Editing Wireless power transfer (section)

== Far-field (radiative) techniques ==
{{anchor|radiative far field}}

[[Near and far field|Far field]] methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). High-[[directivity]] antennas or well-collimated laser light produce a beam of energy that can be made to match the shape of the receiving area. The maximum directivity for antennas is physically limited by [[diffraction]].

In general, [[light beam|visible light]] (from lasers) and [[microwave radiation|microwaves]] (from purpose-designed antennas) are the forms of electromagnetic radiation best suited to energy transfer.

The dimensions of the components may be dictated by the distance from [[transmitter]] to [[Receiver (radio)|receiver]], the [[wavelength]] and the [[Angular resolution#The Rayleigh criterion|Rayleigh criterion]] or diffraction limit, used in standard [[radio frequency]] [[Antenna (radio)|antenna]] design, which also applies to lasers. [[Airy disc|Airy's diffraction limit]] is also frequently used to determine an approximate spot size at an arbitrary distance from the [[aperture]]. Electromagnetic radiation experiences less diffraction at shorter wavelengths (higher frequencies); so, for example, a blue laser is diffracted less than a red one.

The [[Angular resolution#The Rayleigh criterion|Rayleigh limit]] (also known as the [[Ernst Abbe|Abbe diffraction limit]]), although originally applied to image resolution, can be viewed in reverse, and dictates that the [[irradiance]] (or ''intensity'') of any electromagnetic wave (such as a microwave or laser beam) will be reduced as the beam diverges over distance at a minimum rate inversely proportional to the aperture size. The larger the ratio of a transmitting antenna's [[Antenna aperture|aperture]] or laser's exit aperture to the wavelength of radiation, the more can the radiation be concentrated in a [[beamwidth|compact beam]].

Microwave power beaming can be more efficient{{clarify|date=September 2020}} than lasers, and is less prone to atmospheric [[attenuation]] caused by dust or [[aerosol]]s such as fog.

Here, the power levels are calculated by combining the parameters together, and adding in the [[Antenna gain|gains]] and [[Attenuation|losses]] due to the antenna characteristics and the [[Transparency (optics)|transparency]] and [[Dispersion relation|dispersion]] of the medium through which the radiation passes. That process is known as calculating a [[link budget]].

=== Microwaves ===
[[File:Suntower.jpg|thumb|right|An artist's depiction of a [[solar satellite]] that could send energy by microwaves to a space vessel or planetary surface]]

Power transmission via radio waves can be made more directional, allowing longer-distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in the [[Microwave power transmission|microwave]] range.<ref name="Massa2013">{{cite journal |first1=A. Massa, G. Oliveri, F. Viani, and P. Rocca |title=Array designs for long-distance wireless power transmission – State-of-the-art and innovative solutions |journal=Proceedings of the IEEE |date=June 2013 |volume=101 |issue=6 |pages=1464–1481 |doi=10.1109/JPROC.2013.2245491 |last1=Massa |last2=Oliveri |first2=Giacomo |last3=Viani |first3=Federico |last4=Rocca |first4=Paolo |s2cid=2990114}}</ref> A [[rectenna]] may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized.{{citation needed|date=September 2018}} Power beaming using microwaves has been proposed for the transmission of energy from orbiting [[solar power satellite]]s to Earth and the [[Beam-powered propulsion|beaming of power to spacecraft]] leaving orbit has been considered.<ref name=space>{{cite book |last1=Landis |first1=G. A. |title=Laser Power Beaming |chapter=Applications for space power by laser transmission |editor-first1=Jack V. |editor-first2=Edward E. |editor-last1=Walker |editor-last2=Montgomery Iv |year=1994 |volume=2121 |pages=252–255 |doi=10.1117/12.174188 |bibcode=1994SPIE.2121..252L |s2cid=108775324}}</ref><ref>{{cite journal |first1=G. |last1=Landis |title=Space Transfer With Ground-Based Laser/Electric Propulsion |journal=NASA Technical Memorandum |year=1992 |doi=10.2514/6.1992-3213 |url=https://arc.aiaa.org/doi/abs/10.2514/6.1992-3213 |hdl=2060/19930011426 |s2cid=109847404 |hdl-access=free}}</ref>

