Journal of the Korean Physical Society, Vol. 56, No. 1, January 2010, pp. 177∼183

Current Trends in 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 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 Bril- louin scattering DOI: 10.3938/jkps.56.177

I. INTRODUCTION

Laser Fusion Energy (LFE) is one of the most promis- ing sources of clean energy for mankind. A commer- cial laser fusion driver requires a high-energy beam with a total energy of several megajoules per pulse in sev- eral 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 ultra- high energy have been developed for laser fusion research. Fig. 1. Progress of high-energy laser developments in the However, such high-energy lasers need a large active me- world. dia 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 should operate with a high repetition rate around 10 Hz. the system to operate with only a single shot. Diode-pumped solid-state lasers (DPSSLs) are good can- For the commercial use of LFE, however, the lasers didates of high-repetition fusion drivers because of their ability to reduce the thermal loads of the laser media [1, ∗E-mail: [email protected] 4]. Many researchers have developed new laser materials -177- -178- Journal of the Korean Physical Society, Vol. 56, No. 1, January 2010 with high thermal conductivities and fast cooling tech- 2.5 kJ per beam for 1-ns pulse and 6.5 kJ per beam for niques for high-repetition, high-energy pulses. Neverthe- 10-ns pulse. less, DPSSLs do not solve the fundamental thermal prob- In Japan, the Institute of Laser Engineering (ILE) at lems and have limitations in energy scaling with such a Osaka University is the leading institute for high-energy high repetition rate. laser development and fusion research. Researchers at Kong et al. proposed a new concept for a laser fusion ILE have constructed a 12-beam Nd:glass laser, known driver and a beam combination laser system using stimu- as the Gekko XII in 1983, which succeeded the previ- lated Brillouin scattering phase conjugate mirrors (SBS- ously developed Gekko II (2 beams, 0.4 TW), Gekko IV PCMs) [43–57]. This beam combined system can simul- (4 beams, 4 TW), and the Gekko MII (2 beams, 7 TW) taneously achieve a high power, a high energy, and a high with the “KONGOH Project” [12]. The output energy repetition rate by combining many separately amplified of Gekko XII is 500 J per beam at 0.53 µm or 200 J beams without any changes in pump sources, laser mate- per beam at 0.35 µm. In addition, the Gekko laser has a rials, and cooling techniques. This separate amplification petawatt picosecond beam line of 500 J (Gekko PW). Re- does not need a large active media, which means that the cently, a new short-pulse high-power laser, LFEX (Laser whole system can operate with a high repetition rate by for Fast Ignition EXperiment) has been developed for the using previously developed cooling techniques. There- FIREX (Fast Ignition Realization EXperiment) project fore, the beam combination laser system is a promising [13]. The LFEX laser will generate 4-beams of 10-kJ candidate for commercial laser fusion drivers. energies at 1.05 µm in 1-20 picoseconds. Following the FIREX-I project, the FIREX-II project was also pro- posed at the ILE. In the FIREX-II project, ILE re- searchers plan to achieve a 50-kJ/3-ns laser for im- II. HIGH-ENERGY LASERS FOR FUSION plosion and a 50-kJ/10-ps red laser for heating [13]. DRIVERS China has also constructed a high-energy laser facility, named the SG-II facility, at the National Laboratory on 1. Flashlamp-pumped Ultrahigh-energy Solid- High Power Lasers and Physics (NLHPLP) in Shanghai, state Lasers which is an upgraded version of the SG-I [14,15]. The SG-II facility has been in operation since 2000 with over Flashlamp-pumped ultrahigh-energy solid-sate lasers 3000 shots and is currently upgraded with a 24-kJ/ 3- for fusion research have been used for several decades, ns/ 0.35-µm laser beam [15]. This SG-II laser will be as shown in Table 1 [7–27]. In general, the ultrahigh- paired with a pettawatt laser of 1.5 kJ / 2 ps for fast energy facilities have a short-pulse heating laser, as well ignition. Following the SG-II, a SG-III laser facility is as a long-pulse implosion laser, for the fast ignition [5, currently under construction at the China Academy of 6]. Engineering Physics (CAEP) Research Center of Laser In the USA, the Lawrence Livermore National Labo- Fusion (LFRC) [15,16]. The SG-III laser facility is being ratory (LLNL) and the Laboratory for Laser Energetics designed to provide six bundles of 4 × 2 laser beams (LLE) at the University of Rochester are the main laser (48 beams) with a total energy of 150-200 kJ in 3 ns. fusion research facilities. Since the construction of the This Research Center will start operating in 2012. TIL, in 1970, the LLNL has been a leader the prototype facility of SG-III (a bundle of 4 × 2 laser in developing high-energy laser systems, such as Janus, beams), was complete and began operating in 2005 [17]. Cyclops, Shiva, Argus, and [7]. On the basis of The SG-III laser will be upgraded to 400 kJ/ 3 ns/ 0.35 high-energy laser techniques, the National Ignition Fa- µm in a few years. In addition to the SG-III laser, the cility (NIF) under construction at present is becoming SG-IV laser facility with a total energy of 1.5 MJ will be the world’s largest laser facility [7,8]. The goal of the developed and completed in 2020. NIF laser is to obtain 192 beams with a total energy In Korea, developing high-energy lasers started in 1994 up to 3.6 MJ at 1.05 µm (1ω) and 1.8 MJ at 0.35 µm with a Nd:glass laser system named Sinmyung laser at (3ω). The construction of the NIF will be completed in KAIST (Korea Advanced Institute of Science and Tech- 2009, and fusion ignition experiments will start in 2010. nology). This high-energy laser is capable of delivering In the LLE, an upgrade of the OMEGA laser system, 80 J / 40 ps / 1.05 µm [18]. However, laser develop- which is a Nd:glass laser system for direct-drive fusion ment has been stagnant for more than a decade. Re- experiments, was completed in 1995 [9]. This laser sys- cently though, the Korea Atomic Energy Research Insti- tem can deliver 60 beams with a total energy of 37 kJ at tute (KAERI) has developed a 1-kJ Nd:glass laser facil- 0.35 µm. In addition, adjacent to the OMEGA Laser Fa- ity [19,20] and a KAERI Laser Facility (KLF) that uses cility, the construction of the OMEGA EP Laser Facility laser components of Gekko IV from the ILE in Japan was completed in 2008, which contains four high-energy [21]. This KLF laser facility was completed in 2008 and petawatt (HEPW) beamlines with up to 2.6 kJ each in can deliver 4 beam lines of 200 J each. 10-100 ps pulse durations [10, 11]. All four beams will In France, the LULI (Laboratory for Use of Intense also be capable of operating with a long pulse (0.1-10 Lasers) at the CEA (Commissariat `al’Energie Atom- ns). In the long-pulse operation, UV energies can reach ique) has developed and upgraded its laser facilities [22]. Current Trends in Laser Fusion Driver and Beam Combination Laser Systems ··· – Hong Jin Kong et al. -179-

