Bubble fusion, also known as sonofusion, is the non-technical name for a nuclear fusion reaction hypothesized to occur during a high-pressure version of sonoluminescence, an extreme form of acoustic cavitation. Officially, this reaction is termed acoustic inertial confinement fusion (AICF) (see ICF) since the inertia of the collapsing bubble wall confines the energy, causing an extreme rise in temperature. The high temperatures that sonoluminescence can produce raise the possibility that it might be a means to achieve thermonuclear fusion.[1]
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Sono-Fusion covers a wide range of hypotheses and experimental data conducive of the idea that ultrasonic cavitation in liquids (from water to liquid metals) may interfere with nuclear chemistry due to not yet well understood exotic processes happening at high temperatures and pressure in and around collapsing bubbles inside the liquid. The focus of sono-fusion has been mainly about reaching the fusion of deuterium nuclei through ultrasonic cavitation.
US patent 4,333,796,[2] filed by Hugh Flynn in 1978, appears to be the earliest documented reference to a sonofusion-type reaction.
In the March 8, 2002 issue of the peer-reviewed journal Science, Rusi P. Taleyarkhan and colleagues at the Oak Ridge National Laboratory (ORNL) reported that acoustic cavitation experiments conducted with deuterated acetone (C3D6O) showed measurements of tritium and neutron output that were consistent with the occurrence of fusion. The neutron emission was also reported to be coincident with the sonoluminescence pulse, a key indicator that its source was fusion caused by the sonoluminescence.[3]
Shock wave simulations seem to indicate that the temperatures inside the collapsing bubbles may reach up to 10 megakelvins, i.e. as hot as the center of the Sun. A 2008 study has provided data demonstrating bubble temperatures exceeding 100,000K and a pressure dependence that indicated temperatures above 106K could be expected under sonofusion conditions.[4] Although the apparatus operates in a room temperature environment, this is not cold fusion (as commonly termed in the popular press) because the 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 ("seed") the tiny bubbles, whereas most previous experiments started 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 reported 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 claims his interest in bubble fusion began following a post-dinner chat with a friend, Dr. Mark Embrechts, in 1995.
The results were so startling that the Oak Ridge National Laboratory asked two independent researchers, D. Shapira and M. J. Saltmarsh, to repeat the experiment using more sophisticated neutron detection equipment. They reported that the neutron release was consistent with random coincidence.[5][6] A rebuttal by Taleyarkhan and the other authors of the original report said 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. According to Taleyarkhan et al., when properly considering those differences, the results are consistent with fusion.
As early as 2002, while experimental work was still in progress, Galonsky, in a letter to the journal Science[7] expressed doubts about the claim made by the Taleyarkhan team. In Galonsky's opinion, the observed neutrons were too high in energy to be from a deuterium-deuterium (d-d) fusion reaction. In their response (published on the same page), the Taleyarkhan team provided detailed convincing arguments and concluded that the energy was "reasonably close" to that which was expected.
In February 2005 the documentary series Horizon commissioned the collaboration of two leading sonoluminescence researchers, Seth Putterman[8] and Kenneth S. Suslick, 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.[9][10]
In 2011, after many years of further research in the field of sonoluminescence, Seth Puttermann now writes:[11] "At acoustic frequencies of 1MHz, the bubble is smaller and the observed spectrum is now best fit by the transparent plasma model, and a temperature of about one million degrees." And he concludes: "The densities and temperatures which can be accessed with modest sound fields are extraordinary. One is tempted to wonder if bubble acoustics can be used to reach conditions for nuclear fusion…. Some optimism that cavitation will provide a route to fusion is provided by the observation that SL (sonoluminescence) enjoys a big parameter space."
In 2004, new reports of bubble fusion were published by the Taleyarkhan group, saying that the results of previous experiments have been replicated under more stringent experimental conditions.[12][13] 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 following bubble nucleation, whereas this report showed neutron emission many acoustic cycles later. The data, however, was less than stringent insofar as too large a window of measurement was used to determine a coincidence between neutron emission and sonoluminescent light emission. Furthermore, the energy of the detected neutrons was not consistent with neutrons produced from a fusion reaction.
In July 2005, two of Taleyarkhan's students at Purdue University published evidence confirming the previous result. They used the same acoustic chamber, the same deuterated acetone fluid and a similar bubble nucleation system. In this report, no neutron-sonoluminescence coincidence was attempted. Once again, the neutron energies measured were not consistent with those of neutrons produced by a d-d fusion reaction.[14][15] Charges of misconduct were raised, and Purdue University opened an investigation. It concluded in 2008 that Taleyarkhan's name should have appeared in the author list because of his deep involvement in many steps of the research, that he added one author that had not really participated in the paper just to overcome the criticism of one reviewer, and that this was part of an attempt of "an effort to falsify the scientific record by assertion of independent confirmation". The investigation did not address the validity of the experimental results.[16]
In January 2006, a paper published in the journal Physical Review Letters by researchers from Rensselaer Polytechnic Institute reports statistically significant evidence of fusion:[17][18][19] The initial news report, however, shows that the reaction does not always work correctly and it is not known what parameters change to cause the reaction to function properly or not function at all.
