During this project, an experimental campaign was conducted in order to understand the characteristics of the so-called flash-over phenomenon. Therefore, a real solar panel has been tested in a large vacuum chamber under representative conditions. The experiments have been analysed in the light of a FO simplified model developed in the frame of the project and a FO simulator has been developed to represent such phenomenon when testing small solar cells coupons.
Spacecraft in geostationary orbit are subjected to high levels of electrical charging, sometimes up to many kilovolts of negative potentials. Large dielectric areas are exposed to the external environment, with the possibility to create a huge differential potential. This can result in electrostatic discharges (ESD) between surface elements.
This ESD is characterized by a Blow-Off (BO) current emitted to space and a Flash-Over (FO) current which is the surface neutralization. The BO corresponds to the emission of charges stored in the satellite representative capacitance and The FO is a current emitted by the same point (cathodic spot) but collected by the coverglasses (See figure below).
Qualification of solar arrays is performed on such small coupons (typically 2*3 cells) and then requires a set-up representative of in-flight configuration i.e. of the presence of the other cells. Until now the flash-over has been represented in the classical electrical set-up by a capacitance added to those representing the satellite but it is well-known now that this capacitance should be replaced by a "flash-over simulator" i.e. a set-up which could deliver the real shape of the flash-over current which is not yet completely known.
Improving the knowledge of FO characteristics is the main objective of the project in the aim to improve the methods of test of solar array coupons.
The experimental results obtained in the frame of the experimental campaign have shown that during an electrostatic discharge, the Flash Over could propagate and neutralize the total surface of the panel.
It has been shown also that the FO could jump over a gap of 10cm and neutralize another m² of surface.
To improve our capability of analyzing the data, a simplified FO model was developed and used to simulate a large number of cases and to provide parametric studies.
The comparisons with experimental results have allowed us to show that:
- Distinction can be made between the ESD supposed to be created in the cell edge (4500 m/s) or in the interconnector (2500 m/s),
- The location of the ESD triggering is the most important parameter,
- The surface potential before ESD is a first order but the profile has a second order effect,
- There is no primary order effect of the neutralization source (electron or plasma),
- There is no primary order effect of the dielectric materials (CG or CG+Kapton).
Lastly, the model has confirmed that it was difficult to analyze the results as soon as the initial potential profile (electron) was not very well known.
This experiment was conducted by ONERA in IABG large vacuum chamber (WSA Weltraum Simulations Anlage = space simulation facility) on a real solar panel provided by Astrium Ottobrunn.
The panel is 4m*2m with Silicon cells organised in 52 linear strings and covered by CMX coverglasses.
The set-up is schematically described and a photograph is presented on the following figures:
|schematic view of the set-up
click for larger image
|photograph of the panel inside IABG chamber
click for larger image
The set-up allows to reproduce the Inverted Potential Gradient (IPG) situation observed in flight, i.e. while the satellite body (here panel structure and cells) is negatively charged up to several kilovolts, the dielectrics (here coverglasses protecting the cells) build up a differential charge as high as 1000 V. This can be reproduced either with plasma or electrons.
In both cases, measurements of surface potential are made using the moving arm on which two potential probes were placed.
The panel is biased to a high voltage by a power supply and connected to a capacitance Csat. The harness coming from the panel is distributed on 53 corresponding wires (52 strings + honeycomb) and one current probe placed on each wire. These probes measured either an emitted current (on the string where the discharge occurred) or a collected current (on the 51 others).
The core of the project is the experimental campaign. The test plan which was foreseen was completely followed and achieved:
- Step 1 : IPG in electrons,
- Step 2 : IPG in plasma,
- Step 3 : effect of low temperature : -120°C (in electrons),
- Step 4 : same in plasma,
- Step 5: go back to room temperature in electrons (confirmation of steps 1 and 3),
- Step 6 : effect of dielectric capacitance (by adding dielectric foil) – in electrons
125µm of kapton film : thickness*2.25 => C/2.25,
- Step 7 : same in plasma,
- Steps 8 and 9 (reproduce 6 and 7 with a another value) :
25µm of aluminized kapton film : thickness/4 => C*4,
- Step 10 : simulation of gap between panels Addition of the 1m² panel – test in electrons,
- Step 11 : same in plasma.
After the test campaign, a numerical model was developed to analyse and better understand the results. Following this study, a FO simulator was defined, built and tested.
The project is now finished.
Final presentation has been made on 6th of July.
One communication has been done at ESPC in St Raphael (5-10 June 2011).
Some other publications are in preparation.