Nova Laser Technology

In building the Nova laser, we have significantly advanced many technologies, including the generation and propagation of laser beams. We have also developed innovations in the fields of alignment, diagnostics, computer control, and image processIng.•

For further information contact Nova is the world's most powerful at least 10 to 30 times more energetic John F. Holzrichter (415) 423-7454. laser system. It is designed to heat and than Shiva would be needed to compress small targets, typically 0.1 cm investigate ignition conditions and in size, to conditions otherwise produced possibly to reach gains near unity. only in nuclear weapons or in the Because of the importance of such a interior of stars. Its beams can facility to the progress of inertial fusion concentrate 80 to 120 kJ of energy (in research and because of the construction 3 ns) or 80 to 120 TW of power (in time entailed (at least five to seven 100 ps) on such targets. To couple energy years), the Nova project was proposed to more favorably with the target, Nova's the Energy Research and Development laser light will be harmonically converted Administration and to Congress. It was with greater than 50% efficiency from its decided to base this system on the near-infrared fundamental wavelength proven master-oscillator, linear-amplifier­ (1.05-,um wavelength) to green (0.525- chain laser system used on the Argus ,um) or blue (0.35-,um) wavelengths. The and Shiva systems. We had great goals of our experiments with Nova are confidence in extending this to make accurate measurements of high­ neodymium-glass laser technology to the temperature and high-pressure states of 200- to 300-kJ level. As target-physics matter for the weapons program, to data were obtained with the Shiva compress deuterium-tritium (D-T) fusion system and as further computational fuel to densities approaching 200 g/cm3 modeling occurred, it became clear that (1000 times the density of liquid D-T), to the most important demonstrations for determine the ultimate energy required the Nova system would be to generate for efficient ignition and bum of inertial a very high-quality compression fusion targets, l and finally, to make environment with short-wavelength important physics and engineering «O.5-,um) laser light and to determine measurements such as x-ray laser more exactly the laser energy required studies. Figure 1 shows the Nova laser for high-gain fusion reaction.3 facility. As a result, the final Nova laser The Nova concept was born while configuration is a ten-beam system able LLNL's lO-kJ Shiva laser2 was under to provide light output at 1.05, 0.53, and construction in 1975. Physics experiments 0.35 ,um over a wide pulse-duration then being conducted on the range (0.1 ns to greater than 100 ns). Laboratory's Argus laser, together with It incorporates a flexible target theoretical analyses, indicated that a laser configuration for inertial-confinement

8 INERTIAL FUSION

fusion (ICF) direct drive, ICF indirect • Maximum flexibility to permit the drive, x-ray laser, and other target widest variety of weapon physics, ICF, applications employing two target and general physics applications. experimentation areas. These goals were successfully met. As discussed in the article on p. 1, we Two beams of the Nova laser, operating had gained a great deal of laser physics as the temporary Novette facility, have and engineering experience with earlier verified most of the design of Nova's Fig. 1 laser systems-Cyclops, Argus, and laser chain and have demonstrated many Cutaway view of the Nova laser fusion Shiva. The philosophy that guided our of the expected benefits of short­ facility. The space frame at the right Nova design efforts reflected these wavelength target irradiation. supports the ten laser amplifier chains. A system of high-reflectivity mirrors I experiences. The resulting design in corpora ted: ova Laser Design causes the ten laser beams to arrive I 4 simultaneously at the fusion target, • As large an output aperture as The Nova laser system is which is centered in the spherical tar­ I possible to reduce beam count and shown schematically in Fig. 2. get chamber near the top left of the l' N thereby system construction and It consists of a master oscillator and ten picture. maintenance cost. • Maximum fluence through each aperture. This required the development of new laser glasses, propagation techniques, and coatings to sustain the high laser intensity. • Automation of beam alignment and electronic system testing to reduce operating costs and increase the shot rate. • Flexibility to accommodate new technology that would increase performance. • Reuse of Shiva components to reduce cost wherever possible.

~ Output sensor [>-<::J Spatial filters ,"'>--> package -:' -1 III Amplifiers +.L Isolators Space for added D amplifiers

/ Turning mirrors

4 I

Fig. 2 Arrangement of the major optical com­ ponents in a representative Nova beam line. Space provided in the two larger amplifier sections makes it possible to add more amplifiers and increase the beam power at low cost.