Power beaming by microwaves has the difficulty that, for most space applications, the required aperture sizes are very large due to [[diffraction]] limiting antenna directionality. For example, the 1978 [[NASA]] study of solar power satellites required a {{convert|1|km|mi|adj=mid|-diameter}} transmitting antenna and a {{convert|10|km|mi|adj=mid|-diameter}} receiving rectenna for a microwave beam at [[ISM band|2.45&nbsp;GHz]].<ref>{{cite book |doi=10.1109/WCPEC.2006.279877 |year=2006 |last1=Landis |first1=Geoffrey |title=2006 IEEE 4th World Conference on Photovoltaic Energy Conference |chapter=RE-Evaluating Satellite Solar Power Systems for Earth |pages=1939–1942 |isbn=1-4244-0016-3 |s2cid=22181565 |hdl=2060/20070005136 |hdl-access=free}}</ref> These sizes can be somewhat decreased by using shorter wavelengths, although short wavelengths may have difficulties with atmospheric absorption and beam blockage by rain or water droplets. Because of the "[[thinned-array curse]]", it is not possible to make a narrower beam by combining the beams of several smaller satellites.

For earthbound applications, a large-area 10&nbsp;km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1&nbsp;mW/cm<sup>2</sup> distributed across a 10&nbsp;km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants. For comparison, a solar PV farm of similar size might easily exceed 10,000 megawatts (rounded) at best conditions during daytime.

Following World War II, which saw the development of high-power microwave emitters known as [[magnetron|cavity magnetrons]], the idea of using microwaves to transfer power was researched. By 1964, a miniature helicopter propelled by microwave power had been demonstrated.<ref>{{cite web |url=http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0474925 |title=Experimental Airborne Microwave Supported Platform |archive-url=https://web.archive.org/web/20100302204238/http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0474925 |archive-date=2 March 2010 |url-status=dead }}</ref>

Japanese researcher [[Hidetsugu Yagi]] also investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and his colleague [[Shintaro Uda]] published their first paper on the tuned high-gain directional array now known as the [[Yagi antenna]]. While it did not prove to be particularly useful for power transmission, this beam antenna has been widely adopted throughout the broadcasting and wireless telecommunications industries due to its excellent performance characteristics.<ref name="Yagi">{{cite web |url=http://ieee.cincinnati.fuse.net/reiman/05_2004.htm |title=Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi |publisher=Ieee.cincinnati.fuse.net |access-date=4 June 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090611154616/http://ieee.cincinnati.fuse.net/reiman/05_2004.htm |archive-date=11 June 2009}}</ref>

Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at the [[Goldstone Deep Space Communications Complex]] in California in 1975<ref name=autogenerated3>{{cite web |url=http://www.spaceislandgroup.com/solarspace.html |archive-url=https://web.archive.org/web/20220122200330/http://www.spaceislandgroup.com/solarspace.html |archive-date=2022-01-22 |title=Space Solar Energy Initiative |publisher=Space Island Group |access-date=4 June 2009}}</ref><ref name=autogenerated1>{{cite journal |url=http://www.sspi.gatech.edu/wptshinohara.pdf |title=Wireless Power Transmission for Solar Power Satellite (SPS) |edition=Second Draft |first=N. |last=Shinohara |journal=Space Solar Power Workshop |publisher=Georgia Institute of Technology}}</ref><ref name="Brown1984">{{cite journal |last1=Brown |first1=W.C. |title=The History of Power Transmission by Radio Waves |journal=IEEE Transactions on Microwave Theory and Techniques |date=September 1984 |volume=32 |issue=9 |pages=1230–1242 |doi=10.1109/TMTT.1984.1132833 |bibcode=1984ITMTT..32.1230B |s2cid=73648082}}</ref> and more recently (1997) at Grand Bassin on [[Reunion Island]].<ref>{{cite web |url=http://www2.univ-reunion.fr/~lcks/Old_Version/PubIAF97.htm |archive-url=https://web.archive.org/web/20051023080942/http://www2.univ-reunion.fr/~lcks/Old_Version/PubIAF97.htm |archive-date=2005-10-23 |title=POINT-TO-POINT WIRELESS POWER TRANSPORTATION IN REUNION ISLAND |work=48th International Astronautical Congress |location=Turin, Italy |date=6–10 October 1997 |first1=J. D. |last1=Lan Sun Luk |first2=A. |last2=Celeste |first3=P. |last3=Romanacce |first4=L. |last4=Chane Kuang Sang |first5=J. C. |last5=Gatina |publisher=University of La Réunion – Faculty of Science and Technology}}</ref> These methods achieve distances on the order of a kilometer.

