strengths and very close to uniform plane wave simulation within the test volume. These are its strengths, that cannot be outdone by other competing ones viz., the radiating dipole and the hybrid simulators [4].

II. DESIGN

Depending on the NEMP environment being simulated, a designer has to validate the basis of designing an NEMP simulator in line with the checklist [5]. Essentially the checklist aims at finding the followings:

1. Suitability of a given simulator for the EMP environments generated due to the relative positioning of the nuclear detonation viz.: (i) EMP due to underground burst, (ii) EMP due to near surface burst and (iii) EMP due to an exo-atmospheric burst; each has its own rise time and field magnitude characteristics. The simulator to be planned, has to account for one of these EMP environments for simulation.
   

2. Determining all the major dimensions and the other electromagnetic characteristics, this is to estimate the accuracy of the form (spatial, temporal, frequency content, etc.) of the fields, currents, etc. This is to be assessed with and without the presence of the test object and,
   

3. Characterizing the source of the radiation: the typical characteristics are, (i) the nature of the exciting source, e.g. an electrical pulse or CW generator, or a photon generator, (ii) the pulse amplitude, generator impedance and the rep rate requirements.

The guided-wave transmission line NEMP simulator reported here is designed to simulate the electrical characteristics of an ex-atmospheric high altitude nuclear detonation. The pulse shape of the radiated EMP resulting from the high altitude (>40 km above the earth's surface) exponentials. The rise time can be of the order of 10 nanoseconds, as it compares well with the 'unfolding time' of the fission of Uranium . The decay-time matches the duration of the ensuring gamma rays from nuclear detonation. The field intensity decays to a factor of (1/e) in a few hundred nanoseconds. The peak electric field intensity from a high yield nuclear detonation could be about 100 kV/m and gets limited owing to the conductivity imparted to the medium by the drifting secondary electrons and icons. The generally accepted average value of the peak field is about 50 kV/m. The canonical NEMP-Bell lab specified waveform, which describes the above characteristics in its mathematical formulation in time-domain form is,

Here,

(The corresponding values for the IEC waveform are,

).

It is evident from the network theory that the above function resembles the critically damped voltage or current waveforms of a series RLC-circuit, across its resistance. The usual expression denoting the voltage across the resistor, when a charged capacitor is discharged through a series combination of an inductor and resistor, satisfying the critically damped or over damped condition in time-domain form is,


where , is the capacitor voltage before discharge, is the circuit resistance, is the circuit capacitance and is the circuit inductance.

In order, that a transmission line type simulator be purposefully employed for NEMP simulation, it should faithfully reproduce the require wave shape generated from the circuit parameters synthesized using expressions (1) and (2). Additionally, it should propagate this waveshape without distortion while illuminating the test object within its test volume. This can further be elaborated using the combination of a series lumped circuit and the transmission line terminated to its characteristic impedance. This is shown in Fig.1

From the transmission line theory when a generator with its internal impedance, say , and other circuit impedance comprised of lead inductances and resistances, is connected to a transmission line terminated to its characteristic impedance, generally resistive the circuit is equivalent to a series RLC circuit. The effect of the line connecting the termination resistance to the generator can be neglected provided that the line is effectively lossless

and distortionless along its length. The criterion of selecting the terminating resistance gets decided from the transmission line's transverse dimensions, which turn