My thoughts on a reddit discussion
https://www.reddit.com/r/AskPhysics/comments/1lwxxc3/comment/n2jx8gp/?utm_source=share&utm_medium=mweb3x&utm_name=mweb3xcss&utm_term=1&utm_content=share_button
The collapsing of a bubble with sound wave leads to the emission of light in a phenomenon known as sonoluminescnce.
The bubble collapse is rapid and the gas inside the core doesn't have time to exchange heat with the surroundings as it's compressed rapidly leading to what is known as adiabatic compression.
This compression heats up the gas to very high temp. The exact temperatures are inferred from the spectrum of emission which is thought to be a blackbody. But some sophisticated models have also been developed that put the temp in the range 5000k-20000k some even higher.
There's also debate on whether the bubble emission spectrum is truly a blackbody or is it line emission or bremsstrahlung? Personally I think its a mix of all three. The pressures created are intense (Giga pascal range) and they can result in an optically thick plasma. But line emission exists even in stars that definitely are blackbodies so it can't be ruled out in a bubble.
Can it lead to fusion?
Indeed it has been tried and some researchers have reported that fusion does occur inside collapsing bubbles.
https://www.researchgate.net/publication/256548322_Cavitation-Induced_Fusion
https://en.m.wikipedia.org/wiki/Bubble_fusion
https://www.longdom.org/open-access/estimation-of-bubble-fusion-requirements-during-highpressure-high-temperature-cavitation-17489.html
But results have not been reproducible and there has been much debate around their interpretation
http://large.stanford.edu/courses/2015/ph241/chapa2/
Even if it is possible there are a couple of hurdles that would limit its application
1. How do you extract energy out of it? It's surrounded in a water bath that would evaporate leading to cessation of bubble formation.
2. How would you control cavitation process itself? A single bubble is not going to produce much energy. You'll need a lot of them. When there are too many bubbles they tend to coalesce and produce a foam. Multiple bubbles don't behave in the same way as a single bubble. Their cavitation dynamics are different. Core temps are typically lower peaking at 5000k making them unsuitable for fusion.
https://pubs.acs.org/doi/abs/10.1021/jp991524s
However this is a very exciting field of study.
REFERENCES
Radiation Mechanism for a Single Bubble Sonoluminescence
https://www.researchgate.net/publication/239004778_Radiation_Mechanism_for_a_Single_Bubble_Sonoluminescence
SONOLUMINESCENCE: NATURE’S SMALLEST BLACKBODY
https://arxiv.org/pdf/physics/0009057
Bremsstrahlung in single bubble sonoluminescence
https://iopscience.iop.org/article/10.1088/0022-3727/38/14/029/pdf
Molecular emission from
single-bubble sonoluminescence
https://suslick.illinois.edu/documents/nature00877.pdf
Single-Bubble Sonoluminescence
https://www.researchgate.net/publication/2843747_Single-Bubble_Sonoluminescence