# Pulse-forming network > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Pulse-forming_network > Markdown URL: https://mediated.wiki/source/Pulse-forming_network.md > Source: https://en.wikipedia.org/wiki/Pulse-forming_network > Source revision: 1339796895 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) Type of electric circuit A pulse-forming network for an Nd:YAG [laser rangefinder](/source/Laser_rangefinder) The [Shiva Star](/source/Shiva_Star) device at [Air Force Research Laboratory](/source/Air_Force_Research_Laboratory), USA, which generates [pulsed power](/source/Pulsed_power) for high-energy [fusion power](/source/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](/source/Electric_circuit) that accumulates [electrical energy](/source/Electrical_energy) over a comparatively long time, and then releases the stored energy in the form of a relatively [square](/source/Square_wave_(waveform)) [pulse](/source/Pulse_(signal_processing)) of comparatively brief duration for various [pulsed power](/source/Pulsed_power) applications. In a PFN, energy storage components such as [capacitors](/source/Capacitor), [inductors](/source/Inductor) or [transmission lines](/source/Transmission_line) are charged by means of a [high-voltage](/source/High-voltage) power source, then rapidly discharged into a [load](/source/External_electric_load) through a high-voltage [switch](/source/Switch), such as a [spark gap](/source/Spark_gap) or hydrogen [thyratron](/source/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](/source/Klystron) or [magnetron](/source/Magnetron) tube [oscillators](/source/Electronic_oscillator) in [radar](/source/Radar) sets, [pulsed lasers](/source/Pulsed_laser), [particle accelerators](/source/Particle_accelerator), [flashtubes](/source/Flashtube), 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](/source/Lightning) bolts. ## Implementation A PFN consists of a series of high-voltage energy-storage [capacitors](/source/Capacitor) and [inductors](/source/Inductor). These components are interconnected as a "*ladder [network](/source/Electrical_network)*" that behaves similarly to a length of [transmission line](/source/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](/source/Transmission_line). The PFN must be [impedance-matched](/source/Impedance_matching) to the load to prevent the energy reflecting back toward the PFN. ## Transmission-line PFNs Simple charged transmission-line pulse generator A length of transmission line can be used as a pulse-forming network.[1][2] 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](/source/Pulse_generator) (animation, right) a length of transmission line such as a [coaxial cable](/source/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](/source/Characteristic_impedance) *Z*0 of the line.[1] When the power supply is connected, it slowly charges up the capacitance of the line through *R*S. When the switch is closed, a voltage equal to *V*/2 is applied to the load, the charge stored in the line begins to discharge through the load with a current of *V*/2*Z*0, and a voltage step travels up the line toward the source.[2] The source end of the line is approximately an open circuit due to the high *R*S,[1] so the step is reflected uninverted and travels back down the line toward the load. The result is that a pulse of voltage is applied to the load with a duration equal to 2*D*/*c*, where *D* is the length of the line, and *c* is the propagation velocity of the pulse in the line.[1] The propagation velocity in typical transmission lines is generally more than 50% of the [speed of light](/source/Speed_of_light). For example, in most types of [coaxial cable](/source/Coaxial_cable) the propagation velocity is approximately 2/3 the speed of light, or 20 cm/ns. High-power PFNs generally use specialized transmission lines consisting of pipes filled with oil or deionized water as a dielectric to handle the high power stress.[2] A disadvantage of simple PFN pulse generators is that because the transmission line must be matched to the load resistance *R*L to prevent reflections, the voltage stored on the line is divided equally between the load resistance and the [characteristic impedance](/source/Characteristic_impedance) of the line, so the voltage pulse applied to the load is only one-half the power-supply voltage.[1][2] ### Blumlein transmission line Blumlein generator has the advantage that it can generate a pulse equal to the charging voltage *V* A transmission line circuit which circumvented the above problem, producing an output pulse equal to the power-supply voltage *V*, was invented in 1937 by British engineer [Alan Blumlein](/source/Alan_Blumlein)[3] and is widely used today in PFNs.