Table of contents

The theory developed to analyse the effect of a hollow cylindrical collimation channel on absolute calibration is applied to side-on intensity measurements carried out on a cylindrical plasma. The computation shows that the ratio of the absolute calibration integration factor to the side-on measurements integration factor may not be equal to one, as expected. Therefore, this ratio should be computed in order to obtain accurate absolute emission coefficients.

A circuit is described which measures the AC I/V characteristics of a parallel plate capacitor filled with nematic liquid crystal. Using this circuit accurate determination of the anisotropic dielectric constants can be achieved. Because of the simple form of the data the dielectric constants both perpendicular and parallel to the director can be deduced without resorting to asymptotic approximations or curve fitting.

The temperature dependence of the ultra-cold neutron (UCN) transmission losses of polypropylene foils, 25 mu m thick, is investigated and found to decrease sufficiently fast with temperature to make it a useful UCN window material, as suggested by Strelkov's measurements (1977) on polyethylene. The transmission of two 8 mu m clear polypropylene foils increases from (50+or-1)% at room temperature to (85+or-1)% at 77 K. Thin metal coatings reduce this transmission significantly; for example, the same foils coated with 1000 AA of vanadium have a transmission of (28+or-1)% at room temperature and (50+or-1)% at 77 K. The condensation of impurities on the cold windows will also reduce their transmission noticeably and so special care was taken to keep the vacuum as free as possible from condensable gases. Even so, within 24 h an important degradation of the transmission was observed with the clear foils. These mu m foils are strong enough to withstand a pressure difference of 1 atm over a diameter of 67 mm provided that they are cooled down below 77 K, at which temperature the leakage through the bulk of the plastic due to 1 mbar of ⁴He exchange gas becomes negligible.

The design principles of linear induction acceleration of charged particles are discussed with a view to developing a small-scale prototype. Equations are established for the selection of the core and the drive modulator. The development of a modulator along with its drive circuit is described for the small-scale model. An extrapolation of the proposed method for an envisaged large-scale model is suggested.

Reports the investigation of the discharge current and voltage in longitudinal KrF lasers with internal and external capacitive electrodes. Laser action in F₂:Kr:He mixtures in a capacitively coupled discharge was observed with output energies up to 200 mu J and 0.1% efficiency in a discharge tube with diameter 2 mm, length 35 cm and a driving voltage of 84 kV. Factors which influence the operating period were investigated. A maximum lifetime of 10⁴ shots for 3 l of gas mixture was obtained.

Transient absorption in the 450 to 650 nm spectral region has been observed in the discharge channel of a UV pre-ionised mercury bromide laser. To the red side of the HgBr(B-X) gain region, continuum absorption due to the X-A transition in HgBr, and atomic mercury absorption at 546 nm are seen. The X-A continuum is found to be blue shifted by 0.3 eV relative to theoretical predictions, and to overlap the 502-504 nm laser emission region. On the blue side of the gain band, absorption due to the X-B transition and absorption by the B state to higher energy HgBr states are identified through their differing time dependencies. Immediately following the discharge, the HgBr concentration falls rapidly, with a half-life of 1 mu s, through chemical reactions producing molecular bromine. This short lifetime may be a limitation on the optically pumped HgBr laser using UV excitation of the X-B band.

An ultra-high-resolution frequency-offset-locked carbon dioxide waveguide laser spectrometer has been constructed. Three different sample cell configurations have been evaluated, namely: plane-plane etalon, mirror-based two-pass cell and a retro-reflector-based two-pass cell. In the last case peak-to-peak linewidths of 12 kHz have been obtained for the first derivative signal in sulphur hexafluoride at 10.6 mu m.

