Chemical laser
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2008-07-14T02:08:00Z
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A '''chemical laser''' is a [[laser]] that obtains its energy from a [[chemical reaction]]. Chemical lasers can achieve [[continuous wave]] output with power reaching to [[Watt#Megawatt|megawatt]] levels. They are used in industry for cutting and drilling, and in military as [[Directed-energy weapon|directed-energy weapons]].
Common examples of chemical lasers are the [[chemical oxygen iodine laser]] (COIL), [[all gas-phase iodine laser]] (AGIL), and the [[hydrogen fluoride laser]] and [[deuterium fluoride laser]], both operating in the mid-[[infrared]] region. There is also a [[deuterium fluoride-carbon dioxide laser|DF-CO<sub>2</sub> laser]] (deuterium fluoride-carbon dioxide), which, like COIL, is a "transfer laser." The hydrogen fluoride (HF) and deuterium fluoride (DF) lasers are unusual in that there are several molecular energy transitions with sufficient energy to be above the threshold required for lasing. Since the molecules do not collide frequently enough to re-distribute the energy, several of these laser modes will operate either simultaneously, or in extremely rapid succession so that an HF or DF laser appears to be operating simultaneously on several wavelengths unless a wavelength selection device is incorporated into the resonator.
==Origin of the CW chemical HF/DF laser==
The motivation for a chemical laser was born out of the carbon dioxide laser program in the late 1960s and early 1970s. DF had been used as a chemical reaction to excite the carbon dioxide molecule through a near resonant match between one of the DF levels and one of the CO<sub>2</sub> levels. This scheme was used by Navy researchers and their contractors, such as [[Pratt & Whitney|Pratt & Whitney Aircraft]] (a division of [[United Technologies Corporation]]), [[General Electric]], [[Rocketdyne]] (now part of Pratt & Whitney), and [[TRW]] (now part of [[Northrop Grumman]]). This type of laser was called a "transfer laser" by the Navy. Eventually carbon dioxide was eliminated as an intermediary and DF was tried as a stand alone lasing medium. Very quickly, deuterium was dropped in favor of hydrogen, since it is far less costly and more readily available. However, later it was realized that HF produces infrared radiation in the 2.6 to 3.1 μm waveband, a region of the spectrum absorbed by water vapor in the atmosphere. Interest was renewed in DF, which produces radiation in the 3.7 to 4.2 μm band, which passes easily through the atmosphere.
As with many successful inventions, there are many fathers: Don Spencer, Jack Hinchen, George Pimentel, and Bob Freiberg, for example have been credited as being the inventor of the chemical laser, though these researchers worked independently. Clearly it depends on how you define chemical laser, and there were many steps along the way to a purely chemically pumped chemical laser.
A pulsed chemical laser was demonstrated by Jerome V. V. Kasper and [[George C. Pimentel]] by 1965<ref>JVV Kasper and GC Pimentel, Phys Rev Letters 14, 352 (1965) HCl chemical laser, cited in some later reviews as the first chemical laser.</ref>. First, hydrogen chloride was pumped optically so vigorously that the molecule disassociated and then re-combined, leaving it in an excited state suitable for a laser. Then hydrogen fluoride and deuterium fluoride were demonstrated. Pimentel went on to explore a DF - CO<sub>2</sub> transfer laser. Although this work did not produce a purely chemical continuous wave laser, it paved the way by showing the viability of the chemical reaction as a pumping mechanism for a chemical laser. Pimentel was awarded a patent for a scalable overtone HF laser (United States Patent 4,760,582) in 1971.
The [[Laser#Continuous_wave_and_pulsed_lasers|continuous wave]] (CW) chemical HF [[Laser|laser]] was first demonstrated,<ref>D. J. Spencer, T. A. Jacobs, H. Mirels and R. W. F. Gross, “ Continuous-Wave Chemical Laser,” International Journal of Chemical Kinetics, Vol. 1, No. 5, September 1969, pp. 493-494.</ref> and subsequently patented,<ref>D. J. Spencer, H. Mirels, T. A. Jacobs and R. W. F. Gross, “Continuous-Wave Chemical Laser,”
US Patent No. 3,688,215, Aug. 29, 1972.</ref> by researchers at The Aerospace Corporation in [[El_Segundo|El Segundo]], [[California]]. This work was done in parallel with similar work at United Aircraft Research Laboratories (now [[United Technologies Corporation|United Technologies Research Center]]) by J.J. Hinchen.<ref>J.J.Hinchen,J. Chem. Phys., Vol. 59, 1973, p. 233.</ref> Similar work was started up very quickly by James A. Harrington [http://irfibers.rutgers.edu/harrington.html] at the [[University of Alabama in Huntsville]]. These devices used the mixing of adjacent streams of H<sub>2</sub> and F<sub>2</sub>, within an [[Optical_cavity|optical cavity]], to create vibrationally excited HF which [[wikt:lase|lased]]. However, since the fluorine was provided by dissociation of SF<sub>6</sub> gas with a DC electrical discharge, this also fell short of being a purely chemical laser. Later work at the US Army, US Air Force, and US Navy used only chemical reactions to drive a true chemical laser, meaning that only chemical energy from the [[exothermic]] reaction was used in producing the laser beam. This is very important for scaling up to high energy lasers for military or industrial use.