Under experimental conditions, microwave conversion efficiency was measured to be around 54% across one meter.<ref>{{cite journal |last1=Brown |first1=W.C. |last2=Eves |first2=E.E. |title=Beamed microwave power transmission and its application to space |journal=IEEE Transactions on Microwave Theory and Techniques |date=June 1992 |volume=40 |issue=6 |pages=1239–1250 |doi=10.1109/22.141357 |bibcode=1992ITMTT..40.1239B}}</ref>

A change to 24&nbsp;GHz has been suggested as microwave emitters similar to LEDs have been made with very high quantum efficiencies using [[negative resistance]], i.e., Gunn or IMPATT diodes, and this would be viable for short range links.

In 2013, inventor Hatem Zeine demonstrated how wireless power transmission using phased array antennas can deliver electrical power up to 30 feet. It uses the same radio frequencies as WiFi.<ref>{{Cite web |url=https://newatlas.com/cota-ossia-wireless-charging-microwave-phased-array/29217/ |title=Cota system transmits power wirelessly at up to 30 feet |website=newatlas.com |access-date=2018-01-05 |date=30 September 2013}}</ref><ref>{{Cite news |url=https://techcrunch.com/2013/09/09/cota-by-ossia-wireless-power/ |title=Cota By Ossia Aims To Drive A Wireless Power Revolution And Change How We Think About Charging |last=Etherington |first=Darrell |work=TechCrunch |access-date=2018-01-05}}</ref>

In 2015, researchers at the University of Washington introduced power over Wi-Fi, which trickle-charges batteries and powered battery-free cameras and temperature sensors using transmissions from Wi-Fi routers.<ref name=powifi>{{cite arXiv |eprint=1505.06815 |title=Powering the Next Billion Devices with Wi-Fi |last1=Talla |first1=Vamsi |last2=Kellogg |first2=Bryce |last3=Ransford |first3=Benjamin |last4=Naderiparizi |first4=Saman |last5=Gollakota |first5=Shyamnath |last6=Smith |first6=Joshua R. |class=cs.NI |year=2015}}</ref><ref>{{Cite web |url=https://www.technologyreview.com/s/538031/first-demonstration-of-a-surveillance-camera-powered-by-ordinary-wi-fi-broadcasts/ |title=First Demonstration of a Surveillance Camera Powered by Ordinary Wi-Fi Broadcasts |last=arXiv |first=Emerging Technology from the |access-date=2016-09-28}}</ref> Wi-Fi signals were shown to power battery-free temperature and camera sensors at ranges of up to 20 feet. It was also shown that Wi-Fi can be used to wirelessly trickle-charge nickel–metal hydride and lithium-ion coin-cell batteries at distances of up to 28 feet.