A drift-tube mass spectrometer for precision measurements of transport coefficients for ions in gases is described. The apparatus has been used to determine the ratio D_T/ mu between the transverse diffusion coefficient and the mobility for He⁺ ions in the helium at 294 K and 15<or=E/n₀<or=2000 Td, by direct measurement of transverse current density distributions of mass-analysed ions. The apparatus has also been used to determine the drift velocity V_dr and the ratio D_L/ mu between the longitudinal diffusion coefficient and the mobility for Li⁺ ions in argon at 295 K and 40<or=E/n₀<or=400 Td and the ratio D_T/ mu for Li⁺ ions in argon at 294 K and 15<or=E/n₀<or=550Td, using the Townsend method. Good agreement was found between the present experimental results and recent calculations from at initio potentials.

Examines experimentally the axial profiles of the plasma parameters (electron temperature T_e and the electron number density n) of a helium low-pressure gas discharge column sustained by a travelling electromagnetic wave in the presence of a strong magnetic field for two values of magnetic induction corresponding to the magnitudes of omega _c/ omega =2.5 and 5 respectively ( omega _c is the electron cyclotron frequency and omega is the wave angular frequency). It is found that the electron temperature is diminished and the plasma density is increased (accompanied by a decrease of its axial gradient) in comparison with the magnitudes of the same parameters and characteristics of an isotropic plasma column. The measured axial plasma density gradients are compared with theoretically calculated ones and the agreement between theory and experiment is good.

The authors present experimental data for the effective recombination coefficient in a pulsed surface-wave-produced plasma. To determine these data they studied the relaxation of electron density to equilibrium. A 2.45 GHz Surfatron was used in order to couple the HF energy, and three different Pyrex cylindrical tubes of internal radius a=1.5, 2.5 and 4.5 mm, with a corresponding external radius b=a+1.5, were employed. The gas used in the discharge was argon, in the 0.5-12 Torr pressure range. They obtained the total losses by ambipolar diffusion and recombination, divided by pressure, as a function of the quotient between the effective surface wave electric field and gas pressure. At the lower end of the pa range diffusion losses dominate but recombination becomes more important for higher values of pa.

A new broad-band microwave reflectometer in the Q-band (33-50 GHz) with only one antenna is presented. The basic principles of density measurements with the device are outlined for a numerical application to the tokamak TJ-1. Experimental results for the electron density profile are given. Some preliminary results of density fluctuation measurements are also presented.

Strange attractors are plotted for the ionisation instability excited in the weakly ionised positive columns of helium glow discharges. The transition to the turbulent state is experimentally observed. On calculating the correlation integral the periodic instability is revealed to have a low dimension and the turbulent one to have a high but finite dimension.

The influence of argon impurities on the optogalvanic signal induced on neon lines was studied experimentally and theoretically. In particular, the amplitude and sign of the optogalvanic signal due to resonance pumping of the 609.6 nm line of neon were found to be greatly influenced by Penning ionisation of argon atoms by neon atoms excited to one of the 1s states. The model describes the transient effects found experimentally and the pressure and discharge current dependences of stationary optogalvanic signal values.

A theory of the positive column in electronegative gases based on fluid-type momentum equations to describe charged particle motion is presented. It is assumed that quasi-neutrality conditions prevail and the ion inertial terms are neglected. The positive ions are assumed to be created by electron collisions with neutral molecules and the negative ions to be formed by dissociative electron attachment and destroyed by detachment in reactions with neutral species. The mathematical formulation consists of a two-point boundary value problem involving two independent parameters, functions of collisional and transport data, and two eigenvalues. One of these is the central ratio of the negative ion density to the electron density, while the other is related to the ionisation-loss balance and embodies a discharge characteristic for the maintenance field. These eigenvalues and the radial density distributions of the charged species were calculated for a wide range of variation of the independent parameters. An application of the theory to a positive column in oxygen is given as an illustrative example.

