Nuclear Weapon Testing - Radiation Simulation
Radiation, as commonly used in the nuclear weapons arena, applies to neutrons, gamma rays, and x-rays alike. It can also include high-energy beta particles (elec-trons). All of these types of radiation show corpuscular behavior when interacting with matter-the high-energy photons because of their extremely short wavelength. Describing these interactions quantitatively requires the full machinery of relativistic quantum mechanics including the computation of the relevant Feynman diagrams. The particle energies involved range from the upper energy limit of the ultraviolet band, 0.124 keV, to the MeV and tens of MeV associated with the gamma rays and neutrons emitted from a fissioning or fusioning nucleus.
The distinction between x-rays and gamma rays is not fundamentally based on photon energy. Normally, one speaks of gamma rays as having energies between 10 keV and 10 MeV and thinks of even hard x-rays as having lower energies. In fact, the difference between the two phenomena lies in their origin: gamma rays are produced in nuclear reactions while x-rays are an atomic phenomenon produced by electron transitions between discrete atomic levels or by blackbody (thermal) radiation from a heated object. A reasonable upper bound for "x-ray energy" in discussing nuclear phenomenology would be a few hundred keV, associated with the initial stages of fireball formation.
Because actual nuclear tests can no longer be performed, and because above-ground explosions have been prohibited since 1963, the only ways to determine the results of attacks utilize simulators, theoretical models, and the data from earlier U.S. nuclear tests. The integrated use of this information in computer models which can predict the HEMP environment as a function of weapon parameters and explosion geometry is a critical technology requiring protection. In contrast, basic theoretical models lacking actual test results should not be controlled.
Theoretical models of HEMP coupling to generic systems such as cables and antennas are of general scientific interest. Codes associated with the generic coupling of HEMP to systems and which do not reveal specific features of military systems and their responses, performance, and vulnerabilities to HEMP need not be controlled. These codes are similar to those used in electromagnetic compatibility and electro-magnetic interference and the study of lightning. Interest in the synergism between lightning and HEMP will continue.
The United States has been the world leader in HEMP technology since the first articles on the subject appeared in the early 1960's. These scientific papers appeared in the open literature, which allowed the Soviet Union to become active in the field. The general consensus is that Soviet (now Russian) capabilities lag years behind those of the United States. Nonetheless, Soviet interest in pulsed-power, which began under A.D. Sakharov, should call attention to the possibility that some of the Soviet HEMP program was very closely held. HEMP capabilities have been acquired by the European nations, including Sweden and Switzerland. Many of these countries have developed active programs that include the use of simulators operating nearly at the threat level. Papers presented at recent unclassified conferences by participants from the countries of the former Warsaw Pact indicate that they lag significantly behind the West in both simulation and theoretical understanding. Several foreign vendors produce equipment comparable to that available from U.S. sources. France manufactures pulse generators, field sensors, fiber-optic links, transient digitizers, and measurement systems; England manufactures 1-GHz band-width fiber-optic links used mainly in HEMP and conducts high-power microwave research. Switzerland and Israel have also developed test/simulation equipment of high quality.
The upper limit to the frequency of the electromagnetic radiation attributed to HEMP is in the range of a few GHz. Thus, the interactions of the HEMP pulse with systems can be computed using classical electromagnetic theory without the need to include quantum effects. Off-the-shelf equipment suffices for the simulation of HEMP in small volumes. The peak electric field is about 50 kV/m, with a pulse width of several nanoseconds. However, producing equivalent fields over an entire military system such as a tank requires a very large radiating system with feed-point driving voltages in the megavolt range. The combination of antenna feed-point voltage and nanosecond rise time is what gives rise to the connection between HEMP pulsed-power technology and the technology needed to produce appropriate gamma- and x-rays. The production of pulses of neutrons corresponding to those generated by a nuclear weapon is primarily of interest for simulating TREE.
Flash x-ray (FXR) techniques are used to produce hard and soft x-rays. Typically, a high-energy electron beam is dumped onto a target to produce bremsstrahlung ("breaking radiation") photons over a broad range of energies up to the kinetic energy of the incident particles. Calculating the actual spectrum produced in a given target is difficult because thick targets, in which the electrons may interact several times, are required to obtain the desired intensities. This, in turn, raises the importance of nonlinear terms. Ideally, an FXR device should produce the same photon spectrum distributed identically over time as the spectrum from a nuclear device. This is not possible at the present time, but existing simulators provide useful approximations.
Specific technologies used to provide the power pulse include the Z-pinch; Blumlein or coaxial cable pulse-forming and transmission lines; large banks of very high-quality, low-loss capacitors; fast opening and closing gas and liquid switches with very low resistance in the closed state; Marx generators to produce the actual high-voltage pulse, and even Van de Graaff electrostatic generators with high current (for the class of accelerator) output. The switches used are unusual and have few other uses. One, for example, must conduct with a low resistance over a period of 0.4 to 1.0 microsecond, but must open to a high resistance state in times of the order of 10 ns.
Pulsed-power generating and conditioning systems and their associated loads (e.g., vacuum diodes) which convert the pulsed system's electrical output pulse to a photon or particle beam are valuable tools to study the hardness and survivability of critical military systems. The required fidelity of the simulation increases as the size of tested hardware increases because it is important to maintain the correct conditions over the aggregate of components which must function together. Some aspects of systems used in simulators are unclassified, and some border on the classified world. Some devices which may be used to simulate nuclear effects (e.g., the National Ignition Facility to be built at Livermore, or the Particle Beam Fusion Accelerator operating at Sandia National Lab) are also research tools for the broader scientific community.
Of particular importance are NWE simulators that can produce pulses with peak power greater than 25 TW from sources with impedance <0.1 ohm and having vacuum power flow and conditioning that can couple to a radiating load having a circular area less than 500 cm 2. These performance levels exceed the publicly available figures for the SATURN and HERMES III accelerators at Sandia National Laboratory.
Russia has demonstrated strong NWE simulation capabilities, comparable to those of the United States. The UK and France have extensive programs, but less ambitious than Russia's. China has an NWE simulation program, but little is known about its capabilities. Germany has always been a leader in pulsed-power conditioning for basic research applications. Pulsed-power conditioning has been developed in Sweden, primarily to support kinetic energy and particle beam weapons research; in Switzerland, to investigate pro-tection against EMP; and in Israel, primarily for basic research at the Weizmann Institute of Science and for kinetic-energy weapons research at Israel's SOREQ Nuclear Research Center. Germany and Japan use similar technology primarily in support of light ion beams for inertial confinement fusion.
For HEMP simulation, the principal advanced technologies developed in the United States for risetimes less than 2 ns are multiple channel gas switches and multistage circuits in which the last stage charges very rapidly to increase the breakdown field of the output switch and decrease its inductance. The existence of triggered multichannel switches and the use of multistage circuits has been reported widely, but not in the context of EMP simulations. Countries with substantial pulsed-power capabilities (e.g., the UK, France, Russia, and Japan) could easily develop EMP simulators using such technologies.
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