Source-region electromagnetic pulse simulator

A method and apparatus for simulating, in conjunction with a source of ionizing radiation, intense pulsed electromagnetic fields and time varying conductivity caused by the gamma radiation associated with a nuclear detonation. An enclosed space, including the source of ionizing radiation is separated into three spaces, each space separated from the adjacent space by a gas impermeable, radiation permeable barrier. A guided wave structure, pulsed with high voltage pulses in conjunction with the firing of the source of ionization radiation is disposed adjacent to the barrier separating two of the spaces. A gas handling system is provided to introduce a selected non-ionizing gas and a selected ionizing gas into the spaces on either side of the barrier adjacent to the guided wave structure.

BACKGROUND OF THE INVENTION 
This invention relates generally to methods and devices which test the 
ability of equipment and especially electronic equipment, to survive a 
nuclear detonation. More specifically, this invention relates to a method 
and device which simulates, without a nuclear detonation, certain aspects 
of the environment produced by such a detonation. The environment in 
question is the source-region environment, i.e., the area close enough to 
an atmospheric burst that ionization of the air is an essential component. 
The environment is characterized by blast effects, thermal radiation, 
neutron radiation, gamma radiation, intense pulsed electromagnetic fields 
(EMP) and time varying (pulsed) air conductivity. The present invention 
simulates the aspects of gamma radiation, intense pulsed electromagnetic 
fields (EMP) and time varying (pulsed) air conductivity. In the vicinity 
of an atmospheric nuclear detonation, the EMP and air conductivity are 
caused by the gamma flux. Compton electrons, set into motion by the gamma 
rays, act as the sources of both local and radiated fields, and, in 
slowing down, lose energy through the ionization of air molecules. This 
ionization makes the air conductive. 
There are several source region electromagnetic pulse (SREMP) simulators 
one of which is the AURORA Flash X-Ray Facility at the U.S. Army 
Electronics Research and Development Command, HARRY DIAMOND LABORATORIES 
located in Adelphi, Maryland. The AURORA is considered as a self-contained 
SREMP simulator but suffers from several severe limitations. In the 
AURORA, bremsstrahlung is produced in four thick tantalum targets by four 
synchronous 10-MeV electron beams. The bremsstrahlung then produces an EMP 
signal through the same mechanism as does a nuclear detonation. It induces 
Compton electron currents in the AURORA test chamber. As the Compton 
electrons are slowed down, they ionize the air and also produce an 
electromagnetic field in the chamber. However, the EMP produced in the 
AURORA is not a true model of the EMP produced by a nuclear detonation. 
In both AURORA and a true SREMP environment, the early-time electromagnetic 
phenomena can be roughly considered as evolving in two successive phases: 
a. the wave phase (when D.quadrature.&gt;&gt;.sigma.E, during the fast turn-on of 
the Compton drivers); and 
b. the diffusion phase (when .sigma.E&gt;&gt;D). During the wave phase, the 
spherical domain of influence (the volume, surrounding a field point, 
containing sources determining the fields at the field point) has radius 
C(t-t.sub.0), where C is the speed of light and the pulse begins at time 
t.sub.0. During the diffusion phase, the radius of influence contracts as 
.sigma. increases, and when .sigma. levels off, the radius increases as 
.sqroot.t rather than linearly. 
The limitations of the AURORA test facility as a nuclear EMP similator can 
be understood by examining its response in terms of the wave equation for 
the E and H fields. To simplify the examination it is assumed that the 
examination point is far enough away from the AURORA test cell "hot spot" 
(the source of radiation) so that a spatially homogeneous conductivity can 
be assumed. The two wave equations which express the electric and magnetic 
fields in terms of the Compton current J and charge density .rho.: 
##EQU1## 
using conventional notation. If it is assumed that the EMP that is to be 
simulated is in the diffusion phase, then 
##EQU2## 
Another assumption that can be made 
##EQU3## 
though not strictly valid leads to great simplification of the wave 
equations. However, a more thorough treatment involving the 
renormalization of time does justify the qualitative discussion presented. 