Table 1. Currently developed ultrahigh-energy flashlamp-pumped solid-state laser facilities.

Laser Facility Output energy / Pulse duration No. of beams Location 3.6 MJ @ 1.05 µm NIF 1.8 MJ @ 0.35 µm 192 LLNL, USA / 0.1-20 ns OMEGA 37 kJ @ 0.35 µm / 1-3 ns 60 LLE, USA OMEGA EP 2.6 kJ @ 1.05 µm / 1-100 ps 4 LLE, USA 500 J per beam @ 0.53 µm, Gekko XII 200 J per beam @ 0.35 µm 12 ILE, Japan / 0.1-10 ns Gekko PW 500 J @ 1.05 µm / 0.5-1 ps 1 ILE, Japan LFEX 10 kJ @ 1.05 µm / 1-20 ps 4 ILE, Japan SG-II 24 kJ @ 0.35 µm / 3 ns 8 NLHPLP, China SG-III 200 kJ @ 0.35 µm / 3 ns 48 LFRC, China KLF 1 kJ @ 1.05 µm / 8 ns 4 KAERI, Korea LULI2000 2 kJ @ 1.05 µm / ns 1 CEA, France LULI PW 200 J @ 1.05 µm / 1 ps 1 CEA, France LMJ 1.8 MJ @ 0.35 µm / 0.2-25 ns 240 CESTA, France Vulcan 2.5 kJ @ 1.05 µm / 0.1-20 ns 8 CLF, UK

