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Bubble fusion

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Bubble fusion or sonofusion is the common name for a nuclear fusion reaction hypothesized to occur during sonoluminescence, an extreme form of acoustic cavitation; officially, this reaction is termed acoustic inertial confinement fusion (AICF) since the inertia of the collapsing bubble wall confines the energy causing a rise in temperature. The high temperatures producible through sonoluminescence raises the possibility that it might be a means to achieve thermonuclear fusion.

Original experiments

Rusi P. Taleyarkhan (ORNL) and colleagues reported in the March 8, 2002, issue of the peer-reviewed journal Science, that acoustic cavitation experiments conducted with deuterated acetone show measurements of tritium and neutron output that are consistent with fusion; in addition the neutron emission was claimed to be coincident with the sonoluminescence pulse. [1]

Shock wave simulations seem to indicate that the temperatures inside the collapsing bubbles may reach up to 10 megakelvins — as hot as the center of the sun. None of the above measurements have been confirmed by a group outside of Taleyarkhan's and are highly debated, recalling the 1989 cold fusion fiasco. [2] [3] [4] [5] However, New Energy Times has reported a replication by an unrelated group at the University of Texas.[6] Although the apparatus operates in a room temperature environment, this is not cold fusion (as it is commonly termed in the popular press), as the claimed nuclear reactions would be occurring at the very high temperatures in the core of the imploding bubbles.

The researchers used a pulse of neutrons in order to nucleate (i.e., "seed") the tiny bubbles, whereas most previous experiments start with small air bubbles already in the liquid. Using this new method, the team was able to produce stable bubbles that could expand to nearly a millimeter in radius before collapsing. In this way, the researchers stated, they were able to create the conditions necessary to produce very high pressures and temperatures. The sensitivity of the fusion rate to temperature, which is in turn a function of how small the bubbles get when they collapse, in combination with the likely sensitivity of the latter to fine experimental details, may account for the fact that some research workers have claimed to see an effect, while others have not.

Taleyarkhan et al. also prepared identical experiments in non-deuterated (normal) acetone and failed to observe neutron emission or tritium production. Taleyarkhan got the idea of bubble fusion from his friend Dr. Mark Embrechts after a friendly post-dinner chat in 1995.

Oak Ridge replication

These experiments were repeated at Oak Ridge National Laboratory by D. Shapira and M. J. Saltmarsh with more sophisticated neutron detection equipment and they reported that the neutron release was consistent with random coincidence. [7] A rebuttal by Taleyarkhan and the other authors of the original report claimed that the Shapira and Saltmarsh report failed to account for significant differences in experimental setup, including over an inch of shielding between the neutron detector and the sonoluminescing acetone. Taleyarkhan et al. report that when these differences are properly accounted for, the Shapira and Saltmarsh results are consistent with fusion.

In addition, Galonsky has shown that by Taleyarkhan's own detector calibration the observed neutrons are too high in energy to be from a d-d fusion reaction. In a rebuttal comment, Taleyarkhan says the energy is "reasonably close" to that which is expected. [8]

In February 2005, the BBC commissioned a collaboration between Seth Putterman and Ken Suslick (two leading sonoluminescence researchers) to reproduce Taleyarkhan's work. Using similar acoustic parameters, deuterated acetone, similar bubble nucleation, and a much more sophisticated neutron detection device, the researchers could find no evidence of a fusion reaction. This work was reviewed by a team of four scientists, including an expert in sonoluminescence and an expert in neutron detection, who also concluded that no evidence of fusion could be observed. [9]

Claims of replication in 2004, 2005

In 2004, new claims of bubble fusion were made by the Taleyarkhan group, claiming that the results of previous experiments have been replicated under more stringent experimental conditions. [10] [11] These results differed from the original results in that fusion was occurring for a much longer time frame than previously reported; the original report only showed neutron emission from the initial bubble collapse after the bubble nucleation whereas this report shows neutron emission many acoustic cycles later. The data however was lacking in that too large of a window was used for determination of a coincidence between the neutron emission and sonoluminescence light emission. Also, the energy of the detected neutrons were not consistent with neutrons produced from a fusion reaction.[citation needed]

In July 2005, two of Taleyarkhan's students at Purdue University published evidence confirming the previous result. [12] [13] They used the same acoustic chamber, the same deuterated acetone fluid and a similar bubble nucleation system. In this report, no neutron-sonoluminecence coincidence was attempted; also the neutron energy was again not consistent with a neutron produced from a d-d fusion reaction. [citation needed]

A recent report soon to be published in the journal Physical Review Letters claims further evidence of fusion. [14] [15] [16] [17] In this initial news report, however, it shows that the reaction does not always work correctly and it is unknown what parameters change to cause the reaction to function properly versus not function at all.

