siegman@sierra.Stanford.EDU (Anthony E. Siegman) (11/01/89)
From: siegman@sierra.Stanford.EDU (Anthony E. Siegman) [MODERATOR; I'VE BEEN TRYING TO MAIL THIS TO YOU AND GETTING NOTHING BUT BOUNCES FROM att.att.com AND cbnews.att.com. MAYBE OUR MAILER HAS PROBLEMS...?] In response to recent messages regarding laser beam weapons and their power supply requirements: (I was just beginning to compose this msg 10 days ago when the electrical power suddenly went out; everything starting raining down off the shelves; and I went rapidly under my desk . . .) Laser beam weapons have some attractive features, but also several probably insuperable problems. However, the need for a large electrical power supply is not necessarily one of these. There are any number of chemical fuels which when burned produce hot gases in which many or even all of the molecules are formed initially in upper quantum levels, the requisite condition for laser action. If this very hot, high-pressure, highly excited gas is then expanded through a supersonic nozzle or nozzles, one ends up with a cold, low-pressure, rapidly moving, but still highly excited gas laser medium -- a kind of "jet engine" laser medium pumped purely by its own combustion. The supersonically expanded gas coming out of the nozzle can in fact be considerably colder than room temperature so far as its kinetic motion is concerned, as a consequence of the supersonic expansion. This leads to comparatively long upper-state lifetimes and substantially reduced doppler broadening of the molecular transitions. At the same time the population inversion into upper laser levels can be large, leading to substantial laser gain on the molecular transitions. And, following the laser action the spent molecules automatically flow on out of the active laser region and are removed from the system, so they don't hang around and spoil the laser action by reabsorption. Useful examples of such fuel combinations include atomic hydrogen or deuterium plus fluorine or some other halide, leading to lasers such as the HF, DF or HCl lasers which lase over a wide range of wavelengths in the middle infrared; or cyanogen or other fuels which contain carbon, oxygen and nitrogen, and which when burnt lead to powerful gasdynamic CO2 laser action at 10.6 microns in the mid IR. [The clever Israelis even made a small single-shot carbon dioxide laser which used nothing except ordinary gasoline ignited by an automobile spark plug in a thick-walled chamber with a heavily spring-loaded door. This door blew open after ignition, letting the hot gases exhaust through an expansion nozzle. There was enough hot nitrogen and CO2 in the exhaust gases to give a short burst of CO2 laser oscillation at 10.6 microns. What this device could be good for, however, I can't imagine.] In a crude picture of say an HF chemical laser, when an H atom and a fluorine atom come close together in the hot gaseous fuel mixture they attract each other, pull each other toward one another, and the two atoms then in essence "hook together" with a large amount of residual vibrational motion. In quantum terms the two atoms bond together producing an HF molecule in a very highly excited vibrational state. A rough rule of thumb for chemical-gasdynamic lasers is that the combustion of one pound of such fuels can produce enough excited molecules to extract, with proper design, several hundred kilojoules of laser energy output energy -- in a good case, perhaps 500 kJ of output laser energy per pound of fuel burned. Thus, 10 lb of fuel can give you a 1 megawatt laser beam for 5 seconds of total firing time. Lasers in this general range of performance have been built. A one-kilowatt laser beam focused at close range will produce very impressive results, burning through a firebrick or piece of armor plate in a few seconds. A one-megawatt diffraction-limited beam transmitted and focused by, say, a steerable 1-meter-diameter focusing mirror can do the same thing over distances of many kilometers. Attractive features of such beam weapons include: 1) The "photon bullets" travel outward at the speed of light -- about Mach 1 million. A lot better than trying to hit an incoming missile with an outgoing one having about the same velocity and launched from a cold start. 2) You only have to point and aim the final pointing and focusing mirror, not the entire device, making possible rapid retargeting on multiple targets (assuming sufficient tracking and computer power). 3) It really is a "directed-energy" weapon. It can direct nearly all of the energy (but not the mass) of a combustion/explosion process taking place *here* onto a target located way out *there*. Practical problems include: 1) The good chemical laser fuels are almost always horrendously corrosive, flammable, explosive and toxic. The idea of trying to store large quantities of fluorine and hydrogen, for example, and pipe them around a ship, especially one that might come under attack, fills the Navy with well-deserved horror. 2) The optical power levels inside the laser devices themselves are so horrendously high that the high-reflectivity laser mirrors operate just on the verge of self-destruction. Any flaw or blemish or dust particle on the mirror surface causes the mirror reflectivity to decrease or its absorption to increase. As the absorbing spot gets warmer, its absorption goes up, and the situation goes to pot in a runaway fashion. The result is near-instantaneous catastrophic runaway thermal damage which blows the surface off the mirror faster than you can possibly shut things down. The supersonic nozzles are extremely fragile and touchy also. 3) You have to really hit directly on the target to do any good; no "near miss" benefits. 4) The full power output from good chemical lasers is inherently distributed over a very large number of separate infrared wavelengths, corresponding to transitions between many different rotational and vibrational quantum levels of the laser molecules. All the optics therefore has to be very broadband, high-quality, and achromatic over a very wide wavelength range. 5) Constructing a laser of the size needed for a weapon, which will also operate in a single transverse resonator mode so as to produce the necessary "diffraction-limited" output beam, as well as maintaining the necessary mirror alignments (and the delicate nozzles) in the presence of a roaring multi-megawatt combustion process, are very difficult, probably unsolvable technical problems, for a variety of fundamental technical and practical reasons. My personal opinion is that chemical lasers at the multi-megawatt level can surely be built (probably have been built) in the laboratory. _Diffraction-limited_ lasers at the same power level might just barely be accomplished, for short periods of time, in the laboratory, with painstaking adjustment and extraordinary expense. But to think of making hundreds or thousands of such lasers; launching them into space; having them not just work but maintain their performance once they reach orbit; having them stay in operable condition, available for use at short notice, for years or decades; aiming and pointing them remotely; and, not so obviously, being able to have any confidence at all that they will actually work when and if needed -- these are absurd fantasies, not worth taking seriously.
jwtlai@watcgl.waterloo.edu (Jim W Lai) (11/03/89)
From: jwtlai@watcgl.waterloo.edu (Jim W Lai) In article <10842@cbnews.ATT.COM> siegman@sierra.Stanford.EDU (Anthony E. Siegman) writes: >My personal opinion is that chemical lasers at the multi-megawatt >level can surely be built (probably have been built) in the >laboratory. _Diffraction-limited_ lasers at the same power level >might just barely be accomplished, for short periods of time, in the >laboratory, with painstaking adjustment and extraordinary expense. >But to think of making hundreds or thousands of such lasers; launching >them into space; having them not just work but maintain their >performance once they reach orbit; having them stay in operable >condition, available for use at short notice, for years or decades; >aiming and pointing them remotely; and, not so obviously, being able >to have any confidence at all that they will actually work when and if >needed -- these are absurd fantasies, not worth taking seriously. As of 1987 (I know my info is a bit out of date), the MIRACL chemical laser (which burns hydrogen fluoride and deuterium fluoride) achieved power in excess of 1 MW. According to the APS report on SDI, the needed improvement in output from the HF-DF laser was a factor of twenty to be useful for SDI. However, the laser which achieved the best performance cannot be scaled to significantly higher powers. Scaling up a technology may sometimes be a difficult task, if not impossible. -- "Then he ruins everything by talking." Bruce Wayne, _Batman: The Dark Knight Returns_