Method for protecting objects from destruction by laser beams using solid or gaseous protective elements

Solid protective elements such as cesium, thallium or luletium are dispersed in or layered on an object so that when a high energy laser beam strikes the object a highly ionized plasma is provided that absorbs or reflects the laser beam. Alternately, the object to be protected is surrounded with a gaseous protective element such as krypton or xenon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for protecting objects made of 
metals or plastic against high energy laser beams, and to objects so 
protected. 
2. The Prior Art 
Within the past few years research has been commenced and experiments 
conducted to discover ways to protect objects against lasers of high 
output density. However, these activities have been directed to finding 
materials and material constructions which are as resistant as possible to 
high energy laser beams. The results have involved thick, sometimes 
multi-layered protective shields, as well as protective casings, all of 
which considerably increase the weight of the object to be protected. This 
is a big disadvantage for objects such as satellites, space stations and 
very high flying missiles. 
SUMMARY OF THE INVENTION 
It was thus an object of the present invention to create a procedure for 
effectively protecting an object against lasers with high output density, 
without also increasing the weight of the object to be protected. 
Unlike prior techniques, the present invention does not protect an object 
by a "armor" that is as resistant as possible to high energy laser beams, 
but rather provides a material in the radiation path of the laser which is 
converted instantaneously into a highly ionized plasma by the laser beam, 
this highly ionized plasma absorbing and/or reflecting the energy of the 
laser beam and thus preventing the laser beam from penetrating deeply into 
the object to be protected. The invention is applicable to objects made of 
metal, such as steel, aluminum and titanium, and plastic. 
It should be noted that the phenomenon of a highly ionized plasma absorbing 
and reflecting a laser beam is already known from the metal processing 
industry. In this regard, it is known that when processing work pieces 
with laser beams a plasma is formed on certain materials by the 
evaporation of the material, and it has been determined that the plasma 
will absorb a large part of the energy of the laser beam and thus prevent 
a further penetration of the laser beam into the piece. In order to be 
able to penetrate deeply into certain materials by means of a laser beam 
it has thus become necessary to "blow away" this plasma with gases such as 
helium. The present invention utilizes this protective effect of the 
highly ionized plasma for the protection of an object. 
A theoretical analysis of the invention will be explained as follows. A 
high-energy laser beam with an output density above 10.sup.4 W/cm.sup.2 
evaporates material from the surface of a solid material body. Free 
electrons are present in the resulting vapor. Through shock procedures 
they absorb energy from the electromagnetic laser field. The increase of 
energy in the electrons is calculated as follows: 
##EQU1## 
e=elementary charge of the electron F=field strength of the laser field 
m=electron mass 
.omega.=frequency of laser radiation 
M=relative atom mass of the uncharged atoms 
.epsilon.=energy of the electrons 
Y.sub.st =shock frequency of the electrons and atoms. 
The electrons can attain the maximum energy: 
##EQU2## 
When the electron energy becomes as large as the energy .epsilon..sub.ion, 
needed for the ionization of the neutral atoms, there is an avalanche-like 
release of electrons. The resulting highly ionized plasma absorbs or 
reflects the laser radiation to a high degree, which is a result obtained 
theoretically as well as documented by the above-mentioned interference 
with the laser beam in material processing. 
A material which provides protection against an energy rich laser radiation 
also has to permit the rapid formation of a highly ionized plasma. 
Suitable materials can be determined by the equations (1) and (2) as well 
as by .epsilon..sub.ion. In equation (1) the material enters through its 
relative atomic mass M, and that into the second term of the parenthesis. 
With exact consideration .gamma..sub.st is not a constant, but is 
dependent on M, whereby, however, for the qualitative selection of 
suitable materials this dependency can be ignored. It is also possible to 
qualitatively say that the speed of energy increase by the electrons 
increases with rising M. The maximum energy which the electrons can attain 
in a given laser field also increases with M according to equation (2). In 
this it has to be greater than the first ionization energy 
.epsilon..sub.ion of the material. Thus, it can be determined as starting 
criterion for suitable protective materials that the quotient from M and 
.epsilon..sub.ion has to be as large as possible. If one looks at the 
periodic system, the quotient M/.epsilon..sub.ion is particularly high for 
radioactive elements, but they are, understandably, not suitable as 
protective materials for the present purpose. However, they are suitable 
as solid protective materials, particularly in the cited sequence cesium, 
thallium, lutetium, bismuth, lead and other lanthanides. Among the gaseous 
elements xenon and krypton are particularly suitable as radon is not 
stable. 