In 2017, the Federal Communications Commission (FCC) certified the first mid-field radio frequency (RF) transmitter of wireless power.<ref>{{Cite web |url=https://ir.energous.com/press-releases/detail/596/energous-receives-industry-first-fcc-certification-for |title=Energous Receives Industry-First FCC Certification for Over-the-Air, Power-at-a-Distance Wireless Charging :: Energous Corporation (WATT) |website=Energous Corporation |access-date=2018-01-05}}</ref> In 2021 the FCC granted a license to an over-the-air (OTA) wireless charging system that combines near-field and far-field methods by using a frequency of about 900&nbsp;MHz. Due to the radiated power of about 1 W this system is intended for small [[Internet of things|IoT]] devices as various sensors, trackers, detectors and monitors.<ref>{{Cite web |last=Emilio |first=Maurizio Di Paolo |date=2021-11-08 |title=Energous Enables Wireless Power Transfer Solutions at any Distance for U.S. and Europe |url=https://www.eetimes.eu/energous-enables-wireless-power-transfer-solutions-at-any-distance-for-u-s-and-europe/ |access-date=2021-11-11 |website=EE Times Europe}}</ref>

=== Lasers ===
[[File:Laser-powered model airplane.jpg|thumb|A laser beam centered on a panel of photovoltaic cells provides enough power to a lightweight model airplane for it to fly.]]

In the case of electromagnetic radiation closer to the visible region of the spectrum (.2 to 2 [[micrometers]]), power can be transmitted by converting electricity into a [[laser]] beam that is received and concentrated onto [[photovoltaic cell]]s (solar cells).<ref name="FraunhoferISE">{{cite web |url=https://www.ise.fraunhofer.de/en/business-areas/iii-v-and-concentrator-photovoltaics/research-topics/power-by-light |title=Power-by-Light |website=Fraunhofer ISE}}</ref><ref name="OpticalWPT">{{Cite book |doi=10.1109/ICSOS.2011.5783662 |isbn=978-1-4244-9686-0 |chapter=Optical wireless power transmission at long wavelengths |title=2011 International Conference on Space Optical Systems and Applications (ICSOS) |pages=164–170 |year=2011 |last1=Sahai |first1=Aakash |last2=Graham |first2=David |s2cid=18985866}}</ref> This mechanism is generally known as 'power beaming' because the power is beamed at a receiver that can convert it to electrical energy. At the receiver, special photovoltaic laser power converters which are optimized for monochromatic light conversion are applied.<ref name="Bett2008">{{Cite book |doi=10.1109/pvsc.2008.4922910 |isbn=978-1-4244-1640-0 |chapter=III–V solar cells under monochromatic illumination |title=2008 33rd IEEE Photovolatic Specialists Conference |pages=1–5 |year=2008 |last1=Bett |first1=Andreas W. |last2=Dimroth |first2=Frank |last3=Lockenhoff |first3=Rudiger |last4=Oliva |first4=Eduard |last5=Schubert |first5=Johannes |s2cid=21042923}}</ref>

Advantages compared to other wireless methods are:<ref>{{cite news |url=https://www.theguardian.com/science/2009/jan/04/wireless-power-technology-witricity |title=Wireless power spells end for cables |work=The Observer |location=London |date=4 January 2009 |first=David |last=Smith}}</ref>

* [[Collimated]] monochromatic [[wavefront]] propagation allows narrow beam cross-section area for transmission over large distances. As a result, there is little or no reduction in power when increasing the distance from the transmitter to the receiver.
* Compact size: [[solid state laser]]s fit into small products.
* No [[radio-frequency]] interference to existing radio communication such as [[Wi-Fi]] and cell phones.
* Access control: only receivers hit by the laser receive power.

