Publications by Karen Aplin

Variability in long-duration operation of silicon tip field emission devices

Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 24 (2006) 1056-1060

KL Aplin, BJ Kent, L Wang, HF Lockwood, J Rouse, R Stevens

Sources of variability in long-term testing of silicon tip field emission

Technical Digest of the 18th International Vacuum Nanoelectronics Conference, IVNC 2005 2005 (2005) 141-142

KL Aplin, BJ Kent, HF Lockwood, J Rouse, R Stevens, L Wang

Fabrication yield analysis and emission failure investigation of silicon field emission arrays

Technical Digest of the 18th International Vacuum Nanoelectronics Conference, IVNC 2005 2005 (2005) 169-170

L Wang, KL Aplin, SE Huq, R Stevens, BJ Kent, A Malik, IM Loader, GR Thomas

Space applications of micro fabricated field emitters

Technical Digest of the 18th International Vacuum Nanoelectronics Conference, IVNC 2005 2005 (2005) 82-83

BJ Kent, KL Aplin, SE Huq, R Stevens, L Wang

Devices based on the field emission process have many applications as a low power source of electrons. The low mass and volume benefits that are also conferred by micro fabrication are very attractive to the designers of space instrumentation where mass and power have significant cost penalties. However the semi industrial environment used for assembly and test of spacecraft does not easily lend itself to the use of micro fabricated components that are highly sensitive .to dust and molecular contamination and thus mitigation strategies are needed. Equally the long project development times coupled with the requirement for a documentation trail that demonstrates ultra high reliability means that space instruments do not typically use the latest innovative technology even if there are performance advantages. These factors perhaps explain the fact that even though field emitters have been standard components in laboratory devices for many years their use in space based equipment is still limited. In this paper we describe four space based uses of field emitters; two of them are field emitting electron sources and two emit ions. Of the electron emitters one is inside the Ptolemy mass spectrometer which was launched in March 2004 on the Rosetta satellite and for the past year has been travelling on its way to a comet Churymov-Gerasimenko in the outer solar system; and the other is still in preparation to provide charge balance for the LISA system of satellites that will measure gravitational waves from colliding black holes and the first instant of the big bang. The two field emitting ion sources are similar types of microthruster used to provide an exceptionally gravitationally quiet environment in order to carry out high precision physics experiments which can only be carried out in space, free from the effects of gravitational noise present in terrestrial laboratories. These examples are selected to illustrate the overhead involved in developing robust, advanced technologies such as field emitter arrays for use in space instrumentation in which fault tolerance and system redundancy must be built in, as recovery and repair is never an option. © 2005 IEEE.

Aspirated capacitor measurements of air conductivity and ion mobility spectra

Review of Scientific Instruments 76 (2005) 1-6

KL Aplin

Measurements of ions in atmospheric air are used to investigate atmospheric electricity and particulate pollution. Commonly studied ion parameters are (1) air conductivity, related to the total ion number concentration, and (2) the ion mobility spectrum, which varies with atmospheric composition. The physical principles of air ion instrumentation are long established. A recent development is the computerized aspirated capacitor, which measures ions from (a) the current of charged particles at a sensing electrode, and (b) the rate of charge exchange with an electrode at a known initial potential, relaxing to a lower potential. As the voltage decays, only ions of higher and higher mobility are collected by the central electrode and contribute to the further decay of the voltage. This enables extension of the classical theory to calculate ion mobility spectra by inverting voltage decay time series. In indoor air, ion mobility spectra determined from both the voltage decay inversion, and an established voltage switching technique, were compared and shown to be of similar shape. Air conductivities calculated by integration were: 5.3±2.5 and 2.7±1.1 fSm-1, respectively, with conductivity determined to be 3 fSm-1 by direct measurement at a constant voltage. Applications of the relaxation potential inversion method include air ion mobility spectrum retrieval from historical data, and computation of ion mobility spectra in planetary atmospheres. © 2005 American Institute of Physics.

