Thermography methods have found their applications in different fields of human activity. The non-destructive feature of
these methods along with the additional advantage by automated remote control and tests of nuclear installations without
personnel attendance in the contaminated zone are of particular interest. Laser active pyrometry and laser lock-in
thermography for in situ non-destructive characterization of micrometric layers on graphite substrates from European
tokamaks were under extensive experimental and theoretical studies in CEA (France). The studies were aimed to obtain
layer characterization with cross-checking the layer thermal contact coefficients determined by active laser pyrometry
and lock-in thermography. The experimental installation comprised a Nd-YAG pulsed repetition rate laser (1 Hz - 10
kHz repetition rate frequency, homogeneous spot) and a home-made pyrometer system based on two pyrometers for the
temperature measurements in 500 - 2600 K range. For both methods, the layer characterization was provided by the best
fit of the experimental results and simulations. The layer thermal contact coefficients determined by both methods were
quite comparable. Though there was no gain in the measurements accuracy, lock-in measurements have proved their
advantage as being much more rapid. The obtained experimental and theoretical results are presented. Some practical
applications and possible improvements of the methods are discussed.
The laser system is by far the most expensive part of a laser-produced plasma (LPP) EUV source. Thus LPP source designers have not only to optimize the technical performances of the lasers but also cost, efficiency, reliability, redundancy and industrial availability of their approach. Based on the typical conversion efficiencies of present LPP source targets (CE ~ 1- 3%), the laser system will have to deliver an average power of at least 20 kW. This can hardly be obtained with only few laser chains. Starting from this consideration, the EXULITE consortium has first proposed the concept of a spatially multiplexed and modular LPP source using 10 to 20 identical, moderate power laser units which are simultaneously focused on the same spot. This approach increases system redundancy and reduces the development cost of the laser. We will present an original technical approach which allows both to manage the multi-beam attack of the target using a closed-loop control and to collect a maximum of EUV power. The grazing incidence EUV collector is designed such that optimized vacuum pumping and debris mitigation can be obtained. Finally, we can show that modular LPP sources can satisfy the severe EUV dose stability that is commonly requested.
It is very difficult to achieve both high-power and low divergence lasing at once in cw or high-repetition rate lasers because thermal effects limit their efficiency. But as these effects are stable over time, a passive correction is conceivable. In this paper we propose a very simple but effective method to compensate the thermal effects in such lasers, using an aspherical phase-plate inside the resonator. The first results are very promising, as we demonstrated a very stable 418 W, M2=5.7 laser for only 2280 W of total optical pumping power (i.e 2050 W of absorbed pump power).
Since its introduction the thin disk laser based on Yb:YAG material has been thoroughly investigated and first industrial systems are now available. Nevertheless, some problems arise when scaling the power up: The pump set-up is becoming complicated and there could be some difficulties in bonding the thin disk on the cooling finger.
In this paper, we describe a novel pumping scheme solving those two problems. Instead of pumping from one face or on the side of thin disk we propose to use a combination of both using an additional optical element to guide the pump light to the disk. We show that up to 85 % of pump light can be absorbed by the laser material and the pump distribution can be very homogeneous.
FEA calculations show that this configuration is also favourable to a better cooling of disk and reduced deformation in the laser material.
Within the PREUVE project, the GAP of CEA Saclay has developed an EUV source that should meet (alpha) -tool specifications by the end of this year. In particular, a laser-produced plasma source has been developed that uses a dense and confined xenon jet target. Our technical solution is based on a specific target injector design and the use of well adapted nozzle materials to avoid debris formation by plasma erosion. After injection, the xenon is recycled and highly purified to reach a low cost round- the-clock operation. This source provides both high conversion efficiency and low debris flux. These are necessary conditions for its industrial application in the future EUV microlithography. The conception of the so-called ELSA (EUV Lithography Source Apparatus) prototype allows in principal 2 years full operation on the French lithography test bench BEL (Banc d'essai pour la lithographie) that has been developed during PREUVE. In parallel, the EXULITE consortium that is coordinated by Alcatel Vacuum Technology France (AVTF) has started its activities in the frame of the European MEDEA+ initiative on EUV source development. In collaboration with Thales and the CEA, AVTF develops a prototype power source for EUV lithography production tools by the end of 2004. A low cost and modular high power laser system architecture has been chosen and is developed by Thales and the CEA to pump the laser plasma- produced EUV source.
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