Gerrit Kühn

The AEI 10 m prototype interferometer

A 10 m prototype interferometer facility is currently being set up at the AEI in Hannover, Germany. The prototype interferometer will be housed inside a 100 m 3 ultra-high vacuum envelope. Seismically isolated optical tables inside the vacuum system will be interferometrically interconnected via a suspension platform interferometer. Advanced isolation techniques will be used, such as inverted pendulums and geometrical anti-spring filters in combination with multiple-cascaded pendulum suspensions, containing an all-silica monolithic last stage. The light source is a 35 W Nd:YAG laser, geometrically filtered by passing it through a photonic crystal fibre and a rigid pre-modecleaner cavity. Laser frequency stabilisation will be achieved with the aid of a high finesse suspended reference cavity in conjunction with a molecular iodine reference. Coating thermal noise will be reduced by the use of Khalili cavities as compound end mirrors. Data acquisition and control of the experiments is based on the AdvLIGO digital control and data system. The aim of the project is to test advanced techniques for GEO 600 as well as to conduct experiments in macroscopic quantum mechanics. Reaching standard quantum-limit sensitivity for an interferometer with 100 g mirrors and subsequently breaching this limit, features most prominently among these experiments. In this paper we present the layout and current status of the AEI 10 m Prototype Interferometer project.

A spatially resolved relaxation method for pLTE plasma diagnostics in free-burning arcs

We report on a relaxation method to determine the electron-to-gas temperature ratio in the near-cathode region of a low-current free-burning arc operated in argon under atmospheric pressure. We used an intensified CCD camera to record the relaxation process after a fast power-interruption of the discharge with a time resolution better than 1 µs. For the CCD images we applied an ABEL inversion procedure yielding 3D information about the temperature ratio β := Te/Tg with a spatial resolution of 50 µm. We deduced spatially resolved values of β from the relaxation experiment using independently measured values of Te and ne, so we could determine all free parameters in the partial LTE model. (Some figures in this article are in colour only in the electronic version)

Design of the 10 m AEI prototype facility for interferometry studies

The AEI 10 m prototype interferometer facility is currently being constructed at the Albert Einstein Institute in Hannover, Germany. It aims to perform experiments for future gravitational wave detectors using advanced techniques. Seismically isolated benches are planned to be interferometrically interconnected and stabilized, forming a low-noise testbed inside a 100 m^3 ultra-high vacuum system. A well-stabilized high power laser will perform differential position readout of 100 g test masses in a 10 m suspended arm-cavity enhanced Michelson interferometer at the crossover of measurement (shot) noise and backaction (quantum radiation pressure) noise, the so-called Standard Quantum Limit (SQL). Such a sensitivity enables experiments in the highly topical field of macroscopic quantum mechanics. In this article we introduce the experimental facility and describe the methods employed, technical details of subsystems will be covered in future papers.

Status of the AEI 10?m prototype

The AEI 10 m prototype will be an ultra-low displacement noise facility consisting of an L-shaped ultra-high vacuum system with about 10 m long arms, excellent seismic isolation, a well-stabilized high power laser and other advanced interferometry techniques. In the first round of experiments an interferometer to measure at the standard quantum limit of classical interferometry will be set up. This paper describes the status of the AEI 10 m prototype and its individual sub-systems as of April 2012.

Towards a Suspension Platform Interferometer for the AEI 10 m Prototype Interferometer

Currently, the AEI 10 m Prototype is being set up at the Albert Einstein Institute in Hannover, Germany. The Suspension Platform Interferometer (SPI) will be an additional interferometer set up inside the vacuum envelope of the AEI 10 m Prototype. It will interferometrically link the three suspended in-vacuum tables. The inter-table distance will be 11.65 m. The SPI will measure and stabilise the relative motions between these tables for all degrees of freedom, except roll around the optical axis. In this way, all tables can be regarded as one large platform. The design goal is 100 pm/ differential distance stability between 10mHz and 100Hz.

Near-cathode region of a free-burning arc: a spectroscopic investigation

The cathode as an electron emitter possesses different modes of operation to maintain the current forced upon the discharge. We investigated spectroscopically the near-cathode region of a free-burning low-current argon arc operated at atmospheric pressure in order to measure the plasma parameters of that mode of operation that we call the blue-core mode. For better interpretation of the experimental data we applied a simple two-temperature model. As a result, we can describe the arc plasma up to 100 µm near the cathode surface.

Designs of the frequency reference cavity for the AEI 10 m Prototype interferometer

The AEI 10 m Prototype is in its designing phase and will provide a test-bed for very sensitive interferometric experiments, such as the sub-SQL interferometer. It will test new techniques to reach – and even surpass – the Standard Quantum Limit. The experience and knowledge that can be gained from this experiment can be applied to large-scale interferometric gravitational detectors to improve the detector sensitivities. In order for the sub-SQL interferometer to achieve the required sensitivity all limiting noise sources need to be suppressed sufficiently. Noise sources can include seismic noise, thermal noise, and laser noise; laser frequency noise will be the main focus of this document. The laser frequency noise will be suppressed to a level of 10−4 Hz/ at 20 Hz dropping to below 10−6 Hz/ at 1kHz. The proposed design to suppress the laser frequency noise with a ring cavity is described in this paper.

2D display of tungsten impurity in a free-burning arc using laser-induced fluorescence

We present laser-induced fluorescence (LIF) measurements, spatially and temporally resolved, of tungsten impurities eroded from the cathode of a free-burning argon arc. Saturation, but more drastically quenching processes limit a quantitative evaluation of the measurements. Applying a special measuring technique, we were able to determine relative densities and, via a Boltzmann plot, excitation temperatures. A 2D display of the metal impurity can be obtained with a planar LIF arrangement. It shows the distribution of that part of metal component being in a distinct mode of ionization and excitation. As known, the population distribution of the tungsten impurity is dominated by strong demixing effects.

Passive-performance, analysis, and upgrades of a 1-ton seismic attenuation system

The 10m Prototype facility at the Albert-Einstein-Institute (AEI) in Hanover, Germany, employs three large seismic attenuation systems to reduce mechanical motion. The AEI Seismic-Attenuation-System (AEI-SAS) uses mechanical anti-springs in order to achieve resonance frequencies below 0.5Hz. This system provides passive isolation from ground motion by a factor of about 400 in the horizontal direction at 4Hz and in the vertical direction at 9Hz. The presented isolation performance is measured under vacuum conditions using a combination of commercial and custom-made inertial sensors. Detailed analysis of this performance led to the design and implementation of tuned dampers to mitigate the effect of the unavoidable higher order modes of the system. These dampers reduce RMS motion substantially in the frequency range between 10 and 100Hz in 6 degrees of freedom. The results presented here demonstrate that the AEI-SAS provides substantial passive isolation at all the fundamental mirror-suspension resonances.

The AEI 10 m Prototype Interferometer frequency control using the reference cavity and its angular control

The main purpose of the AEI 10 m Prototype is to reach and eventually surpass the Standard Quantum Limit at frequencies ranging from 20 Hz to 1 kHz with a 10 m arm-length Michelson interferometer named the sub-SQL interferometer. The frequency control system uses a 20 m optical path length triangular suspended cavity named the reference cavity, with the goal of suppressing frequency noise of the input laser to a level of ~ 10-4 Hz/ at 20 Hz rolling off to below 6 × 10-6 Hz/ above 1 kHz. It is expected that tight angular control of the reference cavitys mirrors is necessary to reach this stringent requirement.