Neil A. Salmon

Disruptions in JET

In JET, both high density and low-q operation are limited by disruptions. The density limit disruptions are caused initially by impurity radiation. This causes a contraction of the plasma temperature profile and leads to an MHD unstable configuration. There is evidence of magnetic island formation resulting in minor disruptions. After several minor disruptions, a major disruption with a rapid energy quench occurs. This event takes place in two stages. In the first stage there is a loss of energy from the central region. In the second stage there is a more rapid drop to a very low temperature, apparently due to a dramatic increase in impurity radiation. The final current decay takes place in the resulting cold plasma. During the growth of the MHD instability the initially rotating mode is brought to rest. This mode locking is believed to be due to an electromagnetic interaction with the vacuum vessel and external magnetic field asymmetries. The low-q disruptions are remarkable for the precision with which they occur at qψ = 2. These disruptions do not have extended precursors or minor disruptions. The instability grows and locks rapidly. The energy quench and current decay are generally similar to those of the density limit.

Compact real-time (video rate) passive millimeter-wave imager

This paper describes a novel real time mechanically scanned passive millimeter wave imager. This imager produces a field of view of 40 degree(s) X 20 degree(s) with diffraction limited performance and a 25 Hz frame update rate. It is relatively inexpensive because the scene is imaged using 32 direct detection receivers with a frequency of operation from 28 - 33 GHz. The compact antenna uses polarization techniques to fold the beam and is constructed from readily available low cost materials.

Mechanically scanned real-time passive millimeter-wave imaging at 94 GHz

It is well known that millimetre wave systems can penetrate poor weather and battlefield obscurants far better than infrared or visible systems. Thermal imaging in this band offers the opportunity for passive surveillance and navigation, allowing military operations in poor weather. We have previously reported a novel real time mechanically scanned passive millimetre wave imager operating at 35GHz and in this paper a 94GHz variant will be described. This 94GHz imager has a field-of-view of 60° x 30° and has diffraction limited performance over the central two thirds of this field-of-view. It is relatively inexpensive because the scene is imaged using a linear array of direct detection receivers and compact folded optics. The receiver array has been constructed using indium phosphide monolithic microwave integrated circuits (MMICs) allowing high gain and low noise figure to be achieved. The compact optics consist of a polarisation sensitive mirror and a Faraday rotator. readily The mirror is constructed from expanded polystyrene, supporting a printed copper grid etched onto a PTFE/glass fibre substrate. These materials are low cost and readily available. The Faraday rotator is made from a commercial grade plasto-ferrite sandwiched between antireflection coatings. The optics produce a conical scan pattern and image processing is used to generate a raster scan pattern and to perform gain and offset corrections.

First video rate imagery from a 32-channel 22-GHz aperture synthesis passive millimetre wave imager

The first video rate imagery from a proof-of-concept 32-channel 22 GHz aperture synthesis imager is reported. This imager has been brought into operation over the first half of year 2011. Receiver noise temperatures have been measured to be ~453 K, close to original specifications, and the measured radiometric sensitivity agrees with the theoretical predictions for aperture synthesis imagers (2 K for a 40 ms integration time). The short term (few seconds) magnitude stability in the cross-correlations expressed as a fraction was measured to have a mean of 3.45×10-4 with a standard deviation of ~2.30×10-4, whilst the figure for the phase was found to have a mean of essentially zero with a standard deviation of 0.0181°. The susceptibility of the system to aliasing for point sources in the scene was examined and found to be well understood. The system was calibrated and security-relevant indoor near-field and out-door far-field imagery was created, at frame rates ranging from 1 to 200 frames per second. The results prove that an aperture synthesis imager can generate imagery in the near-field regime, successfully coping with the curved wave-fronts. The original objective of the project, to deliver a Technology Readiness Level (TRL) 4 laboratory demonstrator for aperture synthesis passive millimetre wave (PMMW) imaging, has been achieved. The project was co-funded by the Technology Strategy Board and the Royal Society of the United Kingdom.

