Table of contents

Sensors for detecting moving phenomena have to be specified for two conflicting aims: high sensitivity which requires large apertures and wide bandwidth which demands small apertures. By modelling a sensor aperture with a finite-impulse response (FIR) digital filter, the transfer function of arbitrarily shaped and sensitive electrodes can be computed. Dividing the aperture into active and non-active segments according to a binary pulse-compression code gives rise to sensors with a wide bandwidth despite a large overall size.

A versatile system for regulating and displaying the liquid nitrogen level has been developed on the basis of the Archimedes principle. The system consumes almost no energy during its operation in the liquid nitrogen container.

A low-cost, easily implemented optical position transducer with a range of about 10 mm and a precision of about 0.15 mm is described. The transducer uses a commercial photon-coupled interrupt module and a photographic film filter with a transmission which varies linearly with position. Simple and novel means of fabricating such filters and others of arbitrary characteristics by computer generation are described. The transducer has been successfully employed in an undergraduate mechanical resonance experiment.

A detailed description of a smoke generator system for marker nephelometry, laser velocimetry and flow visualisation research in fluid mechanics is presented. In this generator system a low vapour pressure, commercially available oil is vaporised in a preheated chamber and the carrier gas/vapour stream is rapidly cooled to produce an oil condensation smoke with a particle size in the range of 0.2-2.0 mu m. The thermal and process dynamics of the system have sufficiently large time constants to maintain a relatively close control on the smoke concentration from this type of generator process.

The equations for the heat that has passed through each plane from one steady state to another are obtained from the Laplace transform of the differential equation of heat conduction in a solid bounded by two parallel planes. From these equations for the heat flow, a method for the simultaneous determination of specific heat and thermal conductivity of a solid is deduced. This determination is carried out by measuring quantities of heat, and neither measurement of temperature gradient nor comparison with a sample of known thermophysical properties is needed.

A flux calorimeter for the determination of the thermal properties of solids, useful for the measurement of specific heat, thermal conductivity and pyroelectric coefficient, is described. The specific heat of aluminium oxide between 250 and 350 K is determined to show the precision of the measurements. The specific heat and the thermal conductivity of triglycine sulphate are measured near the ferroelectric transition point. From these data, the thermal diffusivity of the sample is calculated. The data agree with those found in the literature.

A description is given of a broadband ratio pyrometer suitable for measuring the temperature of aluminium during processing. Each band, one using a Ge detector and the other a PbS detector, was also used independently as a brightness pyrometer. This tri-modal pyrometer (TMP) has intrinsic precisions of 1 degrees C for the ratio pyrometer, 0.4 and 0.3 degrees C respectively for the Ge and PbS brightness pyrometers over the spectral radiance temperature range of 250 to 600 degrees C. However, for aluminium processing in both rolling and extrusion processes, much greater uncertainties result from the various factors affecting emissivity, e.g. the nature of the process in the plant, the aluminium alloy, the shape of the surface and the temperature of the product. It was found that for some applications the ratio pyrometer was preferable to the brightness pyrometers, but in general it was possible to establish by experiment emissivity functions that allowed the temperature of aluminium to be measured with at least one of the TMP modes with an uncertainty of less than +or-10 degrees C.

A two-colour pyrometer was developed for the measurement of the surface temperature of particles in flight in a DC plasma-spraying jet which is characterised by high temperatures (up to 10000 K), high velocities (more than 500 ms^-1) and very steep radial temperature gradients (3000 K mm^-1) and velocity gradients (300 m s^-1 mm^-1). Measurements of surface temperature have been made on metallic (Ni) or ceramic powders (ZrO₂, Al₂O₃) with mean diameter of 10-100 mu m. The surface temperatures of the particles range from 2000-4000 K and their velocities (measured simultaneously by laser anemometry) are from 50-300 m s^-1. One of the major difficulties of pyrometry in such flows is the plasma radiation, especially in the jet core.