Doubts prompt investigation

A claim as spectacular as the present one naturally arouses a lot of doubt. This culminated in a "special report" published in March 2006 by Nature, that seriously calls into question the validity of the results of the Purdue experiments. [18] They quote Brian Naranjo of the University of California, Los Angeles with the claim that the measured spectrum is consistent with radioactive decay of the lab equipment and hence does not necessarily prove the presence of nuclear reactions.

Doubts about the truthfulness of claims of positive observations have arisen within the Nuclear Engineering faculty of Purdue University. Because of extremely serious concerns, Purdue has initiated a review of the research, to be conducted by Purdue's Office of the Vice President for Research. In a March 9 article headed "Evidence for bubble fusion called into question", Nature journal reported that it had interviewed several of Taleyarkhan's colleagues who suspect something is amiss. [19]

Methods of increasing fusion output

While many of the claims remain unverified at this time, many ideas exist to increase the rate of fusion. The experimental apparatus as conducted in experiments thus far produces energy about seven orders of magnitude lower than that which went into it. However, a number of factors suggest that this is unlikely to remain the case.

Acetone under the temperatures, pressures, and other initial conditions involved is unlikely an optimal solution; several orders of magnitude of efficiency improvement are likely by experimenting with different solutions and laboratory settings. Additionally, using a mix of deuterium and tritium will increase fusion yields by three orders of magnitude (as would simply running the apparatus for long enough, as D-D fusion breeds tritium).

An increase in reaction rate may scale up faster than linearly. As a bubble collapses, shocks bounce inward from the edges, encountering their own reflections and additively combining creating the great heat and pressure to the degree that sonoluminescence or sonofusion can occur. The neutrons from the fusion reaction seed new bubbles nearby, creating a bubble cluster containing over 1,000 cavitating centers which act more powerfully together than they would individually.

One of the most interesting propositions, however, is the potential for a new kind of fusion criticality in sonofusion. Given two acoustic anti-nodes (wherein one is at minimum stress while the other is at maximum stress), neutrons from one node will be released while the other is at maximum stress. Some neutrons will interact with their anti-node, creating a bubble cluster and amplifying the reaction. When it collapses in turn, some of its neutrons will do the same to the original node, leading to a self-sustained nuclear reaction. This possibility is yet to be validated, and still remains theoretical.Template:Citation needed, is this even true?

Sonofusion has some fundamental benefits compared to most other methods of fusion. Shock heating of the fuel leaves the electrons at almost the same velocity as the ions, and thus (due to their much lighter mass) at insignificant temperatures. As energetic electrons are one of the principal energy loss mechanisms in most fusion aparatuses (Bremsstrahlung radiation, recombination losses, line losses, etc), the sonofusion reaction doesn't lose energy as quickly as in such systems. Instead, it behaves largely as if only ions were being dealt with. At the same time, however, it has some fundamental limitations. The amount of dense, energetic area involved in sonofusion is typically tiny, limiting the amount of fusion reactions that can occur (currently about ten per bubble collapse). The biggest limitation, of course, is that the current methodologies used to observe sonoluminescing bubbles require more energy input than would be released form the fusion reaction (if it were to be acheived).

Skepticism at Purdue

An even more recent Nature article[20] reports that colleagues at Purdue are skeptical of Rusi Taleyarkhan's previous findings. While it remains theoretically possible, it would now appear that "bubble fusion" has never been directly observed in nature, and that those who had their doubts regarding the Purdue group were correct to doubt initial claims.

Nature's suggestion of misuse of funds ill-founded?

In July 2006, Nature publicised a claim of Seth Putterman, denied by Taleyarkhan, to the effect that DARPA funds were used to support an experiment reported in Physical Review Letters without the source being acknowledged. This may seem a matter of minor importance. Questions as to Nature's motives [21] have been raised by the fact that, in the article, a conspicuous display of Putterman's arguments headed 'where did the money go?' is immediately followed by a paragraph devoted to 'misuse of federal dollars'. Since Putterman does not himself consider funds were misused, it is unclear why such a paragraph should have been included if there were no intent at all to make the reader think this might have been the case.