[1] In the Blumlein generator (animation, right), the load is connected in series between two equal-length transmission lines, which are charged by a DC power supply at one end (note that the right line is charged through the impedance of the load).[1] To trigger the pulse, a switch short-circuits the line at the power-supply end, causing a negative voltage step to travel toward the load. Since the characteristic impedance *Z*0 of the line is made equal to half the load impedance *R*L, the voltage step is half-reflected and half-transmitted,[1] resulting in two symmetrical opposite-polarity voltage steps, which propagate away from the load, creating between them a voltage drop of *V*/2 − (−*V*/2)= *V* across the load. The voltage steps reflect from the ends and return, ending the pulse. As in other charge-line generators, the pulse duration is equal to 2*D*/*c*, where *D* is the length of the individual transmission lines.[1] A second advantage of the Blumlein geometry is that the switching device can be grounded, rather than located in the high-voltage side of the transmission line as in the typical charged line, which complicates the triggering electronics. ## Uses of PFNs Upon command, a high-voltage switch transfers the energy stored within the PFN into the load. When the switch "*fires*" (closes), the network of capacitors and inductors within the PFN creates an approximately [square output pulse](/source/Square_wave_(waveform)) of short duration and high power. This high-power pulse becomes a brief source of high power to the load. Sometimes a specially designed [pulse transformer](/source/Pulse_transformer) is connected between the PFN and load. This technique improves the [impedance match](/source/Impedance_matching) between the PFN and the load so as to improve power-transfer [efficiency](/source/Electrical_efficiency). A pulse transformer is typically required when driving higher-impedance devices such as klystrons or magnetrons from a PFN. Because the PFN is charged over a relatively long time and then discharged over a very short time, the output pulse may have a peak power of megawatts or even terawatts. The combination of a high-voltage source, PFN, HV switch, and pulse transformer (when required) is sometimes called a "*power modulator*" or "*pulser*". ## See also - [Pulse (signal processing)](/source/Pulse_(signal_processing)) - [Pulse generator](/source/Pulse_generator) - [Pulsed power](/source/Pulsed_power) - [Thyratron](/source/Thyratron) - [Thyristor](/source/Thyristor) - [Triggered spark gaps](/source/Spark_gap#Power-switching_devices) - [Marx generator](/source/Marx_generator) - [Crossatron](/source/Crossatron) - [Pulsed laser](/source/Pulsed_laser) - [Radar](/source/Radar) ## References 1. ^ [***a***](#cite_ref-Haddad_1-0) [***b***](#cite_ref-Haddad_1-1) [***c***](#cite_ref-Haddad_1-2) [***d***](#cite_ref-Haddad_1-3) [***e***](#cite_ref-Haddad_1-4) [***f***](#cite_ref-Haddad_1-5) [***g***](#cite_ref-Haddad_1-6) [***h***](#cite_ref-Haddad_1-7) [***i***](#cite_ref-Haddad_1-8) Haddad, A.; D. F. Warne (2004). [*Advances in High Voltage Engineering*](https://books.google.com/books?id=_ItI3860YAwC&q=blumlein+%22transmission+line&pg=PA602). IET. pp. 600–603. [ISBN](/source/ISBN_(identifier)) [0852961588](https://en.wikipedia.org/wiki/Special:BookSources/0852961588). 1. ^ [***a***](#cite_ref-Mesyats_2-0) [***b***](#cite_ref-Mesyats_2-1) [***c***](#cite_ref-Mesyats_2-2) [***d***](#cite_ref-Mesyats_2-3) Mesyats, Gennady A. (2005). [*Pulsed Power*](https://books.google.com/books?id=Qs40vx3WBlwC&q=blumlein+&pg=PA208). Springer. pp. 13–14, 125. [ISBN](/source/ISBN_(identifier)) [0306486547](https://en.wikipedia.org/wiki/Special:BookSources/0306486547). 1. **[^](#cite_ref-3)** [UK Patent 589127, *Improvements in or relating to apparatus for generating electrical impulses*](http://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&II=3&ND=3&adjacent=true&locale=en_EP&FT=D&date=19470612&CC=GB&NR=589127A&KC=A), Alan Dower Blumlein, filed October 10, 1941, granted June 12, 1947. ## External links - Eric Heine, "*[Conversion](https://web.archive.org/web/20040922063750/http://www.nikhef.nl/~erichn/conversion/conv.html)*". NIKHEF Electronic Department, Amsterdam, the Netherlands. - Riepe, Kenneth B., "*High-voltage microsecond pulse-forming network*". Review of Scientific Instruments Vol 48(8) pp. 1028–1030. August 1977. ([Abstract](https://archive.today/20040827044542/http://content.aip.org/RSINAK/v48/i8/1028_1.html)) - [Glasoe, G. Norris](/source/G._N._Glasoe), Lebacqz, Jean V., "*Pulse Generators*", McGraw-Hill, MIT Radiation Laboratory Series, Volume 5, 1948. --- Adapted from the Wikipedia article [Pulse-forming network](https://en.wikipedia.org/wiki/Pulse-forming_network) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Pulse-forming_network?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.