With these assumptions, the E and H fields inside the AURORA test cell are 
##EQU4## 
Now, considering the diffusion Green function given by 
##EQU5## 
If a value of .sigma.=3.times.10.sup.-4 mho/m, 10-m from the hot spot, and 
a time equal to 10.sup.-7 seconds (roughly the FWHM of the AURORA pulse) 
is substituted in the Green function then 
##EQU6## 
for the exponential factor. In other words, Compton drivers and charges 
within a spherical gaussian "domain of influence" of standard deviation 30 
meters contribute to the local E and H fields in a tactical situation. The 
spatial distribution of source currents in the AURORA test cell has much 
too small an extension to generate a reasonable tactical EMP simulation. 
Another limitation of the AURORA test cell is that the metallic walls of 
the cell short-circuit the E field, an effect seen throughout the cell 
because of the relatively large skin depth of the ionized joint. A third 
limitation is that the radiation pulses rise-time (and hence the 
rise-times of the fields and conductivity) is too long. Because of these 
limitations of existing radiation sources a need for an auxiliary source 
of pulsed fields is needed. One approach has been to place a guided-wave 
structure in the test cell to provide a traveling electromagnetic pulsed 
wave. A large (12 m.times.4 m.times.3 m) transmission line has been 
mounted laterally in the AURORA test cell, see FIGS. 1 & 2 which 
represents prior art efforts in the AURORA. The line is driven by a 100 KV 
pulser which provides the appropriate propagating electric and magnetic 
fields. Simultaneously, AURORA is fired to provide a time-varying pulse of 
ionizing radiation. However, these two effects are not independent, 
resulting in a failure of the system to provide a true modeling of an 
actual nuclear detonation. The time-varying conductivity creates a varying 
load on the pulser-line system and thus distorts the voltage pulse, and 
consequently, the electric field inside the line (the test area). The 
fast-rising load current increases the magnetic field in the line, 
resulting in a negative voltage pulse and a falling E-field; and 
subsequently, as the load impedance rises again, the magnetic field energy 
continues to drive current into it, resulting in a positive voltage pulse 
and a rising and overshooting E-field. This overshooting effect is 
referred to as the inductive kick. 
It is therefore one object of this invention to provide a method to 
simulate the electromagnetic pulse (EMP) created in the near vicinity of a 
nuclear detonation. 
It is another object of this invention to provide a method to simulate the 
electromagnetic pulse (EMP) created in the near vicinity of a nuclear 
detonation that can be utilized in conjunction with a source of 
time-varying pulses of ionizing radiation. 
It is a further object of this invention to provide an apparatus to 
simulate the electromagnetic pulse (EMP) created in the near vicinity of a 
nuclear detonation. 
It is still a further object of the invention to provide an apparatus to 
simulate the electromagnetic pulse (EMP) created in the near vicinity of a 
nuclear detonation that can be utilized in conjunction with a source of 
time-varying pulses of ionizing radiation. 
SUMMARY OF THE INVENTION 
These and other objects, features and advantages of the invention are 
accomplished by a method and apparatus for simulating, in conjunction with 
a source of ionizing radiation, intense pulsed electromagnetic fields and 
time varying conductivity caused by the gamma radiation associated with a 
nuclear detonation. An enclosed space, including the source of ionizing 
radiation is separated into three spaces, each space separated from the 
adjacent space by a gas impermeable, radiation permeable barrier. A guided 
wave structure, pulsed with high voltage pulses in conjunction with the 
firing of the source of ionizing radiation is disposed adjacent to the 
barrier separating two of the spaces. A gas-handling system is provided to 
introduce a selected non-ionizing gas and a selected ionizing gas into the 
spaces on either side of the barrier adjacent to the guided wave 
structure. Also provided are field-shaping busbars for shaping the 
electromagnetic fields in one of the spaces.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, FIGS. 1 and 2 represent a prior art device, 
which is considered a self-contained source region electromagnetic pulse 
(SREMP) simulator. The AURORA test cell, generally at 10, comprises an 
inclosed space filled with air divided into a first space and a second 
space 14 which is used as a test space wherein equipment to be tested is 
placed. Ionizing radiation is produced at one end of the test cell 10 and 
is represented at 16. In the case of the Aurora, bremsstrahlung is 
produced in four thick tantalum targets by four synchronous 10-MeV 
electron beams. The bremsstrahlung then produces an EMP signal through the 
same mechanism as does a nuclear detonation. It induces Compton electron 
currents in the AURORA test chamber 12, 14. As the Compton electrons are 
slowed down, they ionize the air and also produce an electromagnetic field 
in the chamber. The walls of the test cell 10 represent ground planes. 
