Price Reducing Technology for "Pockels" Cell Production
Investigation of Potential Price Reduction of Large-Aperture Pockels Cell Fabrication Technology
Tech Area / Field
- PHY-OPL/Optics and Lasers/Physics
3 Approved without Funding
VNIIEF, Russia, N. Novgorod reg., Sarov
- Tomsk Polytechnical University / Scientific Research Institute of High Voltage, Russia, Tomsk reg., Tomsk\nRussian Academy of Sciences / Institute of Applied Physics, Russia, N. Novgorod reg., N. Novgorod\nSiberian Branch of RAS / Institute of High Current Electronics, Russia, Tomsk reg., Tomsk\nInstitute of General Physics named after A.M. Prokhorov RAS, Russia, Moscow
- Osaka University / Institute of Laser Engineering, Japan, Osaka
Project summaryThe goal of the project is to research whether it is possible to cheapen the construction of large-aperture Pockels cells. Today, Pockels Cell technology, with cell aperture up to 8-10cm, is completely developed and has now reached the stage of industrial manufacture. Such cells are made as the cylinder of electro-optical crystal KDP or Z-cut of DKDP with ring electrodes on its lateral face. These electrodes supply electro-optical crystal with longitudinal electric field. To achieve a reasonably uniform field distribution along the cross section of the crystal (to provide uniformity of Pockels cell aperture)  the crystal-aspect ratio (diameter/length) must be greater than one. Usually, crystal-aspect ratio is selected within the range from 1 to 2 [2,3]. So, the ability to manufacture Pockels Cells with increasing diameter is limited by crystal length, which growths unsatisfactorily. Growing of crystal length leads to the excessive optical absorption of the crystal. For example, the absorption coefficient of KDP crystal is 0.058 1/cm during the wavelength 1.05 microns. The other result of the growth of crystal length is an increase in strain depolarization, which is provided by inside points of inhomogeneous crystal structure. Such inhomogeneous points appear during crystal growth and crystal treating. Of course, another undesirable result is the increasing cost of such large crystals.
All attempts to overcome these difficulties, using liquid  or thin-film conducting  electrodes connected with lateral surfaces of the crystal, remain unsatisfactory, because such electrodes possess neither sufficient electrical conductance, nor sufficient optical-damage threshold.
The first publication to announce the realization of the idea of large-aperture plasma-electrode the Pockels cell (PEPC) appeared in 1984 . Since then, the technology of production manufacture of such plasma electrodes is improving continuously [3,9]. The construction of PEPC is as follows: vacuum regions on each side of the thin KDP or DKDP crystal are filled with working gas. High-current pulse ionizes this gas, forming suitable density plasma. These plasma formations have such a density, to make plasma practically transparent for optical radiation, but to supply high conductivity at the same time. Plasma formations are connected with electro-optic crystal, directly or by quartz plates, and they supply longitudinal electric field like electrodes.
It has been judged to be of interest to research whether it is possible to achieve cheap PEPC manufacturing technology. We suggest two methods of research.
The first one is that all PEPC, as described above, use KDP or DKDP crystals developed using standard technology. Using such technology it is possible to obtain growth speed of KDP and DKDP crystals of about 1mm/day. At the same time, the Institute of Applied Physics of RAS has unique technology of rapid KDP and DKDP crystal growth [7,8]. They achieved a speed of 1mm/hour. So, the time of crystal growth is reduced more than to an order, while crystal characteristics are practically the same. Homogeneity and the optical-damage threshold of rapidly developed crystals is no worse. That is why it is necessary to develop the experimental comparison of characteristics of different speed growth crystals. If the characteristics of rapid growth crystals are not so far from those of standard technology growth crystals or a little worse, a lower cost of one of the main parts of PEPC could be obtained.
Another way of fulfilling the proposed work is connected with the process of plasma electrode formation. It should be mentioned that the High Current Electronics Institute of SD RAS possesses considerable experience of dealing with plasma electrodes used in laser . The PEPC construction used here creates plasma electrodes in a two-stage process. The first stage is a low-current preionization (simmer) discharge; the second stage - a main high-current pulse discharge, which creates plasma electrodes of the requisite density. Preionization discharge is needed to increase plasma density homogeneity under the cross-section of the electrode, and such simmer discharge is initiated by a low-current power supply. Simmer discharge increases crystal heating, leading to a thermal gradient in crystal. This causes nonuniformity of the beam traversing the PEPC. The authors believe that preionization could be supplied with the help of ultraviolet radiation of special spark gaps or by special sounding antenna, supplying HF/VHF field. The authors suppose that these simmer methods would permit not only a decrease in PEPC heating, but they also may permit PEPC construction more easily and cheaply, by rejecting the idea of using powerful permanent magnets. Such magnets are used in the construction of cathode discharge gap .
The project participants are gaining experience in VHF installation development and measurement of its parameters , and they have used and researched different types of simmer discharge source for a considerable time . Moreover, the possibility of using inductive accumulating generators for the creation of plasma electrodes should be investigated. During preliminary research of laser pumping by different types of inductive power sources, the High Current Electronics Institute of SD RAS found that the use of inductive power accumulators helps to create volumetric discharge in a simpler and more homogeneous way. Also, inductive power accumulators must have significant advantages, especially in the creation of large-aperture plasma electrodes.
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