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The French Megajoule Laser Project (LMJ)
Fusion Engineering and Design 44 Ž1999. 43]49
The French Megajoule Laser Project Ž LMJ. Michel L. Andre ´U Direction des Recherches en Ile de France, Commissariat a ` l’Energie Atomique, BP 12, 91680 Bruyeres-le-chatel, France
Abstract This paper describes the goal and the design of the Megajoule Laser to be built in France at the beginning of the next century. Several technical developments are related to this new design and justify the use of new technologies. Most of them are the continuation of a developmental effort by the CEA for several years. Some have come from developments needed for other applications, such as telecommunications. Q 1999 Elsevier Science S.A. All rights reserved. Keywords: Megajoule Laser; Laser components; Target system; Technology development status
1. Introduction The Megajoule Laser Project launched in 1995 by the French Commissariat ` a l’Energie Atomique is designed to deliver 1.8 MJ Ž600 TW. in a cryogenic target ŽFig. 1.. The goal is to demonstrate ignition and produce a target gain of 10 or more. Theoretical and experimental data show that this goal can be obtained using 240 beamlets that are 40 = 40 cm2 each ŽFig. 2. w1x. In order to increase the efficiency of such solid state lasers compared to previous existing facilities, i.e. NOVA in USA, GEKKO 12 in Japan and PHEBUS in France, the general design of each
laser line has been optimized as shown in Fig. 3. Beamlines are gathered by eight in a four by two arrangement. Each of the two amplifiers are located in a laser cavity in order to be crossed four times by the pulse, thus increasing the total efficiency of the laser. The specifications and characteristics of our design, indicated in Fig. 3, represent the target working point compared with the laser perfor-
U
Corresponding author. Tel.: q33 169267876; fax: q33 169267075.
Fig. 1. Main specifications of the LMJ project.
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 2 6 5 - 8
44
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
Fig. 2. Main laser characteristics of the LMJ facility.
mances in terms of Energy vs. Power curves. Depending on the hypothesis used, concerning
for example Frequency Conversion Efficiency, transmission of optics at 0.35 m m, the laser domain is described by the gray area while the arrows show the safety of the target physics design. This design ŽFig. 4. differs slightly from the LLNL-NIF design w2,3x by utilising the demi-tour concept which allows placement of all the amplifiers inside the cavity. The following is a description of the various components of the laser and target system, giving
Fig. 3. Schematic of the working range of the LMJ facility.
Fig. 4. General laser layout showing the pulse injection, the demi-tour driving the pulse four times in the cavity, the large Pockels-cell ŽPEPC. for isolation and the deformable end cavity mirror. A pair of gratings are used to focus the beam on target.
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
the status of the corresponding technology development. 2. Front end The two main components of the front end are the laser sources located in the front end laboratory and the preamplifiers connected to the main laser chains in the laser bays w4x. The pulse is transmitted from the laser sources to the preamplifiers through fiber optics ŽFig. 5.. The laser sources are based on the use of
45
microlasers which have been studied by CEALETI and the phase and amplitude modulators were made by Alenia in Italy ŽFig. 6.. Each of the 240 beamlets have their own source, thus allowing a very flexible adjustment of the pulses in time from beam-to-beam. Each beamlet is also equipped with its own preamplifier module delivering more than 1 J at the entrance of each main chain. The preamplifier is composed of a regenerative amplifier and a four-pass rod amplifier. The base line uses a flashlamp-pumped rod amplifier but
Fig. 5. Schematic of the front end showing the sources located in the front end laboratory and the preamplifier connected to the main amplifier chains in the laser bays.
Fig. 6. Schematic of the source design showing the microlaser source, the phase and amplitude modulator and their controls. The spectral chirping of the pulse is automatically controlled in order to avoid any destructive shots in the final large optics.
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
46
Fig. 7. Schematic of part of a four-by-two amplifier module showing the compactness of the design.
an option is open to use diode pumping in order to obtain a 1-Hz alignment beam up to the KDP crystals of the focusing system. A final choice between flashlamps and diodes will be made taking into account cost issues. 3. Main amplifier chains Their general structure is represented in Fig. 4. The main active components are the two flashlamp-pumped amplifiers. In the LMJ design they are each equipped with nine laser slabs 4 cm thick. Each beamlet cavity contains 18 laser slabs providing a gain of more than 20 000 at 1.05 m m.