9 chains of laser amplifiers (only one of laser research to 100 ns for weapon­ which is shown). The master oscillator effects simulations) and shape (for generates a low-level pulse, typically example, a series of I-ns, "square" 0.1 m], which is preamplified to about pulses, spaced by 5 ns, each more 10 J. The pulse is split into ten beams energetic than the previous one by a and repeatedly amplified and expanded, factor of 10 to 50). The pulses are until, at the exit of the laser chain, amplified in a series of glass-rod each beam is about 10 kJ and 74 cm in amplifiers of increasing diameter (each diameter. Each beam is then directed with a gain of 20 to 25) until the beam to the target chamber by four or five reaches 4 cm in diameter. Spatial-filter turning mirrors. Before the beams enter telescopes further expand the beam to the target chamber, they are each 10 cm, the diameter of the first disk­ diagnosed by sampling a fraction, amplifier section. Each disk-amplifier harmonically converted by passage section has a small signal gain of 4 to 10. through nonlinear KDP crystal arrays, The advantage of the disk amplifiers is and finally focused to a spot on the that they can be scaled to almost Fig. 3 target 0.25 mm in diameter. Figure 3 is a arbitrary beam diameter and still view down Nova's ten amplifier chains. maintain the ratio of high gain to low Nova amplifier chains mounted on their The master oscillator systemS glass thickness that is required for good space frame. Each frame holds five laser chains, each of which is 137 m generates a time bandwidth-limited laser beam propagation. As the beam passes long. pulse of variable width (0.1 ns for x-ray from one amplifier stage to the next, it is expanded to 15 cm, 20 cm, 31.5 cm, 46 cm, and finally 74 cm for propagation to the focusing lens. After each stage (where the gain is a factor of about 10), an isolator or one­ way transmission element is required.6 Otherwise, parasitic reflections from shiny metal parts in the laser chain or from the target cause the laser to break into spontaneous oscillations and cause damage to sensitive components. For the small-diameter sections (up to 5 cm), we use a Pockels-cell isolator. The isolator, which acts as a "light switch," consists of an electro-optical crystal between crossed polarizers. By allowing light to pass for only 10 ns, it prevents the build-up of parasitic oscillations and keeps unwanted, spontaneous emissions generated in the front end of the laser from being amplified to a large enough level to damage the target. All the large­ diameter stages contain Faraday-effect isolators. These devices rotate the polarization plane of backward-reflected light in a direction opposite to that of the forward-propagating laser beam. A polarizer plate in the path of the backward-reflected light rejects the parasitic modes. Most of the isolator components were developed and tested on the Shiva laser system and are being reused on Nova. Spatial-filter telescopes have three primary purposes: (1) to expand the

