With the non degenerate recycling cavities (NDRC) the power and signal recycling mirrors will have to be placed near the suspended injection and detection benches. These are sensitive objects which require a seismic isolation level better than the one needed just for the output mode cleaner (OMC). This will impact the layout of the suspended benches and their seismic isolation. Space will be required, although space will be gained since the NDRC solution provides at the same time part of the beam matching telescope.

According to the reference solution two small mirror suspensions will be hosted onto the benches. The main issue in this case is the room on the bench and this development is highly interlaced with the development of those benches. These systems will certainly require full chain of local controls dedicated to control and recover their position with respect to the sensors placed outside the vacuum chambers. The main issue is related to evade the recoil due to the action on NDRC mirror sitting on benches and the related breadboards. Once the mechanical scheme will be even preliminarily designed a standard optical lever system will be easily deduced. In principle, provided the existence of one folding mirrors equipped with standard payload the angular correction dynamics can be attained by allocating to it the low frequency component.

The most critical bench is the suspended detection bench which is under vacuum and has the prime task of providing the right environment for the output mode cleaner. The layout of this bench has to be changed to accommodate at least:

- The signal recycling mirror. This mirror will have to be suspended, either from a structure attached on the suspended bench or from the marionetta located above the bench. The design of this local suspension will be carried out by Nikhef in collaboration with the Roma-1 group. In addition the required increase of seismic isolation could be given by one or more filter stages. The required control elements will add complexity on the injection and detection benches.

- The larger OMC. Even if its length remains small (below 10 cm), its oven used to control will require a little more space than the current 2.5 cm long OMC.

- The photodiodes for longitudinal and angular controls with associated telescopes. At least the dark fringe photodiodes will be installed on the suspended bench (2 photodiodes and 2 quadrants). Since photodiodes are equipped with motors and come along with shutters, beam dumps and small telescopes, additionnal space will be needed for those items. Even more space will be needed if air tanks have to be used. The extraction scheme of POBS is not designed yet but if it is sent to the same bench 4 more photodiodes will have to fit on this bench.

- As part of the Virgo commissioning activities, baffles and beam dumps have been added to dump the secondary beams produced by the optical elements. Some baffles are directly put on the detection bench and other are just attached around the bench to the vacuum chamber. The upgrade for Advanced Virgo is the opportunity to have a better design which integrates them onto the suspended bench to benefit of the seismic isolation. Vacuum compatible beam dumps will also have to be developed. 

All this will have to be accommodated on the detection bench. It is too early to make any drawing but it is very likely that additional space like the bottom part of the bench will have to be used. Nevertheless, this is a problem which could be solved once the elements to install on the suspended detection bench will be better known. 

This reorganization is a good opportunity to apply to the suspended detection bench some of the improvements foreseen for the clean air distribution in the large mirror vacuum chambers.

Fig: dat plaatje met de IMC end-mirror van NRC.