Drawbacks include:

* Laser radiation is hazardous. Without a proper safety mechanism, low power levels can blind humans and other animals. High power levels can kill through localized spot heating.
* Optical to electrical conversion efficiency of photovoltaic cells is limited. However, special photovoltaic power converters for laser light have demonstrated efficiencies up to 68.9% <ref>{{cite journal |last1=Helmers |display-authors=etal |title=68.9% Efficient GaAs-Based Photonic Power Conversion Enabled by Photon Recycling and Optical Resonance |journal=Phys. Status Solidi RRL |volume=15 |date=2021 |issue=7 |page=2100113 |doi=10.1002/pssr.202100113 |doi-access=free |bibcode=2021PSSRR..1500113H }}</ref>
* Atmospheric absorption, and absorption and scattering by clouds, fog, rain, etc., causes up to 100% losses.
* Requires a direct line of sight with the target. (Instead of being beamed directly onto the receiver, the laser light can also be guided by an optical fiber. Then one speaks of [[power-over-fiber]] technology.)

Laser "power beaming" technology was explored in [[Directed-energy weapon|military weapons]]<ref>{{cite web |last=Skillings |first=Jonathan |url=http://news.cnet.com/8301-11386_3-10024153-76.html |title=Laser weapons: A distant target, CNET news August 23, 2008 1:41&nbsp;pm PDT |publisher=CNET |date=23 August 2008 |access-date=4 June 2009}}</ref><ref>{{cite web |url=http://www.defensetech.org/2006/01/12/laser-weapons-almost-ready-not/ |archive-url=https://web.archive.org/web/20091120185836/http://defensetech.org/2006/01/12/laser-weapons-almost-ready-not/ |url-status=usurped |archive-date=20 November 2009 |title=Laser Weapons "Almost Ready?" Not! |work=Defensetech |publisher=Defensetech.org |access-date=4 June 2009 |date=2006-01-12}}</ref><ref>{{cite web |url=https://www.army.mil/-news/2009/01/30/16279-white-sands-testing-new-laser-weapon-system/ |title=White Sands testing new laser weapon system, US Army.mil, 30 Jan 2009 |publisher=United States Army |date=30 January 2009 |access-date=4 June 2009}}</ref> and [[Laser propulsion|aerospace]]<ref>{{cite web |url=http://www.defensetech.org/2003/11/06/lasers-power-planes-drones/ |archive-url=https://web.archive.org/web/20100119052944/http://defensetech.org/2003/11/06/lasers-power-planes-drones/ |url-status=usurped |archive-date=19 January 2010 |title=Lasers Power Planes, Drones |publisher=Defensetech.org |access-date=4 June 2009 |date=2003-11-06}}</ref><ref>{{cite news |url=http://www.space.com/businesstechnology/051024_spaceelevator_challenge.html |title=Riding a Beam of Light: NASA's First Space Elevator Competition Proves Highly Challenging |first=Roger G. |last=Gilbertson |work=Space.com |date=24 October 2005 |access-date=4 June 2009}}</ref> applications. Also, it is applied for the powering of various kinds of sensors in industrial environments. Lately, it is developed for powering commercial and [[consumer electronics]]. Wireless energy transfer systems using lasers for consumer space have to satisfy [[laser safety]] requirements standardized under IEC 60825.<ref>{{cite journal |last1=Soltani |display-authors=etal |title=Safety Analysis for Laser-Based Optical Wireless Communications: A Tutorial |journal=Proceedings of the IEEE |date=2022 |volume=110 |issue=8 |pages=1045–1072 |doi=10.1109/JPROC.2022.3181968 |url=https://ieeexplore.ieee.org/document/9803253|arxiv=2102.08707 }}</ref>

The first wireless power system using lasers for consumer applications was [[Wi-Charge]], demonstrated in 2018, capable of delivering power to stationary and moving devices across a room. This wireless power system complies with safety regulations according to IEC 60825 standard. It is also approved by the US Food and Drugs Administration (FDA).<ref>{{Cite news |url=https://www.businesswire.com/news/home/20180110006324/en/Wi-Charge-Wins-CES-2018-Innovation-Award#new_tab |title=Wi-Charge Wins CES 2018 Best of Innovation Award |access-date=2018-03-12}}</ref>

