Tomographic imaging of ionisation in gases

A number of electrical tomographic imaging techniques have been developed in the last decade both in the fields of medical diagnostic imaging, where electrical impedance tomography (EIT) is the most common approach, and industrial tomography where a variety of capacitive and inductive regimes have also been investigated. Here, a related but novel approach to the problem of imaging a cross-sectional distribution of ionisation within a volume of gas is described. The tomographic ionisation chamber consists of an insulating cylinder containing a volume of air and supporting a number of equally spaced electrodes surrounding the plane to be imaged. To produce a potential field pattern suitable for ionisation imaging, a potential difference of several hundred volts is applied at one electrode while the rest are maintained at a virtual earth potential. This gives the field pattern (shown here in a figure) which is identical to the familiar equipotential pattern used for EIT with adjacent electrode current drive, except that the equipotential lines and current streamlines are interchanged. To obtain sets of ionisation profiles from which an image can be reconstructed, the current passing through each of the virtually earthed electrodes is measured for each position of the high voltage drive electrode in turn