The LULI2000 facility is a Nd:phosphate system, which nition research. The goal of the HiPER project is to is composed of 2 amplification chains of kJ/ns. This achieve a 200-kJ long-pulse laser combined with a 70 kJ LULI PW system has also been developed for short pulse laser [27]. At present, many countries in Eu- physics experiments. This PW laser system is capable of rope, such as the UK, France, Italy, and Germany, par- delivering 500 J/500 fs. In fact, one of the first steps of ticipate while other countries, such as the USA, Japan, the LULI PW of creating a compressed pulse of 200 J/1 Korea, Canada and China are internally organized. ps has already been achieved [22]. The LULI is also up- grading the LULI 100-TW system with a project called ELFIE (Equipement Laser de Fortes Intensit´eset En- 2. High-repetition Diode-pumped Solid-state ergie). This project began in the middle of 2008, and Lasers 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 Diode-pumped solid-state lasers (DPSSLs) have been line of 5-10 J/50 fs [22]. In addition, the Laser M´egajoule used as commercial fusion drivers because they can (LMJ) Project recently started, with its facility currently achieve high-energy laser pulses with high repetition rate under construction at the CESTA (Centre d’Etudes Sci- [3,4]. For this reason, many researchers in the world have entifiques et Techniques d’Aquitaine) laboratory near developed DPSSLs, as shown in Table 2 [28–42]. Bordeaux [23–25]. The LMJ laser system was designed The Lawrence Livermore National Laboratory (LLNL) to deliver up to 1.8 MJ at a 0.35 µm with 240 beam- in USA first started the MERCURY project for the 100- lets. The duration of the LMJ laser system’s pulse can J level DPSSL using YS:S-FAP slabs [28–31]. The per- be tuned between 0.2 – 25 ns. The Ligne d’Int´egration formance goal of the is to obtain an Laser (LIL), the prototype of LMJ (4 beams), demon- output energy of 100 J at 1.047 µm in 2-10 ns with a strated its performance in 2002 [25], and the LMJ con- 10-Hz repetition rate. At present, the MECURY laser struction is scheduled for the first experiments in 2012. achieves over 60 J of pulse energy at 10-Hz for sev- In the UK, the Central Laser Facility (CLF) of the eral hours [31]. Frequency conversion with the yttrium Rutherford Appleton Laboratory has a Nd:glass laser calcium oxyborates (YCOB) also demonstrated an effi- named Vulcan [26]. The maximum energy of this Vucan ciency of 52% (31.7 J) [31]. Both Japan and the USA laser is 2.5 kJ and is delivered by eight beams. It can be have developed a high-energy DPSSL. The ILE (Institute operated with a long-pulse (100 ps – 20 ns) and a short of Laser Engineering) in Japan proposed a fusion reac- pulse (<500 fs). The short pulse can be achieved using tor, “KOYO,” using a DPSSL, and the conceptual de- the chirped pulse amplification (CPA) and is available in sign was completed for a 4-MJ level Nd:Silica-Phosphate Petawatt and 100-TW beams. Glass DPSSL driver [32–37]. With this DPSSL driver, Recently, the European Union started its High Power the HALNA (High Average-power Laser for Nuclear- laser for Energy Research (HiPER) Project for fast ig- fusion Application) project is in progress at the ILE. At present, the HALNA 100 has been developed for a 100-J -180- Journal of the Korean Physical Society, Vol. 56, No. 1, January 2010

Table 2. High-repetition diode-pumped solid-state laser projects.