Other details include [[Diffraction#Propagation of a laser beam|propagation]],<ref>{{cite web |url=http://www.ieee.org/organizations/pubs/newsletters/leos/oct05/free_space.html |archive-url=https://web.archive.org/web/20081023230609/http://www.ieee.org/organizations/pubs/newsletters/leos/oct05/free_space.html |url-status=dead |archive-date=23 October 2008 |title=Free-Space Laser Propagation: Atmospheric Effects |publisher=Ieee.org |access-date=4 June 2009}}<br />[https://web.archive.org/web/20060317124222/http://www.mellesgriot.com/pdf/CatalogX/X_36_6-9.pdf Propagation Characteristics of Laser Beams – Melles Griot catalog]<br />{{cite book |url={{Google books |id=4NXHYg70qqIC |plainurl=yes}} |title=L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, 2nd ed. (SPIE Press, 2005) |access-date=4 June 2009 |isbn=978-0-8194-5948-0 |year=2005 |last1=Andrews |first1=Larry C |last2=Phillips |first2=Ronald L |publisher=SPIE Press}}</ref> and the [[Quantum coherence|coherence and the range limitation problem]].<ref>{{cite web |author=Dr. Rüdiger Paschotta |url=https://www.rp-photonics.com/coherence.html |title=An explanation of Coherence |publisher=Rp-photonics.com |access-date=4 June 2009}}</ref>

[[Geoffrey A. Landis|Geoffrey Landis]]<ref>{{cite web |url=http://www.islandone.org/Settlements/EvolutionaryPathSPS.html |title=An Evolutionary Path to SPS |publisher=Islandone.org |access-date=4 June 2009}}</ref><ref>{{cite web |url=http://www.geoffreylandis.com/supersynch.html |title=A Supersynchronous SPS |publisher=Geoffreylandis.com |date=28 August 1997 |access-date=4 June 2009}}</ref><ref>{{cite journal |url=http://www.sff.net/people/Geoffrey.Landis/papers.html |title=Papers Relating to Space Photovoltaic Power, Power beaming, and Solar Power Satellites |journal=Astrobiology |volume=1 |issue=2 |pages=161–4 |doi=10.1089/153110701753198927 |pmid=12467119 |access-date=4 June 2009 |year=2001 |last1=Landis |first1=Geoffrey A. |bibcode=2001AsBio...1..161L}}</ref> is one of the pioneers of [[solar power satellites]]<ref>{{cite web |url=http://www.nss.org/settlement/ssp/ |title=Limitless clean energy from space |publisher=Nss.org |access-date=4 June 2009 |archive-date=29 May 2016 |archive-url=https://web.archive.org/web/20160529144039/http://www.nss.org/settlement/ssp/ |url-status=dead}}</ref> and laser-based transfer of energy, especially for space and lunar missions. The demand for safe and frequent space missions has resulted in proposals for a laser-powered [[space elevator]].<ref>{{cite web |url=http://www.spaceward.org/elevator2010-pb |title=Power Beaming (Climber) Competition |publisher=Spaceward.org |access-date=4 June 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090724060700/http://www.spaceward.org/elevator2010-pb |archive-date=24 July 2009}}</ref><ref>{{cite web |url=http://www.spaceelevator.com/ |title=From Concept to Reality |publisher=The Space Elevator |access-date=4 June 2009}}<br />{{cite web |date=31 January 2009 |url=http://crnano.typepad.com/crnblog/2009/01/space-elevator-tethers-coming-closer.html |title=Space Elevator Tethers Coming Closer |publisher=Crnano.typepad.com |access-date=4 June 2009}}</ref>

NASA's [[Dryden Flight Research Center]] has demonstrated a lightweight unmanned model plane powered by a laser beam.<ref>{{cite web |url=http://www.nasa.gov/centers/dryden/news/FactSheets/FS-087-DFRC.html |title=Dryden Flight Research Center, Beamed Laser Power For UAVs |publisher=Nasa.gov |date=7 May 2008 |access-date=4 June 2009}}</ref> This proof-of-concept demonstrates the feasibility of periodic recharging using a laser beam system.

