Characteristics of gamma-ray lasers

[Modax]

I've done some reading recently about how gamma ray lasers might one day be possible, using exotic power sources such as nuclear isomers like 178-hafnium or exotic molecules like "positronium" I realize that these weapons are entirely theoretical at this point and may not ever be practical or even physically possible. However, I am more interested in learning about what the likely characteristics of such weapons would be than in hearing a debate about their plausibility. Therefore I have the following questions:

1. Would gamma ray lasers (grasers) work in an earthlike atmosphere? I know that they would ionize the air they pass through causing blooming, but I remember reading about how ultra-short (nanosecond or picosecond) pulses might allow for increased range. What would the maximum effective range be for a theoretical handheld gamma ray weapon in our atmosphere?

2. What effect does it have on the target? My understanding is that while regular (visible spectrum) laser weapons like the THEL work by the simple transfer of energy, causing damage through heating and vaporization, X-ray and Gamma Ray lasers, on the other hand, are highly penetrating, and do damage by stripping electrons off of atoms in the matter that they pass through. So would an extremely powerful graser burn a hole straight through your body or would it just destroy your DNA and cause your cells to die?

3. What would the primary advantage be (If there is any) to handheld gamma ray weapons as opposed to visible light lasers? Apparently gamma ray lasers are capable of far, far higher power outputs, but tend to be less effective at transferring their energy to the target.

Thanks in advance for your help. Let me reiterate that I'm only interested in a discussion of what characteristics these hypothetical weapons would be likely to have, and how they should be portrayed in realistic science fiction.

[kinnison]

AFAIK, the main problem with making grasers that can be fired more than once is simply; what the heck do you use for the mirrors defining the lasing cavity? X-ray lasers are a little more feasible - after all, X-ray telescopes are in orbit now and therefore the mirrors must exist - but the mirrors only work at extreme grazing angles and therefore the weapon would be quite bulky.

A secondary problem is that the laser cavity would probably have to be lined with quite a lot of lead along the sides. In visible light lasers, part of the process of getting a collimated beam is the leakage of light not perfectly aligned with the beam, this happening all around the sides of the cavity. In a graser working in a similar way, there would be quite a lot of gamma rays emitted all around the sides of the weapon.

However, if by some sort of handwavium you manage to build the laser, it might IMHO be a rather inefficient weapon. Firstly, as you rightly said it would lose energy rather rapidly in ionising the air in the way. Secondly, even extremely large amounts of (hard) X or gamma rays don't kill instantly - it takes several minutes at least. If you managed to dump enough gamma-ray energy into a target to do immediate damage, everything around it would probably be irradiated to lethal levels.

You might get around this by using soft X-rays (near the border with extreme UV, which is arbitrary anyway) which are less penetrating and therefore, counterintuitively, more effective at causing damage.

A gamma-ray laser powerful enough to give the target a dose of radiation that will kill him in a few hours might be quite a nasty assassination weapon, though.

[RedImperator]

3. What would the primary advantage be (If there is any) to handheld gamma ray weapons as opposed to visible light lasers? Apparently gamma ray lasers are capable of far, far higher power outputs, but tend to be less effective at transferring their energy to the target.

None. The shielding around the power generator alone would make it impractically bulky. What would anyone need with very high power outputs in a handheld weapon, anyway?

[Darth Wong]

3. What would the primary advantage be (If there is any) to handheld gamma ray weapons as opposed to visible light lasers? Apparently gamma ray lasers are capable of far, far higher power outputs, but tend to be less effective at transferring their energy to the target.

None. The shielding around the power generator alone would make it impractically bulky. What would anyone need with very high power outputs in a handheld weapon, anyway?

It helps for the "two-legged tank" paradigm that is popular with wanky sci-fi fans.

[Solauren]

3. What would the primary advantage be (If there is any) to handheld gamma ray weapons as opposed to visible light lasers? Apparently gamma ray lasers are capable of far, far higher power outputs, but tend to be less effective at transferring their energy to the target.

None. The shielding around the power generator alone would make it impractically bulky. What would anyone need with very high power outputs in a handheld weapon, anyway?

It helps for the "two-legged tank" paradigm that is popular with wanky sci-fi fans.

The only thing I can think of that they'd be useful for is; it might be easier detect Gamma-Laser fire from orbit, letting you track and locate your weapons (and therefore, your troops).

[RedImperator]

None. The shielding around the power generator alone would make it impractically bulky. What would anyone need with very high power outputs in a handheld weapon, anyway?

It helps for the "two-legged tank" paradigm that is popular with wanky sci-fi fans.

The only thing I can think of that they'd be useful for is; it might be easier detect Gamma-Laser fire from orbit, letting you track and locate your weapons (and therefore, your troops).

Well, there's no chance that could backfire.

