Table of contents

An automatic loop suitable for the measurement of microwave resonator Q factors is described. The technique enables an oscillator to be locked to a predefined point on the resonator curve. An accuracy of better than +or-0.1% has been obtained with the Q loop.

An experimental investigation of the pressure difference across cold traps connected to vacuum chambers with a condensable vapour under low-vacuum conditions is presented. The chamber pressures range from 2.3 to 133 Pa with water vapour pressures ranging from 0 to 500 Pa. The measurements show that a cold-trapped gauge can cause serious errors in the low-micrometre region or where the specific humidity is sufficiently large that a substantial part of the non-condensables may be pumped out of the test chamber.

A CW glow discharge exciting a subsonic, transverse flow CO₂ laser was stabilised by means of a CW auxiliary discharge. The stabilising discharge was maintained between an array of individually ballasted anodes and the cathode of the main discharge. With a simple electrode configuration an extension of stable glow to higher gas pressures than without stabilisation was possible. The maximum power output was 1100 W, at an efficiency of 10% and total gas pressure of 4.3 kPa. Without the auxiliary discharge the respective values were 700 W, 6.3% and 2 kPa.

A Mossbauer spectrometer based on the Motorola MEK 6800D1 microprocessor kit is described and shown to provide a simple, cheap and flexible alternative to conventional multichannel analyser systems.

This note describes the design of a spark calorimeter suitable for routine use. Wherever possible commercially available parts have been used to facilitate construction and minimise cost. Compared with existing methods of calorimetry, the present design possesses the important advantage of enabling the heat released by single sparks to be measured.

A method is described for regulating a low-output-current, high-voltage (16 kV) flyback DC-DC converter without access to the high-voltage side. The voltage error, sensed at the collector of the output transistor where the flyback voltage is directly proportional to the voltage across the output capacitor, controls the duty cycle.

Three simple laser Doppler microscope systems suitable for measurements of material flow in narrow vessels with widths from 10 to 250 mu m or greater are described. Measurements of centre-line flow velocities in model narrow channels with widths in the range 60-250 mu m are reported. Velocities of the order of a few mm s^-1 were measured; considerably slower velocities are also accessible. Such measurements are likely to be of great interest to biologists.

A grazing incidence monochromator has been constructed using the principle of differential reflectivity for order sorting. Some difficulties associated with earlier designs of this type have been solved and various refinements incorporated. The instrument includes a phase grating of the laminar type, as developed for use in the X-ray region. Details of the design and construction of the monochromator together with measurements of performance are reported, and a comparison with the calculated properties of the grating and optical system is made.

A method is described for measuring Gaussian laser beam diameter and, in particular, determining the size of the beam waist in a dual-beam laser Doppler anemometer. Results of measurements carried out by different techniques and theoretical predictions are presented as a check to the validity of that method. The associated problem of the beam waist location is also discussed. The method, called here the 'power ratio method' offers itself as a simple and reliable way of measuring beam diameters and locating beam waists, which, otherwise, are cumbersome and time-consuming.

A cyclic-loading stage for the ETEC Corporation SEM is described. The stage uses servo-controlled electro-hydraulic loading with positive feedback Specimens are loaded symmetric to the specimen centre line. Maximum load is 4893 N, with a maximum loading rate of 4 Hz. Resolution on the specimen is about 50.0 nm for static observation and 100.0 nm for dynamic (TV rate) observation in the secondary-electron mode. Observation of the specimen may also be made using backscattered electrons and in the channelling mode (channelling patterns and channelling contrast). Maximum specimen motion during cyclic loading is 15 mu m.

The potential distribution of a quadrupole lens with circular electrodes is calculated by successive over-relaxation techniques involving the numerical solution of Laplace's equation in three dimensions. The results show that the modified bell-shaped model (parameters: 2z₀=l-1.1a; b=1.44a) is a very close approximation to the potential function for the relatively long quadrupole lens (l>2.2a). The focal properties for electrostatic quadrupoles of lengths 20, 30, 40 and 50 mm, and aperture radius 5 mm, are accurately obtained by using the shadow-image technique. The experimental results are in satisfactory agreement with ones calculated using the above model, for a wide range of lens excitation.

In the field of NMR imaging using selective excitation techniques, a procedure is described whereby image information can be recovered with acceptable fidelity in the presence of large magnetic field non-uniformities. It entails only the running of a reference uniform sample under the same conditions as the unknown and performing a kind of deconvolution of signals. By this means, a number of different kinds of distortion can be compensated, without requiring any detailed knowledge of the particular magnetic field non-uniformity involved.

A field electron emission spectrometer based on a hemispherical analysing element has been designed, built and tested. The instrument is versatile and convenient to operate, has a high sensitivity and a resolution of better than 30 mV. The performance of the instrument has been demonstrated in terms of the energy spectra of a tungsten emitter at 300 and 80K, for which comparative theoretical data exist.

