Mistry, Hemma

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UC BerkeleyBerkeley Scientific JournalTitleLight Wars: The Bright Future of Laser WeaponsPermalinkhttps://escholarship.org/uc/item/2v1656tzJournalBerkeley Scientific Journal, 12(1)ISSN1097-0967AuthorMistry, HemmaPublication Date2009DOI10.5070/BS3121007589Copyright InformationCopyright 2009 by the author(s). All rights reserved unless otherwise indicated. Contact the author(s) for any necessary permissions. Learn more at https://escholarship.org/terms Peer reviewed|UndergraduateeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaW A R  T E C H  • S P R I N G 2 0 0 8  • W A R  T E C HLight Wars: The BrightFuture of Laser Weapons5by Hemma MistryLasers as directed energy weapons: this idea maysound like the ray guns and light sabers of a science fictionwriter’s dreams, but the idea of incorporating lasers intomilitary technology has been pursued since lasers were firstinvented.  The U.S. government has spent millions of dol-lars attempting to make laser weaponry a reality.  In 1983,the Reagan administration initiated an ambitious effort tocreate missile defense systems using lasers in space, nick-named “Star Wars.”  Although these systems were neverrealized, the “Star Wars” program paved the way for futurelaser research.  The Clinton administration continued theeffort for missile defense laser weaponry with an agreementwith the Israeli government in 1996.  Currently there areseveral U.S. military funded programs developing laserweapons.What is a Laser?Lasers can serve as a valuable technology in warfarebecause the light they emit has important and useful quali-ties.  Laser light is monochromatic and coherent, whichmeans that not only does the light have a uniform wave-length, but also the light waves maintain the same phaserelationships over time.  The waves also propagate in a def-inite direction.  These properties allow laser light to carry alarge amount of energythat can be applied in aspecific direction, whichis highly advantageousfor a weapon.The word laseris actually an acronymfor “Light Amplification by Stimulated Emission ofRadiation,” and as the name implies, lasers work by ampli-fying light through the process of stimulated emission.  In1916, Albert Einstein first theorized stimulated emission asone of the three processes by which light can interact withmatter (Perram, et al. 2004).  In this process, a photon oflight hits an electron in an excited, high energy state, caus-ing the electron to fall to a lower energy state which releas-es a photon of light with the same wavelength, phase, anddirection as the incident light.  Mirrors are then used toamplify the emitted light, resulting in a beam of monochro-matic, coherent, directed laser light.This diagram shows the basic components of a laser.The gain medium, or amplifying medium, contains atomswhose electrons are stimulated to emit light.  Mirrors oneither end of the optical cavity reflect the light back severaltimes through the gain medium, amplifying the beam.  Anenergy input is also required to excite the electrons in thegain medium into a high energy state so that stimulatedemission can occur.Military Interest in LasersThe characteristics of laser light give lasers thepotential to make effective weapons.  Because a laser beamis focused in one direction, laser weapons have the poten-tial to be extremely accurate in hitting targets.  In combat,this accuracy would minimize the destruction of surround-ing area that is caused by more imprecise weapons such asexplosives.  The precise aim of the weapon and the mini-mal destruction would also reduce the harm caused to civil-ians and other unintended targets.  Lasers would alsoenable the military to attack targets from far away, providedthat there is an unobstructed path for the laser’s beam totravel between theweapon and target.Since the beam wouldtravel at the speed oflight, the weapon wouldbe effective on thesedistant targets almostinstantaneously. This long-range capability of lasers alsomeans that a laser attack could be carried out covertly fromThe basic components of a laserhttp://cord.org/step_online/st1-5/images/Fig05-01.gifLasers could be used to destroy satel-lites in space, or conversely, space-basedlasers could attack targets on the ground.W A R  T E C H   • S P R I N G  2 0 0 8  • W A R  T E C Ha great distance.Since its invention in 1960 by Arthur Shawlow andCharles Townes, the U.S. military has been interested inapplying laser technology to military use (Perram et al.2004).  For defense purposes, lasers could be implementedin ships, planes, or in ground-based systems to destroymissiles or rockets.  Lasers could also be used to destroysatellites in space, or conversely, space-based lasers couldattack targets on the ground.  Lasers could also be builtinto army vehicles or even planes as offensive weaponsthat could target large areas (Dunn 2005).Progress with Chemical LasersThe mili-tary hasinvestedin thedevelop-ment ofseveraldifferenttypes oflasers,and themain dif-ferences between these lasers are the source of electronsand the source of power.  The most advanced militarylasers currently are chemical lasers, which were inventedin 1965 by George C. Pimentel (Perram et al. 2004).Chemical lasers obtain energy from an exothermic chemi-cal reaction.  