Inertial fusion with the LMJ

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1 Plasma Physics and Controlled Fusion Inertial fusion with the LMJ To cite this article: C Cavailler 2005 Plasma Phys. Control. Fusion 47 B389 View the article online for updates and enhancements. Related content - Prospects and progress at LIL and Megajoule Claude Cavailler, Noël Fleurot, T Lonjaret et al. - Studying ignition schemes on European laser facilities S. Jacquemot, F. Amiranoff, S.D. Baton et al. - The National Ignition Facility George H. Miller, Edward I. Moses and Craig R. Wuest Recent citations - Two-plasmon decay instability of the backscattered light of stimulated Raman scattering K.Q. Pan et al - Beam alignment based on twodimensional power spectral density of a near-field image Shenzhen Wang et al - Improvements in long-term output energy performance of Nd:glass regenerative amplifiers Peng Zhang et al This content was downloaded from IP address on 23/08/2018 at 04:35

2 INSTITUTE OF PHYSICS PUBLISHING Plasma Phys. Control. Fusion 47 (2005) B389 B403 PLASMA PHYSICS AND CONTROLLED FUSION doi: / /47/12b/s28 Inertial fusion with the LMJ C Cavailler CEA/DAM/Ile de France, BP12, Bruyères-le-Châtel, France Received 1 July 2005 Published 8 November 2005 Online at stacks.iop.org/ppcf/47/b389 Abstract The progress of the construction of very large laser facilities LMJ and NIF enables the prediction of inertial fusion achievement. These facilities will open new fields for research: the high energy density physics. Pressures of several 100 Mbars and temperatures of several 100 ev will be reached. Measurements of material properties (EOS and opacities) which have been demonstrated on current or former facilities will be possible at these never reached conditions. Pure hydrodynamics (instabilities) and radiative hydrodynamics astrophysical issues will be addressed. However, ignition and gain as a first proof of Inertial Confinement Fusion is a primary goal. The indirect drive route to inertial fusion has been prepared for many years by CEA (Commissariat à l Energie Atomique). The last ten years were imprinted by a close collaboration between CEA and US-DOE in both the areas of facilities R&D and ignition target physics. The scientific issues are well known: the propagation of laser light through the very long plasma created inside the hohlraum has to be understood and mastered to be sure that less than 10% of laser energy will be backscattered by parametric instabilities. On the other hand, the stability of the capsule implosion has to be matched with the fabrication surface finish so as to avoid shell destruction and extinction of the central hot spot. Recent advances at CEA have allowed a better confidence of reaching ignition using the facility previously specified. These works used the CEA computing capability combined with plasma experiments on existing lasers facilities. Ignition achievement also supposes the realization of suitable cryogenic targets. CEA began the construction of the Laser Megajoule (LMJ), a 240-beam laser facility, at the CEA Laboratory CESTA near Bordeaux. The LMJ is designed to deliver 2 MJ of 0.35 µm light to targets for high energy density physics experiments. Four beams were operated for plasma experiments on the Ligne d Integration Laser (LIL) at CESTA, for the end of 2004, meeting the specifications for LMJ. The realization phase of the LMJ facility was initiated in March 2003 with the start of construction of the building and the target chamber. (Some figures in this article are in colour only in the electronic version) /05/SB $ IOP Publishing Ltd Printed in the UK B389