A multi-ring electrode is used to determine the distribution of the current at the anode of a 200 A arc burning in vacuum and in argon at a pressure of 0.13, 0.65 and 1.3 kPa (1, 5 and 10 Torr).

A method is presented for frequency compensation of a diamagnetic loop that is magnetically coupled to a concentric stainless steel vacuum vessel. In contrast to previous approaches employing active RC filter circuits, this technique can be done entirely with digital computer signal processing. Absolute signal calibration and empirical measurement of the system transfer function are accomplished with a pulsed simulation solenoid. Maximum high frequency response is determined by transfer function noise limitations. Measurements made on the Maryland MIX-1 device show that although the plasma includes hot and cold ions and cold electrons, the diamagnetic loop signal is dominated by the hot-ion kinetic pressure. In particular, hot-ion temperatures deduced from a comparison of diamagnetic loop and ion density (probe) signals compare favourably with independent measurements made with an end loss analyser. Similarities between hot-ion energy and density confinement times indicate that energy confinement is dominated by hot-ion streaming losses.

A two-quadrant divider system is described for accurate analogue computation of the ratio Q=10 N/D. Errors in Q are less than 0.1%, rise time 3 mu s, dynamic range and common-mode rejection 60 dB. Although designed for circular dichroism measurements in the microsecond time range, it has been applied to other measurements needing precision or fast analogue division: spectroscopic source ratioing, laser speckle vibrometry and sound pressure correction.

The impedance and/or frequency ranges of commercial equipment used in electrochemical studies have been extended by interfacing an electrochemical cell and measuring apparatus with an active circuit. Design philosophy and performance of several devices are discussed. The detailed layout of an easily assembled circuit is presented which compensates for the impedance of the connecting cables.

The characteristic equation of a tuned secondary fluxgate sensor is solved analytically using the Senson-Hannaford approximation of a pure second harmonic load current. The gain and bandwidth of the parametric amplification is expressed in terms of the sensor geometry, the core magnetic properties and the excitation level, and they agree with the measurements. The output signal phase and the stability criterion are derived, and the noise properties of the tuned sensor are discussed. Measurements of the signal-to-noise ratio of tuned and untuned sensors show that sensor tuning enhances the noise.

A low-cost microprocessor-based instrument for selective excitation in NMR imaging has been developed. The system is based upon a dual-channel single-sideband modulator (SSBM) and digital waveform synthesiser. The chosen structure makes the instrument capable of generating radio frequency pulse envelopes of arbitrary complexity. It also enables compensation of performance limitations associated with practical SSBMs, to achieve outstanding image spectrum suppression and harmonic distortion levels.

An apparatus has been developed for obtaining high nuclear polarisation by means of microwave-induced optical nuclear polarisation. In this recent variety of dynamic nuclear polarisation (DNP), the sample is doped with centres that are diamagnetic in the ground state, which can be excited to a paramagnetic state via irradiation with light. The nuclei are then polarised as in conventional DNP, using these excited states. The apparatus consists of an electron spin resonance spectrometer with optical detection operating at 75 GHz and at 1.4 K. It is further provided with nuclear magnetic resonance equipment necessary to measure the nuclear spin polarisation.

A new type of integrating parallel output spectrum analyser working in the megahertz frequency range is described. The spectrum analyser is primarily designed for use at the back end of a millimetre-wave radiometer, taking a mixed-down millimetre-wave signal and outputting its spectrum in digital form via an RS232 interface. The new aspect of this spectrum analyser is in the use of a novel type of surface acoustic wave (SAW) component, resulting in a high performance compact instrument.

A high performance Brillouin scattering set-up is described, which consists of a vernier-type tandem Fabry-Perot interferometer (TFPI) for frequency analysis. The optical system is designed in such a way that alignment of the whole set-up can be achieved and maintained very easily. The very good long-term stability allows sampling times of several days. Along with a detailed discussion of the construction the authors give specifications of the optical components and outline some fast and easy alignment procedures for the TFPI as well as for the peripheral optics. Measurements of light scattered from thermally excited bulk and surface waves in NaCl are presented to demonstrate the performance of the system.

