Bomb Pumped Gamma Ray lasers

captinjoehenry wrote:So I know that the current weapons are all bomb pumped x-ray lasers but there was some research into bomb pumped Gamma ray weapons. I would imagine that the shorter wave length and higher energy of a bomb pumped gamma ray weapon might be a good development. Mind you I am not sure on most of the details other than the fact that they were studied.

kzt wrote:Last I knew there is no theoretical mechanism to produce a gamma ray laser.

Impulsively Driven Laser
Andrew Presby found an interesting document entitled "On The Feasibility of an Impulsively Driven Gamma-ray Laser" (1979) at the Federation of American Scientists website. Please note this is for gamma-ray lasers, not x-ray lasers like the discussion above. That is probably why the x-ray laser rods had a maximum length of 5 meters while these graser rods have a length of 0.05 meters.


I wish I'd found the dumb thing years ago when I taking my graduate school lasers class and looking for physics papers on bomb pumped GRASERS. The Nevada experiment described herein sounds suspiciously like the bomb pumped XRASER (xray laser) experiments in the 70s/80s codenamed Excalibur that started the chain of events that got Teller in so much trouble. Thing I cannot figure is that the device described herein seems to produce GAMMA RAYS in the 6-8 MeV range (~0.002 Ångström) which is 10000 times higher photon energy than the stuff I've found in the literature that is available on Excalibur (which was in the ~14 Ångström range).

I've never heard if this worked or not... but there you go.

Andrew Presby

Yeah, I know. The diagram says 100,000 needles and the document says 20,000. Your choice.

Insulating and moderating material between rod assembly and bomb.
The document suggest using Tantalum-180 dissolved in Lithium-7 for the lasing rods, about one part in four thousand. Alternatives are Cobalt-109 and Molybdenum-99.

The design uses the Mössbauer effect, the recoil-free emission and absorption of gamma ray photons by atoms bound in a solid form. This is important. Laser light is coherent light, where all the photons are in perfect lock-step. The trouble with x-ray and gamma-ray emission is that they are powerful enough to make the excited atom recoil in reaction. This throws off the synchronization, so that the beam is not coherent, and thus not a laser beam. The Mössbauer effect prevents this by locking the lasing atoms in a matrix of anchor atoms, thus dealing with the recoil.

It was estimated that the grasing transition energy densities of tens of kilojoules per cubic centimeter. This means a one megajoule graser could fit in a breadbox, sans bomb of course. A laser beam composed of gamma rays impacting on, say, an incoming Soviet nuclear warhead would produce a flood of neutrons generated by gamma-ray/neutron recations, burning a nice hole. And the high-energy Compton-scattered electrons would create an enormous EMP, frying the warhead's electronics.

The document describes a test for the concept. A cylindrical package five centimeters long by five centimeters in radius would be packed with 20,000 lasing needles 25 µ diameter by 5 centimeters long (I assume that µ means micrometre or micron). The needles would be composed of Lithium-7 with 0.025% Tantalum-180. The needles would be aligned in parallel with 100 µ spacing between their axes, and arranges so that the centers of no three needles would be in a straight line.

The rod assembly package would be insulated from the bomb by insulating and moderating material (from the bomb: 15 cm of space, 7 cm of lead, 20 cm of heavy water, 5 cm to the center of the rod assembly). This will ensure that only the proper radiation strikes the assembly, and to allow the assembly to survive for the few microseconds required to create the graser beam. The lead [1] attenuates the gamma radiation from the bomb, [2] slows the debris motion, [3] and blocks the x-rays that would destroy the package. The heavy water moderates the neutron output.

The beam divergence is determined by the aspect ratio, which for this package is on the order of 0.5 milliradian. This is above the diffraction limit (about 8 milliradian).

In the proposed test, a one kiloton device would be detonated to pump the graser. The five centimeter needles have a calculated gain of 2 x 104. About 9% of the nuclear energy in the grasing transition will actually escape the needles, due to the short pathlength for 6.3 keV gamma rays. The energy available is 7.3 x 1016 MeV cm-3, which means the graser beam will be a piddling little 2.6 kilojoules. Keep in mind that is was intended as a test rig, not a functioning weapon.

captinjoehenry wrote:

kzt wrote:Last I knew there is no theoretical mechanism to produce a gamma ray laser.