Spectra of SF₆ circuit breaker arcs during the arc extinction phase following peak currents up to 53 kA have obtained in the wavelength range 510-517 nm. Copper emission lines and irregularly occurring tungsten emission lines from entrained contact material have been identified and the ratio of the number density of tungsten to copper atoms estimated from the line intensities. An approximate radial distribution of both copper and tungsten line intensities has also been obtained. The irregular occurrence of tungsten in the arc plasma column and the restricted extent of the tungsten line emitting cross section are both believed to be a consequence of the complicated evaporation processes associated with the copper-tungsten contact. An approximate correlation is shown to exist between the occurrence of tungsten line emission at the critical instant of current zero when current interruption is sought and a sharp deterioration in the current interrupting ability of the breaker. The implication of this is that the hitherto neglected presence of tungsten at current zero, as distinct from copper, contributes in combination with the already known effects of flow throttling and nozzle ablation in determining the extent of the degradation of circuit breaker performance.

A detailed NMR investigation, supported by DSC and X-ray diffraction measurements, of the combwax of the African bee apis mellifera adansonii is reported. Proton spin-lattice relaxation times in the laboratory and rotating frames, as well as the proton spin-spin relaxation time, have been measured as a function of temperature (333 K>T>110 K). The liquid content of the wax has also been determined as a function of temperature by employing a simple pulse technique. The motional parameters associated with the reorientations of the methyl groups and the chains have been isolated and compared with these parameters for similar motions in Fischer-Tropsch waxes. The T_{1 rho} results revealed a minimum which is associated with the dangling motion of chain ends in the amorphous zone of the wax. The relaxation results strongly suggest that beeswax is branched to a much higher degree than Fischer-Tropsch waxes, including oxidised waxes. The high-resolution ¹³C spectrum of beeswax in the solid state shows that it resembles oxidised Fischer-Tropsch hard wax closely. The major difference is that a higher percentage of carbon atoms are involved in ester groups in beeswax. Oxidised hard wax contains a higher fraction of carbon atoms with double carbon-carbon bonds. The average chain length in beeswax, determined by ebullioscopic methods, is 40 carbon atoms.

For pt.I see ibid., vol.21, p.1421 (1988). Structural aspects and the molecular dynamics of carnauba wax are reported. X-ray diffraction results show that it crystallises in an orthorhombic unit cell. DSC and proton spin-lattice measurements as a function of temperature revealed a premelting solid-solid transition at 335 K. The percentage of carbon atoms in methyl and methylene groups is approximately 12% less than in beeswax, whereas it has a much higher ester and carbonyl content. The average chain length in carnauba wax is 50 carbon atoms. Spin-spin relaxation measurements revealed that the reorientational-translational motion of the chains is much more hindered than in beeswax. The motional parameters associated with the reorientations of the methyl groups have been isolated and compared with these parameters for similar motions in Fischer-Tropsch waxes and beeswax. Spin-lattice relaxation measurements in the rotation frame showed that the activation energy for the dangling motion of the chain ends is approximately 20% higher than in beeswax. The liquid content of the wax has also been determined as a function of temperature by employing a simple pulse technique.

For pt.II see ibid., vol.21, p.1429 (1988). A detailed NMR investigation, supported by DSC and X-ray diffraction measurements, of montan wax is reported. The solid-state high-resolution ¹³C spectrum showed that it has a very high number of carbon atoms occurring in methyl and methylene groups and a relatively low ester, acid and carbonyl content. Laboratory frame spin-lattice relaxation results showed that the methyl groups have a high degree of mobility and that branching of the chains is of minor importance. A reliable value of 34.6+or-2.2 kJ mol^-1 was obtained for the activation energy of the reorientational-translational motion of the chains. The temperature at which T₂, the spin-spin lattice relaxation time, starts to increase with increasing temperature yields tau ₀=(8.7+or-0.6)*10^-14s for this motion. The average chain length of montan wax is 31 carbon atoms. The chain-end motion as revealed by spin-lattice relaxation measurements in the rotating frame is less hindered in montan wax than in the other natural waxes which have been investigated.

