Gravitational waves were detected for the first time in 2015 by the LIGO which, since then, has measured several other events in conjunction with VIRGO. These achievements have stimulated studies for next-generation gravitational telescopes to enlarge the discovery potential of such scientific facilities. Two studies are presently considered: the Cosmic Explorer (CE) and the Einstein Telescope (ET) in USA and Europe, respectively. To increase the detection performance a key parameter is the length of the Fabry-Perrot cavities in which high power laser beams are stored in an ultrahigh vacuum. For both CE and ET, more than 100 km of ~ Ø 1 m vacuum pipes are required, which would represent up to 50% of the total budget of the new experimental facilities if the LIGO and VIRGO’s configurations were adopted. To reduce the cost impact of the vacuum system, unconventional materials, less expensive pipe manufacturing and different surface treatments are scrutinized. In this work, we present measurements performed on low carbon steel (mild steel), having been proposed as an alternative to stainless steel. The outgassing rates of several as-cleaned low carbon steels were measured. Specific hydrogen outgassing rates at room temperature in the 10^-15 mbar l s^-1 cm^-2 were measured for bakeout temperatures as low as 80°C for 48 hours. Water vapour outgassing rates of unbaked samples were similar to or higher than those of stainless steel. To reduce water vapour outgassing, so that a bakeout can be avoided, a silicon coating was proposed. The coating has been produced by chemical vapour deposition with silane as precursor gas; the resulting layer was several hundreds of nanometres thick and resulted in the reduction of the water vapour outgassing rate by a factor 10. Such a value is not low enough to eliminate the need of a bakeout but could open the possibility of temperature treatments below 100°C. Room-temperature specific hydrogen outgassing rates of the Si coated steels in the low 10^-14 mbar l s^-1 cm^-2 were measured after bake-out at 80°C for 48 hours. The hydrogen intake in the studied steels during the coating was investigated by thermal desorption spectroscopy. Optimisation of the mild steel is under study in collaboration with industry to improve vacuum performance and corrosion resistance.

Heating a sample up to 400°C in a vacuum system seems not to be complicated, however when this same sample must travel through several vacuum chambers / load locks before arriving at its test location it becomes a greater challenge.

To overcome this stated challenge that is present at EBL2, a large Extreme Ultra-Violet Lithography (EUVL) test facility at TNO that used for EUVL lifetime experiments, we are in the process of designing a special sample holder that can reach, and control the sample temperature between ambient and 400°C.

The sample holder is being developed as part of the EU program, ID2PPAC. The objective of the project is to investigate EUV-material interaction effects at elevated temperatures. This research will contribute applicable knowledge that will result to better material selection in EUVL applications.

It is expected that the last version of the sample holder will require some logic control elements in the sample holder, this because of the limited pinout (number of electrical connections) of the sample holder. The development and testing sub-assemblies of this last version will be time consuming task because a lot of boundary conditions need to be tested.

To ensure that testing of materials can be tested at an earlier phase than before the completion of the last version of the sample holder, two forerunners will be first designed and manufactured.

A complication originates from the EUV power hitting the sample. This EUV power is a heat source that heats up the sample under test. One version will use a low power heater element to control the temperature, this one however is limited in the amount of EUV power the sample can receive, this to prevent overheating the sample.

The second sample holder will be used when the EUV radiation is higher. The sample holder will control the sample temperature by adjusting the backfill pressure between the sample and a colder temperature-controlled element.

By controlling the backfill pressure, the thermal conductance between the sample and the colder temperature-controlled element will change, this method will enable us to cool, and control the temperature of the sample.

During this presentation we will discuss the need for this sample holder, the design, and results of the two first versions of the sample holder. We expect also to be able to present the final design of the last version of the sample holder.

Europe is going to develop a third-generation underground gravitational wave (GW) observatory, known as the Einstein Telescope (ET). It is designed as a novel equilateral triangle with 10 km long arms and the detectors in each corner. Any two adjacent arms compose two independent interferometers. One interferometer will detect low-frequency gravitational wave signals (LF), while the other will be optimized for operation at higher frequencies.

In order to reduce seismic noise, thermal noise and other systematic noise, the whole system will be 200 to 300 m underneath the ground; the beamline pipes, the suspension towers and the cryostat containing the mirror require ultra-high or high vacuum conditions; and the main optics will partly be cooled to cryogenic temperatures below 20 K. In this way, the GW detecting sensitivity of ET will be significantly increased compared to the current advanced detectors (Virgo, LIGO) and the frequency band will be expanded to lower frequencies. The integral ET vacuum system comprises three different parts: (i) the beamline vacuum characterised by outgassing from the pipe walls, (ii) the tower vacuum characterised by outgassing from the suspension arrangement, and (iii) the cryogenic vacuum systems around the LF mirror.

In this paper, a Test Particle Monte Carlo model has been established with the KIT in-house code ProVac3D, to allow for a system analysis of the cryogenic vacuum area. It assesses the impinging rate of residual gas on the cryogenic mirror, depending on the particle sources from the beamline pipes and from the tower, which are systematically varied. With that, the expected speed of frost formation is estimated, which is critical due to degradations of the optical performance, and helpful information on engineering limits are derived. These simulation results are useful to find how far the cryopump section will influence the condition in the warm beamline pipe and the gas flow rate to the optical mirror. As a second major contribution, a shielding concept around the mirror is presented which reduces the gas load to a level fulfilling the requirements.