Laser Project Current Status Goal Location MERCURY 70 J @ 10Hz 100 J @ 10 Hz LLNL, USA HALNA 33.9 J @ 10 Hz 100 J @ 10 Hz ILE, Japan GENBU Being designed 1 kJ @ 100 Hz ILE, Japan LUCIA 4 J @ 2 Hz 100 J @ 10 Hz LULI, France 150 J @ 0.1 Hz/ 2.5 ns IOQ, Germany POLARIS 1.25 J @ 0.2 Hz 1 PW @ 0.1 Hz/ 150 fs level laser and has achieved 33.9 J at 10 Hz [36], which is a higher rate than the HALNA 10, demonstrated an 8.4-J pulse energy at 10 Hz in 2004 [33–35]. The final goal of the HALNA project is to combine 400 modules with total pulse energy of 10 kJ at a 351-nm and a 12-Hz repetition rate. In addition to the HALNA project, the “GENBU (Generation of ENergetic Beam Ultimate)” laser was conceptually designed at the ILE for a kilojoule-level output energy with a 100-Hz repeti- tion rate [37]. For the GENBU laser, Yb:YAG ceramics will be used as laser materials, and the OPCPA will be adopted for short pulse generation. The LULI (Laboratory of Use for Intense Lasers) in France also started the LUCIA (Laser Ultra-Courts et Intense et Applications) project, which developed a Yb:YAG DPSSL chain of 100 J at 1.03 µm with a 10 Hz repetition rate in the nanosecond region [38–40]. When the frequency is converted, the output energy will be used as a pump source for a pettawatt solid state laser that will use the OPCPA. Thus, high-repetition petawatt pulses of a few tens of femtosecond pulse durations can be obtained. In Germany, the POLARIS (Petawatt Optical Laser Amplifier for Radiation Intensive Experiments) project is in progress at the Institute for Optics and Quantum- electronics (IOQ) [41, 42]. The goal of the POLARIS laser system is to obtain a 1-PW laser pulse at 0.1 Hz in 150 femtoseconds by using the OPCPA from a 150-J DPSSL beam with a 2.5-ns pulse duration.