The selection of the protective material to be used should be done 
according to the above criteria, whereby the cited elements, by themselves 
or in combination, provide high protection. 
The invention will be better understood by reference to the accompanying 
drawings, which depict various preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an outer portion of an object (or material) 10 to be protected 
from a high energy laser beam, the object having an outer surface 10a. In 
the area near the outer surface of the object 10, a protective material 11 
in solid form is dispersed. It can be incorporated in the object by 
sintering, diffusion, alloying, etc. If a laser beam L hits the surface 
10a, the imbedded particles of protective material 11 become 
instantaneously evaporated, and a highly ionized plasma is formed at the 
impact point of laser L. This highly ionized plasma absorbs and reflects 
the energy of the laser beam L with the effect that the laser beam is able 
to penetrate the protected object only during a short initial phase, but 
is prevented from doing so after the formation of the plasma. 
In FIG. 2 a layer 12 of protective material in solid form is applied to the 
surface 10a of the object 10, the layer being applied by plating, 
spraying, painting, etc. Similarly to FIG. 1, the protective material in 
the layer forms a highly ionized plasma immediately after being impacted 
by the laser beam L, which then prevents the penetration of the laser beam 
into the object. 
FIG. 3 shows the use of a gaseous protective material 13. In this 
embodiment a casing 14 is positioned around the object 10 so as to leave a 
space therebetween, and protective material 13 in gaseous form is located 
in the space. The protective effect against the impacting laser beam L 
essentially corresponds to the effect according to FIGS. 1 and 2. The 
casing 14 can be made of the same material for the protective casing 14 as 
object 10, or it can be composed of some other material, or even the solid 
protective material of this invention. 
FIG. 4 shows a composite object in the form of the head of a missile, the 
head including an inner ignition element 10 to be protected and an outer 
casing 14. A protective material 13 in gaseous form surrounds the inner 
ignition element. The protective effect is the same as according to the 
embodiment in FIG. 3. 
FIG. 5 indicates some of the elements which are particularly useful as the 
solid protective materials according to the present invention, the 
elements being identified on the abscissa and the already mentioned 
quotient M/.epsilon..sub.ion being identified on the ordinate. The 
suitability of these elements as a protective material increases with the 
quotient. The elements with the highest quotients are most desirable, 
i.e., to the extent that they are stable. The elements themselves provide 
a high degree of protection if they are embedded in relatively small 
amounts in the surface area of the object to be protected (FIG. 1) or if 
the protective layer formed by them (FIG. 2) is comparatively thin. In any 
case, the increase in weight for the object is slight or even negligible. 
FIG. 6 indicates the elements which are particularly suitable as gaseous 
protective materials according to the invention. The suitability also 
increases with the quotient M/.epsilon..sub.ion. It can be seen that xenon 
and krypton are particularly suitable as protective gases. In this case 
too, the weight increase of the object is slight, particularly if only 
sensitive parts of the object are protected, as shown in FIG. 4. 
It is certainly possible for modifications to be made to the embodiments as 
shown and still fall within the scope of the invention. For example, it is 
possible to employ multiple protective layers, possibly from varying 
protective materials or alternatingly protective materials and material to 
be protected. It is also possible to provide for varying protective 
measures on the same object. For example, the full casing of a rocket can 
be coated with protective material (FIG. 2) and, in addition, the 
particularly vulnerable inner sections containing the ignition element can 
be filled with protective gas (FIG. 4). The protective gas can also 
contain a non-gaseous substance with the highest possible quotient 
M/.epsilon..sub.ion in a finely distributed form, such as a powder which 
is kept in suspension by a blower. It is also conceivable that the casing 
of the object to be protected could be provided with a layer of protective 
material on the inside as well as on the outside. Finally, in certain 
applications, the complete casing of the object to be protected could 
consist of protective material. It is only essential that the protective 
material forms a plasma which absorbs and/or reflects the laser beam 
before the laser beam impacts the endangered parts of the object to be 
protected.