More specifically, let's assume we're firing a laser 50 microns in diameter at a block of steel 10 cubic meters in size and this laser delivers 1 megajoule of energy during the entire length of time it's active. How different is the observable effect on the block if this laser pulse takes 1 second to deliver that megajoule (and then shuts off) vs. doing so in 1 millisecond vs. 1 nanosecond? Does this affect how large a section of the block melts or vaporizes, or how much superheated plasma might be sent flying around the room where this was happening, despite the total amount of energy being applied to the block being the same? (Or how much air ionizes in the laser's path, for that matter)

From my limited understanding of the problem, the vaporization of material impacted by the photons will create plasma that then shields the material immediately behind it from the laser's effects to some degree (by scattering the photons?), and I assume the time interval affects how much time this plasma has to scatter and disperse, but presumably if the plasma hasn't had time to disperse, it simply grows hotter and hotter itself as it absorbs the laser's energy and shouldn't this make it scatter faster anyway? How much would this superheating counteract the smaller window of time for molecules to move (assuming I'm not totally off-base with my understanding here)

The plasma would also transfer heat to the surrounding steel via convection too, yes? Is heat transferred this way rate-limited in some fashion that would mean that slower laser pulses tend to diffuse heat over a larger area of steel than faster pulses? Or is even 1 second far too little time to transfer enough heat to melt an appreciable section of the block not in close contact with the laser itself?

Are there any other major factors here I have overlooked?

My background is not in physics (I'm doing research for a novel, actually), so I apologize for any crudeness or misunderstandings in the above text, and I don't need exact calculations, but even a general understanding of how rate of energy application affects heat diffusion and material ablation would be very helpful.

  • $\begingroup$ You have a bunch of the different parameters. It is a very complex problem, and is material and laser wavelength dependent. $\endgroup$
    – Jon Custer
    Nov 21 '16 at 14:55

It's hard to give a complete answer to your question - it would involve a survey of an entire field. Your initial analysis is reasonable, but "the devil is in the details", and there are lots... You might want to look at this presentation which describes four regimes: fs, ps, ns, and µs. I reproduce one diagram from the presentation:

enter image description here

Usually pulsed ablation (which seems to be what you are describing) is not use for mass removal of material - it is for probing the surface, and sometimes for creating subsurface defects in (semi)transparent materials.

Heating of the bulk happens on the microsecond scale, where conduction of heat can lead to liquefaction of the substrate. Laser machining is used to remove significant amounts of material - usually for precise cutting of intricate shapes in materials that are not easy machined with conventional methods. It is described on Wikipedia, from which I excerpt the following:

There are many different methods in cutting using lasers, with different types used to cut different material. Some of the methods are vaporization, melt and blow, melt blow and burn, thermal stress cracking, scribing, cold cutting and burning stabilized laser cutting.

Vaporization cutting
In vaporization cutting the focused beam heats the surface of the material to boiling point and generates a keyhole. The keyhole leads to a sudden increase in absorptivity quickly deepening the hole. As the hole deepens and the material boils, vapor generated erodes the molten walls blowing ejecta out and further enlarging the hole. Non melting material such as wood, carbon and thermoset plastics are usually cut by this method.

It goes on to describe different methods used to control how much material is removed, and how to prevent it from sticking to the surface again - using double pulsed lasers, blowing a gas across the surface, cracking the material to cause chunks to fall off, etc.

Luckily for you, when you write a novel you only have to be "approximately right"...


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