[Fingolfin_Noldor]

It is important to note that the important thing about a laser in regards to it being a weapon to blind, kill, maim, is not so much its wavelength, but the power the laser medium can deliver. The wavelength that is pretty popular for high energy solid state lasers now is 1.064 micron which is the wavelength at which Nd:YAG lases at. The COIL laser used for the THEL if I am not wrong, lases at 1.3 micron.

At this current point, it is exceedingly hard to produce a gamma ray radiation weapon. A quick google and reading indicates that there are theories out there on how to produce gamma radiation from say hafnium, but experiments are few and research is still considered at the fringe for the moment. But I can immediately see lots of engineering problems from making one. Firstly, I have a feeling that there are no known dielectric mirrors that have coating that can tolerate gamma radiation. It is hard enough to get dielectric mirrors for say 350nm wavelength and these tend to degrade within a month or so of usage. The next option is silver or gold mirrors but gamma rays are incredibly penetrating and losses are bound to be high. Finally, if dielectric coatings cannot tolerate the radiation, then there's no feasible method, aside from gravitational lensing, to focus them effectly.

[Modax]

Thanks guys. I guess this should serve as a reminder that just because a technology is exotic and extremely difficult to engineer doesn't mean it will necessarily be very effective. Clearly, anyone thinking the grasers are some sort of superweapon hasn't got their facts straight.

My understanding of these issues is far from complete. What I have gathered on the internet is that while the energy of an individual photon is determined only by its wavelength (and thus gamma rays have more energy than UV rays, etc.) building more powerful lasers is accomplished by increasing intensity (number of photons emitted per time per unit area?) not by modifying wavelength. So as Noldor mentions, power output is limited by the carrying capacity of the lasing medium, not the energy of the photons themselves. And visible light is the best for direct energy transfer. The only advantage of x-ray/gamma ray lasers would be in space, where they would have a far greater effective range. Is that all right?

[Darth Wong]

The most important thing to remember is that real technologies are designed because someone has a design objective, and that objective is usually a practical one. People don't design and deploy weapons solely because they seem really cool or use the latest technological advancements. They design and deploy weapons in order to defeat a particular defense, or confer a particular tactical advantage.

[fusion]

The most important thing to remember is that real technologies are designed because someone has a design objective, and that objective is usually a practical one. People don't design and deploy weapons solely because they seem really cool or use the latest technological advancements. They design and deploy weapons in order to defeat a particular defense, or confer a particular tactical advantage.

Hafnium pumped lasers do have an advantage of being gamma ray which correlates to a much lower diffraction meaning that the gamma laser will be able stay focus for a longer distance (I know it isn't quite right, but it will suffice) meaning that the gamma laser will be able to hit further target with a smaller lens. A link to a place that explains it better than I do: Link

[Fingolfin_Noldor]

Hafnium pumped lasers do have an advantage of being gamma ray which correlates to a much lower diffraction meaning that the gamma laser will be able stay focus for a longer distance (I know it isn't quite right, but it will suffice) meaning that the gamma laser will be able to hit further target with a smaller lens. A link to a place that explains it better than I do: Link

That would be because the Rayleigh range and thus the beam divergence is very small. But this itself is problematic because you will lots of reflection off the surface of the said lens. Lens are coated with anti-reflection dielectric coating for the purpose of reducing the reflection and maximizing transmission. Fresnel's equations still apply to lens.

[Fingolfin_Noldor]

EDIT: Long Rayleigh Range, defined by pi*w_0^2/lambda, where lambda = wavelength, w0 is the beam diameter.

[RedImperator]

Even if a graser does have exception range, how does an infantryman make use of it? We already have rifles with two kilometer and longer effective ranges, and those are reserved for the best marksmen in the world. I'm not even convinced visible-light lasers will ever supplant firearms as infantry weapons. A graser offers no real practical benefits over a laser, and some major disadvantages?

[Darth Wong]

Even if a graser does have exception range, how does an infantryman make use of it? We already have rifles with two kilometer and longer effective ranges, and those are reserved for the best marksmen in the world. I'm not even convinced visible-light lasers will ever supplant firearms as infantry weapons. A graser offers no real practical benefits over a laser, and some major disadvantages?

As a general rule, one only uses an infantry weapon as powerful as necessary to get the job done. Overpowered weapons come with disadvantages in cost, ammo capacity, weight, size, etc., and macho chest-beating is not a valid military tactical concern.

Having said that, lasers do offer numerous significant advantages over modern rifles besides coolness, if we can make them smaller and lighter (obviously the real sticking point). Part of the reason it's hard to shoot accurately is the recoil of a modern rifle; a laser would have almost no perceptible recoil even at power levels high enough to easily kill a man. Also, the noise of a rifle can give away your location more easily than a laser pulse would; there is actually a system which uses sound from several microphones to triangulate the position of a sniper in order to direct return fire. Also, one of the reasons we need such superb marksmen to get these long-range shots is the fact that wind speed and gravity drop are such big factors at such long distance. Not so for a laser. Even the finger movement of pulling the trigger on a modern gun can throw the gun off its aim, but with a laser, you could have a much smaller push-button movement to fire it, or you could even have a time-delay shot, like a camera on a timer, which shoots one second after you pull the trigger so you can settle down.

