D. Smith

University of Birmingham, Birmingham, ENG, United Kingdom

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Publications (149)300.15 Total impact

  • C V Goodall · D Smith
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    ABSTRACT: A diversity of opinion exists at the present time concerning the interpretation of single Langmuir probe characteristics, in particular with respect to the location of space potential and the determination of charged particle number densities from the characteristics. This work describes measurements made with very small Langmuir probes in gaseous afterglow plasmas at room temperature where conditions for a study of this kind are expected to offer particular advantages. A comparison is made of the values of electron densities obtained from the characteristics using six methods which are commonly used. Good agreement is found to exist between those deduced from the orbital limited characteristics in the accelerating region for electrons and those calculated from the probe current at the inflexion point of the characteristics. That these are also in tolerable agreement with those obtained using the intersecting tangents method is thought to be fortuitous.
    No preview · Article · Dec 2002 · Plasma Physics
  • D Smith · A G Dean · N G Adams
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    ABSTRACT: Measurements are described of the temporal and spatial variations of the space charge fields in afterglow plasmas together with parallel measurements of electron density (ne), positive ion density (n+), negative ion density (n-), electron temperature (Te) and mass-analysed wall currents of both positive and negative ions. In the case of the plasmas containing electrons and positive ions only, it has been shown that the spatial variation of plasma potential (Δ Va) is related to gradients in the charged particle densities and to the electron temperature via diffusion theory, and no complications in the sampling of positive ion wall currents with an orifice probe arise. For plasmas in which significant concentrations of negative ions exist, the previously observed `trapping' of negative ions within the plasma occurs, as manifest by a lack of negative ion wall current at early afterglow times followed later by a sudden onset. The probe measurements indicate a continuous diffusive loss of electrons and positive ions during the afterglow, whereas the negative ions remain within the plasma until ne has fallen to less, similar 10−2n- at which point an avalanche loss process of the remaining electrons begins which rapidly leads to the establishment of a negative-ion/positive-ion plasma. During the afterglow period, a progressive collapse of the wall potential and the ambipolar field occurs from those characteristic of an electron-dominated plasma to those of a negative-ion-dominated plasma. The sudden appearance of negative ion wall current is closely correlated with the avalanche collapse of the ambipolar field. The significance of these observations to mass-spectrometric sampling from afterglow plasma is discussed.
    No preview · Article · May 2002 · Journal of Physics D Applied Physics
  • D Smith · M J Copsey
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    ABSTRACT: Mass spectrometric observations of helium afterglows by ion sampling from the walls of a large discharge vessel are described. It is shown that in very pure cataphoretically-cleaned helium in the pressure range 03 to 2 torr the only important processes occurring in the late afterglow are ambipolar diffusion of atomic and molecular ions and electrons, and conversion of atomic to molecular ions by three-body collisions with neutral helium atoms. The atomic and molecular ion mobilities corresponding to the observed diffusion rates are respectively 112 ± 04 cm2 v-1 s-1 and 159 ± 04 cm2 v-1 s-1, in acceptable agreement with several previous mobility measurements, and the value obtained for the three-body conversion coefficient, 85 ± 3 torr-2 s-1 (pressure reduced to 0 °C) is in good agreement with a recent similar measurement by other workers in a higher pressure range. The wall temperature of the discharge vessel during these measurements was 295 °K. Measurements carried out in helium samples which were not subjected to cataphoretic purification demonstrated the serious effect of small amounts of impurity on the decay rate of the molecular helium ion.