Here is a section from Atomic Rockets about bomb pumped gamma ray lasers:

Impulsively Driven Laser
Andrew Presby found an interesting document entitled "On The Feasibility of an Impulsively Driven Gamma-ray Laser" (1979) at the Federation of American Scientists website. Please note this is for gamma-ray lasers, not x-ray lasers like the discussion above. That is probably why the x-ray laser rods had a maximum length of 5 meters while these graser rods have a length of 0.05 meters.


I wish I'd found the dumb thing years ago when I taking my graduate school lasers class and looking for physics papers on bomb pumped GRASERS. The Nevada experiment described herein sounds suspiciously like the bomb pumped XRASER (xray laser) experiments in the 70s/80s codenamed Excalibur that started the chain of events that got Teller in so much trouble. Thing I cannot figure is that the device described herein seems to produce GAMMA RAYS in the 6-8 MeV range (~0.002 Ångström) which is 10000 times higher photon energy than the stuff I've found in the literature that is available on Excalibur (which was in the ~14 Ångström range).

I've never heard if this worked or not... but there you go.

Andrew Presby

Yeah, I know. The diagram says 100,000 needles and the document says 20,000. Your choice.

Insulating and moderating material between rod assembly and bomb.
The document suggest using Tantalum-180 dissolved in Lithium-7 for the lasing rods, about one part in four thousand. Alternatives are Cobalt-109 and Molybdenum-99.

The design uses the Mössbauer effect, the recoil-free emission and absorption of gamma ray photons by atoms bound in a solid form. This is important. Laser light is coherent light, where all the photons are in perfect lock-step. The trouble with x-ray and gamma-ray emission is that they are powerful enough to make the excited atom recoil in reaction. This throws off the synchronization, so that the beam is not coherent, and thus not a laser beam. The Mössbauer effect prevents this by locking the lasing atoms in a matrix of anchor atoms, thus dealing with the recoil.

It was estimated that the grasing transition energy densities of tens of kilojoules per cubic centimeter. This means a one megajoule graser could fit in a breadbox, sans bomb of course. A laser beam composed of gamma rays impacting on, say, an incoming Soviet nuclear warhead would produce a flood of neutrons generated by gamma-ray/neutron recations, burning a nice hole. And the high-energy Compton-scattered electrons would create an enormous EMP, frying the warhead's electronics.

The document describes a test for the concept. A cylindrical package five centimeters long by five centimeters in radius would be packed with 20,000 lasing needles 25 µ diameter by 5 centimeters long (I assume that µ means micrometre or micron). The needles would be composed of Lithium-7 with 0.025% Tantalum-180. The needles would be aligned in parallel with 100 µ spacing between their axes, and arranges so that the centers of no three needles would be in a straight line.

The rod assembly package would be insulated from the bomb by insulating and moderating material (from the bomb: 15 cm of space, 7 cm of lead, 20 cm of heavy water, 5 cm to the center of the rod assembly). This will ensure that only the proper radiation strikes the assembly, and to allow the assembly to survive for the few microseconds required to create the graser beam. The lead [1] attenuates the gamma radiation from the bomb, [2] slows the debris motion, [3] and blocks the x-rays that would destroy the package. The heavy water moderates the neutron output.

The beam divergence is determined by the aspect ratio, which for this package is on the order of 0.5 milliradian. This is above the diffraction limit (about 8 milliradian).

In the proposed test, a one kiloton device would be detonated to pump the graser. The five centimeter needles have a calculated gain of 2 x 104. About 9% of the nuclear energy in the grasing transition will actually escape the needles, due to the short pathlength for 6.3 keV gamma rays. The energy available is 7.3 x 1016 MeV cm-3, which means the graser beam will be a piddling little 2.6 kilojoules. Keep in mind that is was intended as a test rig, not a functioning weapon.

So bomb pumped gamma ray lasers are definitely a possibility even with a warhead yield as low as one kiloton. So I would assume that the much higher yield available to Honorverse would allow for some seriously impressive bomb pumped gamma ray lasers that could represent a major increase in the destructive potential of every missile system currently employed.