In close cooperation with the Lawrence Livermore National Laboratory, we have carefully studied the design of the amplifier in order to improve the total gain per laser slab and the gain uniformity ŽFig. 7.. A very compact design using shaped reflectors has been optimized and is going to be tested in the Amplifier Laboratory built in Livermore ŽFig. 8.. Two 4 = 2 modules have been built, one by LLNL and one by CEA. Testing was completed in 1997 in order to verify the gain uniformity and the wave front distortion of the beam. 4. Power conditioning Four-hundred megajoules are stored in a capacitor bank located close to the amplifiers ŽFig.
Fig. 8. Schematic and image of the amplifier module to be tested at LLNL.
Fig. 9. Schematic of one circuit of the power conditioning of LMJ.
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
9.. Three main development efforts were conducted in this field: 1. Compact capacitors, which are able to store more than 750 Jrl, were designed and tested by the LCC Company in France. They have the advantage of being self-safe in case of an accidental discharge inside the capacitor. 2. Research is still ongoing on high energy commuters, our goal is to commute 250 kAmp. 3. Long reliable flashlamps Ž2 m. have been
47
designed and built by the Vermetal company. They survived more than 10 000 shots without noticeable degradation.
5. Beam transport and focusing system The general design of the facility has been chosen in order to decrease the optical pathlength between the output of each beamline and the focusing system on the target. This has been
Fig. 10. Beamlines arrangement in an in-line building.
Fig. 11. Schematic of the focusing design using two diffraction gratings in order to filter the parasitic frequencies. The second grating working at 0.35 m m is also the beam focusing device on the target.
48
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
obtained with an in-line building where two laser bays are symmetrically arranged on both sides of the target building ŽFig. 10.. The beam transport is obtained using four mirrors per beamlet in an arrangement which allows easy changing of the irradiation geometry from indirect drive to direct drive experiments. The short pathlength Ž- 40 m. avoids the risk of Raman scattering during the propagation in air. The focusing system is based on an original arrangement using diffraction gratings. This design has been selected in order to filter the remaining 1.05 m m and 0.53 m m light before they could enter the target chamber ŽFig. 11.. Individual beams are propagated and focused in a twoby-two arrangement. The development of such gratings is in process. We demonstrated recently on a 5-cm sample, a diffraction-limited focal spot. An efficiency of 94% has been measured and the laser damage fluence at 0.35 m m is that of fused silica.
6. Target chamber and diagnostics The 10-m inside diameter target chamber shown in Fig. 12 is a double wall aluminum vessel able to sustain the total energy radiated during 20-MJ high yield experiments w5x. The X-ray fluence Ž1 Jrcm2 . on the inside wall will be sustained by an inner wall for which different materials are being evaluated. The design of the target chamber takes into account great flexibility in terms of irradiation arrangements. The diagnostics needed for ignition characterization are under development. Diagnostics Insertion Systems have been designed and a prototype is been tested at the Phebus facility. They will ´ have great flexibility in the diagnostics organization for each group of experiments and between each high energy shot in order to replace diagnostic components located close to the target. 7. Controls The LMJ facility will be driven by a complex control system which is based on a main supervision associated with specific controls related to each function of the facility. The supervision is devoted to the exchanges between the facility itself and the facility users. The ergonomic aspects have been carefully studied taking into account the experience acquired at the Phebus facility. Fig. 13 shows the interfaces between the supervision and the specific controls. 8. Ligne d’integration laser (LIL)
Fig. 12. Target chamber inside its building. The use of gratings which deviate the 0.35-m m beam by an angle of approx. 508, allows the placement of an intermediate protective wall between the target chamber and the target chamber room walls. This intermediate wall will protect neutron-sensitive components during high-yield shots.