10 INERTIAL FUSION

diameter of the beam, (2) to smooth it earn Propagation with the filter, and (3) to project it The performance per unit cost down the chain to prevent unwanted _ B of any laser system increases diffraction. A spatial filter consists of rapidly with increasing laser fluence.8 a pair of lenses and a pinhole located Most of the costs of these lasers are in a vacuum tube between them at the either somewhat independent of aperture lens focal points. The first lens focuses (fixed) or proportional to beam area the parallel-propagating rays of the (aperture size). Fixed costs are associated laser beam through the pinhole. Any with the building, the target systems, misdirected rays (the cause and result the space frame, the computer, and the Fig. 4 of local beam non uniformity) come to a diagnostic system. Variable costs that are Interchain peak and average fluences focus either nearer to or farther from the proportional to beam area are associated as a function of distance along the first lens and therefore do not pass with optical elements, amplifiers, spatial laser chain for a 10.5-kJ laser pulse through the pinhole located at its filter, isolators, etc. By increasing 1 ns long. The sizes and locations of .. focal point. The second, larger lens the beam fluence through the baseline the spatial filter lenses are indicated recollimates the beam diverging from the along the top of the figure. Entrance­ system, we obtain more total energy lens surfaces will suffer damage if the pinhole to match the larger diameter of with essentially the same components. peak fluence exceeds the indicated the next amplifier section. This optical Major limits on beam power (discussed limits. system also acts as a telescope to project and magnify the image of the beam as it 20r------, passes to a larger-diameter section without creating deleterious diffraction ripples. This laser architecture, consisting 15 of a series of amplifiers of increasing diameter, prevents damage to any of the "'E many optical surfaces by light of too ...,~ Final turning mirror high an intensity. At the output ends of CIi u 10 I: damage threshold the larger amplifier sections, the peak Q) energy fluence approaches the damage u:::::l threshold for uncoated glass. The most threatened components in the laser are 5 Polarizer damage the input lenses to the final spatial filters. threshold After the beam has been expanded and • • • • • • • smoothed by the spatial filter, its peak • • energy fluence is well below the damage O.. ~.-~~ ~~~------~------~------~ threshold for coated lenses and safely o 50 100 150 below the damage threshold of the Distance along laser chain, m harmonic conversion system at the output (Fig. 4) . The performance of this propagation strategy was modeled in detail by the fast Fourier-transform code MALAPROP.? The predictive success of this comprehensive code is the result of the fine mesh size permitted by LLNL's t Cray computers, the accurate modeling of diffraction and saturation, and our spatial noise model, which mimics the Fig. 5 effects of small scattering sites and - 4.50 Full computer simulation (using the - 3.50 correlates well with independent - 2.50 MALAPROP code) displaying the uni­ I statistical observations of noise sources. - 1.50 formity of a 12.8-kJ laser pulse 3 ns - 0.90 Figure 5 is a full computer simulation of long as it appears entering the 0.00 '" harmonic conversion array at the target the variation of the uniformity of a 1.00 ~e,~ ~\\'- chamber. Profiles toward the back of 12.8-kJ beam with time over the life - 40 - 20 0 20 40 the figure are the first to arrive at the of a 3-ns pulse. Beam diameter, cm target.

11 in the article beginning on p. 1) are Plifiers ca tastrophic damage thresholds and Disk amplifiers and other cumulative nonlinear, self-focusing ~ components through the aberrations. Important engineering 15-cm-diam stage are similar to (or concerns related to aperture usage reused) Shiva laser components. include the efficiency with which the Although optimized for Shiva's shorter aperture is filled (the beam fill fac tor), pulse operation, these components are the transmissivity of optical elements, acceptable in these smaller aperture and the efficiency of the harmonic sections of Nova. The output amplifier conversion. Additional issues affecting stages (the 20-, 31.5-, and 46-cm-diam cost are the beam amplification efficiency amplifiers) were specially designed for (energy extracted from the gain medium) the Nova laser to improve electrical and the electrical efficiency with which efficiency and to meet propagation amplifier gain is produced (discussed constraints on the ratio of gain to glass below). thickness. A goal of the research program Figure 6 shows one of the 46-cm-diam supporting the Nova laser was to amplifiers in its partly assembled maximize each of these influential rectangular housing. The two elliptical propagation parameters. Technologies glass disks are tilted at Brewster's angle developed to this end include new to permit almost zero transmission loss. phosphate laser glasses characterized by The disks are paired to compensate for Fig. 6 high gain and a low nonlinear index of beam offset caused by refraction in each Partly assembled rectangular laser­ refraction, new neutral-solution and tilted plate. The amplifier housing is disk amplifier (46-cm-diam aperture). sol-gel antireflection coatings and lined on the top, bottom, and both sides The amplifier disks are elliptical for over-coated thin-film reflectors, new with silver-plated reflectors to provide placement at an angle to the circular apodization techniques to operate with a tight optical coupling between the disks beam. Each disk is divided in half (indi­ fill factor of over 80%, new antireflection and the flashlamps. One set of the cated by the vertical band) and clad around its edges with colored glass to coatings to reduce transmission loss in flashlamps, backed by a silver reflector, minimize unwanted spontan ~ous laser the crystals used in harmonic conversion, is visible in Fig. 6 behind the two action, which would reduce the amount efficient (up to 80%) harmonic­ elliptical glass disks. A shield of flat of energy available for amplifying light conversion techniques, and new beam­ glass between each row of flashlamps traveling in the beam direction. Trans­ focusing techniques to minimize the and the amplifier disks helps protect the verse flashlamps and their reflectors appear at the rear. The interior parts of number of lens surfaces and their disk surfaces from damage should a the optical cavity are silver-plated for complexity for multiwavelength target flashlamp explode. These shields also high reflectivity. focusing. preserve beam quality by keeping turbulent convection currents of flashlamp-heated air out of the beam path. Scrupulous clean-room techniques are used throughout the assembly, maintenance, and operation of these amplifiers (the technician's costume is lint-free) to keep the optical surfaces free of scattering centers, such as minute dusk specks, which are the main source of beam nonuniformity and of potential optical damage. The aperture in the final 46-cm amplifier is so large that it posed two special problems: the possibility of spontaneous internal laser action within the plane of the disk and a size exceeding that for economical fabrication. To solve both problems, we split the disk in half (the vertical band visible across the middle of each disk in Fig. 6) to shorten the transverse gain path and to

