Improved Ar(II) transition probabilities

Precise Ar(II) branching ratios have been measured on a high current hollow cathode with a 1-m Fourier transform spectrometer. Absolute transition probabilities for 11 Ar(II) lines were calculated from these branching ratios and lifetime measurements published by Mohamed et al. For the prominent 4806 Å line, the present result is Aik = 7.12×107s-1 ±2.8%, which is in excellent agreement with recent literature data derived from pure argon diagnostics, two-wavelength-interferometry, and Hβ-diagnostics when using the theoretical data of Vidal et al

Oscillator strengths of Ti II from combined hook and emission measurements

It is demonstrated that a large set of accurate oscillator strengths of Ti II can be determined from a combination of hook and emission measurements without any assumption concerning the plasma state. Modified cascaded arcs and hollow cathode discharges have been used as plasma light sources for both hook and emission measurements. The relative f values have been converted to an absolute scale by means of literature data. The overall uncertainties of the f values are about 13-25%. Comparisons with other experimental and theoretical data are made which indicate excellent to fair agreement. Only for one of the published data sets has a wavelength-dependent discrepancy of up to a factor of two been found

Inversion of the Coupling Absorption at the Two-Photon Resonance in a Coupling-Probe-Spectroscopy Experiment

Using probe and coupling lasers, a system characterized by electromagnetically induced absorption was investigated. A switch of the EIA peak of the coupling laser to a dip was measured as function of the laser intensities

New design of electrostatic mirror actuators for application in high-precision interferometry

We describe a new geometry for electrostatic actuators to be used in sensitive laser interferometers, suited for prototype and table top experiments related to gravitational wave detection with mirrors of 100 g or less. The arrangement consists of two plates at the sides of the mirror (test mass), and therefore does not reduce its clear aperture as a conventional electrostatic drive (ESD) would do. Using the sample case of the AEI-10 m prototype interferometer, we investigate the actuation range and the influence of the relative misalignment of the ESD plates with respect to the test mass. We find that in the case of the AEI-10 m prototype interferometer, this new kind of ESD could provide a range of 0.28 μm when operated at a voltage of 1 kV. In addition, the geometry presented is shown to provide a reduction factor of about 100 in the magnitude of the actuator motion coupling to the test mass displacement. We show that therefore in the specific case of the AEI-10 m interferometer, it is possible to mount the ESD actuators directly on the optical table without spoiling the seismic isolation performance of the triple stage suspension of the main test masses

Gravitational wave detectors on the ground and in space

Small prototypes of gravitational wave detectors have been under development for over 30 years. But it is only now that we have the necessary technology available to build large instruments with good sensitivity. After several years of construction, the first ground-based interferometers will go into operation in 2001 and a space-based detector is expected to be launched in 2010. These instruments will complement each other because the gravitational wave spectrum extends over many decades in frequency. Ground-based detectors can only observe the audio-frequency regime above 1 Hz, while sources in the low-frequency regime are only accessible from space because of the unshieldable background of local gravitational noise on the ground

LISA and its pathfinder

On astronomical scales, gravity is the engine of the Universe. The launch of LISA Pathfinder this year to prepare the technology to detect gravitational waves will help us 'listen' to the whole Universe

LISA - an ESA cornerstone mission for a gravitational wave observatory

The European Space Agency has selected LISA, a gravitational wave observatory, as a cornerstone mission in its future science program Horizons 2000. This observatory will complement the development of ground-based gravitational wave detectors currently under construction. A spaceborne detector will enable the observation of low-frequency gravitational waves in a frequency range from to which is totally inaccessible to ground-based experiments. This frequency range is unique in that it is expected to contain signals from massive black holes, galactive binary stars, as well as the most violent events in the Universe. LISA will attain this low-frequency sensitivity by employing laser interferometric distance measurements over a very long baseline of . Three of these baselines form an equilateral triangle with spacecraft at each vertex. The cluster of spacecraft is in an Earth-like orbit around the Sun trailing the Earth by . The spacecraft contain infrared light-emitting Nd:YAG lasers and freely floating test masses made from a special platinum - gold alloy with vanishing magnetic susceptibility. The spacecraft are being kept centred on their test masses by using drag-free technology and field-emission electric propulsion, thus letting the test masses follow purely inertial orbits

LISA mission overview

More than 80 years ago, Einstein has predicted that accelerated masses will emit gravitational waves, propagating distortions of the spacetime fabric. The gravitational wave spectrum of known and expected sources covers many decades in frequency. While sources in the audio-frequency regime above 1 Hz are accessible to ground-based detectors, sources in the low-frequency regime can only be observed from space because of the unshieldable background of Newtonian gravitational noise. LISA is a laser-interferometric gravitational wave detector in space designed to observe gravitational wave signals from galactic as well as cosmological sources in the frequency range from 0.1 mHz to 1 Hz. LISA comprises a cluster of three spacecraft at the corners of an equilateral triangle of 5 Mio km size. The cluster is in an earth-like heliocentric orbit trailing the earth by 20 degrees. Each spacecraft carries lasers and free-flying proof masses and is kept on a purely inertial orbit by drag-free technology using field emission electric propulsio

Laser power stabilization for second-generation gravitational wave detectors

We present results on the power stabilization of a Nd:YAG laser in the frequency band from 1 Hz to 100 kHz. High-power, low-noise photodetectors are used in a dc-coupled control loop to achieve relative power fluctuations down to 5×10−9 Hz−1/2 at 10 Hz and 3.5×10−9 Hz−1/2 up to several kHz, which is very close to the shot-noise limit for 80 mA of detected photocurrent on each detector. We investigated and eliminated several noise sources such as ground loops and beam pointing. The achieved stability level is close to the requirements for the Advanced LIGO gravitational wave detector