Saturday, February 24, 2024

Directed Energy Weapons: High-Power Microwaves

Dr Anil Kumar Maini

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Types of HPM weapons

As mentioned earlier, HPM weapons are generally categorised as narrow-band HPM and ultra-wide-band HPM. Narrow-band HPM weapons produce an HPM output in a narrow band of frequencies with the bandwidth equal to only a few per cent of the centre frequency. These weapons are capable of generating relatively-higher output power levels as compared to ultra-wide-band HPM weapons.

Narrow-band HPM systems are effective only on a given class of targets that would absorb the frequency emitted by the system. Therefore in the case of a narrow-band HPM, knowledge of absorption by the target material as a function of frequency and aspect angle is an advantage. Frequency absorption data may be generated by scanning the target with a tunable low-energy microwave source and evaluating the reflected signals for missing frequencies. Missing frequencies in this case are those that are heavily absorbed by the target material.

Narrow-band HPM weapons have the advantage of better transmission characteristics and fewer problems with fratricide. Limitations include their susceptibility to countermeasures such as target hardening and prior knowledge of the threat required to choose the optimum microwave frequency.

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Ultra-wide-band HPM weapons, on the other hand, radiate over a broad frequency range but deliver comparatively less microwave energy at specific frequencies. While, narrow-band HPM weapons are capable of defeating only well-defined targets or class of targets, ultra-wide-band weapons are intended for use against a wide range of targets.

Ultra-wide-band HPM weapons do not require any prior information on the target’s absorption characteristics and provide a broad capability range. Due to comparatively lower radiated microwave power and poorer transmission characteristics than narrow-band HPM weapons, these have shorter operational ranges.

Components of an HPM weapon system

As outlined earlier, an HPM weapon system essentially comprises a pulsed power source, a source of microwave energy and a transmitting antenna (Fig. 1). The pulsed power source further comprises a power supply and a pulse generator.

Pulse power generators. Pulse power generators that drive HPM sources are generally required to deliver short, intense electrical pulses of 1MV or more with pulse duration up to 1µs. One way to generate the required pulses is by using capacitor banks that transform a slowly-rising low-voltage signal into a fast-rising high-voltage sigPulse power generators. Pulse power generators that drive HPM sources are generally required to deliver short, intense electrical pulses of 1MV or more with pulse duration up to 1µs. One way to generate the required pulses is by using capacitor banks that transform a slowly-rising low-voltage signal into a fast-rising high-voltage signalnal.

fig 3
Fig. 3: Marx generator MG30-3C-100NF

A common capacitor bank configuration is Marx bank (Fig. 2) where the capacitors in the bank are connected in parallel during the charging process and switched to a series connection during discharge.

The series connection multiplies the voltage by the number of capacitors in Marx bank. Resistive charging is slow and therefore limits repetition rate. Inductors could also be used in place of resistors. Inductor charging is preferred for higher repetition rates of a few hertz or more. If Marx bank has n capacitors with each charged to a voltage V from a DC power supply, voltage delivered to the load during discharge would theoretically be equal to n×V.

Spark gaps are used as switches and breakdown voltage of spark gaps is kept higher than the voltage V across each capacitor. Initially, all capacitors are in parallel and are charged to a voltage V. Spark gaps are in open state. In order to initiate discharge, the first spark gap is externally triggered to the breakdown state connecting the first two capacitors in series and thereby raising the voltage across the second spark gap to 2V.

The second gap also goes to breakdown state and the process continues till a voltage pulse with amplitude equal to n×V is applied across the load.

One such Marx generator designed to drive HPM sources is APELC MG20-22C-2000PF from Applied Physical Electronics LC. This megavolt-class Marx generator with its 18-ohm source impedance is specifically designed to drive low-impedance loads. With a 50kV charge voltage it delivers a 500kV, 1.1kJ pulse into a matched load with a peak power of more than 12GW. This generator uses low-impedance, parallel-switched topology, which makes it well-suited for a wide variety of HPM applications.

Another Marx generator from the same company is MG30-3C-100nF (Fig. 3) that is capable of storing a maximum of 1.8kJ and can deliver 300kV to a matched load. This Marx generator has low impedance of 33ohms and is axially compact in order to drive HPM antennae on remote platforms. Maximum peak power and repetition rate specifications are 5GW and 10Hz, respectively.

The other method is to use a flux-compression generator (Fig. 4). In such a generator, a magnetic coil is compressed either by explosive or magnetic forces leading to rapidly-rising current pulse. In an explosively-driven flux-compression generator, chemical energy of the explosives is to be partially converted into the energy of an intense magnetic field surrounded by a correspondingly large electric current.

Explosively-driven flux-compression generators are for one-time use only as the equipment gets destroyed during operation.

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