    No preview · Article · May 2002 · Journal of Physics B Atomic and Molecular Physics
  • D Smith · N G Adams
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    ABSTRACT: This paper is concerned with an investigation of the plasma generated during the impact of hypervelocity microparticles with a metal target. A laboratory source of hypervelocity micron-sized iron particles is first described which utilizes a 2 MV Van de Graaff generator, followed by a discussion of the techniques used to determine the mass and velocity of the particles. On impact of the iron projectiles on a slatted molybdenum target, charge is generated, extracted from the region of impact by suitably biasing a gridded electrode, and subsequently detected using a conventional wideband electronic amplifier or an electron multiplier. It is shown statistically that during impact equal numbers of electrons and positive ions are produced - indicating plasma generation - and that the total charge released (Q) may be described empirically in terms of the mass (m) and velocity (v) of the particle by a simple power law relationship of the kind Qαmαvβ, with β = 32 ± 01 over the complete velocity range investigated (005 to 10 km s−1) and with α = 085 for v>1 km s−1 and 133 for v<1 km s−1. It is also shown that the charge extracted from the plasma reaches a maximum a few microseconds after impact and subsequently decays exponentially with a time constant of several micro-seconds. A crude mass analysis of the positive ions generated during the impact using a simple time-of-flight mass spectrometer has indicated that, over the velocity range investigated, the dominant ions in the spectrum are characteristic of the projectile and not the target material. From considerations of the relative abundance of the metal ions in the spectra, together with the known relative metal atom concentrations in the iron projectiles, the temperatures of the plasmas have been estimated (from Saha equilibrium considerations) to be of the order of several thousands of degrees Kelvin. In a detailed discussion, attempts are made to consider how the kinetic energy of the projectile is dissipated - eventually producing plasma - in terms of available theoretical models of hypervelocity impact. It is shown that the most satisfactory model of this very complex interaction appears to be one in which the relative importance of shock-wave propagation into the projectile and target materials is considered. Finally, a discussion of the properties and the temporal behaviour of the impact-produced plasma is presented in order to assess the relative importance of diffusion and recombination processes, thus to assist in the interpretation of the experimental results obtained in this study.
    No preview · Article · May 2002 · Journal of Physics D Applied Physics
  • D Smith · I C Plumb
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    ABSTRACT: This paper describes the results obtained from a comparative study of the electron currents (ie) and positive-ion currents (i+) which flow to a small cylindrical Langmuir probe in pure rare-gas afterglow plasmas at low pressures. It is shown that within a predictable restricted range of charge density, linear plots of ie2 and i+2 against the probe-plasma potential difference could be obtained for positively and negatively biased probes respectively; which is in qualitative agreement with the Langmuir theory of orbital-limited-current collection. However, the values of the electron density (ne) and the apparent positive-ion density (n+) deduced from the slopes of the above plots were not equal; n+ always exceeding ne and related empirically to it via the mass of the positive ion m+ (in amu) thus: n+ = (1+007m+½)ne. It is suggested from additional experimental evidence that the values for ne are accurate and the discrepancy arises as a result of the inadequate theoretical description of the positive-ion currents flowing to the negatively biased probe. The data points of Tonks and Langmuir (1929) and of Shaeffer (1971) obtained in arc plasmas are shown to be consistent with the present results. The significance of these results to plasma diagnostics using probes is discussed.
    No preview · Article · May 2002 · Journal of Physics D Applied Physics
  • D Smith · C V Goodall · M J Copsey
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    ABSTRACT: Measurements are described of slow electron density decay rates in pure helium afterglow plasmas, using a very small single Langmuir probe, a technique not hitherto applied to the study of ambipolar diffusion. Values are obtained for the rate of conversion of He+ ions to He2+ ions and for the respective mobilities of these ions at a wall temperature of 295 °K, which are in reasonable agreement with values obtained by other workers using different techniques. This agreement is considered to indicate that depletion problems, which are expected to be most severe in slowly decaying plasmas, have been overcome by the utilization of very small probes. Evidence is also presented for the existence of elevated electron temperatures at times as late as 10 ms in the afterglow, and these results, together with those above, are critically discussed and compared in detail with the complementary mass spectrometric observations described in I.