Another problem with Nature's stance (the journal has stated "[we] believe that we have nothing to apologise for, and nothing to correct") is that the accounting details obtained by Putterman do not appear necessarily to support his conclusions. Despite the damaging effects that the publication of a flawed allegation is likely to have had, the journal is still at this time refusing to publish a clarification.[21]

Ethical dimension

The ethical aspect of Nature's coverage of bubble fusion is complementary to that addressed in the above, and has equally been a cause of concern. Taking the Code of Ethics of the Society of Professional Journalists[22] as the norm, one may note the following in relation to the article concerned:

  1. According to the code, journalists should 'test the accuracy of information from all sources and exercise care to avoid inadvertent error'. As noted above, the accounting details listed in the article do not appear to support Putterman's possibly damaging conclusions: however, they are presented in the article in a way that suggests that they do.
  2. Journalists should also 'make certain that headlines ... do not misrepresent. They should not oversimplify or highlight incidents out of context.' Merely posing the question 'Where did the money go?', which appears very conspicuously in the article, raises in the reader's mind the idea of fraud, a suggestion that the article itself does not in any way support.

See also Science, Journalism and Nature.[23]

References

  1. ^ R. P. Taleyarkhan, C. D. West, J. S. Cho, R. T. Lahey, Jr. R. Nigmatulin, and R. C. Block, Evidence for Nuclear Emissions During Acoustic Cavitation, Science 295, 1868 (2002). (available online)
  2. ^ D. Shapira, M. J. Saltmarsh. Comments on Reported Nuclear Emissions during Acoustic Cavitation, 1 March 2002. (available online (PDF))
  3. ^ R. P. Taleyarkhan, R. C. Block, C. D. West and R. T. Lahey Jr., "Comments on the Shapira and Saltmarsh Report" 2 March 2002. (available online (PDF))
  4. ^ F. Becchetti, Evidence for Nuclear Reactions in Imploding Bubbles, Science 295, 1850 (2002).
  5. ^ D. Kennedy, To Publish or Not to Publish, Science 295, 1793 (2002).
  6. ^ New Energy Times Newsletter 18 (Bubble Fusion Discoverer Taleyarkhan Strikes Back)
  7. ^ D. Shapira, M. J. Saltmarsh. Nuclear Fusion in Collapsing Bubbles — Is it There? An Attempt to Repeat the Observation of Nuclear Emissions from Sonoluminescence, Phys. Rev. Lett. 89, 104302 (2002).
  8. ^ A. Galonsky. Tabletop Fusion Revisited Science 297, 1645 (2002).
  9. ^ C. Murry. An Experiment to Save the World BBC Horizons, aired Feb 2005. (BBC News story) (Transcript of the program)
  10. ^ [1]
  11. ^ R. P. Taleyarkhan et al., Additional Evidence of Nuclear Emissions During Acoustic Cavitation, Physical Review E 69, 036109, 22 March 2004.(abstract available online)
  12. ^ [2]
  13. ^ Y. Xu, A. Butt Confirmatory Experiments for Nuclear Emissions During Acoustic Cavitation Nuc. Eng. Des. 235, 1317 (2005).
  14. ^ [3]
  15. ^ R. P. Taleyarkhan, C. D. West, R. T. Lahey, Jr., R. I. Nigmatulin, R. C. Block, and Y. Xu. Nuclear Emissions During Self-Nucleated Acoustic Cavitation, Phys. Rev. Lett. 96, 034301 (2006). (abstract available online)
  16. ^ January 31, 2006, Sciencedaily: Using Sound Waves To Induce Nuclear Fusion With No External Neutron Source Quote: "...The experiment was specifically designed to address a fundamental research question, not to make a device that would be capable of producing energy, Block says...To verify the presence of fusion, the researchers used three independent neutron detectors and one gamma ray detector. All four detectors produced the same results: a statistically significant increase in the amount of nuclear emissions due to sonofusion when compared to background levels..."
  17. ^ (Received 19 September 2005; published 27 January, 2006), Physical Review Letters: Nuclear Emissions During Self-Nucleated Acoustic Cavitation Quote: "...Statistically significant nuclear emissions were observed for deuterated benzene and acetone mixtures but not for heavy water. The measured neutron energy was <=2.45 MeV, which is indicative of deuterium-deuterium (D-D) fusion. Neutron emission rates were in the range ~5×103 n/s to ~104 n/s and followed the inverse law dependence with distance..."
  18. ^ [4]
  19. ^ [5]
  20. ^ Nature. March, 2006. Bubble fusion: silencing the hype
  21. ^ a b Nature on the Attack. Includes links to correspondence and to Nature article.
  22. ^ [6] SPJ Ethics Code
  23. ^ Science, Journalism and Nature, New Energy Times article.

See also

  • Sonoluminescence — the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.
  • Cold Fusion — research continues into cold fusion into the 21st Century.
  • List of energy topics
  • "Chain Reaction" the fictionalized tale of a discovery and attempted cover-up of a practical fusion reactor.
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