlamp-pumped ultrahigh-energy solid-sate lasers for fusion research have been used for several decades, as shown in Table 1 [7–27]. In general, the ultrahighenergy facilities have a short-pulse heating laser, as well as a long-pulse implosion laser, for the fast ignition [5, 6]. In the USA, the Lawrence Livermore National Laboratory (LLNL) and the Laboratory for Laser Energetics (LLE) at the University of Rochester are the main laser fusion research facilities. Since the construction of the long path laser in 1970, the LLNL has been a leader in developing high-energy laser systems, such as Janus, Cyclops, Shiva, Argus, and Nova [7]. On the basis of high-energy laser techniques, the National Ignition Facility (NIF) under construction at present is becoming the world’s largest laser facility [7, 8]. The goal of the NIF laser is to obtain 192 beams with a total energy up to 3.6 MJ at 1.05 µm (1ω) and 1.8 MJ at 0.35 µm (3ω). The construction of the NIF will be completed in 2009, and fusion ignition experiments will start in 2010. In the LLE, an upgrade of the OMEGA laser system, which is a Nd:glass laser system for direct-drive fusion experiments, was completed in 1995 [9]. This laser system can deliver 60 beams with a total energy of 37 kJ at 0.35 µm. In addition, adjacent to the OMEGA Laser Facility, the construction of the OMEGA EP Laser Facility was completed in 2008, which contains four high-energy petawatt (HEPW) beamlines with up to 2.6 kJ each in 10-100 ps pulse durations [10, 11]. All four beams will also be capable of operating with a long pulse (0.1-10 ns). In the long-pulse operation, UV energies can reach
2.5 kJ per beam for 1-ns pulse and 6.5 kJ per beam for 10-ns pulse. In Japan, the Institute of Laser Engineering (ILE) at Osaka University is the leading institute for high-energy laser development and fusion research. Researchers at ILE have constructed a 12-beam Nd:glass laser, known as the Gekko XII in 1983, which succeeded the previously developed Gekko II (2 beams, 0.4 TW), Gekko IV (4 beams, 4 TW), and the Gekko MII (2 beams, 7 TW) with the “KONGOH Project” [12]. The output energy of Gekko XII is 500 J per beam at 0.53 µm or 200 J per beam at 0.35 µm. In addition, the Gekko laser has a petawatt picosecond beam line of 500 J (Gekko PW). Recently, a new short-pulse high-power laser, LFEX (Laser for Fast Ignition EXperiment) has been developed for the FIREX (Fast Ignition Realization EXperiment) project [13]. The LFEX laser will generate 4-beams of 10-kJ energies at 1.05 µm in 1-20 picoseconds. Following the FIREX-I project, the FIREX-II project was also proposed at the ILE. In the FIREX-II project, ILE researchers plan to achieve a 50-kJ/3-ns blue laser for implosion and a 50-kJ/10-ps red laser for heating [13]. China has also constructed a high-energy laser facility, named the SG-II facility, at the National Laboratory on High Power Lasers and Physics (NLHPLP) in Shanghai, which is an upgraded version of the SG-I [14, 15]. The SG-II facility has been in operation since 2000 with over 3000 shots and is currently upgraded with a 24-kJ/ 3ns/ 0.35-µm laser beam [15]. This SG-II laser will be paired with a pettawatt laser of 1.5 kJ / 2 ps for fast ignition. Following the SG-II, a SG-III laser facility is currently under construction at the China Academy of Engineering Physics (CAEP) Research Center of Laser Fusion (LFRC) [15,16]. The SG-III laser facility is being designed to provide six bundles of 4 × 2 laser beams (48 beams) with a total energy of 150-200 kJ in 3 ns. This Research Center will start operating in 2012. TIL, the prototype facility of SG-III (a bundle of 4 × 2 laser beams), was complete and began operating in 2005 [17]. The SG-III laser will be upgraded to 400 kJ/ 3 ns/ 0.35 µm in a few years. In addition to the SG-III laser, the SG-IV laser facility with a total energy of 1.5 MJ will be developed and completed in 2020. In Korea, developing high-energy lasers started in 1994 with a Nd:glass laser system named Sinmyung laser at KAIST (Korea Advanced Institute of Science and Technology). This high-energy laser is capable of delivering 80 J / 40 ps / 1.05 µm [18]. However, laser development has been stagnant for more than a decade. Recently though, the Korea Atomic Energy Research Institute (KAERI) has developed a 1-kJ Nd:glass laser facility [19,20] and a KAERI Laser Facility (KLF) that uses laser components of Gekko IV from the ILE in Japan [21]. This KLF laser facility was completed in 2008 and can deliver 4 beam lines of 200 J each. In France, the LULI (Laboratory for Use of Intense Lasers) at the CEA (Commissariat `a l’Energie Atomique) has developed and upgraded its laser facilities [22].
Current Trends in Laser Fusion Driver and Beam Combination Laser Systems · · · – Hong Jin Kong et al.
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Table 1. Currently developed ultrahigh-energy flashlamp-pumped solid-state laser facilities. Laser Facility NIF OMEGA OMEGA EP Gekko XII Gekko PW LFEX SG-II SG-III KLF LULI2000 LULI PW LMJ Vulcan
Output energy / Pulse duration 3.6 MJ @ 1.05 µm 1.8 MJ @ 0.35 µm / 0.1-20 ns 37 kJ @ 0.35 µm / 1-3 ns 2.6 kJ @ 1.05 µm / 1-100 ps 500 J per beam @ 0.53 µm, 200 J per beam @ 0.35 µm / 0.1-10 ns 500 J @ 1.05 µm / 0.5-1 ps 10 kJ @ 1.05 µm / 1-20 ps 24 kJ @ 0.35 µm / 3 ns 200 kJ @ 0.35 µm / 3 ns 1 kJ @ 1.05 µm / 8 ns 2 kJ @ 1.05 µm / ns 200 J @ 1.05 µm / 1 ps 1.8 MJ @ 0.35 µm / 0.2-25 ns 2.5 kJ @ 1.05 µm / 0.1-20 ns
The LULI2000 facility is a Nd:phosphate system, which is composed of 2 amplification chains of kJ/ns. This LULI PW system has also been developed for plasma physics experiments. This PW laser system is capable of delivering 500 J/500 fs. In fact, one of the first steps of the LULI PW of creating a compressed pulse of 200 J/1 ps has already been achieved [22]. The LULI is also upgrading the LULI 100-TW system with a project called ELFIE (Equipement Laser de Fortes Intensit´es et Energie). This project began in the middle of 2008, and the main goals of this project are to enhance the output energy to 100 J and to develop a short-pulse OPCPA (Optical Parametric Chirped Pulse Amplification) beam line of 5-10 J/50 fs [22]. In addition, the Laser M´egajoule (LMJ) Project recently started, with its facility currently under construction at the CESTA (Centre d’Etudes Scientifiques et Techniques d’Aquitaine) laboratory near Bordeaux [23–25]. The LMJ laser system was designed to deliver up to 1.8 MJ at a 0.35 µm with 240 beamlets. The duration of the LMJ laser system’s pulse can be tuned between 0.2 – 25 ns. The Ligne d’Int´egration Laser (LIL), the prototype of LMJ (4 beams), demonstrated its performance in 2002 [25], and the LMJ construction is scheduled for the first experiments in 2012. In the UK, the Central Laser Facility (CLF) of the Rutherford Appleton Laboratory has a Nd:glass laser named Vulcan [26]. The maximum energy of this Vucan laser is 2.5 kJ and is delivered by eight beams. It can be operated with a long-pulse (100 ps – 20 ns) and a short pulse (
