National Ignition Facility (NIF) / KDP Crystals
The very high-energy Neodymium (Nd)-glass lasers used for inertial confinement fusion in NIF, employ large plates of nonlinear crystals as electro-optic switches and frequency converters in the laser beam array. The crystals substantially increase the laser's energy output. The inertial confinement fusion lasers under construction in the U.S. and France have an aperture of approximately 40 cm x 40 cm. They require three different types of single crystal boules with linear dimensions in the range of 50-100 cm per side.
Potassium dihydrogenphosphate, KH2PO4, (KDP) and its analogs are the only nonlinear crystals currently used for these applications due to their unique physical properties. These properties include transparency over a wide region of the optical spectrum, resistance to damage by laser radiation, and relatively high nonlinear efficiency, in combination with reproducible growth to large size and easy finishing.

The main limitation in the growth of such large crystals by traditional techniques is a relatively slow growth rate of 0.5 - 1 mm/day. One to two year growth cycles for sizable KDP crystals that are produced by traditional means are normal. Traditional methods face a number of considerable challenges that include construction and reliable performance of the crystallizer, overcoming an inherently high risk of failure, and protecting against crystal defects during the long growing periods. The process typically results in low yields and high costs of the final crystals. These reasons stimulated development of new techniques to accelerate growth without sacrificing optical quality of large KDP crystals.

Research on the rapid growth of KDP crystals began in the early 1980s when the world's largest laser of that time, Nova, which had 10 laser beams, was being built at LLNL. New, rapid-growth techniques explored radical modification of standard crystallization equipment. It was thought that the simple crystallizers which were widely used in industry and scientific research would not be suitable for rapid growth for several reasons. First was the limitation in the mass of crystals grown from a closed volume of solution using a temperature reduction method. Second, it seemed impossible to achieve the hydrodynamic conditions and control of supersaturated solution concentrations needed to avoid defect formation and spontaneous nucleation. Alternative designs were proposed, but they could not solve these problems. As a result, the 27 cm x 27 cm plates of KDP crystals that were used in the Nova project were grown by traditional techniques.

Research, which began at Moscow State University in Russia, demonstrated that KDP crystals could be grown in standard crystallizers using an approach that was fundamentally similar to the traditional solution-growth process without observing spontaneous nucleation or visible crystal defects. This approach shortened the growth time by one to two orders of magnitude from conventional methods. The process was further refined and scaled-up for production of crystals for Nova's successor, the lasers in NIF and for the Laser Megajoule Project (LMJ) in France. The NIF laser system with its 192 beams will produce 1.8 MJ, which is 40 times the energy of Nova. The NIF lasers have an aperture of approximately 41 cm x 41 cm and require about 600 crystal plates of both KDP and its analog DKDP, a mono deuterated KDP analog. The crystals are used in Pockels cells, and also in flat lenses of two types. Type I using KDP. and Type II using DKDP. These two types are used for frequency conversion, i.e. shifting the laser beam frequency from infrared to the higher energy ultraviolet region.

Fig. 1 Schematic of the Nd-Glass inertial fusion laser used in NIF.
The Pockels cell is a kind of optical switch, which allows the beam to pass four times through the main amplifiers. This device uses the polarization property of a KDP electro-optic crystal to produce an "on/off" effect by rotating the polarization. The Pockels cell allows light to either pass through or to reflect, hence the "switching". The NIF Pockels cell essentially traps the laser light between two mirrors as it makes four one-way passes through the main cavity amplifiers before being switched out and greatly amplifying the energy of the beam. The switching is very rapid, on the order of 102 nanoseconds.

Depending on the type of application: switching or frequency conversion, the boules needed to yield crystal plates of this size must be about four times the size of the boules that were used for Nova. In order to achieve economically useful yields, crystals grown for NIF and LMJ, the crystals should exceed 50 cm in all three dimensions (Fig.2).
Fig. 2. The minimum size of KDP single crystal boules (in cm) needed for plates of different types for the laser arrays in the NIF project.
Successful development of the rapid-growth technique provided the technology to produce these crystals in record time in nearly 100% yield. A schematic of the apparatus used to grow the crystals follows.
Fig. 3. General schematic of a crystallizer used for rapid crystal growth: (1) growth tank; (2) air-sealed lead; (3) stirrer; (4) heater; (5) thermocontroller; (6) water bath; (7) platform with the seed; (8) growth solution; (9) water cup seal.

The newly developed crystal growth techniques have been successfully applied to KDP, and two KDP analogs, mono deuterated KDP (DKDP) and ammonium dihyrogenphosphate (ADP). Development required new scientific discoveries and new technical advancements. Furthermore, research that explored relatively high supersaturation and high growth rates revealed many phenomena that were previously obscure or unnoticed. Extensive research in the growth of crystals under a wide rage of conditions, including temperature ranges from 5 to 80°C, supersaturation levels of up to 40%, growth rates exceeding 50 mm/day, and crystal sizes of up to 90 cm, have extended scientific knowledge about crystal growth, their physical properties and their optical quality.

The process of research is remarkably similar across widely ranging scientific disciplines. Even though questions under study might be totally unrelated and the scientific fields seemingly totally different from each other, the general methods remain the same. An educational NASA project, Mars Student Imaging Project and the growth of KDP crystals are but two examples of the sameness of how research is actually done, despite being very different discipline-wise.

 

Introduction | Energy/Nuclear Energy | Inertial Confinement Fusion
National Ignition Facility/KDP | Solutions & Crystal Growth | Glossary