III. BEAM COMBINATION LASER SYSTEM Fig. 2. Conceptual schemes of the beam combination laser USING SBS-PCMS FOR A FUSION fusion drivers using stimulated Brillouin scattering phase con- DRIVER jugate mirrors: (a) wavefront dividing scheme, and (b) ampli- tude dividing scheme (AMP: optical amplifier; QWP: quarter wave plate; FR: Faraday rotator; SBS-PCM: stimulated Bril- A beam combination laser that uses stimulated Bril- louin scattering phase conjugate mirror). louin scattering phase conjugate mirrors (SBS-PCMs) is a promising candidate for fusion drivers. The conceptual schemes of beam combination laser fusion drivers, pro- beam splitters in the amplitude-dividing scheme. Both posed by Kong et al., are shown in Fig. 2 [43–54]. Figure schemes include a series of cross-type amplifier stages. 2(a) is the wavefront-dividing scheme, and Figure 2(b) Each cross-type amplifier has SBS-PCMs on both sides is the amplitude dividing scheme. In beam combination and is insensitive to the misalignments of the optical laser systems, the main beam is divided into many sub- components because the reflected phase conjugate waves beams for separate amplification, either by using prisms return on exactly the same path as the incident beam in the wavefront-dividing scheme or by using polarizing [55]. Therefore, this cross-type beam combined system Current Trends in Laser Fusion Driver and Beam Combination Laser Systems ··· – Hong Jin Kong et al. -181- is very profitable for alignment, maintenance, and repair when it is applied to a fusion driver. The SBS-PCMs on the right side of each cross-type amplifier stage per- form as optical isolators. With the SBS isolators, the leaked beams that occurred on the left side amplifiers due to birefringence cannot go back to the laser oscilla- tor because their energies do not exceed the threshold values for SBS generation [55, 56]. On the left side of each cross-type amplifier stage, the array amplifier can increase the beam’s energy with double pass optical am- plification when it is divided by some sub-beams. The SBS-PCMs in this array amplifier are used as reflectors, instead of conventional mirrors. These SBS-PCMs can compensate for the thermally induced wavefront distor- tions while self focusing can occur in the active media with the generation of phase conjugate beams. There- fore, a diffraction-limited high-quality beam can be ob- tained at the output. The divided sub-beams are recom- bined again after double-pass amplification and become the input beam of the next amplifier stage. Using many amplifier stages of beam combination, the high-energy laser output needed for the fusion can be obtained. For a coherent beam combination, however, phase con- Fig. 3. Two types of self-phase-controlled SBS mirrors: trol of the SBS waves is required because the SBS wave is (a) concentric type, and (b) confocal type. generated by thermal noise and has a random phase [57]. In order to solve this problem, Kong et al. proposed a new phase control technique, “self phase control,” which ture applications by combining separately amplified laser can control the phase of the SBS wave by just locat- beams of ∼100 J. ing a concave mirror behind the conventional SBS mir- ror [30,31]. This concave mirror reflects the front part of the incident pulse, and a standing wave is generated by the front part of the pulse and the ensuing incident IV. CONCLUSION pulse. The standing wave produces a weak density mod- ulation by electrostriction, and the acoustic wave starts This paper presents the current status of high-energy at one nodal point of this standing density modulation laser facilities in the world, which have developed flash- so that the phase of the acoustic wave is locked. Fi- lamp-pumped ultrahigh-energy solid-state lasers for fu- nally, the phase of the SBS wave from this acoustic wave sion research and high-repetition diode-pumped solid- is no longer random and is locked. By using this self- state lasers to act as commercial fusion drivers. However, phase-control method, the phase of the SBS wave can they have limitations in raising the beam energy and be- be independently controlled and locked by adjusting the ing able to operate at the high repetition rate needed for position of the concave mirror. Consequently, a high- a fusion driver. energy beam can be obtained by increasing the number A beam combination laser system that uses SBS- of combining beams. PCMs for a fusion driver has been proposed by Kong Figure 3 shows two different types of self-phase- et al.. This system has many advantages. It can operate controlled SBS mirrors. Figure 3(a) is a concentric type, with a high repetition rate around 10 Hz without any and Figure 3(b) is a confocal type. In a concentric type limitations of energy scaling by using separate amplifi- SBS mirror, the reflected beam at the uncoated concave cations. A high-quality beam can also be obtained by mirror generates a standing wave while the lens focuses using phase conjugate mirrors. Furthermore, the whole on the ensuing incident beam. In a confocal-type SBS system, which is composed of cross-type amplifier stages, mirror, a high-reflectivity coated concave mirror plays is easy to align, maintain, and repair. The main prob- the roles of both the lens and the uncoated concave mir- lem of the beam combination system is the random phase ror in a concentric type. In the beam combination ex- characteristic of the SBS wave. However, it is possible periments, both types of self-phase controlled mirrors ef- to control the phase of the SBS wave by using a self- fectively stabilize the phase difference between the SBS phase control technique proposed by Kong et al. The waves [45–54]. Phase stabilization of the four-beam com- principle of this phase control technique was experimen- bined laser system was also demonstrated recently [58]. tally demonstrated. Consequently, the proposed beam This beam combined system is expected to obtain a much combination laser system is a promising candidate for higher energy with a high repetition rate of 10 Hz in fu- commercial laser fusion drivers. -182- Journal of the Korean Physical Society, Vol. 56, No. 1, January 2010

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