Effect of the troposphere on surface neutron counter measurements

Advances in Space Research 35 (2005) 1484-1491

KL Aplin, RG Harrison, AJ Bennett

Surface neutron counter data are often used as a proxy for atmospheric ionisation from cosmic rays in studies of extraterrestrial effects on climate. Neutron counter instrumentation was developed in the 1950s and relationships between neutron counts, ionisation and meteorological conditions were investigated thoroughly using the techniques available at the time; the analysis can now be extended using modern data. Whilst surface neutron counts are shown to be a good proxy for ionisation rate, the usual meteorological correction applied to surface neutron measurements, using surface atmospheric pressure, does not completely compensate for tropospheric effects on neutron data. Residual correlations remain between neutron counts, atmospheric pressure and geopotential height, obtained from meteorological reanalysis data. These correlations may be caused by variations in the height and temperature of the atmospheric layer at ∼100 hPa. This is where the primary cosmic rays interact with atmospheric air, producing a cascade of secondary ionising particles. © 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

A field effect spacecraft neutralizer for the LISA Pathfinder mission

Classical and Quantum Gravity 22 (2005)

BJ Kent, KL Aplin, L Wang, SE Huq, R Stevens, A Malik, D Nicolini

We outline the design, construction and testing of a field effect neutralizer, which provides a source of up to 6 mA of electron emission to maintain charge neutrality for the LISA Pathfinder mission spacecraft. The low mass, low power neutralizer uses silicon field emitter arrays and has been engineered for integration into the LISA Pathfinder micro-propulsion system. The silicon emitters are constructed using micro-fabrication techniques and electron beam lithography to ensure uniformity and precise control of emitter location. Control of the manufacture processes, plasma-enhanced vapour deposition, wet and dry plasma etch and various coatings, has been optimized to achieve robust reproducible devices suitable for space use. © 2005 IOP Publishing Ltd.

Absorption of infra-red radiation by atmospheric molecular cluster-ions

Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 775-783

KL Aplin, RA McPheat

Results from previous laboratory experiments indicate that both the protonated water dimer H3O+(H2O), and molecular cluster-ions, X+(H2O)n absorb infra-red (IR) radiation in the water vapour continuum region between 4 and 14μm (2500-714 cm-1). Protonated water clusters are a common species of atmospheric molecular cluster-ion, produced by cosmic rays throughout the troposphere and stratosphere. Under clear-sky conditions or periods of enhanced atmospheric ionisation, such as solar proton events, the IR absorption by atmospheric ions may affect climate through the radiative balance. Fourier transform infrared spectrometry in a long path cell, of path length 545 m has been used to detect IR absorption by corona-generated positive atmospheric cluster ions. The column concentration of ions in the laboratory spectroscopy experiment was estimated to be ∼1013 m-2; the column concentration of protonated atmospheric ions estimated using a simple model is ∼1014 m-2. Two regions of absorption, at 12.3 and 9.1 μm are associated with enhanced ion concentrations. After filtering of the measured spectra to compensate for spurious signals from neutral water vapour and ambient carbon dioxide, the strongest absorption region is at 9.5-8.8 μm (1050-1140 cm-1) with a fractional change in transmissivity of 0.03±0.015, and the absorption at 12.5-12.1 μm (800-825 cm-1) is 0.015±0.008. © 2005 Elsevier Ltd. All rights reserved.

A field effect spacecraft neutraliser for fundamental physics missions

(2004) 423-428

BJ Kent, D Nicolini, SE Huq, KL Aplin, R Stevens, L Wang, A Malik

We describe the design, construction and testing of a field effect emitter array, a low power source of electrons providing up to 6mA, for spacecraft neutralising. The emitter array has been designed and engineered to be suitable for integration into the micro propulsion system of fundamental physics missions such as LISA Pathfinder. The silicon emitters are fabricated using micro fabrication techniques involving electron beam lithography to ensure uniformity and precise control of emitter location on each array. Process control of the remaining techniques including plasma enhanced vapour deposition, wet and dry plasma etch, and various coatings has been important in achieving robust reproducible devices suitable for space use.

