Acoustic emission analysis method could help. Consult, please, Mr. A. P.  Kubyshking of Laser Institute in Shatura

I have always believed that this method is used for the investigation of solids, and will not help to measure the vapor pressure. Is not it ?

Well, "acoustic emission" is connected with solids, maybe I should use other expression - noise analysis?. As I know, Mr. Kubyshkin is skilled experimentator; he explored various acoustic effects connected with laser beam interaction with materials and he may asses if the idea of using plasma flow from keyhole accompanied with acoustic sizzling could give some information about pressure in the laser-made channel.

I'm not familiar with Kubyshkin, although once worked at the same institute. Through acquaintances I learned that Kubyshkin is not in affiliation with Laser Institute now and his traces are lost. But I'll try to find him. In any case many thanks for the advice.

If you only concern the average pressure inside the keyhole, from this point of view, I think you can try to do thrust measurement. the total thrust produced by the plasma jet is approximately equal to the pressure times keyhole cross section. the thrust measurement should be very careful because most thrust transfer is not suitable for steady force. And meanwhile, you'd better to record the hole image which can present data of active thrust surface.

I like this sentence. Many thanks. I think you're absolutely right, claiming that it is necessary to measure the thrust force of the steam jet. However, much will solve the experimental skills in this case. It should be noted that it is impossible to use the recoil pressure measurement, which is mainly determined by hydrodynamic thermocapillary disturbances, rather than by vapor pressure.