Pulse-forming network

thumb|A pulse-forming network for an Nd:YAG [[laser rangefinder]]

thumb|The [[Shiva Star device at Air Force Research Laboratory, USA, which generates pulsed power for high-energy fusion power experiments. Each of the 6 radial arms is a pulse-forming line delivering a pulse of energy to the center, whose capacitors store a total of 10 MJ of energy and can create microsecond pulses of 120 kV and 6 million amperes.]]

A pulse-forming network (PFN) is an electric circuit that accumulates electrical energy over a comparatively long time, and then releases the stored energy in the form of a relatively square pulse of comparatively brief duration for various pulsed power applications. In a PFN, energy storage components such as capacitors, inductors or transmission lines are charged by means of a high-voltage power source, then rapidly discharged into a load through a high-voltage switch, such as a spark gap or hydrogen thyratron. Repetition rates range from single pulses to about 10⁴ per second. PFNs are used to produce uniform electrical pulses of short duration to power devices such as klystron or magnetron tube oscillators in radar sets, pulsed lasers, particle accelerators, flashtubes, and high-voltage utility test equipment.

Much high-energy research equipment is operated in a pulsed mode, both to keep heat dissipation down and because high-energy physics often occurs at short time scales, so large PFNs are widely used in high-energy research. They have been used to produce nanosecond-length pulses with voltages of up to 10⁶–10⁷ volts and currents up to 10⁶ amperes, with peak power in the terawatt range, similar to lightning bolts.

Implementation

A PFN consists of a series of high-voltage energy-storage capacitors and inductors. These components are interconnected as a "ladder network" that behaves similarly to a length of transmission line. For this reason, a PFN is sometimes called an "artificial, or synthetic, transmission line". Electrical energy is initially stored within the charged capacitors of the PFN by a high-voltage DC power supply. When the PFN is discharged, the capacitors discharge in sequence, producing an approximately rectangular pulse. The pulse is conducted to the load through a transmission line. The PFN must be impedance-matched to the load to prevent the energy reflecting back toward the PFN.

Transmission-line PFNs

thumb|Simple charged transmission-line pulse generator

A length of transmission line can be used as a pulse-forming network. This can give substantially flat-topped pulses at the inconvenience of using of a large length of cable.

In a simple charged transmission-line pulse generator (animation, right) a length of transmission line such as a coaxial cable is connected through a switch to a matched load R_L at one end, and at the other end to a DC voltage source V through a resistor R_S, which is large compared to the characteristic impedance Z₀ of the line. and is widely used today in PFNs.