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reduce the size of each plate. We then fabrication restrictions while maintaining clad the edge of each half disk, all the a desirable ratio of gain to glass way around, with a copper-doped glass thickness. that matches the index of refraction of the disk's phosphate glass. This edge omputer Controls cladding absorbs spontaneously emitted Without an extensive and laser light before it can undergo Csophisticated computer control reflection and be amplified to a network, it would be impossible to significant degree. Left uncontrolled, control a complex laser system such as such a buildup of spontaneous emission Nova, with its 1600 high-power electrical would deplete the excited-state circuits driving 5000 flashlamps and population in the glass before the desired 32 Faraday rotators, together with pulse arrived to sweep the energy from hundreds of moveable mirrors, electronic the amplifier. control functions, and diagnostic sensors. A split disk produces a split beam, as Figure 7 outlines the Nova control Fig. 7 seen in the beam profiles in Fig. 5. By system, in which fifty LSI-ll/23 using apodizing spatial filters and image­ microcomputers are linked with four Diagram of the Nova control system, showing the use of multiple and relaying techniques, we can control VAX-ll/780 computer systems. Common interchangeable minicomputers potential diffraction problems associated hardware and software provide the (VAX-11/780) and microcomputers with obscuration of the beam's center flexibility to transfer operations and (LSI-11/23). The operator control con­ line. Segmented-disk technology is functions from one computer to another soles, also interchangeable, are of the crucial to Nova and to future laser in the network. touch-panel type. We use optical fibers extensively to avoid electrical interfer­ systems because it allows us to expand, Nova's control system was designed ence from high-current discharges (as almost arbitrarily, the diameter of a to satisfy control and data-acquisition from flashlamps) when transmitting glass disk amplifier without material- functions in four areas: computer data and control information.

Operator Hardcopy consoles unit

VAX analysis computer Novanet To VAX Building 381 .------_------....~ development system VAX control co computers Video One-way data '----"'"",,,'-r'

~ ______-6·I_ I--______~ ~1 analysis system ~ ? • Low-speed status

Multiport memory Optical fiber @ ..,. I Novabus ~-~ ~- LSI-11/23 Event fibers "Pink" "Red" 10 [JI logger .. Power­ video video Novalink conditioning sources sources Device/ fibers devices 0 cameras o o Alignment and diagnostic devices and computers