Space 12 and space 14 are roughly separated by a guided-wave structure 16. 
The guided-wave structure 16 is made of a conductive mesh, represented at 
18, which can accomodate a high voltage electromagnetic pulsed wave. The 
guided-wave structure, also called a transmission line, is driven by a 100 
kv pulser (not shown) represented at 20, which provides the appropriate 
propagating electric and magnetic fields. 
FIGS. 3 and 4 represent a first embodiment of the present invention. The 
AURORA test cell 10, the ionizing radiation source 16 and the walls 
representing ground planes are as described in the discussion associated 
with FIGS. 1 and 2. The enclosed space within the test cell is separated 
into three spaces, a first space 22, a second space 24 and a third space 
26. Space 22 and 24 are separated by a gas impermeable, radiation 
permeable barrier 26. Space 24 and 26 are likewise separated by a gas 
impermeable radiation permeable barrier 28. Space 24 and space 26 are 
roughly separated by a guided-wave structure 30. The guided-wave 
transmission line 30 is made of a conductive material, preferrably a mesh 
or screen like material, represented at 32, which can accomodate a high 
voltage electromagnetic pulsed wave. The guided-wave structure is driven 
by a 100 kv pulser (not shown) represented at 34, which provides the 
appropriate propagating electric and magnetic fields. Gas handling means 
are provided to control the type of gas in spaces 24 and 26. For example, 
pump 38 inputs a selected gas into space 24 and exhaust pump 40 exhausts 
unwanted gases from space 24. Likewise pump 42 inputs a selected gas into 
space 26 and exhaust pump 44 exhausts unwanted gases from space 26. 
FIGS. 5 and 6 represents an alternate embodiment of the present invention. 
Like numerals represent like components shown in FIGS. 3 and 4. This 
embodiment shows the spaces 24 and 26 rearranged within the test cell and 
FIG. 6 graphically illustrates one use of the test cell, i.e., a 
representation of a tank and its equipment being tested for nuclear 
effects. Also shown in this embodiment are field shaping bushbars 46 which 
are used to adjust the fields produced within the test cells. Appropriate 
voltage sources (not shown) are used to apply a voltage to the bushbars 46 
and are within the known state-of-the-art. 
The operation of the present invention will be described in conjunction 
with the FIGS. representing the prior art and the embodiments of the 
present invention. As described above FIGS. 1 and 2 represent prior art 
devices, for use in the AURORA Flash X-Ray Facility at the U.S. Army 
Electronics Research and Development Command, Harry Diamond Laboratories 
at Adelphi, MD. Theoretically, the most straight forward simulator would 
be one that produced the appropriate gamma pulse and spectrum over the 
appropriate volume, and the proper fields and conductivity would follow. 
However, the AURORA and other test facilities are insufficient for three 
reasons (1) its pulse rise is too slow by about a factor of ten, (2) the 
irradiated volume (20 m.times.15 m.times.5 m) is too small and (3) the 
metal walls of the test cell reduce the fields by shorting them out. 
Because of these limitations there is a requirement for an auxiliary 
source of pulsed fields. Ideally, the conductivity would be provided by 
the ionizing radiation pulse, the pulsed fields would be provided by the 
auxiliary pulser and the two could be regarded as independent of each 
other. To properly simulate coupling to relatively large systems, a 
traveling electromagnetic pulsed wave is required. A large guided-wave 
structure such as a transmission line has been provided in the AURORA. 
