Consider laser cutting. Will 1 Joule of energy penetrate deeper into material if it is delivered at 100 Watts rather than 50 Watts?

Consider alternative: suppose you can to cut up to 6 mm thick PMMA acrylic plate with continuous-wave laser at power of 50 W and linear energy density of 5 J/mm. Would the same linear energy density but at higher power of 150 W allow to cut thicker acrylic plates?

  • $\begingroup$ This depends on the timescale you're considering: shrinking from ten seconds to a second won't change much, but if you shrink the pulse length enough (well under a picosecond) while keeping the pulse energy constant, then yes, the dynamics can change. Is that the sort of scale you were envisioning? $\endgroup$ – Emilio Pisanty Apr 5 '17 at 22:16
  • $\begingroup$ No, I am only considering the case of going from 1 W to 100 W, while keeping energy delivered to the surface at some constant value. $\endgroup$ – Abyr Valg Apr 5 '17 at 23:15

Short answer - sure. Take the limiting case of a laser pointer. 5 mW for 200 seconds delivers 1 Joule - and it won't cut at all. As the instantaneous power goes up and the pulse duration shortens, you increase the probability of locally heating your sample before the heat can be conducted away.

Above a certain power limit, the opposite may happen. The power becomes so great that you generate a plasma above the sample, and that plasma might shield your sample from some of the subsequent laser power. But that is only going to happen in really extreme cases.

You can find more information at this link to a Q&A on Researchgate which specifically addresses the ablation threshold.

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    $\begingroup$ I fixed the title and encourage you to do so in the future when you encounter unclear titles. $\endgroup$ – DanielSank Apr 5 '17 at 21:40
  • $\begingroup$ @DanielSank - you are right, I should have just done that. Thanks for the nudge. $\endgroup$ – Floris Apr 6 '17 at 13:53

Due to Conservation of Energy, the amount of energy is conserved with time at any classical (non-quantum) isolated "Universe" during any finite period of time regardless of the rate at which that energy is produced or consumed. In other words, it's important to know the period of time that your 100 Watts power is in effect. Since it won't be available longer than 0.01 second, you are limited to its effects during this period of time. So your question is whether the "penetration depth" (assuming that the word is understood as the physical work being done on that piece of plate in the form of breaking the bonds between molecules rather than reaching the goal of cutting it through) of 1 joule of energy during a period of 0.01 second can be greater than that of a period of 0.02 second. The answer would be No. With the modifications made to the question, we need engineers as well as physicists to answer it correctly unless you specify the question is more of physics or more of engineering.


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