AIN coatings on silicon field emitters and oxidised gates to enhance reliability for space applications

Technical Digest of the 17th International Vacuum Nanoelectronics Conference, IVNC 2004 (2004) 160-161

L Wang, KL Aplin, BJ Kent, SE Huq, R Stevens, GR Thomas, IM Loader, CM Collingwood, A Malik, HO Blom

The development of coatings for silicon field emission arrays used as an electron source to maintain spacecraft charge neutrality was described. The neutralizer specification included 6mA emission current at 0.2 W/mA and the instrument was required to operate for a lifetime of more than 6000 hours. In order to make the field emission device more resistant to ion bombardment and thermal failure, the effects of adding a layer of AlN, and thermal oxidation of the chromium gate electrode were investigated. A semi-automatic conditioning process, optimized for AlN coated devices was applied to each array before running at constant currents. It was found that once the device has been fully conditioned, the emission characteristic are permanent.

Modelling studies of charged particle interactions for a space application

Institute of Physics Conference Series 178 (2004) 221-226

KL Aplin, VP Tarakanov

An electron emitter neutraliser to prevent spacecraft charging from the use of positive ion thrusters is under development at the Rutherford Appleton Laboratory. A particle-in-cell code, KARAT, has been used to simulate laboratory experiments, Laboratory test results have been successfully obtained with simulations, for model verification. Neutraliser-ion thruster interactions in space have also been modelled. An ion-propelled spacecraft rapidly charges to kilovolt potentials, which can be dissipated by an electron-emitting neutraliser. The effect of background particle concentrations on neutralisation has also been studied.

Atmospheric ion spectra and the rate of voltage decay of an aspirated cylindrical capacitor

Institute of Physics Conference Series 178 (2004) 155-160

KL Aplin

Aspirated cylindrical capacitors are commonly used instruments for atmospheric ion measurements, more of which are needed for climate and pollution studies. However the classical theory of one operating mode of this instrument is based on an approximation, which may introduce errors in ion measurement. A modification to the classical theory of the instrument is consequently proposed. Applying this modification enables ion mobility spectrum information to be extracted from the rate of voltage decay of the aspirated capacitor in air. It can also be used to improve air conductivity measurements.

Reliability tests of gated silicon field emitters for use in space

Journal of Physics D: Applied Physics 37 (2004) 2009-2017

KL Aplin, CM Collingwood, BJ Kent

Neutralizes are required to prevent spacecraft charging from satellite ion propulsion. This paper discusses the development of a gated silicon tip field emitter (FE) neutralizer, specified to deliver 6 mA, with each tip emitting a mean current of 7 nA. It is important to investigate factors affecting the lifetime of field emitter arrays for a space application, as longevity and reliability are both critical requirements. Semi-automated procedures to prepare 400 arrays, each consisting of 765 FEs, for life tests are described with failure conditions strictly defined by mission constraints. Results of 25 life tests on 72 arrays driven to failure at constant emission current are summarized, and a case study of one test is presented. Two of the three failure mechanisms identified are consistent with thermal failure and damage by ion bombardment. Reduced field enhancement from tip erosion caused by ion bombardment is a common explanation for FE failure. However, scanning electron microscope examination of tip apex diameters showed no significant relationship between array failure and apex geometry. The third failure mechanism was associated with short-lived arrays and may be caused by manufacturing defects. Substantial intrinsic variability was observed in the arrays tested, even with the rigorous production standards required for space applications. Arrays without manufacturing defects had lifetimes of thousands of hours.

Optimization of silicon field-emission arrays fabrication for space applications

Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 22 (2004) 1407-1410

L Wang, R Stevens, E Huq, I Loader, B Kent, K Aplin, J She

The optimization of fabrication of silicon field-emission arrays was carried out on 4 in. silicon wafers for the field-emission electric propulsion neutralizer applications. The optimization of a mixture of SF6, O2 and Ar for silicon tip etches was done to improve the uniformity and process repeatability. A thin aluminum nitride (AlN) film was coated on gated emitter arrays to enhance the field at the tip and to protect tips from ion bombardment. A statistical analysis of emission characteristics shows a narrow distribution of the turn-on voltage from array to array and achieve a few milliamperes current emission by connecting several arrays in parallel.