13 • Power conditioning, including images, calorimeters, high-voltage capacitor-bank activation, laser firing, power supplies, interlocks, 20-kV and system timing. digitizers, transient digitizers, and • Alignment of the individual laser remote image memories). The control components and the target. required for these elements ranges from • Laser diagnostics (measurements simple status monitoring to the of beam energy and uniformity). demands of closed-loop alignment • Target diagnostics (measurements through the digital image processing of of temperatures and densities achieved two-dimensional beam profiles. in the target implosion). As an example of the system's remote, When designing the Nova system closed-loop alignment capability, in 1979, we made use of many new consider the task of positioning each of advances in digital controls. These over 70 spatial-filter pinholes scattered included high-speed (lO-MHz) optical­ throughout the laser bay. If done fiber communication links that manually, this task would require a crew interconnect the distributed computers of technicians and would take several and devices, an advanced control-system hours. In our system, a solid-state language, named Praxis (developed vidicon camera forms an image of the jointly with Bolt, Beranek, and Newman, output beam and sends it to the control Inc.), high-speed parallel processors for system. The control system automatically automatic image processing and beam processes the image and issues positioning, and touch-sensitive panels commands to stepper motors throughout overlaying color graphics displays on the the laser bay to adjust the positions of operations control consoles. the various pinholes, completing the The control system was organized alignment in only 30 minutes. into four main control and data acquisition subsystems and a fifth unifying subsystem (central controls) armonie Conversion that integrates functions and centralizes A crystal of potassium operations. The system supports more H dihydrogen phosphate (KDP), than 5000 individual control elements properly cut and oriented in a laser and sensors (stepping motors, video beam, can con'V'ert up to 80% of the incoming light to a second or higher harmonic, thereby increasing its frequency. 9 However, applying this technique and diagnosing the results on beams of the size and intensity of Nova's required considerable innovation. Conventional approaches to the design of harmonic converters capable of generating both second- and third­ harmonic light would require three assemblies containing crystals of three different thicknesses. We developed a system that consists of a single crystal array3 containing nine sets of two crystals positioned back to back (Fig. 8). Fig. 8 This arrangement increases the An assembled harmonic conversion ar­ interchangeability of parts and ray of KDP crystals sandwiched be­ significantly reduces costs. To generate tween transparent windows of fused the second-harmonic frequency, the silica. Two of these arrays, appropri­ array is oriented so that the two crystals ately aligned and irradiated with polar­ function independently, producing the ized 1.0S-JlIT1 light from the laser, can harmonically converted light in two provide either O.S3-pm (green) or O.3S-JlIT1 (blue) light for more effective orthogonally polarized components, one coupling to the ICF target. from each crystal. To generate the third

14 INERTIAL FUSION

harmonic, we simply rotate the assembly neutron and x-ray target yields, and it about the beam direction by 0.17 rad includes a basic set of target diagnostic (10 deg) and tum the crystals about instrumentation. 6 mrad (0.25 deg) to the proper phase­ Nova's primary target chamber is an matching angle, thereby converting two­ aluminum sphere 4.6 m in diameter. Its thirds of the incoming infrared (l.05-jim) diameter was originally chosen to beam to green (0.53-jim) light in the first support 20 final-focusing f/4 lenses 74 cm crystal. The second crystal mixes the in diameter at their correct focal distance remaining infrared light with green light (Nova was originally designed with 20 to produce third-harmonic blue light laser beams). However, as the final (0.35 jim). Efficient conversion is ten-beam design with harmonic­ obtained over a wide range of input conversion arrays requires the same focal beam intensities for second-harmonic distance, it was not necessary to change generation and over a smaller but still the chamber size. The walls of the adequate range for third-harmonic chamber are 13 cm thick to support generation. Figure 9 shows a atmospheric pressure, the weight of the harmonically converted Nova-like beam crystal arrays, and the focusing-lens drive produced by the Novette laser. mechanisms while maintaining accurate The size of Nova's beams poses alignment. The chamber is made of special problems for harmonic aluminum to prevent the buildup of conversion. Growing KDP crystals 74 cm long-lived radioactivation products in diameter is out of the question; associated with other commonly used it takes months to grow 27-cm-diam structural metals. Aluminum has only a crystals, and 74-cm-diam crystals would take years. Our solution was to assemble slices cut from 27-cm-diam crystals into a 3-by-3 mosaic array with a 74-cm-diam aperture. The growth, cutting, alignment, final finishing, and crystal coating technologies were all either scaled up dramatically to handle the volume or were newly invented. Two of the most significant innovations are the use of a diamond cutting tool on a precision lathe to cut each KDP crystal surface to its final smoothness, its correct angle with crystal axes, and its required flatness. A second important innovation is the sol-gel that is used to provide an antireflection coating on the crystals, raising their output by 12%. The sol-gel is applied as a thin liquid layer that dries to form the coating. Without the development of these finishing and coating technologies, it would have been impossible meet the Nova cost, performance, or schedule goals. he Target System The Nova target system is Tdesigned to accommodate a wide variety of experimental programs. It Fig. 9 provides a variety of target-focusing and beam-diagnostic options at each of three Output of the Nova-like laser chain at (a) the fundamental wavelength wavelengths in two target chambers, it (1.05/1m) and (b) the first harmonic is designed to operate with moderate (0.53 pm).