This energy excites the electrons in the gainmedium, which is usually a low pressure gas.  For exam-ple, the reaction of fluorine atoms with hydrogen mole-cules produces hydrogen fluoride molecules and releas-es excess energy.  These molecules of hydrogen fluo-ride become the gain medium, and the excess energyis used to raise electrons into an excited state.Chemical lasers are the most developed andmost advanced lasers used for military application.They are also capable of generating the highest powerof any kind of laser – over 1 megawatt.  The mostadvanced chemical laser defense system currently is theTactical High Energy Laser (THEL), which is a deuteri-um-fluoride laser.  In 1996, Northrop Grumman, a corpo-ration providing defense technology, received $89 millionto design and build this system for the US and Israeli gov-ernments.  The system shot down its first Katyusha rocketin 2000, and since then it has successfully shot down mul-tiple test rockets, mortar shells, and other missiles (Hecht2006).Despite its operational successes, the THEL hasserious drawbacks.  The whole laser system is about thesize of six buses placed side-by-side, and the developershave so far failed to reduce the system to a reasonablesize, which means that its portability would be an issue formilitary use.  Another concern is the THEL’s enormousfuel requirement and furthermore, the storage and supplyof this fuel.  The performance of the THEL is also unsatis-factory, considering the time and money spent building it.After investing over $300 million in this program, thePentagon decided to drop THEL in September 2005because of these problems, as writer William J. Broadexplains in an article on the program (New York Times,July 30, 2006).A chemical laser program that is showing somepromise, however, is the Airborne Laser (ABL), alsodeveloped by Northrop Grumman.  The goal of ABL is tofit a Boeing 747 airplane with an oxygen iodine laserweapon so that it can detect and shoot down missiles soonafter they are launched.  However, like THEL, ABL is notprogressing as hoped – the team has yet to install the laserin the aircraft.   Operational setbacks mean that the pro-posed date for the first missile test, 2008, will probably befurther postponed.Solid-State Lasers Hold PromiseWhile scientists are struggling to successfullyincorporate chemical lasers into missile defense systems,solid-state lasers are holding much more promise.  Insteadof chemicals, these lasers use a ruby or crystal to producelight.  Most commonly, these lasers use crystal yttriumaluminum garnet (YAG).  Ions of usually neodymium orytterbium are dropped into the crystal, and the process ofstimulated emission occurs on these atoms.  The energyused to excite the electrons in these ions comes from elec-tricity, specifically flashlamps or diodes.Solid-state lasers hold several advantages overchemical lasers.  Because they are powered by electricity,there is no need for expensive and bulky chemical fuels.They can also produce wavelengths of light that transmitwell through the atmosphere, unperturbed by interference.However, solid-state lasers can only operate at a fractionof the power of chemical lasers.  The military would liketo see a 100kW solid-state laser because this is the energyrequired to destroy mortars and artillery rockets(Shachtman 2006).The build-up of heat is a problem in solid-statelasers because the diodes produce a lot of heat but onlyThe Tactical High Energy Laser (THEL) at the U.S.Army Missile Range at White Sands, New Mexicohttp://www.afa.org/magazine/dec2002/1202attack6.jpgThe goal of ABL is to fit a Boeing 747airplane with a laser weapon so that it candetect and shoot down missiles soon afterthey are launched.6W A R  T E C H  • S P R I N G  2 0 0 8  • W A R  T E C Hhttp://tr.wikipedia.org/wiki/Absintheperform well atroom tempera-ture.  As thelaser runs, heatbuilds up in thecrystal whichmust beremoved.  Thiscooling problemplaces a limit onthe power of thelaser because aspower output ofthe diodesincreases, moreheat is producedwhich must beremoved.  Mostlasers are con-tinuously cooledusing a coolantsuch as water toremove wasteheat.  Continuous cooling, however, creates problems of itsown.  Because the inside of the laser is at a much highertemperature than the surface, a temperature gradient is cre-ated which causes stress damage to the equipment (Parker2002).Heat capacity lasers are solid-state lasers that sepa-rate the cooling process from lasing.  These lasers operatein cycles: for about ten seconds the laser will fire, followedby a longer period of cooling (thirty seconds to a few min-utes). Then, the laser will fire again.  The laser can now beoperated at higher power, meaning that a more powerfulbeam can be produced.  This cooling process was a break-through in the development of solid state lasers made bythe Solid-State Heat Capacity Laser (SSHCL) team at theLawrence Livermore National Laboratory (LLNL) (Parker2002).  Their laser is currently the highest power solid-statelaser in the world, and it reached this 67 kW energy level inFebruary 2007, approaching the 100kw goal (LawrenceLivermore National Laboratory 2007).The U.S. is currently funding the Joint High PowerSolid-State Laser (JHPSSL) program, also under NorthropGrumman.  