3 B390 C Cavailler Figure 1. Gold hohlraum with D-T fusion target for the indirect drive. 1. Introduction The development of experimental facilities for studying inertial fusion and high-energy-density physics has spanned several decades since the advent of lasers. Soon after the invention of the laser, in the early 1960s, it was postulated that energetic laser beams might be used to compress and heat a capsule containing deuterium and tritium to the point where fusion ignition and burn could occur. Energetic laser pulses were also found to provide a means for inducing extreme temperatures, pressures and shock environments in materials and for producing a variety of plasma conditions for scientific study [1]. By 1995 the amount of progress made in understanding how to generate inertially confined fusion and high energy densities in material was sufficiently advanced to allow a serious review of CEA fusion concepts; the LMJ project which is a key part of the French defense programme called Programme Simulation devoted to laboratory experiments on the behaviour of materials under very high temperature and pressure conditions began. It also has applications in the field of astrophysics and fundamental physics [2]. 2. Achieving ignition with LMJ In inertial confinement fusion (ICF) ignition occurs when energy production and alpha particle deposition from the central hot spot are sufficient to initiate a self-sustaining burn wave that propagates into the surrounding main fuel. Target gain is defined as the ratio of thermonuclear energy produced to driver energy on target. The first key of inertial confinement fusion is the target. The CEA baseline target remained unchanged for several years. Target design has to be a trade-off addressing the different physics issues and be robust against experimental and technological uncertainties. LMJ uses the so-called indirect drive configuration which consists of a gold hohlraum cylinder of 6.2 mm diameter and 10.4 mm length as presented in figure 1. The laser beams arranged in three cones on each side of the hohlraum are focused through the holes on the internal wall of the cylinder and are converted into x-rays characterized by a radiation temperature in the range of 3 to 4 MK. These x-rays drive the implosion of the capsule made of two layers, an ablator and a cryogenic DT, leading to a hot spot and ignition. DT has to be maintained at 18.2 K by cryogeny [3] Laser plasma interaction (LPI) To succeed with a fusion experiment the physicist must be able to introduce some amount of laser energy into the hohlraum in order to produce an isotropic radiation field of 300 to 350 ev.

4 Inertial fusion with the LMJ B391 When crossing the window at the laser entrance hole (LEH) the laser beam experiences LPI through a very long plasma. Along this propagation, parametric instabilities occur backscattering a fraction of laser energy. This fraction has to be mastered to less than 10% using smoothing techniques on the laser beams. The study of laser propagation through the plasma needs large calculation capability. New massively parallel calculation capacity with the new CEA computer called TERA and the improved modelling allow French physicists to calculate laser propogation through realistics plasmas. The TERA 1 computer with 2400 processors has been operational since The new TERA 10 computer with 9000 processors will be available in The efficiency of laser conversion into x-rays from short wavelengths was demonstrated in the 1980s (see e.g. the French LULI results (1980) [4]). This is the reason why the baseline LMJ design was based on laser light conversion to the third harmonic (3 ω 0 ) Hohlraum energetics CEA has good confidence in the 2D predictions of hohlraum energetics. They were confirmed by a lot of experimental results. Laser energy of 1.4 MJ was introduced into the hohlraum produces 1.03 MJ of x-rays (74%). These x-rays lead, after the wall and LEH losses, to 0.16 MJ in the capsule, which can be described as 11% efficiency. To achieve cryogeny it might be necessary to decrease the gas density in the target but, as a consequence, more laser energy is needed (when ρ decreases from 0.8 to 0.6 mg cc 1 laser energy must increase by 3%) Implosion The implosion stability of the capsule remains a key issue for ignition. We must limit hydrodynamic instabilities during the implosion which could ruin the imploding shell and perturb the hot spot. In both cases ignition could fail. A better understanding of perturbation growth (theory and calculation) will enable us to tune the capsule design to an achievable technology. We now have reasonably good calculations of perturbation growth during the implosion and of perturbed yield with different initial conditions concerning the target surface roughness. Moreover, 3D calculations will be soon available with TROLL, a 3D hydrocode on the TERA 10 computer. The capsule stability is sensitive to DT gas fill which depends on cryogenic temperature and DT ageing. The LMJ baseline target with a uniformly doped ablator and a 0.5 mg cc 1 gas fill did not have enough margin to ignite so we have recently developed a graded dopant design which significantly improves the capsule stability. 3. LMJ performance requirements A dimensional analysis of the indirect drive FCI implosion allows us to plot iso-safety curves regarding LPI and hydro-instabilities in the P laser, E laser diagram presented in figure 2. The grey area presents the region where ignition could be achieved as planned by LLNL (NIF) or CEA (LMJ). This area corresponds to power and energies described by integrated calculations achieved during the last 10 years by the two laboratories. To secure ignition taking into account laser backscattering and beam phasing (for tuning isotropy of irradiation) leads to the yellow region which requires LMJ to be able to produce 1.8 MJ of UV light and 550 TW of peak power.

