A 1 Megajoule gamma ray laser would be required to initiate DT fusion burn.
The trouble is that this means assembling a dense cloud of positronium in a quantum state known as a Bose-Einstein condensate (BEC). How to do this without the positronium annihilating in the process was unclear.
Now a team led by Christoph Keitel of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, suggests that ordinary lasers could be used to slow the annihilation by 200 times. The trick is to tune the lasers to exactly the energy needed to boost the positronium into a higher energy state, in which the electron and positron orbit farther from one another. That makes them much less likely to annihilate.
Arxiv - A method is proposed to manipulate the annihilation dynamics of a dense gas of positronium atoms employing superradiance and subradiance regimes of the cooperative spontaneous emission of the system. The annihilation dynamics is controlled by the gas density and by the intensity of the driving strong resonant laser field. In particular, the method allows to increase the annihilation lifetime of an ensemble of positronium atoms more than hundred times by trapping the atoms in the excited state via collective radiative effects in the resonant laser and cavity fields.
Ordinary atoms can only form a BEC when cooled gradually to within a fraction of a degree of absolute zero. By contrast, due to quantum effects, positronium will form a BEC at close to room temperature.
Gamma rays are quanta of light of very short wavelength and very high energy—on the order of a million times more energetic than visible light photons, and at least 10 times more energetic than X-ray photons.
Xray and gamma ray lasers are discussed here (14 pages) Xray lasers have been produced already.
Air Force study (University California at Riverside) Positronium Annihilation Gamma Ray Laser (42 pages)
The idea being explored is to use an annihilation gamma-ray laser to enable ignition of a fusion burn for actinide-free production of energy.
A possible solution would be to use a gamma ray laser to initiate burn in fuel that has been compressed via shock compression. The advantage over the use of x-ray lasers is that the superior penetrability of annihilation gammas would allow penetration of higher atomic number materials used for compression.
A prerequisite for the gamma laser is generating and controlling a large number of positrons at high density. We have made progress in this area by improving our positron beam to produce dense pulses of positrons such that we were able to produce the first dipositronium molecules.
The PI proposed reviving the impact fusion concept by using an annihilation gamma ray laser to ignite a fusion reaction in a compressed DT target, as indicated in Figure 1. The cubic inch of compressed DT shown in Figure 1 would burn in about 1 ns to yield 1,000 GJ of energy, ten times the current engineering break-even value. The process can be repeated every 1000 sec to yield a 1 GW reactor as indicated in Figure 2 or a single DT detonation could be used to set off a 1 megaton blast using a cubic meter of liquid D2.
Advantages of the proposed approach. The advantages of photons with energies of several hundred keV, loosely termed “gamma-rays”, over optical energy photons for inflicting damage on a distant target or for igniting fusion reactions are:
* Gamma rays penetrate a target to a thickness of roughly 10 g/cm2 and so impart up to two orders of magnitude greater impulse for a given energy compared to visible or infrared photons, thus leading to the fissure of large objects.
* Gamma rays are not significantly deflected by the atmosphere or its fluctuations, although absorption by the air limits the range at sea level to approximately 100 m if no means if employed for making a transparent gamma-ray channel through the atmosphere.
* The small size of the gamma-ray laser would be advantageous for steering and portability.
* A small annihilation gamma-ray laser would be fuelled by stored antimatter (positrons), which would leave no trace of radioactivity, although a GJ device might need to be based on energy derived from fusion.
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