In 2005, three Russian scientists Smorodov E., Galiakhmetov R., Ilgamov M. conducted the experiments on shock compression of deuterium bubbles in glycerine. The results were published in monograph "Physics and chemistry of cavitation" (2008), where they demonstrated the existence of the neutron yield, which indicates the initiation of nuclear-fusion reaction in deuterium plasma. This enables the creation of fundamentally new types of reactors for nuclear fusion and the creation of new types of neutron sources.
In November 2006, in the midst of charges leveled at Taleyarkhan as regards his research standards, Dr. Edward R. Forringer and undergraduates David Robbins and Jonathan Martin of LeTourneau University presented two papers at the American Nuclear Society Winter Meeting that reported replication of neutron emission during a visit to the meta-stable fluids research lab at Purdue University. Their experimental setup was similar to the preceding experiments in that it used a mixture of deuterated acetone, deuterated benzene, tetrachloroethylene and uranyl nitrate. Notably, however, it operated without an external neutron source and used two types of neutron detectors. They claimed a liquid scintillation detector measured neutron levels at 8 standard deviations above the background level, while plastic detectors measured levels at 3.8 standard deviations above the background. When the same experiment was performed with non-deuterated control liquid, the measurements were within one standard deviation of background, indicating that the neutron production had only occurred during cavitation of the deuterated liquid.[20][21][22]
There is presently a growing body of evidences indicating that ultrasonic cavitation interferes with nuclear processes.[23][24]
Reports as spectacular as the above arouse a lot of doubt. In March 2006, Nature published a "special report" "silencing the hype" that called into question the validity of the results of the Purdue experiments.[25] The report quotes Brian Naranjo of the University of California, Los Angeles to the effect that spectrum measured in these sonofusion experiments is consistent with radioactive decay of the lab equipment and hence does not reliably demonstrate the presence of nuclear reactions.[26]
The response of Taleyarkhan et al., published in Physical Review Letters, attempts to refute Naranjo's hypothesis as to the cause of the neutrons detected.[27]
Doubts at Purdue University's Nuclear Engineering faculty as to whether the positive results reported from sonofusion experiments conducted there were truthful prompted the university to initiate a review of the research, conducted by Purdue's Office of the Vice President for Research. In a March 9, 2006 article entitled "Evidence for bubble fusion called into question", Nature interviewed several of Taleyarkhan's colleagues who suspected something was amiss.[28]
On February 7, 2007, the Purdue University administration determined that "the evidence does not support the allegations of research misconduct and that no further investigation of the allegations is warranted". Their report also stated that "vigorous, open debate of the scientific merits of this new technology is the most appropriate focus going forward."[29][30] In order to verify that the investigation was properly conducted, House Representative Brad Miller requested full copies of its documents and reports by March 30, 2007.[31]
In June 2008, a multi-institutional team including Taleyarkhan published a paper in Nuclear Engineering and Design to "clear up misconceptions generated by a webposting of UCLA which served as the basis for the Nature article of March 2006", according to a press release.[32]
On July 18, 2008, Purdue University announced that a committee with members from five institutions has investigated 12 allegations of research misconduct by Rusi Taleyarkhan. It concluded that two allegations were founded—that Taleyarkhan had claimed independent confirmation of his work when in reality the apparent confirmations were done by Taleyarkhan's former students and was not as "independent" as Taleyarkhan implied, and that Taleyarkhan had included an additional colleague's name on one of his papers who had not actually been involved in the research ("the sole apparent motivation for the addition of Mr. Butt was a desire to overcome a reviewer's criticism," the report concluded).[16][33] Purdue University had previously said, in a press release in July 2005, that Butt's replication was independent from Taleyarkhan.[34]
Taleyarkhan appealed the conclusions in the report, but this was rejected. He said the two allegations of misconduct were trivial administrative issues and had nothing to do with the discovery of bubble nuclear fusion or the underlying science, and that "all allegations of fraud and fabrication have been dismissed as invalid and without merit — thereby supporting the underlying science and experimental data as being on solid ground".[35] A researcher questioned by the LA Times said that the report had not clarified whether bubble fusion was real or not, but that the low quality of the papers and the doubts cast by the report had made him lose all the credibility that he formerly had among the scientific community.[36]
On August 27, 2008 he was stripped of his named Arden Bement Jr. Professorship, and forbidden to be a thesis advisor for graduate students for at least the next 3 years.[35][37]
It should be noted however that the Kimberly-Clark corporation who "has a long-standing relationship with Purdue University,",[38] and who had accumulated a wide range of expertise and know-how in the use of ultrasonic cavitation in its industrial processes for the treatment of emulsions and stabilization of chemical compounds or monitoring of chemical reactions filed a patent application on the 5th of December 2007 for An ULTRASONIC TREATMENT CHAMBER FOR INITIATING THERMONUCLEAR FUSION.[39] This patent application US 2009/0147905 A1 was published on June 11, 2009. It quotes Taleyarkhan's research and is partly based on some of the ideas already developed in the 1978 patent from Hugh G. Flynn. This patent application, signed by a Kimberly-Clark research team of 8 members, covers a wide range of experimental devices for ultrasonic treatment of liquids of such various natures as organic solvents and liquid metals under a wide range of temperature, pressure, electric fields, etc.
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