5 B392 C Cavailler 700 _ P laser (TW) Increasing safety regarding hydro instabilities Increasing safety regarding LPI LMJ specifications 345 ev LMJ baseline design 300 ev NIForiginal design 300 ev 260 ev E laser (MJ) FCI2 integrated simulation + 10% backscattering and other contingencies _ Figure 2. LMJ laser operating region. Cavity Amplifier L4 Cavity SF Transport amplifier L3 L2 L1 M1 PEPC Cavity amplifier Polarizer Transport Transport amplifier Front End PAM L-Turn mirror Target Bay Laser bay: Length = 127 m, Width = 9m, Height = 12 m Figure 3. General laser layout showing pulse injection, L-Turn providing a passive method for a four-pass cavity operation, large Pockels-Cell (PEPC) for isolation and deformable end cavity mirror M1. To reach that purpose CEA has chosen a glass laser which has a wavelength of µm (1 ω). Frequency conversion with KDP (1 ω to 2 ω) and DKDP (1 ω +2ω 3 ω) crystals is necessary to produce the UV light required by the physicists. Another limitation is the threshold damage in the optics 3ω and the maximum size imaginable of these optics. These considerations have led the LMJ Project to design 240 beams delivering 8.2 kj of UV light for each beam. 4. LIL/LMJ facility description The LMJ has been described in previous papers. It has a multipass amplification structure (figure 3), the 18 amplifier laser glass slabs being arranged in two amplifiers within a four-pass cavity whose end-cavity mirror is a deformable mirror (M1) in order to correct wavefront distortion. The front-end pulse (Pre-Amplifying Module: PAM, up to 1 J [5]) is injected in the transport spatial filter and the four passes are obtained with a passive optical arrangement called demi-tour (L-turn). In order to optimize the laser-target coupling, different beam smoothing techniques will be used starting with the so-called Longitudinal-SSD naturally provided by the 0.5 nm bandwidth

6 Inertial fusion with the LMJ B393 Figure 4. LMJ focusing system uses a grating lens to tilt the UV beam towards the target chamber centre. The unconverted 1 ω and 2 ω light is transmitted in a beam dump and stays outside the target chamber. Figure 5. LIL laser bay (spatial filter on the left side, amplifier located next). and the gratings used to focus the beam on target. To prevent the remaining 1 ω and 2 ω light from entering the target chamber, focusing is achieved using a pair of gratings, one on each side of the two frequency conversion crystals. The second grating deflects and focuses the 3 ω light at the centre of the target chamber, letting the other wavelengths be absorbed outside of the chamber (figure 4). 5. LIL performances In order to prepare the LMJ construction, a prototype called LIL (Ligne d Intégration Laser) was built from 1998 to A photograph of the laser bay and amplification section with the cavity spatial filter and the transport amplifier is shown in figure 5. The target bay with the 5 m target chamber and a plasma diagnostic inserter is shown in figure Laser performances LIL quadruplet commissioning has been achieved and the following results have been demonstrated: Ultraviolet energy goal exceeds the requirement with 9.5 kj at 9 ns on a single beam [6] (equivalent to more than 2.2 MJ on LMJ).

7 B394 C Cavailler Figure 6. LIL target chamber with an insertable plasma diagnostic. Figure 7. 3 ω high energy quad focal spot measurement of LIL (CPP + 14 GHz). Power has already been demonstrated at the 2 TW level (close to 500 TW LMJ equivalent ). Beam synchronization achieves the 30 ps requirement for the 4 beams of the quadruplet. Quad UV focal spot size at the target chamber centre (far field imaging) has been at high energy with a continuous phase plate (CPP) and 14 GHz smoothing: 700 µm has been already achieved. Beam pointing stability was controlled in the 25 µm range (x and y axis), consistent with the overall 50 µm LMJ specification. Figure 7 presents this focal spot for 5 shots, the energy of which was adjusted between 2.5 and 4.8 kj (duration between 0.7 and 5 ns). These results demonstrate and fulfil the goal. Figure 8 presents the good influence of the smoothing effect as expected. All these results are presented in [7] LIL plasma experimental program LIL plasma diagnostics. The LIL plasma diagnostics have built as a coherent set that allows the realization of the first experimental program on LIL including the characterization of LMJ beam smoothing. They are presented in figure 9.