    No preview · Article · May 2002 · Journal of Physics B Atomic and Molecular Physics
  • D Smith · I C Plumb
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    ABSTRACT: A detailed appraisal of the collisionless sheath single cylindrical Langmuir probe technique applied to the study of electron densities and temperatures in afterglow plasmas is described. It is shown that for positive probe potentials (V) no significant disturbance (`pseudodepletion') of the plasma occurs until the electron current to the probe (ie) approaches the natural diffusive positive ion current flowing to the reference electrodes. This is indicated by departures from linearity of ie2-V plots obtained in the orbital-limited current régime. Departures from linearity have also been observed due to probe sheath expansion resulting in departures from cylindrical geometry above a critical value of V. It is shown that, for values of V below those critical values signifying the onset of pseudodepletion and sheath expansion, meaningful probe characteristics can be obtained in afterglows from which accurate electron densities can be obtained. Also described are the effects which arise from temporal changes of electron temperature, and the resulting changes in floating potential Vf. The latter has been measured as a function of the electron temperature (obtained from plots of the electron retarding region) and is shown to substantiate the original Langmuir theoretical expression for Vf.
    No preview · Article · May 2002 · Journal of Physics D Applied Physics
  • A G Dean · D Smith · N G Adams
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    ABSTRACT: The single Langmuir probe technique has been used to determine the relaxation rates of the electron temperature, Te, in repetitively produced, time-resolved afterglow plasmas. Comprehensive data is presented for krypton plasmas together with preliminary data for neon and argon plasmas. The electron temperature cooling curves for a given gas pressure, p0, have been interpreted in terms of a characteristic time, tau en, for electron cooling via electron-neutral collisions. Spatial measurements of Ie indicated that no significant Ie gradients exist in the plasmas during the periods over which cooling curves were obtained; it is also argued that there was no significant elevation of the gas temperature above the wall temperature during the afterglow. In all three gases good agreement is obtained with previously published data.
    No preview · Article · Apr 2001 · Journal of Physics B Atomic and Molecular Physics
  • D Smith · C V Goodall · N G Adams · A G Dean
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    ABSTRACT: Measurements are described of the ion and electron decay rates in pure argon and argon-oxygen afterglow plasmas. Mass-spectrometric investigations of the temperature dependence of the rate coefficient for the Ar+/O2 charge-exchange reaction over the vessel wall temperature range 180 to 580 degrees K showed the rate coefficient to decrease from 7.05*10-11 cm3 s-1 to 2.18*10-11 cm3 s-1. A Langmuir probe was used to determine the dissociative recombination coefficients alpha of Ar2+ ions and O2+ ions with electrons in these afterglows at 295 degrees K. alpha (Ar2+) was found to be dependent on the power associated with the ionizing pulse initiating the discharge, the value increasing from 4.7*10-7 cm3 s-1 to 12.6*10-7 cm3 s-1 as the power was decreased. Measurements of alpha (O2+) in the argon-oxygen afterglow yielded a value of (2.1+or-0.3)*10-7 cm3 s-1 in good agreement with those values obtained by other workers using independent techniques.
    No preview · Article · Mar 2001 · Journal of Physics B Atomic and Molecular Physics
  • N G Adams · D Smith
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    ABSTRACT: The design and appraisal of a mass spectrometer, based on the focusing properties of mutually perpendicular electric and magnetic fields, is described. This field geometry spatially separates ions as a linear function of mass along a focal plane. This allows the spectrometer to be used in applications requiring the simultaneous monitoring of many ion species of independently varying intensities, such as from transient or pulsed ion sources, a feature not associated with other spectrometers. This instrument incorporates a novel detection system comprising a channel multiplier plate which considerably enhances sensitivity and a 50 element slatted collector array which separately detects the individual ion signals in the chosen mass range 20-70 u. To date a detailed experimental appraisal in the scanned E field mode, in which the resolved ion current spectrum is swept across one of the collectors, has provided data on the detailed mass peak profiles, the resolving power of the analysing E*B field and the efficiency and dispersion of the detector system. The data provided by this investigation has allowed a detailed appraisal to be presented concerning the modifications required in order that the instrument can be operated in the constant E field mode in which the E field is fixed and the resolved ion current is distributed amongst the 50 collectors.