Scientists from the Chinese Academy of Sciences have developed a proof-of-concept of utilizing a dual-wavelength laser to wirelessly charge portable devices or UAVs.<ref>{{cite journal |title=A coupled model on energy conversion in laser power beaming |journal=Journal of Power Sources |volume=393 |pages=211–216 |date=15 May 2018 |doi=10.1016/j.jpowsour.2018.05.010 |last1=Wu |first1=Chen-Wu |last2=Wang |first2=Jyhwen |last3=Huang |first3=Chen-Guang |url=http://dspace.imech.ac.cn/handle/311007/77622 |bibcode=2018JPS...393..211W |s2cid=104165547}}</ref>

=== Atmospheric plasma channel coupling ===
{{See also|Electrolaser}}

In atmospheric plasma channel coupling, energy is transferred between two electrodes by electrical conduction through ionized air.<ref name="Nawaz">{{Cite journal |last1=Nawaz |first1=Suddiyas |title=Wireless Power Transmission |url=https://www.academia.edu/9539890 |website=Academia 2015 |access-date=31 December 2015}}</ref> When an electric field gradient exists between the two electrodes, exceeding 34 kilovolts per centimeter at sea level atmospheric pressure, an electric arc occurs.<ref name="Ray_2009">{{Cite book |title=An Introduction to High Voltage Engineering |last=Ray |first=Subir |publisher=PHI Learning |year=2009 |isbn=978-8120324176 |pages=19–21 |url=https://books.google.com/books?isbn=812032417X}}</ref> This atmospheric [[Electrical breakdown|dielectric breakdown]] results in the flow of electric current along a random trajectory through an ionized [[plasma channel]] between the two electrodes. An example of this is natural lightning, where one electrode is a virtual point in a cloud and the other is a point on Earth. Laser Induced Plasma Channel (LIPC) research is presently underway using ultrafast lasers to artificially promote development of the plasma channel through the air, directing the electric arc, and guiding the current across a specific path in a controllable manner.<ref>{{cite web |title=Electrolaser |website=WiseGeek website |publisher=Conjecture Corp. |year=2015 |url=http://www.wisegeek.com/what-is-an-electrolaser.htm |access-date=25 October 2015}}</ref> The laser energy reduces the atmospheric dielectric breakdown voltage and the air is made less insulating by superheating, which lowers the density (<math>p</math>) of the filament of air.<ref name="Scheller">{{cite journal |last1=Scheller |first1=Maik |last2=Born |first2=Norman |last3=Cheng |first3=Weibo |last4=Polynkin |first4=Pavel |year=2014 |title=Channeling the electrical breakdown of air by optically heated plasma filaments |journal=Optica |volume=1 |issue=2 |pages=125–128 |doi=10.1364/OPTICA.1.000125 |bibcode=2014Optic...1..125S |doi-access=free}}</ref>