8 Inertial fusion with the LMJ B395 (CPP+14GHz) Y (pixels) 0 Y (pixels) X (pixels) D 90% E = µm 3% Io = ( , ) µm Intensite LIGNE C mes =15.2% X (pixels) D 90% E =703/ 702 / 706 µm 3% Io = (850,918) / (857 / 891) / (836/ 904) µm Intensite LIGNE C mes =10.7% Tableau image (u.a.) Tableau image (u.a.) lineout ( 1pix) 1pix <-> 6.8 µm X (pixels) X (pixels) Figure 8. 3 ω high energy quad focal spot: smoothing effect. Figure 9. LIL target diagnostics. These diagnostics can be distributed in three main groups: The energy diagnostics; these are used to measure the backscattered, transmitted and deviated laser light as well as the x-ray energy emitted by the target. The Raman-Brillouin backscatter spectrometer (figure 10) measures the energy coming back from the target and going through the 351 nm grating focusing the incident laser on target. The x-ray imagers; these are mainly based on microscopes with distances to the target in the range of 400 to 700 mm. A total of six imagers have been built for LIL. Three imagers

9 B396 C Cavailler fibres Figure 10. The Raman-Brillouin backscatter diagnostic. Filters : 100µm Al 20µm Cu U.a. Idem avec l'abcisse ds plan source Moyenne_H ic Moyenne_V il microns Horizontal lineout Vertical lineout kev range X-RAY FOCAL SPOT AT 9.5 kj IN 9 ns ic 30 1 g g il100 1 g µm Figure 11. Static pinhole camera. are installed in an SID which is the LIL main inserter allowing accurate alignment on a target of a three metre long diagnostic. the static pinhole imager (figure 11) was used to image the first plasmas created on LIL. the two multi-imagers microscopes (figures 12(a) and (b)) based on the KB technique are realized owing to six mirrors among which four are parallel and two are in an orthogonal position. Each one produces eight images that are recorded on the four strip lines of a gated camera with a 100 ps time resolution. The soft x-ray imager

10 Inertial fusion with the LMJ B397 (a) (b) Figure 12. (a) The multi-kb x-ray imagers. (b) A hole closing observed with the multi-kb soft x-ray imager. (100 ev to 1 kev) has a 10 µs spatial resolution in a 0.8 cm field of view and a magnification of 8. The hard x-ray imager (1 kev to 5 kev) has a 30 µ resolution in a 2 mm field of view and a magnification of 4, its total length being superior to 3 m; the associated SID cart includes a telescopic extension. The three other imagers are installed in a French version of the SIM (the LLNL Nova manipulator). The x-ray spectrometers. Two broadband power spectrometers have been built. The first one (DMX) covers the 50 ev 20 kev range with 12 channels equipped with mirrors and coaxial diodes and 8 high energy channels among which three have an AsGa detector. The second spectrometer has 12 measurements channels which can be easily adapted as filtration (scintillator and photomultiplier) or fluorescence channels (fluorescence target, scintillator and photomultiplier) in the kev range. Two high resolution spectrometers of the Johan type complete the diagnostics set. The alignment method has been generalized to all the diagnostics requiring a precise alignment (the x-ray imagers and the broadband spectrometers). The pointing and focusing adjustments are realized owing to two optical pointers whose spots are superimposed on the target. The pointers are composed of a photodiode emitting at 630 nm with a tunable intensity of an optical fibre and of a lens. The obtained accuracy is 30 µs in the focal plane and 500 µs in the distance to the target. Even if debris protections, EMI shielding and contamination control are considered, the environmental conditions of the LIL target diagnostics are not a critical criteria of their design. Nuclear vent and EMI protections have been applied. A limited amount of beryllium filters is used which are set in insertable cassettes. The diagnostics are connected to nuclear vent. Double grooves allow contaminated materials removal. The diagnostics critical elements can be transported in airtight boxes, and to assure radiologic control the diagnostics are equipped with specific ports. All these diagnostics are described in detail in [11] First LIL physics experiment. Following the laser commissioning of LIL, the first plasma experiments were conducted in December 2004 and January 2005 in order to test the experimental capability of the facility [12]. The goal of the plasma jet experiment was to observe the creation of a plasma jet on the axis of a conical gold target irradiated by the UV incident quadruplet beam (figure 13) and validate our simulation models.