    No preview · Article · Feb 2001 · Journal of Physics E Scientific Instruments
  • I C Plumb · D Smith · N G Adams
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    ABSTRACT: Measurements of the temporal variations of the concentrations of O+2 ions, O+4 ions and electrons in krypton-oxygen afterglow plasmas at 180 K are described. From mass spectrometric observations of the time varying O+2 ionic wall current the rate coefficient for the forward ternary association reaction O+2+O2+Kr to or from O+4+Kr was measured to be (8.3+or-1.6)*10-30 cm6s-1 at 180 K. This result together with the known equilibrium constant for this reaction enabled gas and charge density conditions to be determined for which the O+4 ion was rapidly established as the dominant positive ion species in the afterglow. Electron-positive ion recombination coefficients, alpha , were deduced from measurements of the temporal decay rate of the electron number densities. From early afterglow time observations alpha (O+2) was measured to be (3.5+or-1.0)*10-7 cm3s-1, and from late afterglow time observations alpha (O+4) was measured to be (1.8+or-0.6)*10-6 cm3s-1. Possible ionospheric implications of the O+4 ion result are briefly discussed.
    No preview · Article · Feb 2001 · Journal of Physics B Atomic and Molecular Physics
  • D Smith · A G Dean · I C Plumb
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    ABSTRACT: Measurements of the conversion reaction coefficients, beta , for the three body conversion of krypton and xenon atomic to molecular ions in pure rare gas afterglow plasmas have been made. The values for beta at 300 K are (342+or-15) Torr-2s-1 in krypton, and (447+or-22) Torr-2s-1 in xenon. Preliminary investigations in the case of krypton indicate that beta is essentially the same at gas temperatures of 180 K and 300 K, but that there is a marked decrease in beta as the gas temperature is increased to 510 K. These results, coupled with the previous measurements, in this laboratory, of beta in helium, neon and argon have allowed an assessment to be made of the validity of several theoretical models. It is shown that good agreement exists between the experimental values and those calculated using the theoretical approach of Mahan (1965).
    No preview · Article · Feb 2001 · Journal of Physics B Atomic and Molecular Physics
  • D Smith · N G Adams
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    ABSTRACT: As a continuation of earlier work (see abstr. A86600 and B37521 of 1974) a modified multicollector detector system for a cycloidal focusing magnetic mass spectrometer is described. The results of an experimental appraisal of the detector are presented together with the resolution and sensitivity range of the spectrometer-detector combination.
    No preview · Article · Feb 2001 · Journal of Physics E Scientific Instruments
  • M J Church · D Smith
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    ABSTRACT: Measurements are reported of the decay of ionisation in positive-ion/negative-ion flowing afterglow plasmas. When one or both of the ion species is molecular, conditions of pressure and ionisation density can be readily established under which mutual neutralisation (ionic recombination) dominates over ambipolar diffusion and hence the recombination coefficient alpha i can readily be determined. When both ionic species are atomic the recombination rate is so slow that ambipolar diffusion is seen to dominate over ionic recombination under the conditions of the experiment, and it has only been possible to determine upper limits for alpha i.
    No preview · Article · Jan 2001 · Journal of Physics D Applied Physics
  • D Smith · N G Adams · A G Dean · M J Church
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    ABSTRACT: The application of the single Langmuir probe techniques to the measurement of electron number densities and temperatures in helium flowing afterglow plasmas is described. Data are presented relating to electron loss by both ambipolar diffusion and recombination, which illustrates the value and quality of the diagnostic technique. Results are also presented which illustrate how metastable atom densities can be readily determined by exploiting the Penning effect, and also probe determinations of electron temperature which demonstrate the role of metastable atoms in elevating the plasma electron temperature. In conclusion, the potential value of the probe-flowing afterglow combination is discussed with reference to the determination of positive and negative ion densities and ionic mass, of ion-molecule reaction rates and electron temperature relaxation rates.