15 single isotope with a very low cross to develop many new approaches to the section for neutron capture, and the problems of isolation, amplification, resulting radioactive isotope has a half­ stray-beam suppression, control, and life of only 2.3 minutes. Hence, the only targeting. Construction was completed in lingering radioactivity will come from December 1984, on schedule and within impurities and alloying elements and will the budget of $176 million. Compared to be at almost unmeasurable levels, far previous LLNL lasers, Nova is four times below minimum laboratory and more cost effective (measured on a government standards. joules-per-dollar basis) in constant Figure 10 shows the target chamber dollars. During Nova's activation period, installed in the Nova system, with the which extends into the spring of 1985, five west beams equally spaced around the laser will be tuned to meet the west axis. The five symmetrically performance milestones (most of which placed east beams, entering from the were demonstrated with the Novette other side, are positioned so that each system). Weapon-physics and ICF aims between the west-beam ports. This experiments will begin in the summer of arrangement prevents one beam from 1985. The laser innovations and firing into the other. The five-beam inventions demonstrated here are already overlap spot at the common focus is less being extended to meet the needs for than 250 pm in diameter, including larger, more efficient, higher-average­ allowances for alignment, positioning, power laser systems.lO ~ and verification tolerances. ummary The Nova laser system, although S it incorporates many techniques and devices developed for the Argus and Shiva lasers, surpasses all previous lasers Key Words: Argus; frequency conversion; ICF; in power and wavelength flexibility. Its inerti al confinement fu sion; KDP; MALAPROP; advanced technology made it necessary Novanet; plasma shutter; Shi va.

Fig. 10 The Nova target chamber, a massive aluminum sphere 4.6 m in diameter with walls almost 13 cm thick. The large flanges carry the frequency-conversion arrays and the final-focusing lenses with their positioning mechanisms.

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Notes and References 7. W. W. Simmons, J. T. Hunt, and W. E. 1. J. L. Emmett, J. H. Nuckolls, and I. E. Wood, Warren, "Light Propagation Through Large "Fusion Power by Laser Implosion," Sci. Laser Systems," IEEE}. Quantum Electron. Amer., 230, 24 (1974). QE-17, 1727 (1981). 2. D. R. Speck et al., "The Shiva Laser Fusion 8. J. F. Holzrichter, D. Eimerl, E. V. George, J. B. Faci lity," IEEE J. Quantum Electron. QE-17, Trenholme, W. W. Simmons, and J. T. Hunt, 1599 (1981). Physics of Laser Fusion, Vol. Ill. High-Power 3. J. H. Nuckolls, "The Feasibility of Inerti al Pulsed Lasers, Lawrence Livennore National Confinement Fusion," Phys. Today 35, 24 Laboratory, Rept. UCRL-S2868, Rev. 1 (1982). (1982). 9. M. A. Summers et al., "A Two-Color 4. W. W. Simmons, Engineering Design, Conf- Frequency Conversion System for High Power 81 1040 (1 982). Lasers," IEEE/OSA Conf. on Laser S. D. J. Kuizenga, "Oscillator Development for Engineering and Optics (CLEO), Tech. Dig., 30 the Nd:Glass Laser Fusion Systems," IEEE, }. (June 1981). Quantum Electron. QE-17, 1694 (1981). 10. J. L. Emmett, W. F. Krupke, and J. B. 6. G. Leppelmeier and W. W. Simmons, Trenholme, The future Development of High­ Semiannual Report, Lawrence Livermore Power Solid State Laser Systems, Lawrence National Laboratory, Rept. UCRL-S0021-73-1, Livennore National Laboratory, Rept. p. 78, and UCRL-S0021-73-2, p. SO (1973). UCRL-S3344 (1982).

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