Of course, that still doesn't explain why would need a laser of a particular wavelength, if a different type of laser would still get the job done.

[Fingolfin_Noldor]

I have serious doubts that you could ever place such a device in the hands of an infantry soldier. The power requirements are rather too large and would drain off the power rather quickly, never mind the weight issues.

[Modax]

Are you sure? Look at current energy storage tech: My laptop normally uses about 20 watts and the battery lasts for about 4 hours. That equates to 288 kilojoules. My battery weighs just over half a kilo, so 500 kilojoules per kilogram is a reasonable estimate for current batteries.

The muzzle energy of a .50 BMG bullet is about 20 kilojoules. Lasers are less efficient on a per-joule basis than bullets, but lets assume that 20 kilojoules delivered in a short pulse will kill at long distances. With an optimistic 30% efficiency for our future laser, you're looking at about 8 high energy pulses per 1 kg battery pack, which is acceptable for a sniper weapon.

Since Lithium-ion batteries can't unload their energy that rapidly, you'd probably have to have a two-stage system where the battery charges up an ultracapacitor big enough to hold the energy for a single shot. From what I've read, there is still a lot of room for energy storage tech to improve, so it isn't the biggest issue in these weapons. The BIG problems are power/weight and cooling.

[Fingolfin_Noldor]

Are you sure? Look at current energy storage tech: My laptop normally uses about 20 watts and the battery lasts for about 4 hours. That equates to 288 kilojoules. My battery weighs just over half a kilo, so 500 kilojoules per kilogram is a reasonable estimate for current batteries.

The muzzle energy of a .50 BMG bullet is about 20 kilojoules. Lasers are less efficient on a per-joule basis than bullets, but lets assume that 20 kilojoules delivered in a short pulse will kill at long distances. With an optimistic 30% efficiency for our future laser, you're looking at about 8 high energy pulses per 1 kg battery pack, which is acceptable for a sniper weapon.

Since Lithium-ion batteries can't unload their energy that rapidly, you'd probably have to have a two-stage system where the battery charges up an ultracapacitor big enough to hold the energy for a single shot. From what I've read, there is still a lot of room for energy storage tech to improve, so it isn't the biggest issue in these weapons. The BIG problems are power/weight and cooling.

You aren't forgetting the incredibly complex cooling equipment are you that needs power as well? If you have 30% optical efficiency, that means 70% of the energy put in is going to be radiated as heat.

As a rule, the biggest problem right now to putting lasers into a tank is the cooling equipment which I can assure you is fairly non-trivial and a glorious mangle of heat pipes and tubes delivering coolant or water to draw away the heat. I can assure you all that copper for heat dissipation is going to run into a few kilograms.

[Modax]

Yes, I understand; I mentioned that in the last sentence of my post. There either needs to be a major breakthrough in photonics and materials science, or else they will have to have bulky heat exchangers and cooling tech. Such a laser would be more like a heavy anti-materiel rifle like an M82 than an assault rifle.

[Littlefoot]

Im no scientist like the others who have posted here, but from what has been said about the nature of grasers and lasers, and from a military prospective if lasers with heat dissapation problems are going to be scaled down to the infantry level, the most likely early footmobile versions would probably be along the lines of the old .30 cal water cooled machine guns of WW1. Bulky and heavy, but for a weapons team very much an offensive as well as defensive weapon system. As for the grasers in a sci-fi, given the dificulties already stated, it may be best to leave them in the realm of a special aplications weapon for spec-ops units or as an airship mounted radiation delivery system used for area/material denial. If you have never trained for an NBC environment, you cant know the special horror of the MOP suit, running and gunning in a mask, or Decon. Given that this is radiation we are talking about, unless this sci-fi civ has the resources to field large numbers of infantry in fully enclosed and shielded armor suits, the infantry are screwed. Aditionally, you dont get a dust-off in a high radiation/chem/bio zones, so the psych value is high on the enemy. Does anyone know if the graser will be visible? if not, then double the terror caused becaus the first anyone will know that they are under fire, they will be dead men walking.

[Sky Captain]

I can see grasers only being useful on a combat spacecraft where weapons with extreme range are really needed, also the ability to kill the crew on enemy space station with highly penetrative gamma rays while leaving hardware relatively intact can be an advantage. On the ground there is no need for such a range and since in a futuristic sci fi scenario most infantry level combat is likely to happen inside buildings there is hardly a need for your average infantryman to have a laser with multi kilometer range.