    No preview · Article · Jan 2001 · Journal of Physics D Applied Physics
  • N G Adams · M J Church · D Smith
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    ABSTRACT: A study of plasma and neutral gas flow dynamics has been carried out in pure helium flowing afterglow. The measured plasma flow velocity nu p is observed in effect to be radially independent, in contrast to the instantaneous neutral gas flow velocity profile nu g(r) which is approximately parabolic with some slip at the walls. This observation has led to a new and simplified approach to the flow dynamics based on the concept of a radially independent plasma flow velocity from which, in conjunction with the instantaneous charged particle velocity radial profile (assumed to be equal to nu g(r) and a fundamental mode Bessel radial distribution of charged particle (plasma) density, an averaged plasma flow velocity nu p has been calculated which is shown to be accurately equal to nu p in the approximate pressure range 0.3-0.9 Torr. nu p (hence nu p) is shown to be related to the mean neutral gas flow velocity nu 0 according to nu p approximately 4/3 nu 0.
    No preview · Article · Jan 2001 · Journal of Physics D Applied Physics
  • N G Adams · D Smith · D Grief
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    ABSTRACT: Measurements are reported of the reactions of the ground (3P) and metastable electronically excited (1D2 and 1S0) states of Xe2+ ions with several atoms (Ar and Xe) and molecules (H2, N2, O2 and CO2) at 300K. The data were obtained using a selected-ion-flow-tube (SIFT) apparatus. State selection was achieved by introducing suitable 'chemical-filter' gases into the flow tube. The rate coefficients for the reactions with the molecules were large with the exception of some of the reactions of the 1S0 state. Conversely, the atom reactions were generally slow except for the 3P state reaction with Ar in which a favourable pseudo-energy resonance must occur. All the molecular reactions proceeded via single-charge transfer and in the 1D2 reactions with molecules, dissociation of the product ion was usually observed.
    No preview · Article · Jan 2001 · Journal of Physics B Atomic and Molecular Physics
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    ABSTRACT: Thermal-energy reaction rate coefficients and product ion distributions have been measured for reactions of both the ground state and metastable electronic states of O2+, NO+ and O+ with several neutral species, using a selected-ion flow tube. In general the excited-ion reaction rates are fast, frequently approaching the Langevin limit. Collisional quenching occurs for the reactions of NO+* with N2, O2 and H2 and the quenching rates have been determined. The ion source also provided a substantial yield of doubly charged O2 permitting some measurements of reaction rates of O22+.
    No preview · Article · Jan 2001 · Journal of Physics B Atomic and Molecular Physics
  • N.G. Adams · D Smith
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    ABSTRACT: The reactions of He+ ions with N2, O2, CO2, and CH4, and of C+ and N+ ions with O2 are presented. The measurements were carried out in the SIFT apparatus (selected ion flow tube), which involves the injection of a mass-selected positive-ion beam into a flowing neutral gas into which a reactant gas is introduced at a position downstream in the flow. Data acquisition and analysis are made by the flowing afterglow technique. The results for the He++N2 reaction agree with the previously established branching ratio. For the other reactions, considerable differences are apparent between the product distributions obtained and those previously reported and an explanation is attempted. A brief discussion of the reaction mechanism is presented for each reaction in the light of the determined product distributions, and the anticipated development of the SIFT technique for ion-molecule reaction mechanisms is outlined.
    No preview · Article · Jan 2001 · Journal of Physics B Atomic and Molecular Physics
  • D Smith · A G Dean
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    ABSTRACT: Measurements have been made, at gas temperatures of 300K of electron temperature, Te, relaxation rates (2000K>or approximately=Te>or approximately=350K) in pulsed flowing afterglow plasmas of pure argon and argon-nitrogen and argon-oxygen mixtures. Momentum transfer cross sections were derived, which are in good agreement with previous data. Values of G, the fraction of energy transferred per collision, were deduced using previously published results for momentum transfer cross sections. These G values show a marked variation with Te as expected and are interpreted in terms of energy transfer to rotational states of the molecules. The G values over the complete Te range are less than theoretically predicted in the case of nitrogen, but greater than predicted in the case of oxygen.
    No preview · Article · Jan 2001 · Journal of Physics B Atomic and Molecular Physics