A simple stroboscopic pulse generator is described which can be used with an electro-optic modulator for real-time holographic interferometry of vibrating objects. This unit makes it possible to optimise the characteristics of the stroboscopic pulse quickly and conveniently and to vary its phase relative to that of the vibration.

For mass spectrometers with magnetic scanning the m/e ratio is directly proportional to the square of the magnetic field intensity, i.e. m/e varies as B². In the device described a voltage signal precisely representing the ratio m/e is obtained by means of two Hall probes placed in parallel between the poles of the electromagnet. The first Hall probe is fed by a constant DC supply, while the second is fed by a current directly proportional to the output voltage of the first Hall probe. Since both Hall probes experience the same magnetic field the output voltage of the second Hall probe is proportional to B².

A time-of-flight mass spectrometer was designed to determine the size distribution of metal clusters produced by inert-gas aggregation. Antimony clusters grown in argon gas range in size from a few atoms up to several hundreds of atoms. The cluster size distributions are variable depending on the experimental conditions, such as rate of evaporation, gas pressure and cell geometry.

A simple device is designed and demonstrated for the accurate measurement of ion or neutral beam profiles in instruments where the beams may be scanned. The device is self-contained, compact and, requiring no electrical connections, is suitable for sample stages where samples may be exchanged without breaking the vacuum. The method is appropriate to beams with diameters in the range 10-500 mu m.

Digital techniques are presented for filtering particle scatter contributions to laser Rayleigh scattering signals. The filter methods follow from criteria based on particle scatter characteristics and allow differentiating between molecular and particle scatter contributions to the measured signal. Use of the techniques reported allow for Rayleigh scattering measurements in flows where particles are present. Measurements of mean and RMS mole fraction distributions are presented for a free jet and the results for the mean compare with those obtained from a sampling probe and indicate uncertainty in the mean to be within 2% of unity. For the RMS results, particle scatter is shown to lead to overprediction when the Rayleigh signals are unfiltered but the problem can be circumvented to a significant degree by the methods described. It is concluded that particle scatter contributions to laser Rayleigh scattering signals cannot, in some instances, be associated with any criteria which allow their removal.

A triple cell constructured and applied for spectral, photophysical and photochemical studies over a wide temperature range is described. Its advantages are shown over the traditional solution where a measuring cell or a tube is placed inside a Dewar.

The performance of a Doppler monochromator using a multi-anode microchannel plate detector has been evaluated. The system uses a photon-counting technique directly on an array of 20 curved anodes. A resolving power of 15000 has been obtained.

A four-wire probe and a novel signal interpretation procedure were developed for the simultaneous measurement of three instantaneous velocity components in a three-dimensional, turbulent, flow field. The new probe consists of four sensors slanted at 50 degrees with respect to the centreline axis and arranged in a double V configuration; each V configuration is perpendicular to the other. The implementation of this technique and the inclusion of the fourth wire remove ambiguity and resolve most of the problems affecting the performance of the three-wire probe and, in particular, prevent the occurrence of conditions of non-uniqueness. The new technique was tested in turbulent pipe flow and in a swirling jet issuing into a co-flowing stream. Some of the results obtained in the swirling flow are compared with previous measurements, which were obtained using a five-tube pressure probe. The agreement between both sets of data is shown to be very good.

The design of a low cost and fast analogue buffer memory based on a charged-coupled device analogue shift register is described for extending the time resolution of multichannel analysers down to 20 ns.

Describes the design of a simple circuit which gives a direct reading of the magnetic field by converting the frequency of an NMR magnetometer. The circuit uses n-bit BCD programmable frequency dividers to perform the conversion to a high degree of precision.

