The multi-disciplinary character at Onera

Plasma Control for Electromagnetism

A plasma is mainly a cloud of charged particles (electrons and ions), mostly in a gas. Due to this fact, interaction of some plasma with an electromagnetic wave can be quite significant when the charge density is sufficient. The most popular uses of plasma (screens, lighting) do not require high electron density and are low in energy consumption. In order for plasma to have significant effect on an electromagnetic wave, the electronic density must be increased by several orders of magnitude and therefore energy consumption becomes a dimensioning parameter.

Studies conducted as part of the Plasma Control for Electromagnetism federating research project, had three main areas of application:

radar stealth
antennas, lenses and in general radiating systems
protection against high-power microwaves

The planned scientific objectives were the production of effective plasma, i.e., with sufficient electronic density (> 1012 electrons/cm3), to model them and conduct experiments that will help in establishing the feasibility of these plasmas and validate the models.

The discharges studied (glow discharges and continuous arcs) are all produced by applying high voltage between an anode and a cathode. Only the first two application topics are taken up here.

Stealth application

The stealth application is split into two sub-themes:

masking equipment (antennas, optics) by producing plasma inside a cavity (radome). The possibility of selecting the gas and the pressure largely simplifies plasma production.
masking the external scattering points on an aircraft. Here, the gas is given and it is necessary to deal with the air flow.

For the second application, protection of air intakes is preferred, by creating a discharge inside the duct. Studies were done on a specifically developed test bench (CHYPRE bench), made up of a conduit of 5 cm x 10 cm sections, which combines three functions, creation of plasma, generation of air flow (0 < v < 150 m/s) and propagation of an electromagnetic wave (around 2 GHz).

Figure 1 - CHYPRE bench

We were able to demonstrate that a simple filament discharge in the centre of the conduit could attenuate a wave crossing the plasma twice (back and forth) by at least 3 dB for consumed power of a few hundred Watts. A purely thermal model of the discharge was developed (Elenbaas-Heller relationship), and its operation highlighted the essential role played by stabilization of the power supply on stability of the discharge.

In presence of air, the discharge gets blown, and we let it slide along an electrode which is like a blade, at the end of which it extinguishes. We have demonstrated that it is possible to regenerate it immediately, ensuring permanent presence of plasma in the conduit. In presence of air flow, the plasma maintains its absorption performances.

Prise de vue
Figure 2 - Sliding discharge

Antenna Application

Two fields of study were preferred

a "lens" field: a plasma column is developed in a cylindrical chamber (ref. Scientific image Plasma lens). Waves are deviated by the plasma, and we can control this deviation by modifying plasma's characteristics by adjusting the high voltage. One interest in using plasma is the switching speed that can be fast, less than a microsecond.
the second field pertains to the use of capillary tube networks in which plasma is created.

Figure 3 – Stacking of tubes
more or less filled with plasma

Figure 4 – Radioelectric measurement
on a capillary tube

This structure, an assembly of dielectric tubes and conductors, is a meta-material that can be given specific transmission, reflection, refraction or diffraction properties of electromagnetic waves. Furthermore, these properties can be controlled at the speed at which plasma's characteristics are modified, i.e. better than one microsecond. The very low radioelectric losses in this structure give us an extremely competitive concept in comparison to the currently available solutions.

Prospects

The level of control obtained on plasmas, with respect to their production with low power consumption budget and possibilities of acting on electromagnetic waves and the availability of validated models, means that we can consider several applications in the field of stealth and antennas (controllable meta-materials). Plasma protection against high power microwaves, a topic dealt with at a lower level, however offers bright perspectives given the accessible switching speeds and electromagnetic field shielding capabilities

Key words

Plasma, radar stealth, antenna, High Power Microwaves

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