This is represented by 16 in FIGS. 1 and 2. The transmission line 16 is 
mounted laterally in the AURORA test cell 10 and is driven by a 100 kv 
pulser, represented at 20. Simultaneously, the ionizing radiation source 
16 is fired to provide a time-varying pulse of ionizing radiation. 
However, it has been found that these two effects are not independent. The 
time-varying conductivity creates a varying load on the pulser-line system 
and thus distorts the voltage pulse, and consequently the electric field 
within the test area 14. The fast-rising load current increases the 
magnetic field in the line, resulting in a negative voltage pulse and a 
falling E-field; and subsequently, as the load impedance rises again, the 
magnetic field energy continues to drive current into it, resulting in a 
positive voltage pulse and a rising and overshooting E-field which is 
referred to as the "inductive kick". This field distortion, since it 
depends on changes in current, is proportional to the relative change and 
rate of change, during the radiation pulse, of the load resistance, i.e., 
it is related to the air conductivity. It is not desired to entirely 
eliminate the change in load resistance since the change in air 
conductivity is an essential feature of the environment to be simulated. 
However, the effect upon the system can be minimized by allowing only the 
test area to be ionized. To effectuate this requires two parallel 
transmission lines, a master line which comprises the test area 14 and a 
slave line which is prohibited from experiencing the ionization which 
decreases the degrading effect upon the system. The present invention 
minimizes the relative change in loading by making the slave-line 
impedance as low as possible. Then, since only the master-line impedance 
is time-varying, the time-variance of the parallel combination is 
minimized, according to the formula: 
##EQU7## 
The simulator of the present invention utilizes the diverse behavior of 
specific gases when exposed to ionizing radiation. The simulator as shown 
in FIGS. 3-6 has the interior space separated into chambers or spaces 24, 
26 and a gas-handling system by which the chambers or spaces can be 
exhausted and filled with selected gases. Among the most important 
microscopic parameters by which gases can be characterized are atomic 
number, photon range, electron range, electron elastic and inelastic 
cross-sections, electron attachment rate, electron-ion and ion-ion 
recombination rates, avalanching rates, electron mobilities, and ion 
mobilities. For example, SF.sub.6 and O.sub.2 have high electron 
attachment rates, and hence are not strongly ionized; N.sub.2 has a low 
attachment rate and is strongly ionized; CO.sub.2 has high inelastic 
electron cross-sections so that electrons thermalize fast, inhibiting 
avalanching, gases with high atomic numbers have low photon ranges and 
hence tend to become quite conductive; inert gases (rotationally symmetric 
molecules) have Ramsauer-Townsend "transparencies" at about 0.7 eV which 
dominate the thermalization process. 
The preferred embodiment utilizes SF.sub.6 as the non-ionizing gas in space 
24 and either air, N.sub.2 or Xe as the ionizing gas in space 26. The 
selection of gases provides considerable flexibility and control over the 
loading of the line and the conductivity wave form in the test space. 
An important feature of the master-slave line, as compared to the prior art 
system, is that, due to the lower impedance of the double line, much more 
energy is drawn from the pulser in order to serve the same test space. 
Most of this energy is consumed in the slave line, where the fields are 
not used directly for simulation, but serve the purpose of maintaining the 
line voltage at its proper value. The achievement of greater EMP waveform 
fidelity is obtained at the cost of more stringent pulser requirements. 
There is a limiting conductivity beyond which, for desired waveforms with 
frequency content above a certain level, the present invention becomes 
inappropriate. This is due to the self-sheilding effect which arises 
whenever conduction current dominates displacement current and the medium 
becomes primarily diffusive. Under these conditions the fields at a point 
are determined by source currents and charge densities only within a 
sphere of influence whose radius is approximately given by the diffusion 
length: 
##EQU8## 
where T is a "time of interest", say 100 ns. When this radius is of the 
order of the transmission line dimensions, the line no longer functions as 
desired. However, below this limit the present invention closely 
approximates the EMP of an actual nuclear detonation. 
While the invention has been described with reference to the accompanying 
drawings, it is to be clearly understood that the invention is not to be 
limited to the particular details shown therein as obvious modifications 
may be made by those skilled in the art. The embodiments of the invention 
should only be construed within the scope of the following claims.