Optical fibre sensors are attractive devices that can bring substantial advantages over conventional sensing approaches
for fission Material Testing Reactors (MTRs), such as high accuracy capabilities with limited intrusiveness and the
ability to withstand high temperature. In the framework of the Joint Instrumentation laboratory (JIL), CEA and
SCK CEN have joined their resources to develop, in particular, an OFS prototype with the aim to measure dimensional
changes on nuclear materials irradiated in MTRs.
We briefly present the objectives and the workplan of that project, in which the first phase addressed an analysis of the
different measurement systems considered towards the specific environmental conditions encountered in a fission
reactor. Among them, radiation is responsible for the biggest error source through the density change of silica glass due
to neutron-induced compaction. The analysis has leaded us to focus mainly on an Extrinsic Fabry Perot design based on
low coherence interferometry. As part of the current development, we present the results of table top experiments that
allow appreciating the variation with different parameters of the response, especially the modulation of the signal
returned. That permits to set partially the design and brings some tolerances data. A home made signal conditioning
allows to extract the cavity length and then the change in the dimension of the sample to test.
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.
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.