Louis R wrote:The author of "An Introduction to Modern Starship Armor Design" speculates that the RMN's Mk73 laser assembly generates radiation at ~125keV. This is some 20 times the energy of the tested device, and 2-3x my guesstimate, and it's the energy range Honorverse warships are designed to withstand. Lower-energy weapons aren't going to serve much purpose. Higher-energy weapons certainly would, but the referenced article states that there is no reason to believe that stimulated-emission grasers are or ever will be physically possible. [And repeatedly makes the point that most '-asers' in the Honorverse _don't_ use stimulated emission physics]

Louis R wrote:That doesn't necessarily follow: you _can't_ make those needles larger - self-absorption is already soaking up better than 90% of the emission - so you need to use a lot more of them, making dispersion, targeting and alignment orders of magnitude more difficult. To the point where I would judge it completely impractical to build such a warhead. You can't simply focus more of the warhead output onto a small number of small emitters because past a certain point [which I strongly suspect that 1kT nudet was approaching] dumping more energy into the package just destroys it more quickly, and eventually it goes poof before you get any useful output at all.

Of course, there's also the problem that this _isn't_ a graser package in Honorverse terms anyway. There are two distinct ways to define 'gamma radiation'. The original, physical definition is that it is electromagnetic radiation generated by processes within the nucleus, while x-rays are generated by processes involving the electrons outside the nucleus. The alternative definition, currently used primarily by astronomers, makes it a section of the electromagnetic spectrum harder than x-rays, which are themselves somewhat arbitrarily defined as harder that ultraviolet radiation. There are transitions in the way photons interact with matter at the boundaries that affect instrument and detector design, but the borders are rather fuzzy to say the least. What matters here is that in the Honorverse 'gamma ray' is defined as having a wavelength <1pm, or alternatively energy > ~1.2MeV [which is actually a lot higher than the current value].

The device below is clearly being described by the physical definition since it produces radiation at 6.3keV. This would be fairly typical of a bomb-pumped graser. There are doubtless much more energetic metastable nuclear transitions that could populate an upper laser level, but not a lot of them if the Mossbauer effect is a necessary element. Also, there simply aren't enough hard photons generated by a nudet of any size to create a useful population inversion at high energies - this is a matter of radiation physics, not subject to being 'fixed' by handwavium. Without doing a lot of searching for data that, for the most part, is unpublished even if anybody has bothered to measure it, I can't be sure, but my guess is that the highest energy nuclear transitions that would generate useful output would be in the 40-50keV range, and I may well be optimistic on that.

The author of "An Introduction to Modern Starship Armor Design" speculates that the RMN's Mk73 laser assembly generates radiation at ~125keV. This is some 20 times the energy of the tested device, and 2-3x my guesstimate, and it's the energy range Honorverse warships are designed to withstand. Lower-energy weapons aren't going to serve much purpose. Higher-energy weapons certainly would, but the referenced article states that there is no reason to believe that stimulated-emission grasers are or ever will be physically possible. [And repeatedly makes the point that most '-asers' in the Honorverse _don't_ use stimulated emission physics]

captinjoehenry wrote:Here is a section from Atomic Rockets about bomb pumped gamma ray lasers:



So bomb pumped gamma ray lasers are definitely a possibility even with a warhead yield as low as one kiloton. So I would assume that the much higher yield available to Honorverse would allow for some seriously impressive bomb pumped gamma ray lasers that could represent a major increase in the destructive potential of every missile system currently employed.

kzt wrote:

Louis R wrote:The author of "An Introduction to Modern Starship Armor Design" speculates that the RMN's Mk73 laser assembly generates radiation at ~125keV. This is some 20 times the energy of the tested device, and 2-3x my guesstimate, and it's the energy range Honorverse warships are designed to withstand. Lower-energy weapons aren't going to serve much purpose. Higher-energy weapons certainly would, but the referenced article states that there is no reason to believe that stimulated-emission grasers are or ever will be physically possible. [And repeatedly makes the point that most '-asers' in the Honorverse _don't_ use stimulated emission physics]

That is Andrew, the same one cited above in the thread.

His employer doesn't mind his SF hobby, but apparently doesn't want his name attributed to it formally.