Table of contents

The principles of aperture synthesis are outlined and the implementation of these principles in some algorithms is examined with reference to radio astronomy. Progress made in applying these techniques at optical frequencies in terrestrial and spacecraft instruments is briefly reviewed.

The rapid development of data acquisition and processing products provides an opportunity for the use of the fast Fourier transform (FFT) in a digital phase sensitive detector (PSD), where the measurement quality is largely dependent on the signal sampling. The error analysis on this scheme is related to the general error analysis of FFT, but there exist some essential differences between the two. The authors study the optimization of, and present several rules for, the signal sampling in a PSD, using FFT. The results are useful in reducing the influence of stationary and periodic interferences.

A high precision rotator with accurate mechanical phase control has been constructed. The system is primarily designed for the rotation of polarizers, diameters up to 100 mm, to produce amplitude modulation of broad band radiation in acousto-optical instruments for radiation intensity measurement. Rotation speed may be phase-locked at 2, 4 or 8 Hz to a crystal oscillator or over the range 0.625-10.5 Hz to an external signal. Air bearings are used for low friction and noise: acoustic output, at 80 mm, is only 16 mu Pa at 4 and 8 Hz. A high torque motor gives fast rotor acceleration, 57 rad s^-2, and broad-band phase control electronics (analogue and digital) utilizing encoding marks on the rotor circumference give good stability over the range of speeds without external adjustment. Outputs at 128 and 4 pulses rev^-1 are provided for synchronization of external apparatus such as phase sensitive detectors. The standard deviation, at 4 Hz (240 RPM), in phase determined between pi /2 pulses (4 per rev) is 1 mrad and in frequency, measured over 100 s, is 0.8 mu Hz (2*10^-7).

A fast-response temperature sensor for measuring atmospheric temperature was constructed and is described. The sensor was based on the thermocouple principle, connected to a thermocouple conditioner (AD595): the cold junction was compensated via an electrical reference and the signal amplified. This reference compensation was built into the sensor itself. The time constant of the thermocouple was decreased by rolling out a circular wire. The mean temperatures measured by the sensor were compared with those measured by an accurately calibrated Pt 100 resistance thermometer. The agreement between both sensors for outdoor measurements gave a standard error of estimate of 0.20 K. The fast outdoor temperature excursions around the running mean, measured by the sensor, were compared with those measured by a fast-response sonic thermometer. The agreement of the temperature variances between both sensors was better than 2% (standard error of estimate 0.05 K) and was dependent on measuring height and mean windspeed. The 3 dB point of the instrument was about 2 Hz.

Low-frequency ESR imaging requires field configurations which are difficult to obtain using traditional electromagnets. Multipolar magnets represent a convenient way to obtain both the main magnetic fields and two of the three gradients necessary for a 3D image reconstruction. The authors present the first experimental implementation of this design in the form of a 16-pole electromagnet of cylindrical symmetry, with an aperture of 27 cm and a length of 60 cm. To achieve a given field profile the currents at each pole must be individually controlled and the whole experiment must be under computer control. The power supply consists of 16 bipolar sections that deliver 10 A through a load of 3.7 Omega . The field is sensed by Hall probes at the 16 pole positions and this information is used to correct the field. Details are provided of the power supply and of the interface between power supply and computer.

A multi-channel accumulator is described, which provides reduced data acquisition times compared with existing multi-channel scalers and multi-detector systems. The instrument has been implemented using a single application-specific integrated circuit and a memory array, so that it may be mounted as a single-card plug-in for a system controller. Tests indicate that the instrument will accumulate data from the detector at rates corresponding to up to 25.6 million pulses per second. This represents a fivefold improvement over the previous best reported count rate. The application of the accumulator in a high resolution electron energy-loss spectrometer is considered although the specification is sufficiently general to allow its use in other experimental apparatus.

The high efficiency, high-repetition-rate operation of an all-solid-state exciter for KrF excimer lasers was investigated. A 12.2 mu s duration current pulse was compressed to 100 ns duration with a three-stage magnetic pulse compressor. An energy of 11.4 J stored in the primary capacitor was transferred, with an energy transfer efficiency as high as 71%, to the 3.0 atm (300 kPa) laser gas mixtures. The maximum KrF laser energy obtained was about 100 mJ with an overall efficiency of 0.87%. An average laser power of 7.1 W was obtained at 100 PPS (6.5 W at 140 PPS). Interface conditions of the exciter and the laser head were also investigated in terms of laser output by changing the output pulse duration and the total value of the peaking capacitors in a charge-transfer-type circuit.