Equations describing the deformation of a twisted nematic liquid crystal in an electric field in terms of the local mean molecular orientation (director) have been derived by Deuling (1974). In addition Berreman (1973) has developed a method of describing the optical properties of such a cell. In this paper their model has been extended to take account of the degree of orientational disorder of the liquid crystal molecules. An electrical equivalent circuit of the cell is then introduced along with a method of computing the liquid crystal capacitance so that the fraction of the cell voltage appearing across the liquid crystal may be computed. Finally a comparison is made between the experimentally measured light transmission and that predicted by the theory with and without the molecular disorder correction. The overall agreement between theory and experiment is improved considerably when the molecular disorder correction is included.

Ultrasonic and static compression experiments were performed in the frequency range 50 kHz to 1 MHz in order to investigate the mechanical behaviour of silica aerogels as a function of internal gas pressure p_G and external stress P_ext. The measurement of longitudinal and transverse sound velocities allows the Young's modulus and the Poisson's ratio for aerogels of different densities to be determined upon variation of p_G and P_ext. For low-density aerogels ( rho approximately=100 kgm^-3) the authors found a decrease in sound velocity upon evacuation. Surprisingly, the sound velocity decreases upon uniaxial compression with small loads. The most important finding from the static compression experiments is that low-density aerogels display creeping with a time constant of about 45 min.

The 110 degrees C thermoluminescence (TL) peak of unfired, synthetic quartz (Sawyer Research Products) has been found to be strongly superlinear with the dose of excitation. Quadratic and nearly cubic dependences have been observed with both beta - and gamma -irradiation. Firing the samples at 300 degrees C or higher increased the sensitivity substantially and removed the superlinearity. The fired samples have been found to have a pre-dose (sensitisation) effect similar to natural quartz, however, the pre-dose effect is weaker than in many of the other types of quartz. A model of competition during the heating phase, previously suggested to explain a quadratic dose dependence, is now shown to be capable of explaining the very strong superlinearity observed with the unfired sample. Furthermore, the model can also be used to account for the observed shifts in the temperature of the TL peak, the linearity and sublinearity of the fired samples, and their higher sensitivity.

Describes detailed probe measurements of the saturated ion current and the floating potential of the ECR oxygen plasma created in an inhomogeneous magnetic field at pressures from 6*10^-2 to 4*10^-1 Pa. The electron plasma density decreases from the ECR region in the direction of decreasing magnetic field. The floating potential is positive. It is also shown that the surface potential of a flat floating probe depends on the probe material.

The current oscillations in gas-filled diodes were investigated in sub-normal glow discharge. It was found that the frequency of these oscillations, in a given circuit, depended only on the negative value of dU/dI. Also it was found that, in the same circuit, the semiconductor components with equal values of dU/dI generated oscillations with equal frequencies.

The electron velocity distribution function has been measured using a flat one-sided probe at the axis of a plasma gap in a helium discharge positive column for the pressure range 0.2-1.0 Torr and discharge currents 50-500 mA. Moments of the electron velocity distribution function and hence the electron convective velocity for different discharge regimes are determined. The convective velocity is shown to be determined according to discharge conditions either by a pair of elastic-atom collisions or by collective processes of electron collisions.

A magnetic field of 5 T was used to orient a concentrated solution of poly( gamma -benzyl L-glutamate) in 3-chloro-2-chloromethyl-1-propene. The direction of the field was parallel to the casting surface. After evaporating most of the solvent, the films were irradicated with ultraviolet light from a high-pressure mercury lamp which polymerised the remaining solvent. Areas of high-quality uniformly oriented films were obtained.

The dynamic behaviour of the coercive force of particulate recording media is investigated theoretically. It is shown that as regards the value of H_c the field sweep is equivalent to an effective time determined partially by the particle volume. This is in agreement with an empirical relation arising from recent experimental data.

For original paper see P. Mialhe et al., ibid., vol.21, p.383 (1988). The authors show that an explanation recently given for the compensation law (Meyer-Neldel rule) is incorrect.