Nineteenth century Parisian smoke variations inferred from Eiffel Tower atmospheric electrical observations

Atmospheric Environment 37 (2003) 5319-5324

RG Harrison, KL Aplin

Atmospheric electrical measurements provide proxy data from which historic smoke pollution levels can be determined. This approach is applied to infer autumnal Parisian smoke levels in the 1890s, based on atmospheric electric potential measurements made at the surface and the summit of the Eiffel Tower (48.7°N, 2.4°E). A theoretical model of the development of the autumn convective boundary layer is used to determine when local pollution effects dominated the Eiffel Tower potential measurements. The diurnal variation of the Eiffel Tower potential showed a single oscillation, but it differs from the standard oceanic air potential gradient (PG) variations during the period 09-17UT, when the model indicates that the Eiffel Tower summit should be within the boundary layer. Outside these hours, the potential changes closely follow the clean air PG variation: this finding is used to calibrate the Eiffel Tower measurements. The surface smoke pollution concentration found during the morning maximum was 60±30μgm-3, substantially lower than the values previously inferred for Kew in 1863. A vertical smoke profile was also derived using a combination of the atmospheric electrical data and boundary layer meteorology theory. Midday smoke concentration decreased with height from 60μgm-3 at the surface to 15μgm-3 at the top of the Eiffel Tower. The 19th century PG measurements in both polluted and clean Parisian air present a unique resource for European air pollution and atmospheric composition studies, and early evidence of the global atmospheric electrical circuit. © 2003 Elsevier Ltd. All rights reserved.

Meteorological effects of the eclipse of 11 August 1999 in cloudy and clear conditions

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 459 (2003) 353-371

KL Aplin, RG Harrison

Integrated atmospheric measurements were made at two sites in the UK during the total solar eclipse of 11 August 1999. Sensitive high-resolution meteorological observations revealed dynamical atmospheric effects despite the presence of cloud. Short-term eclipse-related changes dominated over temperature, wind speed and wind directions associated with the synoptic conditions, and 35 min period gravity waves were detected in surface-pressure observations. Theoretical calculations for solar radiation changes were compared with observations made during the 97% partial eclipse in clear skies at Reading, Berkshire, and used to predict the radiative changes expected at Camborne, Cornwall, during totality. At Camborne, a vertical array of sensitive fine-wire thermometers was operated with a high-resolution sonic anemometer, logged at 5 Hz. Despite cloudy conditions, which reduced the eclipse-induced thermal perturbations, small temperature and wind changes were detected which were consistent with theory. The cold outflow from the umbra was unambiguously detected for the first time in cloudy conditions.

Optimisation of silicon fed fabrication for space application

Proceedings of the IEEE International Vacuum Microelectronics Conference, IVMC 2003-January (2003) 83-84

L Wang, R Stevens, SE Huq, I Loader, BJ Kent, KL Aplin, JC She

Ionisation of the atmosphere

Weather 57 (2002) 358-

K Aplin

Whorl of confusion [4]

New Scientist 176 (2002) 28-

K Aplin

Mid-nineteenth century smoke concentrations near London

Atmospheric Environment 36 (2002) 4037-4043

RG Harrison, KL Aplin

Measurements of atmospheric electricity began at Kew Observatory, London (51°28′N, 0°19′W) in 1843, with recording apparatus installed by Lord Kelvin in 1861. The measured electric potential gradient (PG) at Kew has always been influenced by smoke pollution, causing a December PG maximum and July minimum. Theory links PG variations with aerosol concentrations, and the 20th century smoke measurements made at Kew permit smoke concentrations to be retrieved from 19th century PG data. Absolute calibration of the 1862-1864 PG is achieved by considering changes in the global electric circuit, for which the geomagnetic aa-index is used as a proxy. The mean annual PG in 1863 is estimated as 363±29Vm -1 , and the mean smoke concentration found is 0.17±0.05mgm -3 . Diurnal variations in smoke pollution differ between the seasons, and change in their character after the advent of motor traffic. Copyright © 2002 Elsevier Science Ltd.