When I worked with the laser at my last job, one thing that was a problem was trying to get an idea of the beam quality- how uniform the intensity was across the beam- I finally developed a quick way to get a high contrast image of the beams. I took a block of copper and painted one face with carbon. I placed it in the beam path so the beams would hit the black face. I then turned the laser on at full power- three 500 Watt beams hit the block- the carbon coating explosively vaporized where the beams hit. I turned the laser off after a few seconds and removed the block. Where the beams hit there was bare copper, the visible oxidation of the copper was very sensitive to intensity- it was a very quick and effective way of visualizing the beam.

Each of the 500 Watt beams was 1 cm in diameter, but they did not vaporize the copper. You need a lot of concentrated power to do that...