DICTAT was developed to provide a practical engineering tool to address problems of internal dielectric charging.

Electrical charging of dielectric materials in the magnetosphere is a major cause of satellite anomalies. Where surface charging is concerned, there are a number of software tools (e.g. NASCAP [Rubin et al., 1980] and EQUIPOT [Wrenn and Simms, 1990]) which enable satellite designers to model the extent of the problem and to make satellites more resistant to this effect. For the internal charging problem a useful scientific tool is provided by the ESA-DDC code [Soubeyran and Floberhagen, 1994]. DICTAT Was developed developed to provide a practical engineering tool to address problems of internal dielectric charging.

DICTAT calculates the electron current that passes through a conductive shield and becomes deposited inside a dielectric. From the deposited current, the maximum electric field within the dielectric is found. This field is compared with the breakdown field for that dielectric to see if the material is at risk of an electrostatic discharge.

The user can select whether the simulation should use a constant spectrum (as in a laboratory test), or a changing environment experienced over an orbit. In the latter case, the user must supply details of the orbit but does not need to know the environment experienced by spacecraft in that orbit. The tool has, in-built, a position-dependent worst-case model of electron fluxes in the outer radiation belt. The model is named FLUMIC (Fluence Model for Internal Charging). This model gives electron spectra as a function of L, B/B0, fraction of solar cycle and fraction of year. If the structure does not discharge in this model environment, then it should be safe.

If the structure is predicted to exceed the breakdown threshold, then the tool will suggest changes to the shield and dielectric thicknesses that would bring the structure safely below the threshold.

In order to produce a fast user friendly tool, the code uses analytical approximations to calculate electron transport through the shield and deposition in the dielectric. This is not the most accurate way this could be done. However, a more critical complexity in the internal charging problem lies in the changes that take place in dielectric conductivity in space. Here the tool includes all the important physical processes: radiation, temperature and electric field effects on conductivity. Because the best equations describing these effects are primarily empirical and approximate, this aspect of the calculation does not justify a more detailed current deposition analysis.

Although the main output of the code is a statement on whether the dielectric is likely to discharge or not, this assessment is based purely on whether the maximum electric field exceeds the breakdown threshold for the dielectric. Other types of discharge are possible, particularly discharge from a high-potential surface across a gap. The probability of this discharge depends on the details of the geometry of the structure and the surrounding structures. The code makes no assessment of these. However, the code does output the surface potential which should enable the user to assess whether a surface discharge is a possibility. Typically a surface potential of hundreds of volts should be a cause for concern.

Finally, as for most simulation programs, DICTAT results depend strongly on the quality of the input data. Dielectric material properties, e.g. bulk conductivity and breakdown electric field, are hard to obtain and may vary by orders of magnitude from one sample to the next. The tool is supplied with a list of representative parameters for a number of materials but the user may need to change these to worst-case values or as the result of a well conducted experimental measurement.

Updated on: 24/02/2022
Created on: 22/02/2022
Version Control System: svn
Owner: ESA