11 B398 C Cavailler Figure 13. Experimental set-up showing the conical target and the plasma diagnostic. Figure 14. Experimental results showing the grid at 0.4 ns and the plasma jet 2 ns later. The plasma created by the impact of the laser beam on the gold is expanding towards the cone axis where its density increases. It generates a jet some nanoseconds after the laser beam extinction, and decreases progressively in intensity. During the collisions, the jet irradiates x-rays in the 100 ev to 3 kev. The laser pulse duration was selected to be 300 ps. Simulations were made with UV laser peak power between 1 and 4 TW. The plasma diagnostic was set perpendicularly to the cone axis. It consists in the multi Kirkpatrick-Baez x-ray imager delivering 8 images with a time frame of 100 ps and in the spectral bandwidth <1 kev. Two of these eight images are shown in figure 14: the first one, recorded at to +400 ps, just after the extinction of the UV laser beam provides the x-ray image of a grid (used as a spatial reference) heated by the plasma and attached to the target. the second image, recorded 2 ns later, indicates that the grid which has cooled is no longer seen and shows that the jet is visible in front of the target. The distance travelled by the jet on the image allows us to deduce a supersonic speed of 600 km s 1.

12 Inertial fusion with the LMJ B399 Figure 15. Geometry of target irradiation by the 60 quadruplets (left) and cryogenic D-T target inside its cavity (right). 6. LMJ facility 6.1. Fusion target irradiation In order to achieve fusion, LMJ will use the so-called indirect drive configuration in which 60 quadruplets similar to the LIL one will irradiate the D-T fusion target (some millimetres in diameter [6]) located at the centre of a cylindrical cavity (centimetre size). Thirty of them (arranged in 3 cones of 10 quadruplets, respectively, at 30, 40 and 59 with respect to its vertical axis: figure 15) will enter through the north pole window of the cavity, and the 30 others will enter symmetrically through the south pole window Target chamber With a diameter of 10 m and a thickness of 10 cm of aluminium, it will provide 260 holes; 80 of them can be used as laser windows, giving the maximum flexibility for experimental arrangement of the 60 quadruplets, either in the indirect drive configuration (implosion of the micoballoon induced by x-rays created inside a gold cavity during the interaction of the laser beams on its internal surface) or in the alternative option called direct drive (where the beams directly hit the microballoon to implode it). Figure 16 shows the target chamber realization, and figure 17 shows this chamber in the LMJ environment with its cryogenic target positioner (left), beam alignment reference (right) and plasma diagnostic inserters. A 40 cm thick borated concrete shield deposited on the target chamber will help protect the equipment from neutrons and gamma rays created during the fusion process Target bay The target bay (figure 18) is a 33 m concrete cylinder, divided in 20 angular sectors inside which are located the frequency conversion and focusing systems. At the centre of the target bay, the target chamber is supported by large space frames which also hold the plasma diagnostics as well as the cryogenic target positioner and the beam alignment reference. The stability of these structures has to be carefully designed to guarantee the 50 µm accuracy of beam pointing on target. Surrounding the target bay, a second concrete wall separates the target area from the four laser bays and forms the switchyard area which allows the dispatching of the 60 quadruplets to

13 B400 C Cavailler Figure 16. Construction of the target chamber in Bordeaux on the megajoule site. Figure 17. Cryogenic target and plasma diagnostics. Figure 18. Target bay and laser beam switchyard.

