Editing Pulse-forming network

[[File:Pulse forming network for Nd YAG laser.jpg|thumb|A pulse-forming network for an Nd:YAG [[laser rangefinder]]]]
[[File:Shiva star.jpg|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 wave (waveform)|square]] [[pulse (signal processing)|pulse]] of comparatively brief duration for various [[pulsed power]] applications. In a PFN, energy storage components such as [[capacitor]]s, [[inductor]]s or [[transmission line]]s are charged by means of a [[high-voltage]] power source, then rapidly discharged into a [[external electric load|load]] through a high-voltage [[switch]], such as a [[spark gap]] or hydrogen [[thyratron]]. Repetition rates range from single pulses to about 10<sup>4</sup> per second. PFNs are used to produce uniform electrical pulses of short duration to power devices such as [[klystron]] or [[magnetron]] tube [[electronic oscillator|oscillators]] in [[radar]] sets, [[pulsed laser]]s, [[particle accelerator]]s, [[flashtube]]s, 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 10<sup>6</sup>–10<sup>7</sup> volts and currents up to 10<sup>6</sup> amperes, with peak power in the terawatt range, similar to [[lightning]] bolts.

==Implementation==
A PFN consists of a series of high-voltage energy-storage [[capacitor]]s and [[inductor]]s. These components are interconnected as a "''ladder [[Electrical network|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 matching|impedance-matched]] to the load to prevent the energy reflecting back toward the PFN.

==Transmission-line PFNs==
[[File:Charge line animation.gif|thumb|Simple charged transmission-line pulse generator]]

A length of transmission line can be used as a pulse-forming network.<ref name="Haddad">{{cite book |last= Haddad |first= A. |author2= D. F. Warne |title= Advances in High Voltage Engineering |publisher= IET |date= 2004 |pages= 600–603 |url= https://books.google.com/books?id=_ItI3860YAwC&q=blumlein+%22transmission+line&pg=PA602 |isbn= 0852961588}}</ref><ref name="Mesyats">{{cite book |last= Mesyats |first= Gennady A. |title= Pulsed Power |publisher= Springer |date= 2005 |pages= 13–14, 125 |url= https://books.google.com/books?id=Qs40vx3WBlwC&q=blumlein+&pg=PA208 |isbn= 0306486547}}</ref> 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''<sub>L</sub> at one end, and at the other end to a DC voltage source ''V'' through a resistor ''R''<sub>S</sub>, which is large compared to the [[characteristic impedance]] ''Z''<sub>0</sub> of the line.<ref name="Haddad" /> When the power supply is connected, it slowly charges up the capacitance of the line through ''R''<sub>S</sub>. 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''<sub>0</sub>, and a voltage step travels up the line toward the source.<ref name="Mesyats" /> The source end of the line is approximately an open circuit due to the high ''R''<sub>S</sub>,<ref name="Haddad" /> 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.<ref name="Haddad" /> The propagation velocity in typical transmission lines is generally more than 50% of the [[speed of light]]. For example, in most types of [[coaxial cable]] the propagation velocity is approximately 2/3 the speed of light, or 20&nbsp;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.<ref name="Mesyats" />

A disadvantage of simple PFN pulse generators is that because the transmission line must be matched to the load resistance ''R''<sub>L</sub> to prevent reflections, the voltage stored on the line is divided equally between the load resistance and the [[characteristic impedance]] of the line, so the voltage pulse applied to the load is only one-half the power-supply voltage.<ref name="Haddad" /><ref name="Mesyats" />

=== <span id="blumlein"></span> Blumlein transmission line===
[[File:Blumlein transmission line animation.gif|thumb|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]]<ref>[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 UK Patent 589127, ''Improvements in or relating to apparatus for generating electrical impulses''], Alan Dower Blumlein, filed October 10, 1941, granted June 12, 1947.</ref> and is widely used today in PFNs.<ref name="Haddad" /> 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).<ref name="Haddad" /> 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''<sub>0</sub> of the line is made equal to half the load impedance ''R''<sub>L</sub>, the voltage step is half-reflected and half-transmitted,<ref name="Haddad" /> resulting in two symmetrical opposite-polarity voltage steps, which propagate away from the load, creating between them a voltage drop of ''V''/2&nbsp;−&nbsp;(−''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.<ref name="Haddad" /> 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 wave (waveform)|square output pulse]] 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]] is connected between the PFN and load. This technique improves the [[impedance matching|impedance match]] between the PFN and the load so as to improve power-transfer [[electrical efficiency|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)]]
*[[Pulse generator]]
*[[Pulsed power]]
*[[Thyratron]]
*[[Thyristor]]
*[[Spark gap#Power-switching devices|Triggered spark gaps]]
*[[Marx generator]]
*[[Crossatron]]
*[[Pulsed laser]]
*[[Radar]]

==References==
{{Reflist}}

==External links==
*Eric Heine, "''[https://web.archive.org/web/20040922063750/http://www.nikhef.nl/~erichn/conversion/conv.html Conversion]''". NIKHEF Electronic Department, Amsterdam, the Netherlands.
*Riepe, Kenneth B., "''High-voltage microsecond pulse-forming network''". Review of Scientific Instruments Vol 48(8) pp.&nbsp;1028–1030. August 1977. ([https://archive.today/20040827044542/http://content.aip.org/RSINAK/v48/i8/1028_1.html Abstract])
*[[G. N. Glasoe|Glasoe, G. Norris]], Lebacqz, Jean V., "''Pulse Generators''", McGraw-Hill, MIT Radiation Laboratory Series, Volume 5, 1948.

[[Category:Pulsed power]]