A corona preionized pulser-sustainer, transverse, electric, atmospheric CO₂ laser has been developed. This small-aperture laser can produce 180 mJ optical pulses of 5 mu s duration from a discharge volume of 15.6 cm² with an efficiency of 12.4%. The laser was driven by a simple inexpensive solid-state exciter.

A simple and easy-to-operate real-time wavemeter that can discern the 10 Hz wavelength jitter of a pulsed laser is constructed. Only a single set of solid fused silica etalon and a 1024-channel diode array is needed to detect the wavelength jitters for various lasers. An optical fibre is used for easy alignment and operation without uncovering the wavemeter for various light sources. A method of determining the centre of an asymmetric etalon fringe, which sometimes has an ambiguous peak and is likely to change in intensity, is discussed in detail. The wavemeter is used to detect the relative wavelength change of a scanning dye laser, an injection-seeded Nd-YAG laser and a commercial He-Ne laser, and is tested by an iodine-stabilized He-Ne laser. The results are analysed and discussed in relation to the characteristics of the lasers and the capabilities of the wavemeter.

The authors present a novel wide bandwidth pulse transmission technique for the study of the interactions between near-plane-wave ultrasound and thin films (down to 50 mu m) of adhesive polymer set between glass substrates. Acoustically thick transducers are clamped in coaxial alignment on either side of the glass substrates and short (less than 10 ns) acoustic transients are made to reverberate to and fro in the test bond. The signal received consists of time-resolvable and successively dispersed reverberations from the bond layer. It is digitized at 1 GHz and approximately corrected for the effects of transducer insertion and transient radiation coupling between the transducers. Frequency domain methods are then applied to estimate absorption coefficient, propagation velocity and the real and imaginary parts of the plane-wave elastic modulus, all as functions of frequency. Preliminary data obtained by this technique indicate that a number of adhesives display plane-wave velocity dispersion and absorption as a function of frequency that can be modelled by a relaxation process with a single time constant. A simple spring-dashpot model for an anelastic solid provides a mechanistic equivalent to the observed relaxation.

Void fraction measurements are made by the gamma -radiation attenuation method in mercury-air two-phase flow systems. To measure the void fraction with an accuracy of the order of 5%, the required gamma -ray source and the strength of the source is experimentally found for mercury-air two-phase flow systems of pathlength up to 80 mm using ¹³⁷Cs and ⁶⁰Co. This is achieved by comparing the simulated void fraction and the measured void fraction. Since the dynamic fluctuations affect the accuracy of the measurement, the dynamic effect is analysed by simulating void fraction fluctuations of different magnitude. If the fluctuation magnitude is known then the required correction can be applied to the measured void fraction. The measured void fraction is compared with the predicted values by a theoretical model available for the upward liquid metal-gas two-phase flow in pipes.

The authors describe a floating bridge technique and a high-voltage apparatus for carrying out interrupted-field time-of-flight (IFTOF) transient photoconductivity measurements with applied voltages up to 1 kV. Switching transient voltages appearing as a result of the removal and reapplication of the applied bias voltage, which otherwise would obliterate the normal TOF photocurrent signal, have been reduced to an undetectable level by using a floating Schering bridge network with two nulling capacitors. The bias voltage is applied from a single triggerable floating high-voltage switch which utilizes n-channel TMOS transistors and can deliver up to 1 kV in a switching time of approximately 150 ns. By switching the gain of an amplifier during the interruption time, even very small recovered photocurrent signals can be measured with respect to the pre-interrupted signal. The technique has been successfully applied to measure both hole and electron trapping lifetimes in a-Se:0.5%As electroradiographic photoreceptors, and to determine the range of the charge carriers. The distinct advantage of the present apparatus, as well as its shortcomings, over the complementary bias and single nulling capacitor method, which was described previously, are also addressed.

A device for cleaving crystals mounted on a standard sample holder end is described. The cleaver is fixed on a CF 35 UHV flange that can be placed anywhere along the path of a rotation-translation motion feedthrough for sample transfer.

The authors describe a compact, low-temperature STM (scanning tunnelling microscopy) design whereby the scanning tip crosses a TE-mode K-band cavity at a high E-field region of the cavity, defined by an iris and a movable end-wall microwave plunger.

It is desirable to know the mechanical and physical properties of thin film metallizations used as interconnects in microelectronic circuits. These properties are best determined from films that have been removed from actual device substrates. Removing intact, free-standing CVD tungsten thin films (1.0 mu m) from a phosphosilicate-glass covered silicon substrate is possible. The method involves an annealing treatment and wet chemical separation treatment using hydrofluoric acid. The resulting films are suitable for mechanical testing.

It is shown that in collision experiments an enhanced signal to noise ratio can be achieved in principle through the simultaneous measurement of several cross sections. The measurements of differential, total and coincidence cross sections are discussed.