Precision mechanics and mechatronics

Vibrations in the mirrors should only come from gravitational waves.

This means that vibration dampers must isolate the entire installation from all sources of disturbances, and in particular seismic movements. Typically, three different types of vibrations must be reduced:

• Horizontal vibrations (via a chain of pendulums)
• Vertical vibrations (via elastic elements of maragin steel and inertial bodies)
• Rotational vibrations, where control is required in the tilt, yaw and roll directions.

The control is both active and passive. Passive control is performed using heavy concrete platforms, inverted pendulum systems and multiple levels of damping systems. The mechanical systems are designed for natural frequencies with a period of 5 to 10 seconds, so that these are outside the measurement range of the ET. The local damping systems traditionally consist of leaf springs, damping cables and stiff combinations of e.g. thin metal.

These components can be long (1 to 5 meters) and are composed of different materials such as stainless steel and aluminum alloys with different natural frequencies and mechanical properties (e.g. stiffness) depending on the location where they are used.

The active control systems must be able to detect and damp extremely small vibrations (on the order of 10-15m) at low frequency (periods of 5 to 10 seconds). Active control is required for six degrees of freedom (6 DoF ), which makes the control systems for the ET much more complex compared to existing gravitational wave detectors.

To achieve the required observational sensitivity, the instruments of the Einstein Telescope must consist of ultra-precise components. The instruments must be drastically isolated from all sources of disturbances, and more particularly from seismic movements.

For example, about 200 isolation systems are needed in the ultra-high vacuum to dampen vibrations for the central and recycling mirrors, input and output mode cleaners , optical measurement tables, mirrors for the optical filter cavities , and so on. The orders of magnitude for the components we are talking about are as follows.

• More than 5000 leaf springs
• >1000 cryogenic suspension wires
• >1000 inertial sensors
• >5000 displacement sensors
• >5000 actuators

The damping systems are needed for all six interferometer systems. Three of the interferometers need the vibration dampers at room temperature. In addition, there are three interferometers that operate at cryogenic temperatures, and whose damping systems must also be able to operate at cryogenic conditions.

Below is a list of requirements for the mechatronic components used in the Einstein Telescope . The list is not intended to be exhaustive, but it gives an impression of the complexity of the ET and the challenges that must be overcome.

• The components must be able to withstand ultra-high vacuum conditions.
• The components must be baked and cleaned before use under ultra-high vacuum. No impurities, water or gas residues are allowed. Connections and welds must be very clean for these ultra-high vacuum components. Possible impurities are for example chemicals that can corrode the pure silicon suspension wires, or water molecules that can create a thin ice film on the mirrors.
• The presence of hydrocarbons must be avoided at all costs. These become very volatile under high vacuum and can stick to the mirror, resulting in black spots or scattering of the laser beams. This is not an everyday requirement. For example, classic lubricants for engines and moving parts are absolutely out of the question.
• Some mechatronics systems must operate at cryogenic temperatures down to 10K to 20K, in addition to the need to maintain operation under ultra-high vacuum.
• Mechatronic systems operating at cryogenic temperatures must not come into contact with the outside world to avoid thermal bridges. Power and control lines must follow a strict sequence from colder to warmer stages .
• The mechatronic systems operating close to the mirror (e.g. in the vacuum towers) must be free from any possible vibrations. They must not have direct connection to the outside world (e.g. power and signal cables) to prevent noise from entering the towers via the connection. The connections must follow the strict sequence of the low vibration (mirror) stages to the outside environment.
• Components must operate at ultra low powers to avoid heat dissipation into the cryogenic environment.
• For the actuators coils are preferred, so that direct mechanical contact and heat exchange can be avoided. The coils must be protected against electromagnetic waves induced by the external environment, such as eddy currents or Schumann waves.
• The mechatronics systems must be able to operate at extremely low distances (up to 10-18m) and at low frequencies (0.1 to 10 Hz), which exhausts the possibilities that can be achieved with classical sensors.
• The displacement sensors must be custom made based on the individual specifications for each of the stages, and for use under cryogenic conditions.
• A special type of maragin steel, commonly used in aerospace, is preferred for the sheaths. It is expected that these elements will be difficult to find.

It is expected that research into new control systems and sensors will find its way into other highly demanding industries, such as aerospace, automotive or space applications.