The authors describe a simple and economical device used to monitor the orientation of the gondola in a stratospheric flight from Sicily to Spain. The solar sensor furnished the orientation of the gondola in daytime with an accuracy of +or-40'.

Giotto encountered Comet Halley on 14 March 1986, about four weeks after the comet's perihelion passage when it was most active. Giotto passed the comet nucleus at a distance of 600 km on the sunward side. Its scientific payload-comprising a narrow-angle camera, three mass spectrometers for the compositional and isotopic analysis of the neutral gas, ions and dust particles in the cometary coma, various dust impact detectors, a photopolarimeter and a set of plasma instruments-worked flawlessly and returned a wealth of data. The mission's scientific objectives, the spacecraft and the mission from launch to encounter are described. After the encounter Giotto was retargeted to return to the Earth. An Earth swing-by manoeuvre on 2 July 1990 will allow Giotto to encounter Comet Grigg-Skjellerup in July/August 1992.

The Giotto magnetometer experiment employs a low-mass (1.357 kg), low-power (818 mW) instrument in a dual magnetometer configuration using flux gate sensors of the ring core type. It has provided accurate vector magnetic field measurements on its way to and near Comet Halley, working flawlessly from switch-on on 22 August 1985 to the formal end of the mission on 15 March 1986.

The RPA-Copernic plasma experiment for the Giotto mission consists of two sensors, the RPA1-EESA spectrometer and the RPA2-PICCA electrostatic mass analyser. RPA1-EESA measures the three-dimensional distributions of electrons between 10 eV and 30 keV. These electron measurements should contribute to the definition of the properties of the cometary plasma and its interaction with the solar wind. This sensor includes an electrostatic symmetric quadrispherical analyser of novel design and provides 4 pi electron measurements with high sensitivity and good energy, angular and time resolution. With a 360 degrees *4 degrees field of view and with 17 fast-counting MCP sectors, a three-dimensional distribution is obtained every 2 s (half a spin of the space probe). The experiment includes three microprocessors. They interface RPA1-EESA with RPA2-PICCA and with the spacecraft, and perform extensive onboard data processing. One of the processors is designed to sort the three-dimensional electron distribution into two-dimensional pitch angle distributions in real time. The symmetry axis of the electron momentum flux tensor is computed on board. This symmetry axis is generally aligned with the magnetic field direction.

A lightweight energetic particle detector system (EPONA/EPA) is described which was designed to operate in those novel environmental conditions (i) characterised by the varying solar aspect angles and temperatures pertaining during the cruise phase of the Giotto spacecraft and (ii) during the dust bombardment characterising closest approach ( approximately 600 km) to the nucleus of Comet Halley. Representative data, illustrating the functioning of EPONA during both the Giotto cruise phase and at comet encounter, are presented. The instrument has a wide range of applications to other space experiments where reliable plasma diagnostics are required. Data monitoring is also discussed together with data reduction.

The dust impact detection system (DIDSY) comprises six independent subsystems with the primary aim of registering the impacts of all particulates of significant mass incident on the Giotto probe during the post-perihelion encounter with Comet Halley in 1986. Mounted on Giotto's front dust shield, the detectors determine the mass distribution of the dust with a dynamic range of some 16 decades of mass and an ultimate spatial resolution of some 70 km. An additional detector located on the rear shield monitors the dust particles that are able to penetrate the front dust shield. Sensing techniques comprise diaphragm-mounted piezoelectric elements for large and intermediate masses, film capacitance discharge sensors for intermediate masses and impact charge sensors for the smallest masses. They are controlled and the data processed by a microprocessor-based system that allows the wide range of anticipated flux rates to be handled. Measurement principles and sensor characteristics are described.

