Aviation Week & Space Technology
08/09/2004, page 24
Douglas Barrie
London
Michael A. Taverna
Paris
France explores characteristics of plasma fields in relation to low-observable technology
Furtive Exploration
The French Defense Ministry is working on the development of active stealth technology based on plasma-field generation, applicable to combat aircraft and high-altitude cruise missiles. It believes the practicality of such an approach will be determined in the next 2-3 years.
State-owned aerospace and defense research agency Onera is examining the use of plasma as part of a broader research program into active stealth. Work has been underway since early 2003. Despite a difficult environment for research funds there is continuing support for low-observable efforts in France as it is considered a strategic defense technology.
Initial generations of stealth technology were passive, relying on airframe shaping or radar absorbent material to reduce a platform's radar cross section. Active stealth requires the generation of some form of field or counter wave-form to lower detectability.
Both the U.S. and Russia also continue to look at plasma technology for stealth applications, with research believed to have been underway as far back as the late 1980s. A number of other European countries, including the U.K., are also interested in this area.
The U.S. and Russia also, apparently, had been interested in plasma technology for other reasons as well. The approach was viewed as a means of achieving high Mach flight without having to cope with the normal aerothermodynamic issues. The technology was so interesting that at one point it was investigated, some say, under the auspices of the Strategic Defense Initiative. Russian interest is harder to gauge. Photographic evidence now years old had led some in the government to conclude that brightly lit objects seen in the images could be Russian aircraft or missiles involved in plasma experiments.
The Onera research is considering the generation of plasma both around the airframe to mask radio-frequency (RF) energy bright spots and within enclosed spaces, such as inside the aircraft's radome. One of the characteristics of a plasma field is the absorption of RF energy.
A conventional planar array radome can be rotated 90 deg. when not in operation to minimize RF reflectivity, but this technique cannot be used with a fixed phased-array fighter antenna.
In the case of the latter a small plasma field would be generated within the radome to mask the antenna. This technique is also being pursued in Russia for use on its fifth-generation fighter. The 2nd Central Scientific Research Institute is studying this active stealth technology to reduce the reflectivity of fighter radar.
Sustaining a plasma field within an enclosed space, rather than in the free airstream surrounding an airframe, is considerably less of a technical challenge. It also requires less energy.
The Onera work is at the initial technical feasibility stage, and the research agency is investigating the potential for cooperation with university teams in Europe and the U.S.
Laboratory teams have already built a functioning plasma antenna, but are still working on the source itself, the design of which presents considerable engineering obstacles. For example, it must have a very high ionization density--1012 electrons per cubic centimeter--without giving off telltale heat or oscillations.
For external airframe use it must also be possible to keep the field stable in high-speed flight, in sufficient volume, without making undue demands on onboard power systems.
Another potential use for a plasma field would be to reduce the radar cross section of a high-flying cruise missile. Again this technology has already been looked at in Russia. The Chelomei (now NPO Mashinostroenia) 3M-25 Meteorit (AS-X-19) strategic cruise missile was intended to be fitted with a plasma system--dubbed Marabu--to improve its stealth characteristics. The program was canceled in the early 1990s.
Plasma-field generation is one of a number of active stealth techniques that could at some stage be applied to France's ASMP-A strategic cruise missile, now nearing completion of the development stage.
Onera is intimately involved in developing other active stealth technologies. There are clear indications some of these are considerably more advanced than plasma in terms of development. These active stealth techniques are also aimed at platforms such as unmanned combat aerial vehicles, combat aircraft and missiles. However, engineers declined to discuss them.
Onera is actively collaborating with Germany, Italy, Sweden and other continental European countries on radar signature management and other sensitive defense technologies. Cooperation with the U.K., however, is virtually nil. Britain is restricted in carrying out any stealth research in a European context as a result of a bilateral agreement with Washington.
Alongside developing low-observable technologies Onera is also working on counter-stealth systems such as surface-wave radar (SWR). This type of radar holds out the promise of detecting cruise missiles flying at low altitude.
Stanley W. Kandebo contributed to this report from New York.
Work in this area is already at an advanced stage, drawing on the existing Nostradamus over-the-horizon radar demonstrator. Engineers say they are ready to build an SWR demonstrator, which could be operational in two or three years. France recently concluded an agreement with Singapore to explore cooperative research in this area.