The analysis of the HF laser performance is complicated due to the need to simultaneously consider the [[Fluid_dynamics|fluid dynamic]] mixing of adjacent supersonic streams, multiple non [[Chemical_equilibrium|equilibrium]] [[Chemical_reaction|chemical reactions]] and the interaction of the [[Active_laser_medium|gain medium]] with the optical cavity. The researchers at The Aerospace Corporation developed the first exact analytic ([[Flame_sheet|flame sheet]]) solution,<ref>R. Hofland and H. Mirels, “Flame Sheet Analysis of CW Diffusion Type Chemical Laser. 1 Uncoupled Radiation,” AIAA Journal, Vol. 10. No. 4, April 1972, pp. 420-428.</ref> the first numerical computer code solution<ref>W. S. King and H. Mirels, “ Numerical Study of a Diffusion Type Chemical Laser,” AIAA Journal, Vol. 10, No. 12, Dec. 1972, pp. 1647-1654.</ref> and the first simplified model<ref>H. Mirels, R. Hofland and W. S. King, “ Simplified Model of CW Diffusion Type Chemical Laser,” 10th AIAA Aerospace Sciences Meeting, San Diego, Calif.,January 17-19, 1972. (Also AIAA Journal, Vol. 11. No. 2, February 1973, pp. 156–184.)</ref> describing CW HF chemical laser performance.
Chemical lasers stimulated the use of wave-optics calculations for resonator analysis. This work was pioneered by E. A. Sziklas (Pratt & Whitney Aircraft) and A. E. Siegman (Stanford University.) An example of an early paper on this subject is
E. A. Sziklas and A. E. Siegman, "Mode calculations in unstable resonator with flowing saturable gain. II. Fast Fourier transform method," Appl. Opt., vol. 14, pp. 1873--1889, August 1975. Part I of this was a companion paper that dealt with Hermite-Gaussian Expansion and has received little use compared with the Fourier Transform method which has now become a standard tool at United Technologies (SOQ), Lockheed-Martin (LMWOC), SAIC (ACS), Boeing (OSSIM), tOSC, MZA (Wave Train), and OPCI. Most of these companies competed for contracts to build HF and DF lasers for DARPA, the U.S. Air Force, the U.S. Army, or the U.S. Navy throughout the 1970s and 1980s. General Electric and Pratt & Whitney dropped out of the competition in the early 1980's leaving the field to Rocketdyne (now ironically part of Pratt & Whitney - although the laser organization remains today with Boeing) and TRW (now part of Northrop Grumman.)
Based on this work, chemical laser models were developed at SAIC by R. A. Wade, at TRW by D. Bullock, and Rocketdyne by D. A. Holmes. Of these, perhaps the most sophisticated was the CROQ code at TRW, outpacing the early work at Aerospace Corporation.
==Performance==
Studies<ref>N. Cohen, "A Review of Rate Coefficients for Reactions in the H2-F2 Laser System," TR-0172(2779)-2 Sept 1971. The Aerospace Corp., El Segundo, Ca.</ref> led to the design of efficient high-power experimental CW HF laser devices. Power levels up to 10 [[Watt#Kilowatt|kW]] were achieved by The Aerospace Corporation researchers. DF lasing was obtained by the substitution of D<sub>2</sub> for H<sub>2</sub>.
The TRW Systems Group in [[Redondo Beach, California]], subsequently received US Air Force contracts to build higher power CW HF/DF lasers. Using a scaled-up version of an Aerospace Corporation design, TRW achieved 100 kW power levels. General Electric, Pratt & Whitney, & Rocketdyne built various chemical lasers on company funds in anticipation of receiving DoD contracts to build even larger lasers. Only Rocketdyne received contracts of sufficient dollar amounts to continue competing with TRW. TRW produced the [[MIRACL]] device for the U.S. Navy that achieved megawatt power levels. The latter is believed to be the highest power continuous laser, of any type, developed to date (2007).
TRW also produced a cylindrical chemical laser (the Alpha laser) for DARPA, which had the advantage, at least on paper, of being scalable to even larger powers. However, by 1990, the interest in chemical lasers had shifted toward shorter wavelengths, and the chemical oxygen-iodine laser (COIL) gained the most interest, producing radiation at 1.315 μm. There is a further advantage that the COIL laser generally produces single wavelength radiation, which is very helpful for forming a very well focussed beam. This type of COIL laser is used today in the [[Boeing YAL-1|ABL]] (Airborne Laser, the laser itself being built by Northrop Grumman) and in the [[Advanced tactical laser|ATL]] (Advanced Tactical Laser) produced by Boeing. Meanwhile, a lower power HF laser was used for the [[THEL]] (Tactical High Energy Laser) built in the late 1990s for the Israeli Ministry of Defense in cooperation with the U.S. Army SMDC. It holds the distinction of being the only fielded high energy laser to demonstrate effectiveness in fairly realistic tests against rockets and artillery. The MIRACL laser has demonstrated effectiveness against certain targets flown in front of it at White Sands Missile Range, but it is not configured for actual service as a fielded weapon. This may soon change with ABL and ATL, if current plans and funding hold out.
==Future Variants==
With advances in Super-Alloy construction the [[USAF]] is currently working on a new type of chemical laser using cold fuel as its energy source with advantages of cooling and controlled firing. The Cold Chemical Laser or [[CCL]] is believed to use liquid nitrogen or liquid hydrogen as its fuel source in which the fuel is compressed to over 1000[[psi]] and ignited similar to a Jet engine and the hot compressed gas is ejected through a barrel or nozzle at extremely high speed and power output in excess of the 300Kw range and is hot enough at around 4,500 degrees C to melt even [[Stainless Steel]] and due to its extreme force of ejection of heated gas would be most effective against aircraft and light to Medium Tanks. However this design comes at a cost of the price of fuel (Liquid Nitrogen or Liquid Hydrogen). Also the weapon or device would proberly use a classified form of Ceramic-Titanium construction. The weapon would proberly be the equilivant of a 150mm shell at close range and the [[CCL]] bears similar weapon to the [[PPC]] (Particle Projectile Cannon) in the mechwarrior game series. If current prices become lower in the near future it is likely the weapon or device will be built in around 2014.
==References==
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