Taking into account the large performance gap between a facility like Phebus and the LMJ, from 8 to 1800 kJ, and considering the evolution of the technologies that are going to be used, it strongly necessitates building, as a first step of the project, a representative prototype of the LMJ. This prototype, called LIL, is a complete beamline of the LMJ from the laser source up to the focusing system. It is one-thirtieth of the total LMJ facility. Its goal is to validate in real shape the technical concepts to be used. It will also be used in order to prepare the activation, operation and
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
49
Fig. 13. Schematic of the general design of the LMJ control system.
maintenance of the LMJ. People will be trained on the LIL prior to operation of the LMJ. On another hand, the LIL, which will be able to deliver 60 kJ of UV light, will be associated with a target chamber in order to perform physics experiments which should start as early as the beginning of 2002. This experimental facility will last in parallel with LMJ and will be used to prepare diagnostics and experiments on this much larger facility, The LIL will also allow the preparation of possible evolutions of the LMJ, like for example short pulses if the fast ignitor concept demonstrates that it will be interesting. 9. Project planning After the LIL has been activated and has fully demonstrated its performance, the LMJ facility will be built in two phases. The first one, consisting of one-third of the total energy and power, will be operating in 2005 and the second, with total performance available, will be completed in 2010. In order to reach this goal. the LMJ building must start construction in 2002. The site se-
lected for both LIL and LMJ is a CEA research center located in the south-east of France, near the city of Bordeaux. References w1x Status of the LWJ Project, in: M.L. Andre ´ ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., Second Annual International Conference, Paris. October 22]25, 1996. w2x Status of the National Ignition Facility Project, in: W.H. Lowdermilk ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., Second Annual International Conference, Paris, October 22]25, 1996. w3x Conceptual Design of the National Ignition Facility, in: J.A. Paisner et al. ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., First Annual International Conference. Monterey, 31 May-2 June, 1995. w4x The Megajoule Front End Laser system overview, in: P. Estrailler et al. ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., Second Annual International Conference, Paris, October 22]25, 1996. w5x Target Conceptual Design Issues of the French Laser Megajoule Facility ŽLMJ., in: D. Schirmann ŽCEA., M. Tobin ŽLLNL. ŽEds.., Proceedings of the 12th topical meeting on the Technology of Fusion Energy, June 16]20, 1996, Reno, Nevada.
The French Megajoule Laser Project Ž LMJ. Michel L. Andre ´U Direction des Recherches en Ile de France, Commissariat a ` l’Energie Atomique, BP 12, 91680 Bruyeres-le-chatel, France
Abstract This paper describes the goal and the design of the Megajoule Laser to be built in France at the beginning of the next century. Several technical developments are related to this new design and justify the use of new technologies. Most of them are the continuation of a developmental effort by the CEA for several years. Some have come from developments needed for other applications, such as telecommunications. Q 1999 Elsevier Science S.A. All rights reserved. Keywords: Megajoule Laser; Laser components; Target system; Technology development status
1. Introduction The Megajoule Laser Project launched in 1995 by the French Commissariat ` a l’Energie Atomique is designed to deliver 1.8 MJ Ž600 TW. in a cryogenic target ŽFig. 1.. The goal is to demonstrate ignition and produce a target gain of 10 or more. Theoretical and experimental data show that this goal can be obtained using 240 beamlets that are 40 = 40 cm2 each ŽFig. 2. w1x. In order to increase the efficiency of such solid state lasers compared to previous existing facilities, i.e. NOVA in USA, GEKKO 12 in Japan and PHEBUS in France, the general design of each
laser line has been optimized as shown in Fig. 3. Beamlines are gathered by eight in a four by two arrangement. Each of the two amplifiers are located in a laser cavity in order to be crossed four times by the pulse, thus increasing the total efficiency of the laser. The specifications and characteristics of our design, indicated in Fig. 3, represent the target working point compared with the laser perfor-
U
Corresponding author. Tel.: q33 169267876; fax: q33 169267075.
Fig. 1. Main specifications of the LMJ project.
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 2 6 5 - 8
44
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
Fig. 2. Main laser characteristics of the LMJ facility.
mances in terms of Energy vs. Power curves. Depending on the hypothesis used, concerning
for example Frequency Conversion Efficiency, transmission of optics at 0.35 m m, the laser domain is described by the gray area while the arrows show the safety of the target physics design. This design ŽFig. 4. differs slightly from the LLNL-NIF design w2,3x by utilising the demi-tour concept which allows placement of all the amplifiers inside the cavity. The following is a description of the various components of the laser and target system, giving
Fig. 3. Schematic of the working range of the LMJ facility.
