The energy--partitioning, energy--coupling (EPEC) experiments at the National Ignition Facility (NIF) will simultaneously measure the coupling of energy into both ground shock and air--blast overpressure from a laser--driven target. The source target for the experiment is positioned at a known height above the ground--surface simulant and is heated by four beams from NIF. The resulting target energy density and specific energy are equal to those of a low--yield nuclear device. The ground--shock

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... stress waves and atmospheric overpressure waveforms that result in our test system are hydrodynamically scaled analogs of seismic and air--blast phenomena caused by a nuclear weapon. In what follows, we discuss the motivation for our investigation and briefly describe NIF. Then, we introduce the EPEC experiments, including diagnostics, in more detail. Number of fissions × atoms/fission ∝ Mass of material (2a) or Yield × atoms/fission ∝ Mass of material (2b) Since measuring the yield of nuclear test explosions has been done using seismic and other means for decades it might seem that the measurement of the yield of a low--yield nuclear explosion would be straightforward. However, the measurement of yields of explosions done as part of a nuclear testing program were generally done when weather conditions were favorable and the placement of the test device, whether underground or above the surface, were chosen to facilitate a good measurement of the yield. Accurately measuring the yield of a nuclear detonation in an unknown placement in a structured environment is a much more challenging problem. For any nuclear detonation near the surface of the earth, most of the energy is expressed in a combination of ground shock and air blast. For explosions slightly under the surface, the majority of the energy will be in the ground shock while for an explosion slightly above the surface, most of the energy will be in the air blast. But the sum of these two components of the energy should be, to first order, independent of the placement of the device. The US entered into a moratorium on nuclear testing in 1992 and signed the Comprehensive Test Ban Treaty (CTBT) in 1996. (However, the US has yet to ratify the CTBT.) Thus, there is at the present time no capability to generate new data on the effects of a nuclear explosion in a structured environment. However, the development of high--powered lasers to support Inertial Confinement Fusion (ICF) and experiments to support the Stockpile Stewardship Program provide the capability to pursue new approaches to generating data to support the investigation of nuclear weapons effects in structured environments. The concept of scaled experiments is not new. In "The Effects of Nuclear Weapons" page 101--102, Glasstone and Dolan [1] identifies the basic scaling laws for nuclear weapons (see also Appendix A): Theoretically, a given pressure will occur at a distance from an explosion that is proportional to the cube root of the energy yield. Full--scale tests have shown this relationship between distance and energy yield to hold for yields up to (and including) the megaton range. Thus, cube root scaling may be applied with confidence over a wide range of explosion energies.