14 Inertial fusion with the LMJ B401 Figure 19. The west wing of LMJ with its two laser bays (15 chains or 30 quadruplets) surrounding the target bay. the target chamber through a set of 6 turning mirrors for each beam. This bioshield (2 m thick) protects the facility workers and will isolate the laser hardware from these nuclear radiations during ignition shot. Special attention is required when designing the instruments placed inside the target bay to take into account the nuclear background. We must verify if they can continue to operate under such harsh environment conditions. They will have to operate not only during gain shots but also for weaker neutron yield to control the laser initial conditions (energy, power, pointing conditions,...) and close to the plasma to diagnose it (temperature, density, timing and yield of the nuclear emission) [13, 14]. Similar conditions will also be present in ITER and a common approach to these nuclear vulnerability subjects that has recently begun must be fruitful for both sides and must be enhanced in the future Laser bays Four laser bays are arranged around the target area. 15 laser chains (amplifying each 8 beams in the same mechanical structure: 30 quads total) are located in the two laser bays of the west wing of the building, (figure 19); the other 15 are positioned symmetrically in the east wing. The capacitor banks are represented on each side of the two laser bays. They will store a total of 480 MJ delivered to the flash lamps illuminating the laser slabs of the amplifiers LMJ building The LMJ building design takes into account constraints related to the target bay (especially plasma diagnostics and cryogenic target). They are quite important in terms of mechanical stability, temperature control and access for maintenance. Cleanliness control of all optical components and their mechanical supporting structures is also a major technical challenge. The LIL prototype has shown that this difficulty has already been mastered. The construction of the LMJ building started in Located 150 metres away from the existing LIL building, the four laser bays will be located on both sides of the m 2 target chamber bay (figures 20 and 21).

15 B402 C Cavailler Figure 20. Artist s view of the future LMJ facility. Figure 21. General view of the construction site. The laser and target bay concrete halls are under construction. Work on the target chamber supporting structures started as planned in May They will be ready to introduce to the chamber inside the building in summer Conclusions Plasma diagnostics have been installed in LIL and have been used with success in the first plasma experiments. In parallel, the LMJ building construction stays on schedule. The target chamber, which is now under construction will be introduced into the building during summer The first light of LMJ with 240 beams is scheduled for the beginning of 2011 and the first fusion experiments during In France, we begin to have, a convergence between design robustness, target fabrication (cryogeny) and laser performances that allows us be confident of achieving inertial fusion with the LMJ. Acknowledgments The author is indebted to J Tassart, P A Holstein, J Giorla, N Fleurot, J L Bourgade, J P Le Breton of the Direction des Applications Militaires of CEA for their valuable contributions to this paper.

16 Inertial fusion with the LMJ B403 References [1] Tarter C B 2001 Inertial fusion and high-energy density science in the United States Inertial Fusion Sciences and Applications (Elsevier) [2] Campbell E M et al 2000 Inertial fusion science and technology for the 21st century Compte Rendus de l Académie des Sciences série IV, Tome 1, no 6, August 2000 [3] Holstein P A et al 1999 Target design for the LMJ Proc. Inertial Fusion Sciences and Applications 99 (Bordeaux, September 1999) [4] Labaune G C and Fabre E et al 1982 Phys. Rev. Lett [5] Estraillier P et al 1996 The megajoule front end laser system overview Proc. Solid State Lasers for Application to Inertial Confinement Fusion (ICF) (Paris, October 1996) [6] Schirmann D and Tobin M 1996 Target conceptual design issues of the French laser megajoule facility (LMJ) Proc. 12th Topical Meeting on the Technology of Fusion Energy (Reno, June 1996) [6] Di Nicola J M et al 2003 IFSA Conf. [7] Di Nicola J M et al 2005 IFSA Conf. submitted [8] Le Breton J P et al 2004 Inertial Fusion Sciences and Applications ANS pp [9] Bourgade J L et al 2001 RSI, 72 No 1 Part II, pp [10] Le Breton J P et al 2004 Chocs no 29 CEA, Paris pp [11] Le Breton J P et al 2004 Plasma Diagnostics for the LMJ Laser Integration Line (LIL) ECLIM (ROME, 6 10 September 2004) [12] IFSA 2005 Conf. on First Physic Experiment of LIL submitted [13] Bourgade J L Rev. Sci. Instrum. submitted [14] Bourgade J L 2004 New constraints for plasma diagnostics development due to the harsh environment of LMJ class lasers 23rd Symp. on Fusion Technology (Venise, September 2004)