Sampling circuitry on the lines of that in a sampling oscilloscope has been interfaced with a scanning electron microscope (SEM). This sampling SEM allows the examination of very fast, repetitive voltage waveforms with rise times of the order of 200 ps or less.

A compact mechanical system for use in secondary-emission studies is described. Several specimens may be accommodated and cleaved in vacuo. The yield and the energy spectrum of the secondaries may be studied for angles of incidence of the primary-electron beam between 0 and 90 degrees . The robustness of the system allows the cleavage of mechanically strong specimens.

The design of an automatic apparatus for broadband complex admittance measurements on MOS structures is considered in detail. An apparatus is described which operates in a frequency swept mode over the range 10 Hz-100 kHz. It can measure admittances having very small loss angles with a total uncertainty in G_m/ omega C_m of the order 10^-4 over the full frequency range. On typical MOS devices this allows the accurate determination of interface-state parameters at densities as low as 2*10⁹ states cm^-2 eV^-1.

For a quantitative description of the fast, self-sustained, high-pressure gas discharges used in transversely excited gas lasers the waveforms of current and voltage must be known accurately. An integrated system is described measuring currents and voltages without any interference by using the Faraday and Kerr effects. The sensitivity of the current measurement is 20 mV kA^-1. The Kerr cell can be used with different sensitivities within the range 10-150 kV. The rise time of each detector system is 5 ns.

The important design features of a Couette cell capable of measuring birefringence, viscosity and permittivity changes in shear are discussed. Described are the details of construction of such a cell which can perform one or all of these functions. The cell is capable of operating in the range of shear rates up to about 3*10⁵ s^-1 for dielectric and viscosity determinations and up to about 10⁴ s^-1 for double-refraction measurements. This paper concentrates primarily on the details pertinent to dielectric measurements.

A digital control system using TTL logic elements is described for use with a 1.4 MW impulse generator. The unit is fully programmable and records on magnetic tape the data obtained from breakdown studies of long spark gaps in a form suitable for on-line computer processing. The precautions necessary for operating solid state components in the hostile environment of a high-voltage laboratory include the use of optical links and battery-operated power supplies.

An automatic apparatus is described for the measurement of volumetric flow rates in the 3-3000 cm³ min^-1 with an accuracy and a reproducibility of 0.25%. The apparatus can be used at temperatures up to 60 degrees C.

The measurement of sorption and desorption time lags associated with transport of gases through membranes presents characteristic difficulties. Apparatus and related experimental procedures are described which permit accurate determination of four time lags.

Investigations have shown that three-electrode electrostatic lenses whose axial potential distribution is given by Phi (z/a)= Phi (0)exp(K₀ tan^-1(z/a)^m) are simple lenses whose paraxial ray paths can be rigorously obtained in the form of hypergeometric functions. Based on these solutions, the important focal quantities and the spherical and chromatic aberrations can be expressed analytically simply in terms of the halfwidth a_h of the potential distribution and the ratio Phi (0)/V of the mid field potential to the accelerating potential for various three-electrode lenses specified by m. The important parameters m and a_h and their dependence upon electrode geometry are derived approximately from electrostatic potential theory with the aid of experimental measurements. Optimum lenses with minimum focal length, together with their spherical and chromatic aberration constants, are described and discussed in terms of practical design.

Power leakage measurements were made in the vicinity of a microwave cavity with a Narda isotropic radiation monitor 8315 A. This monitor used a probe calibrated at 2.45 GHz and capable of reading power densities of up to 20 mW cm^-2. In the first part of the investigation sealed fused-silica tubes (6.5 mm outer diameter) filled with various low-pressure inert gases were used together with two ³/₄-wave Broida cavities with one having a small aperture capable of accepting a 7 mm outer diameter tube and the other, having a larger aperture, accepting 10 mm outer diameter tubes. The second part of the investigation was to place the cavities on to an EDT blank which was connected to a vacuum system. This was done since microwave discharge, using the appropriate inert gas, is normally run as part of the degassing procedure in the preparation of EDTS.

Resistors of 10⁹-10¹² Omega , used in picoammeters for ion current determination in mass spectrometry, for example, commonly do not obey Ohm's law exactly, for their resistance tends to diminish with increasing voltage drop. According to manufacturers' figures the effect is typically of the order of 0.03% V^-1 for a 10¹¹ Omega resistor, averaged over 1-100 V. In high-precision measurements these figures are not sufficiently accurate, and the authors have therefore developed a method of determining the nonlinearity of such resistors over a range of 1-10 V with a precision of about 0.01% of relative resistance.

The modification presented here for use in conjunction with a Philips PW 1050 goniometer ensures that the shutter is positively closed by mechanical constraints before access is gained to the potentially hazardous areas. It is simple to construct, retains the original convenience of operation and requires little modification of the original equipment. The principle of operation has been applied to other tube shields and equipment. The device has been exhaustively tested by workers of various levels of competence and, while complying with the Act, has been found convenient in use.