active control of nuclear-enhanced radiation belts 2006 ada518538

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Active Control of Nuclear-Enhanced Radiation Belts

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SIMULATION, COMPUTING, AND MODELING 2006 NRL REVIEW

Active Control of Nuclear-Enhanced

Radiation Belts

G.I. Ganguli, M. Lampe, W.E. Amatucci, andA.V. StreltsovPlasma Physics Division

Introduction: A high-altitude nuclear detonationby a hostile power could ood the Earths radiationbelts with MeV killer electrons, which would remaintrapped by the geomagnetic feld or years. This orders-o-magnitude enhancement o the normal radiationbelts would disable low-Earth-orbit satellites withinweeks,1 thereby degrading military capabilities andseverely impacting the economy. This threat has beencalled a potential Pearl Harbor in space. Developmento an eective countermeasure is a national priority.NRL recognized the importance o this threat early onand initiated a comprehensive 6.1 program in FY04 thatincludes theory, computer simulation, and laboratory

experiment. We are developing quantitative physicsmodels that are needed as a precursor to space-basedtests. The ultimate objective is to avoid catastrophicradiation damage to space assets by restoring the radia-tion belt to its natural state within a week.

Background: It has been known since the 1960sthat the natural particle population in the radiationbelt is controlled by a eedback process: electromagneticwaves are amplifed by the loss-cone electron distri-bution naturally present in the radiation belt, and thewaves in turn scatter and precipitate particles. Thereis every reason to believe that the killer nuclear-gener-ated electrons could be similarly precipitated, i it werepossible to introduce a sufciently energetic spectrumo waves with the right wavelength and requency.Mainline remediation eorts emphasize the use owhistler waves transmitted rom a space-based antennaand amplifed by the ree energy available within theradiation belt. A technical panel in 2001 assessed theavailable technology and concluded that remediationcould be achieved within a week, with only a modestinvestment o energy rom a ew satellite-based anten-

nas emitting at 6 kW, i 20 dB amplifcation o theappropriate waves could be assured. However, whilethere is clear experimental evidence or sporadic whis-tler amplifcation and secondary emission,2 the circum-stances necessary to trigger the process are not wellunderstood or predictable. Nonlinear processes appearto be at work that depend sensitively on the wave

requency, require triggering waves above a thresholdstrength, and also require the presence o naturalmagnetospheric inhomogeneities known as ducts. Theparametric dependences and scaling laws are unknown.Nor is there any predictive capability or the operationo a high-intensity space-based antenna system withinthe nonlinear magnetospheric plasma medium. We arecurrently addressing these issues.

Current Research:To understand whistlerdynamics in nuclear-enhanced radiation belts, weneeded to develop wholly new types o ast numericalsimulation models using techniques such as elimina-

tion o the displacement current and enorcement oquasi-neutrality. These models have yielded valuableinsights. For example, Fig. 4 shows a test that illus-trates saturation o whistler amplifcation by magnetictrapping. The study is now being extended to includeinhomogeneities and wave packets o fnite size, whichlead to de-trapping and, hence, extended amplifca-tion and triggered emission. Figure 5 illustrates aull-wave simulation o whistler propagation in ductswith diameter comparable to the whistler wavelength.Earlier ray tracing models are inappropriate or thesenarrow ducts, which could be sel-generated by a large-amplitude whistler, thereby enabling amplifcation andunattenuated propagation over large distances, even inthe absence o natural ducts.

These theoretical investigations o nonlinear whis-tler optics are complemented by experiments in theNRL Space Chamber. We have investigated the reso-nance-cone propagation o low-power whistlers andthe sel-ducting o large-amplitude whistlers to developthe technique or lossless long-distance propagation.Figure 6(a) shows propagation o small-amplitudewhistlers in the Space Chamber plasma, illustrating

FIGURE 4

Computer simulation showing electron densityin a 2-D section of phase space (pitch angle vs electron gyration phase ) that illustratesthe phenomenon of phase trapping.

1 2 3 4 5 6

y

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2006 NRL REVIEW SIMULATION, COMPUTING, AND MODELING

FIGURE 6

(a) Experimental measurements of whistler wave resonance cone width as a functionof wave frequency. (b) Demonstration of whistler wave self-ducting with transmittingantenna located at r= 0 (solid) and r= 10 cm (dashed).

FIGURE 5

Computer simulation showing wave ducting within a tube of reduced electron density. The upper curveshows the wave amplitude essentially constant. In the absence of a duct, the wave would expand and thusfall off in amplitude.


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SIMULATION, COMPUTING, AND MODELING 2006 NRL REVIEW

the low-requency cuto at the lower-hybrid resonantrequency and the confnement o wave power withinthe lower-hybrid resonance cone. Figure 6(b) demon-strates the sel-ducting o high-power whistler waves.The data show collimation o the whistler radiationpattern two meters downstream, with the transmittingantenna located on the chamber axis (solid line) and

10 cm o axis (dashed line).

A New Remediation Concept: We are alsodeveloping a radically di erent approach or rapidremediation based on massive injection o energy overa short duration. I a large quantity o neutral vaporsuch as lithium is released rom a satellite moving inthe equatorial plane, the vapor is photo-ionized overa period o about an hour. The resulting ions, movingat~7 km/s normal to the geomagnetic feld, spin up

into a highly unstable ring distribution, which emitslarge amplitude electromagnetic waves in the desiredrequency and wavelength range. These waves can pre-cipitate the killer electrons within hours, as opposedto a week. The process in eect taps the orbital kineticenergy to create a giant ion magnetron in the radia-tion belt. A ton o neutral lithium would carry 10 GJ

o energy, vastly more than could ever be introducedthrough electromagnetic antennas.

[Sponsored by ONR]

References1 D.G. Dupont, Nuclear Explosions in Orbit, Sci Am.290(6),

100, 2004.2 R.A. Helliwell, VLF Wave Stimulation Experiments in the Mag-

netosphere rom Siple Station, Antarctica, Rev. Geophys.26,551, 1988.

active control of nuclear-enhanced radiation belts 2006 ada518538

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