Scene simulation for passive and active millimetre- and submillimetre-wave imaging for security scanning and medical applications

The benefits of moving up in frequency from the millimetre wave region towards a frequency of 1 THz are those of smaller systems and better diffraction limited image resolutions. Limitations will be examined, considering effects such as the absorption in the atmosphere, various materials, the human body and fundamental radiometric noise limitations. The physics behind these considerations will be examined and results and artefacts presented using scene simulation. Conclusions are that above 500 GHz high atmospheric absorption severely limits imager to subject distances to a few hundred metres. The effect of absorption is poor subject illumination and high signal attenuation between the subject and the imager. These limitations may be over come partially, for short imager to subject distances (less than a few hundred metres), by using active illumination with narrow bandwidth radiation sources. However, transmit powers rise steeply with imager to subject distance and radiation frequency, lying typically between 1 mW and 1 W over the frequency band 500 GHz to 1 THz, for a radiation bandwidth of 1 GHz and an imager to subject distance of 20 m. Similar systems analysis for medical applications indicates that the high attenuation in human tissue limits probing or penetration distances of the radiation. Radiometric photon noise, electrical properties of human tissue and irradiation power restrictions (1 mW/cm2) limit maximum diagnostic depths to between two and one millimetres between the frequencies of 100 GHz and 1 THz.

Millimeter-Wave and Terahertz Imaging in Security Applications

The relatively short wavelength of mm-wave and THz radiation coupled with good transmission through many dielectric materials allows images to be formed of concealed objects. This chapter gives an overview of the detectors, their associated circuitry, and system developments over the past 10 years, focussing on personnel security screening. We will discuss the phenomenology of imaging at these wavelengths, introduce the reader to the basic architectures being used and developed for image forming instruments, show examples of systems, and also discuss the feasibility of spectroscopic THz imaging for security screening applications.

Polarimetric scene simulation in millimeter-wave radiometric imaging

This paper describes the general requirements and an approach to scene simulation in millimetre wave radiometric imaging that is based on multi faceted semitransparent layered media in the earth’s three-dimensional geometry. The driving attributes in this field are essentially the transparency of clothing for security scanning and the transparency of fog, cloud, rain and dust for all weather flight. Out-door illumination and the physics of the interaction of millimetre waves with the atmosphere and obscurants are discussed, together with the interaction of millimetre waves with multi layer material surfaces, giving rise to transmission, reflection and emission. The physics of these interactions are discussed in the context of computer graphics. These considerations enable a powerful polarimetric modelling capability to be developed that can be used to simulate all scenarios, including artificial or burst illumination, from in-doors to imaging from satellites.

Interferometric aperture synthesis for next generation passive millimetre wave imagers

This paper discusses the phase effects in the near-field associated with aperture synthesis imaging. The results explain why in some regions of the near-field it is possible to use Fourier transform techniques on a visibility function to create images. However, to generate images deep inside the near-field alternative processing techniques such as the G-matrix method are required. Algorithms based on this technique are used to process imagery from a proof of concept 22 GHz aperture synthesis imager [1]. Techniques for generating synthetic cross-correlations for the aperture synthesis technique are introduced and these are then validated using the image creation algorithms and real data from the proof of concept imager. Using these data the phenomenon of aliasing is explored. The simulation code is then used to illustrate how the effects of aliasing may be minimised by randomising the locations of the antennas over the aperture. The simulation tool is used to show how in the near field the technique can provide a range resolution in 3D imaging of a couple of millimetres when operating with a wavelength of 13 mm. Moving to illustrate the quality of images generated by a next generation aperture synthesis imagers, the software is extended to systems with hundreds of receiver channels.

Minimising the costs of next generation aperture synthesis passive millimetre wave imagers

This paper examines the sourcing of low cost components for next generation passive millimetre wave (PMMW) aperture synthesis imagers. Splitting the elements of the imager into antennas/receivers, analogue to digital converters (ADCs), digital signal processors (DSP) and a host computer, technologies are identified that can minimise the cost of these in future systems. It is established that the follow-on aperture PMMW imagers can be constructed at relatively low cost, using a combination of low frequency (< 30 GHz) satellite receiver technology, high-speed clocked comparators, DSP (both Field Programmable Gate Arrays (FPGAs) and Graphical Processor Units (GPUs)) and the latest personal computers that use high-speed high lane count PCI Express Bus technology.

Operational issues for passive millimeter-wave imaging systems

Passive millimeter wave (mm-wave) imaging systems have attracted an increasing interest over the past years due to their superior poor weather performance compared with visible and infrared systems. In the UK the Defence Evaluation and Research Agency Malvern developed its first mm-wave radiometers in the late 1950s. These systems were bulky and had poor spatial resolution and low thermal sensitivity, but the considerable advances in semiconductor solid state devices have allowed the size and weight of imagers to be reduced. Advantage can also be taken of sophisticated on-line signal processing and of complex theoretical modeling and analysis. This paper examines the merits of the different operating frequencies in terms of atmospheric transmission vs. resolution and also discusses issues such as image processing. High quality images are presented to demonstrate the potential of this emerging technology.