In 2005, a 27kW beam was produced, andalthough it has not achieved the power level of LLNL’slaser, progress is being made.  JHPSSL also aims for apower level of 100kW, and this laser could be used inships, aircraft, or ground-based systems for missile defense(Northrop Grumman 2007).The Future of Free-Electron LasersSolid-state lasers and chemical lasers have certainlimitations on the power and quality of beam that they canachieve.  A laser that bypasses this problem and allows forincreased tunability is the free-electron laser.  Free-electronlasers work by sending a relativistic beam of electrons, orelectrons moving close to the speed of light, through aseries of alternating magnetic fields, called a wiggler orundulator.  The wiggler causes the electrons to oscillate at acertain frequency depending on the strength and location ofthe magnets, and the electrons then emit light depending ontheir velocities.  Free-electron lasers were first created in1977 by John Madey at Stanford University (Dylla andCorneliussen 2005).Unlike chemical and solid-state lasers, free-electronlasers can theoretically be tuned to any wavelength becausethey do not rely on the quantum structure of atoms to pro-duce light.  In chemical and solid-state lasers, the processof stimulated emission can only produce certain discretewavelengths of light because electrons are bound to discreteenergy levels.  Since the transition between energy levels ofthe electron determines the wavelength of the emitted pho-ton, these lasers can only operate at specific wavelengths.However, in a freeelectron laser there is no gain medium and the electrons are “free”; therefore, the emitted photonsare not restricted by the discrete energy levels associatedwith atomic structure (Dylla and Corneliussen 2005).  Theabsence of a gain medium also means that the laser will notgenerate a large amount of heat, so cooling is not an issue.The electrons in this laser do, however, need to be acceler-ated, and this requires an accelerator which can be large,heavy, and expensive.The U.S. Navy has invested in the Free-ElectronLaser (FEL) program at the Thomas Jefferson NationalAccelerator Facility. This laser has achieved 14.2kW ofpower, and progress is being made in improving this figure(Jefferson Lab 2007).  Of the three lasers, freeelectron lasers have the most optimistic future as weapons becauseof their advantages over chemical and solid state lasers;however, their development is still in the early stages and itmay be a decade or more before they are operational.While the most powerful and most advanced laser weaponssystems involve chemical lasers, solidstate and free-electron laser programs promise much more excitingresults.  With the continued advancement of these militaryfunded programs, the dream of futuristic laser weaponryThe Solid-State Heat Capacity Laser at LLNLhttp://www.llnl.gov/PAO/news/news_images/bigpix/sshcl.jpghttp://www.star.le.ac.uk/~rw/courses/lect4313_fig27.gifA free electron laser.  Electrons are accelerated in a linear acceler-ator, and then sent through the undulator, a series of alternatingmagnetic fields, in which laser light is created.7could be a reality within the next decade.References1. Dunn, Richard J. III.  2005.  Operational Implications ofLaser Weapons.  Northrop Grumman.  http://www.analysis-center.northropgrumman.com (accessed October 15, 2007).2. Dylla, H. Frederick, and Steven T. Corneliussen.  2005.Free-Electron Lasers Come of Age.  Jefferson Lab.http://www.jlab.org/news/articles/2005/fel.pdf (accessedOctober 15, 2007).3.  Heat Capacity Laser. 2007.http://llnl.gov/nif/psa/sshcl.html (accessed October 10,2007).4. Hecht, Jeff.  2006.  Lasers advance slowly into the bat-tlefield. New Scientist 2566: 26-27.5. Jefferson Lab Free-Electron Laser Program. 2007.http://www.jlab.org/FEL (accessed October 10, 2007).6. LaFortune, K.N., R.L. Hurd, S.N. Fochs, M.D. Rotter,P.H. Pax, R.L. Combs, S.S. Olivier, J.M. Brase, R.M.Yamamoto.  2007.  Technical challenges for the future ofhigh energy lasers.  Proceedings of SPIE - TheInternational Society for Optical Engineering 6454.http://firstsearch.oclc.org (accessed October 15, 2007).7. Northrop Grumman. 2007.  http://www.northropgrum-man.com (accessed October 10, 2007).8. Parker, Ann.  2002.  World’s Most Powerful Solid-StateLaser. Lawrence Livermore National Laboratory.http://www.llnl.gov/str/October02/Dane.html (accessedOctober 15, 2007).9. Perram, Glen P., Michael A. Marciniak, and MatthewGoda. 2004.  High energy laser weapons: technologyoverview.  Proceedings of SPIE - The International Societyfor Optical Engineering 5414. http://firstsearch.oclc.org(accessed October 10, 2007).10. Rincon, Paul. 2007.  Record Power for military laser.BBC News. http://news.bbc.co.uk (accessed October 18,2007).11. Shachtman, Noah. “Attack at the Speed of Light.”Popular Science May 2006.12. U.S. Military Scores Laser Success. 2002.  BBC News.http://news.bbc.co.uk/2/hi/technology/2407807.stm(accessed October 15, 2007).W A R  T E C H   • S P R I N G 2 0 0 8  • W A R  T E C H8

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