A Brief Review of Bubble Fusion X. Fan1 1School of Physics, Peking University, Beijing, People's Republic of China (Dated: May 17, 2012) Bubble fusion is an alternative approach to controlled fusion, and it is a usage of sonoluminescence. This process occurs when an acoustically trapped and periodically driven gas bubble in liquid collapses so strong that the temperature inside the bubble reaches an extreme value. This article shows the significance of bubble fusion, then introduces the basic mechanism of sonoluminescence, and then reviews the research history, and finally gives an outlook on bubble fusion. INTRODUCTION Bubble fusion is one of the possible approaches to controlled fusion. Bubble fusion, also known as sono- fusion, officially called "acoustic inertial confinement fu- FIG. 1: Left to right: apparition of bubble; slow expansion; sion (AICF)", is the fusion reaction possibly occurs in a quick and sudden contraction; possible fusion event. high-pressure and high-temperature process called sono- luminescence. The inertia of a collapsing bubble wall will confine the energy in a small space, and leads to an ex- of ultrasonic wave or sound wave with enough intensity treme rise in both temperature and pressure, which can induces a bubble within a liquid to collapse quickly. We lead to light that can be watched with naked eyes, even can even see the flash of light with our naked eyes. possibly reach the requirement of the fusion reaction. The flash of light lasts very short, about 100ps long. Sonoluminescence is mainly studied during the 1990s, The bubbles are quite small when emit the light, about while the application to bubble fusion is mainly studied 1mm in diameter. The temperature inside the bubble during the last 10 years. changes with the experimental conditions including the composition of the liquid and gas. The estimate of the maximum temperature at the bubble collapse has been THE SIGNIFICANCE OF BUBBLE FUSION revised downward during a decade of research from ear- ly estimates of 108 K to the more modest present-day estimates which cluster around104K. As everyone knows, nowadays, the lack of energy is In nuclear research, there is a condition needed for fu- perhaps the biggest problem for human being to develop sion to occur, which is called Lawson criterion. The three our civilization, especially after when the fossil fuels are main perimeters are temperature, density, and the con- used up within only several hundreds of years. Controlled finement time. Lawson criterion says that the triple prod- fusion is the most feasible way to supply enough energy uct of these three perimeters has the minimum required for human beings, which is universally acknowledged. value. The high temperature that sonoluminescence can There are many approaches to this goal. The t- reach make some scientists believe that this is a possible wo main approaches are magnet confinement (Tokamak) approach to controlled fusion. and laser containment. However, all approaches to con- trolled fusion encountered huge difficulties and chal- lenges. The timeline of the worldwide largest Tokamak MECHANISM OF SONOLUMINESCENCE device ITER (International Thermonuclear Experimen- tal Reactor) says that it will start the deuterium-tritium The basic dynamics of the motion of the bubble can operation in 2026, which is a long time from this momen- be described by the Rayleigh-Plesset equation as an ap- t, and we are completely not sure whether it will succeed. proximation: The laser containment is even worse. Because of this sit- uation, alternative approaches to the controlled fusion, 3 1 R_ 2γ such as bubble fusion, also deserve a careful treatment. RR¨ + R_ 2 = (p − P − P (t) − 4µ − ) 2 ρ g 0 R R This equation describes the motion of the radius of the THE PROPERTIES OF SONOLUMINESCENCE bubble R as a function of time t. The Rayleigh-Plesset equation can give us a good estimation of the motion Sonoluminescence is the phenomenon that the emission until the final stage of the collapse. In order to study of short bursts of light from imploding bubbles in a liquid the details at the final stage of the collapse, we need to when excited by sound. It can be observed when a beam analysis the process more deeply. 2 The exact mechanism of the phenomenon of sonolu- SUMMERY AND OUTLOOK minescence is not fully understood. A major difficul- ty is that the direct measurement of the state of the matter inside the bubble is extremely hard to oper- ate. Theories include: hotspot, bremsstrahlung radia- Bubble fusion is an alternative approach to controlled tion, collision-induced radiation and corona discharges, fusion developed in recent several years, and it now re- nonclassical light, proton tunneling, electrodynamic jets mains immature. The mechanism cannot be fully under- and fractoluminescent jets (now largely discredited due stood by theories developed by now, and further theories to contrary experimental evidence). need to be studied. The experiment shows clues to fu- sion reaction, however the contradictory results tell us that we should find out what is the exact environment RESEARCH HISTORY ON BUBBLE FUSION that can trigger a fusion reaction. The earliest documented reference to bubble fusion is Professor Yian Lei at Peking University point out that, from US patent, by Hugh Flynn in 1978. the former equipment can only generate bubbles whose In 2002, Rusi P. Taleyarkhan and his colleagues at the size cannot be controlled exactly. He developed a new Oak Ridge National Laboratory (ORNL) first observed device that can generate bigger and size-controlled bub- evidence that can prove the existence of bubble fusion. bles, and hoped to get higher temperature and pressure. He used deuterated acetone(C3D6O) in the experiment, The experiment is still in process. and he observed consistent tritium and neutron output If bubble fusion is understood more clearly and proved from the device, which show that fusion reaction occurred to be a feasible approach to controlled fusion in the fu- inside the bubble. It is a really startling result, however, ture, then we can step forward to the final goal: to make this result remains controversial. it to be the ultimate energy source for human beings for- Then two independent researchers D.Shapira and ever. M.J.Saltmarsh were asked to repeat the experiment with more sophisticated equipment, while they reported that the neutron release was consistent with random coinci- dence. In 2005, two leading sonoluminescence researchers Seth Putterman and Kenneth S.Suslick repeated Tale- [1] R. P. Taleyarkhan, \Evidence for Nuclear Emissions Dur- yarkhan's experiment with similar parameters, however ing Acoustic Cavitation", Science 73, 1868(2002) they found no evidence of a fusion reaction. [2] Michael P. Brenner, \Single-bubble sonoluminescence", On the other hand, some following experiments con- Reviews of Modern Physics 74, 425(2002) [3] Sascha Hilgenfeldt, \A Simple explanation of light emis- firmed the previous result by Taleyarkhan, one of them is sion in sonoluminescence", Nature 398, 402(1999) by two of Taleyarkhan's students at Purdue University in [4] Seth J. Putterman, \Sonoluminescence: Sound into 2005. In 2006, researchers from Rensselaer Polytechnic Light", Scientific American, 32(1995) Institute published a paper in Physical Review Letters [5] http://en.wikipedia.org/wiki/Bubble fusion reports statistically significant evidence of fusion. [6] http://en.wikipedia.org/wiki/Sonoluminescence.