As expected, a wide range of ion species and velocity distributions were found as the Giotto spacecraft traversed the coma of Comet Halley. The outer coma was characterised by the interaction between solar wind and cometary plasmas; the inner coma by the outflow of cometary neutrals and their ionisation products. The resultant demands on instrument dynamic range had precluded the use of a single sensor for measurements of ion composition. The Giotto ion mass spectrometer (IMS) therefore consisted of two sensors: one optimised for the outer coma and the other for the inner coma, with each obtaining complementary information in the region for which it was not optimised. Both sensors featured mass-imaging characteristics, thereby permitting simultaneous measurements of several ion species by means of multi-detector arrays. Resultant mass per charge resolution was >or=20. In addition to mass per charge, the energy per charge and the elevation and azimuth of the incident ions were measured. In-flight data show that the IMS met its scientific goals for the Halley encounter.

Describes the technique used to record time delay and waveform measurements for the Giotto radio science experiment of ESA's mission to Comet Halley. The data were taken by using either two-way measurements (during pre- and post-encounter) or one-way measurements (during encounter with Comet Halley), the downlink of the radio signal of the Giotto spacecraft being received at 8.4 GHz by the 64 m tracking stations of NASA's Deep Space Network (DSN). The waveform measurements were obtained at a sampling frequency of 50 kHz with an open-loop receiver assembly at DSN station Canberra as used for the Voyager/Uranus fly-by. Performance and calibration data are given as relevant to the radio subsystems on the ground and aboard Giotto.

The implanted ion spectrometer (IIS) is a dedicated instrument for the analysis of ionised cometary particles and their interaction with the solar wind. The particle identification principle used in IIS is based on ion deflection in an electrostatic analyser in combination with time-of-flight techniques. The constructed particle identifier function considers the ionic charge as a known quantity, since ionisation by solar UV radiation is the dominant process. The instrument uses novel principles to accommodate a low-power/low-weight state-of-the-art spectrometer. A multisensor system with miniature time-of-flight detectors together with the spacecraft spin motion provide a uniform, though not contiguous, angular coverage of the unit sphere. New detector multiplexing techniques permit the operation of IIS with maximum sensitivity without sacrificing time or angular resolution because of duty cycle effects. IIS is an integral but physically separate part of the solar wind ion analyser JPA in the Giotto payload. The sensor system and the operation of IIS are described. The flight performance is illustrated by observations in the undisturbed solar wind and near the comet.

The heavy ion analyser RPA2-PICCA is designed to identify the composition and distribution of thermal positive ions in the coma of Comet Halley. The instrument is an electrostatic analyser which takes advantage of the large relative fly-by velocity and the fact that the ions in the inner coma should have low temperatures and be predominantly singly charged. Thus the E/Q (energy/charge) measurements can be directly related to the mass distribution of the ions. Mass identification will be provided in the range from 10 to 203 amu (atomic mass units) with a mass resolution of either Delta m=0.4 amu or Delta m=1 amu dependent on the ion mass.

The three-dimensional velocity distribution of positive ions in the neighbourhood of Comet Halley was measured by an instrument which included two complementary sensors. The fast ion sensor measured the energy/charge distribution from 10 eV/q to 20 keV/q once per revolution of the spacecraft. It obtained the characteristics of the solar wind flow near the comet. The implanted ion sensor measured the energy/charge distribution from 90 eV/q to 90 keV/q with discrimination into five mass groups in a period of 32 spacecraft revolutions. These observations provided the angular distribution of the cometary ions formed by the ionisation of gas molecules sublimed from the cometary nucleus. The relation between the raw count rates and the plasma parameters for the fast ion sensor is derived on the basis of a complete energy angle scan of the sensor in a calibration source. The accuracy of the analysis was tested by two techniques using data collected during the mission.

The Halley multicolour camera is a high-resolution imaging system on board Giotto. The fast spin of this spacecraft (15 RPM) required a unique and highly specialised design complex fully autonomous operation. Design criteria and trade-offs and operational aspects are emphasised in this instrument description. Actual performance numbers are given whenever available. The imaging quality is demonstrated by sample images taken from the Earth and the comet.