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May 03, 2010

The Path to Bose Einstein Condensate Positronium and Gamma Ray Lasers

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Here is a powerpoint presentation on the from 2009 on the work to create a Bose-Einstein condensate of Positronium (24 pages). This work recently made progress as researchers isolated a collection of “pure” or spin polarized positronium atoms for the first time. Positronium is a short-lived system in which an electron and its anti-particle are bound together. [wikipedia]










Steps Listed from 2009
1: Improve Ps-Ps scattering measurement 
   More signal
   Measure thermalization rate
2: Ps2 spectroscopy in vacuum 
   Unambiguous detection of the molecule
   Obtain binding energy from thermodynamics
   Measure excited state energy intervals/lifetimes
3: Precision spectroscopy of Ps 
   2-photon Doppler free measurement of 1S-2S interval
   Measurement limited by lasers not counting statistics
4: Production of Ps Bose-Einstein condensate 
   New Ps conversion materials
   Laser cooling of Ps
   Study Ps BEC phase diagram
   Create Ps “atom laser”

Gamma Ray Lasers

The US Air Force Research Lab has funded the University of California at Riverside (who made the positronium progress) to develop a gamma ray laser (june 2009, 42 pages)
The goal of the work reported here is the perfection of a powerful laser based on the coherent annihilation of a Bose-Einstein condensate of positronium, the hydrogen-like atoms formed from bound electron-positron pairs. We have made progress in several areas that takes us closer to our goal. Specifically we report (I) advances in positron storage and manipulation techniques; (II) Development of laser systems for cooling positronium atoms and measuring their velocity distribution; (III) A theoretical estimate of the ignition threshold for DT fuel heated by a burst from an annihilation gamma ray laser; and (IV) A new concept for more rapid laser cooling of light atoms including positronium.

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 practical fusion energy source is the only way to energy sufficiency for the United States of America that will eliminate the strangle-hold of foreign oil. The three approaches presently on the table are close to scientific break even, but are far from being practical. Plasma fusion occurs at such low densities that the experimental reactor has grown too large to be competitive. Laser fusion obtains very high fuel densities but requires the use of laser powers that do not seem to be scalable. The impact fusion concept proposes to create high densities with high speed projectiles, but so far the densities that can be achieved are not sufficient to initiate burn.

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 positrons for a laser would have to be in the form of positronium that has formed a Bose-Einstein condensate. Achieving this state will require improved laser diagnostics and we report here the development of cooling and measuring laser systems that will be needed for this task.

To have an idea whether the proposed laser ignited fusion might be practical, we have made a preliminary estimate of the threshold conditions for initiating ignition of a DT fusion burn. At ten times liquid density the threshold would occur for 0.5 MJ energy deposited at the center of a spherical fuel pellet, corresponding to the energy that would be obtained from the annihilation of about 10^19 positrons. While this is about one billion times as many as the record number of stored positrons, there is as yet no reason why it should not be possible to develop the technology to this point.

When such large numbers of positronium atoms are used, the critical temperature for Bose-Einstein condensation will be higher than room temperature. However, at the early stages where the underlying physics is being established, we will probably be necessary to use laser cooling to achieve a positronium Bose-Einstein condensate. Since positronium only lives for a few hundred nsec while it is being cooled, it is essential that the cooling be as efficient as possible. With this in mind, we have invented and report here a new method for cooling positronium that should allow us to cool from a four times higher starting temperature.
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The specific goals are:
* Form a Bose-Einstein condensate of spin-polarized triplet positronium atoms, which is a prerequisite for making an annihilation gamma ray laser, and observe its properties.
* Develop the necessary antimatter technology components including a positron storage device for storing and delivering 10^13 positrons in a single 100 ns burst.
* Observe the stimulated emission of annihilation gamma rays which is the precursor to lasing.
* Make a positronium annihilation gamma ray laser delivering 1 Joule gamma ray pulses.
* Increase the positron storage to 10^16 positrons and the laser energy to 1 kJ.
* Assess the prospects for scaling the technology to a system capable of delivering 1 MJ annihilation gamma ray laser pulses from a few nano-grams of positronium (10^19 atoms).

Advantages of the proposed Gamma Ray Laser 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.

Plan of work.

The first annihilation laser will be made in the following steps:
1.Attain Bose-Einstein condensed (BEC) positronium.
2. Make a source capable of delivering 10^12 slow positron per second on a 1 mm target.
3.Develop a multiple trap for storing and releasing 10^13 positrons.
4. Observe stimulated annihilation.
5. Make 1J annihilation gamma ray laser pulses. [this one joule laser has to be scaled up about one million times to enable the nuclear fusion system described above]

A three year project to get through step 1 would proceed via the following tasks.
A. Year 1: Make a system for producing brightness enhanced 10 ns pulses of 10^7 5 keV positrons in a 10 micrometer diameter spot.
B. Year 1: Develop a method for making cavity structures in porous silica for containing BEC positronium.
C. Year 2: Make a BEC positronium target chamber with 4K cooling and optical access.
D. Year 3: Develop a laser system for detecting the BEC state via the disappearance of Doppler broadening.
E. Year 3: Characterize the positronium BEC by measuring the condensate fraction as a function of time, temperature and density.

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