Fig. 4. General laser layout showing the pulse injection, the demi-tour driving the pulse four times in the cavity, the large Pockels-cell ŽPEPC. for isolation and the deformable end cavity mirror. A pair of gratings are used to focus the beam on target.
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
the status of the corresponding technology development. 2. Front end The two main components of the front end are the laser sources located in the front end laboratory and the preamplifiers connected to the main laser chains in the laser bays w4x. The pulse is transmitted from the laser sources to the preamplifiers through fiber optics ŽFig. 5.. The laser sources are based on the use of
45
microlasers which have been studied by CEALETI and the phase and amplitude modulators were made by Alenia in Italy ŽFig. 6.. Each of the 240 beamlets have their own source, thus allowing a very flexible adjustment of the pulses in time from beam-to-beam. Each beamlet is also equipped with its own preamplifier module delivering more than 1 J at the entrance of each main chain. The preamplifier is composed of a regenerative amplifier and a four-pass rod amplifier. The base line uses a flashlamp-pumped rod amplifier but
Fig. 5. Schematic of the front end showing the sources located in the front end laboratory and the preamplifier connected to the main amplifier chains in the laser bays.
Fig. 6. Schematic of the source design showing the microlaser source, the phase and amplitude modulator and their controls. The spectral chirping of the pulse is automatically controlled in order to avoid any destructive shots in the final large optics.
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
46
Fig. 7. Schematic of part of a four-by-two amplifier module showing the compactness of the design.
an option is open to use diode pumping in order to obtain a 1-Hz alignment beam up to the KDP crystals of the focusing system. A final choice between flashlamps and diodes will be made taking into account cost issues. 3. Main amplifier chains Their general structure is represented in Fig. 4. The main active components are the two flashlamp-pumped amplifiers. In the LMJ design they are each equipped with nine laser slabs 4 cm thick. Each beamlet cavity contains 18 laser slabs providing a gain of more than 20 000 at 1.05 m m.
In close cooperation with the Lawrence Livermore National Laboratory, we have carefully studied the design of the amplifier in order to improve the total gain per laser slab and the gain uniformity ŽFig. 7.. A very compact design using shaped reflectors has been optimized and is going to be tested in the Amplifier Laboratory built in Livermore ŽFig. 8.. Two 4 = 2 modules have been built, one by LLNL and one by CEA. Testing was completed in 1997 in order to verify the gain uniformity and the wave front distortion of the beam. 4. Power conditioning Four-hundred megajoules are stored in a capacitor bank located close to the amplifiers ŽFig.
Fig. 8. Schematic and image of the amplifier module to be tested at LLNL.
Fig. 9. Schematic of one circuit of the power conditioning of LMJ.
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
9.. Three main development efforts were conducted in this field: 1. Compact capacitors, which are able to store more than 750 Jrl, were designed and tested by the LCC Company in France. They have the advantage of being self-safe in case of an accidental discharge inside the capacitor. 2. Research is still ongoing on high energy commuters, our goal is to commute 250 kAmp. 3. Long reliable flashlamps Ž2 m. have been
47
designed and built by the Vermetal company. They survived more than 10 000 shots without noticeable degradation.
5. Beam transport and focusing system The general design of the facility has been chosen in order to decrease the optical pathlength between the output of each beamline and the focusing system on the target. This has been
Fig. 10. Beamlines arrangement in an in-line building.
Fig. 11. Schematic of the focusing design using two diffraction gratings in order to filter the parasitic frequencies. The second grating working at 0.35 m m is also the beam focusing device on the target.
48
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
obtained with an in-line building where two laser bays are symmetrically arranged on both sides of the target building ŽFig. 10.. The beam transport is obtained using four mirrors per beamlet in an arrangement which allows easy changing of the irradiation geometry from indirect drive to direct drive experiments. The short pathlength Ž- 40 m. avoids the risk of Raman scattering during the propagation in air. The focusing system is based on an original arrangement using diffraction gratings. This design has been selected in order to filter the remaining 1.05 m m and 0.53 m m light before they could enter the target chamber ŽFig. 11.. Individual beams are propagated and focused in a twoby-two arrangement. The development of such gratings is in process. We demonstrated recently on a 5-cm sample, a diffraction-limited focal spot. An efficiency of 94% has been measured and the laser damage fluence at 0.35 m m is that of fused silica.