This new process is being explored for use as a laser lightning rod and as a means to trigger lightning bolts from clouds for natural lightning channel studies,<ref name="Rakov">{{cite book |last1=Rakov |first1=Vladimir A. |last2=Uman |first2=Martin A. |title=Lightning: Physics and Effects |publisher=Cambridge Univ. Press |date=2003 |pages=296–298 |url=https://books.google.com/books?id=TuMa5lAa3RAC&q=laser&pg=PA296 |isbn=978-0521035415}}</ref> for artificial atmospheric propagation studies, as a substitute for conventional radio antennas,<ref name="Stahman_1964">{{cite web |last1=Stahmann |first1=J. R. |title=LASER TYPE ULTRA-VIOLET RADIATION FEASIBILITY FOR LIGHTNING AND ATMOSPHERIC PROPAGATION STUDIES |url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0609217 |archive-url=https://web.archive.org/web/20160126215236/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0609217 |url-status=dead |archive-date=26 January 2016 |website=DEFENSE TECHNICAL INFORMATION CENTER OAI |publisher=LIGHTNING AND TRANSIENTS RESEARCH INST ST PAUL MN |access-date=16 January 2016 |date=Oct 1964}}</ref> for applications associated with electric welding and machining,<ref name="Lawrence">{{cite book |last1=Lawrence |first1=Jonathan R. |last2=Waugh |first2=D. |title=Laser Surface Engineering: Processes and Applications |publisher=Elsevier |date=2014 |pages=456–460 |url=https://books.google.com/books?id=n297AwAAQBAJ&q=%22laser+guided+discharge%22&pg=PA456 |isbn=978-1782420798}}</ref><ref name="Forestier">{{cite journal |last1=Forestier |first1=B. |last2=Houard |first2=A. |last3=Revel |first3=I. |last4=Durand |first4=M. |last5=André |first5=Y. B. |last6=Prade |first6=B. |last7=Jarnac |first7=A. |last8=Carbonnel |first8=J. |last9=Le Nevé |first9=M. |last10=de Miscault |first10=J. C. |last11=Esmiller |first11=B. |last12=Chapuis |first12=D. |last13=Mysyrowicz |first13=A. |title=Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament |journal=AIP Advances |date=March 2012 |volume=2 |issue=1 |pages=012151 |doi=10.1063/1.3690961 |bibcode=2012AIPA....2a2151F |doi-access=free}}</ref> for diverting power from high-voltage capacitor discharges, for [[directed-energy weapon]] applications employing electrical conduction through a ground return path,<ref name="Giulietti">{{cite book |doi=10.1007/978-3-642-03825-9_6 |chapter=On Lightning Control Using Lasers |title=Progress in Ultrafast Intense Laser Science |series=Springer Series in Chemical Physics |year=2010 |last1=Kasparian |first1=Jérôme |last2=Wolf |first2=Jean-Pierre |volume=98 |pages=109–122 |isbn=978-3-642-03824-2}}</ref><ref name="Franklin">{{cite book |last1=Franklin |first1=Steve |title=Non-Lethal Weapon Handbook |publisher=Digital Services |date=2015 |pages=161–162 |url=http://www.rottys.eu/download-pdf-non-lethal-weapon-handbook-book-by-digital-services.pdf}}</ref><ref name="Quick_2012">{{cite web |last1=Quick |first1=Darren |title=U.S. Army weapon shoots lightning bolts down laser beams |url=http://www.gizmag.com/laser-induced-plasma-channel/23117/ |website=Gizmag |publisher=Gizmag Limited |access-date=16 January 2016 |date=28 June 2012}}</ref><ref name="Kaneshiro">{{cite web |last=Kaneshiro |first=Jason |title=Picatinny engineers set phasers to 'fry' |website=news Archives |publisher=US Army official website www.mil.gov |date=21 June 2012 |url=https://www.army.mil/article/82262/Picatinny_engineers_set_phasers_to__fry_/ |access-date=25 October 2015}}</ref> and [[Radio jamming|electronic jamming]].<ref name="Clerici">{{cite journal |last1=Clerici |first1=Matteo |last2=Hu |first2=Yi |last3=Lassonde |first3=Philippe |last4=Milián |first4=Carles |last5=Couairon |first5=Arnaud |last6=Christodoulides |first6=Demetrios N. |last7=Chen |first7=Zhigang |last8=Razzari |first8=Luca |last9=Vidal |first9=François |last10=Légaré |first10=François |last11=Faccio |first11=Daniele |last12=Morandotti |first12=Roberto |title=Laser-assisted guiding of electric discharges around objects |journal=Science Advances |date=June 2015 |volume=1 |issue=5 |pages=e1400111 |doi=10.1126/sciadv.1400111 |pmid=26601188 |pmc=4640611 |bibcode=2015SciA....1E0111C}}</ref>