6. Target chamber and diagnostics The 10-m inside diameter target chamber shown in Fig. 12 is a double wall aluminum vessel able to sustain the total energy radiated during 20-MJ high yield experiments w5x. The X-ray fluence Ž1 Jrcm2 . on the inside wall will be sustained by an inner wall for which different materials are being evaluated. The design of the target chamber takes into account great flexibility in terms of irradiation arrangements. The diagnostics needed for ignition characterization are under development. Diagnostics Insertion Systems have been designed and a prototype is been tested at the Phebus facility. They will ´ have great flexibility in the diagnostics organization for each group of experiments and between each high energy shot in order to replace diagnostic components located close to the target. 7. Controls The LMJ facility will be driven by a complex control system which is based on a main supervision associated with specific controls related to each function of the facility. The supervision is devoted to the exchanges between the facility itself and the facility users. The ergonomic aspects have been carefully studied taking into account the experience acquired at the Phebus facility. Fig. 13 shows the interfaces between the supervision and the specific controls. 8. Ligne d’integration laser (LIL)
Fig. 12. Target chamber inside its building. The use of gratings which deviate the 0.35-m m beam by an angle of approx. 508, allows the placement of an intermediate protective wall between the target chamber and the target chamber room walls. This intermediate wall will protect neutron-sensitive components during high-yield shots.
Taking into account the large performance gap between a facility like Phebus and the LMJ, from 8 to 1800 kJ, and considering the evolution of the technologies that are going to be used, it strongly necessitates building, as a first step of the project, a representative prototype of the LMJ. This prototype, called LIL, is a complete beamline of the LMJ from the laser source up to the focusing system. It is one-thirtieth of the total LMJ facility. Its goal is to validate in real shape the technical concepts to be used. It will also be used in order to prepare the activation, operation and
M.L. Andre´ r Fusion Engineering and Design 44 (1999) 43]49
49
Fig. 13. Schematic of the general design of the LMJ control system.
maintenance of the LMJ. People will be trained on the LIL prior to operation of the LMJ. On another hand, the LIL, which will be able to deliver 60 kJ of UV light, will be associated with a target chamber in order to perform physics experiments which should start as early as the beginning of 2002. This experimental facility will last in parallel with LMJ and will be used to prepare diagnostics and experiments on this much larger facility, The LIL will also allow the preparation of possible evolutions of the LMJ, like for example short pulses if the fast ignitor concept demonstrates that it will be interesting. 9. Project planning After the LIL has been activated and has fully demonstrated its performance, the LMJ facility will be built in two phases. The first one, consisting of one-third of the total energy and power, will be operating in 2005 and the second, with total performance available, will be completed in 2010. In order to reach this goal. the LMJ building must start construction in 2002. The site se-
lected for both LIL and LMJ is a CEA research center located in the south-east of France, near the city of Bordeaux. References w1x Status of the LWJ Project, in: M.L. Andre ´ ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., Second Annual International Conference, Paris. October 22]25, 1996. w2x Status of the National Ignition Facility Project, in: W.H. Lowdermilk ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., Second Annual International Conference, Paris, October 22]25, 1996. w3x Conceptual Design of the National Ignition Facility, in: J.A. Paisner et al. ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., First Annual International Conference. Monterey, 31 May-2 June, 1995. w4x The Megajoule Front End Laser system overview, in: P. Estrailler et al. ŽEd.., Solid State Lasers for Application to Inertial Confinement Fusion ŽICF., Second Annual International Conference, Paris, October 22]25, 1996. w5x Target Conceptual Design Issues of the French Laser Megajoule Facility ŽLMJ., in: D. Schirmann ŽCEA., M. Tobin ŽLLNL. ŽEds.., Proceedings of the 12th topical meeting on the Technology of Fusion Energy, June 16]20, 1996, Reno, Nevada.