I'd put it an other way:
Scientists and engineers who have years of experience rarely say "it can't be done". They rather say "I don't see how to do it". Usually because they learned the hard way to be cautious with what humans achieve.
In my case as a young engineer, when R. Reagan started the Strategic Defense Initiative and its directed energy weapons, I answered "impossible because of basic physical limitations". Meanwhile such weapons are operational.
More recently, I understood from electromagnetism how the mind-reading machines are built, then the machines that inhibit briefly the brain's operation to let people fall and hurt themselves, and quite recently, the machines for artificial tinnitus. All this is well explainable by hard standard science despite it looks far-fetched - the spooks' code for mind-reading machines is adequately "science-fiction". This makes me shy of telling "can't be done".
As for Jean-Pierre Petit, I've never-ever read one single scientific argument against his claims. But very often words like "conspirationist" and "waco". Exactly like if making unusual claims lets him look like a madman to most observers, whatever his explanations and justifications. Or: exactly like if nearly every reader and pseudo-scientist used non-scientific methods to decide what to believe.
For sure, JPP is a rare specialist for MHD, he was a research director at the French public research council, and he still publishes top-quality papers in peer-reviewed astronomy journals.
There, I don't find figures about how much energy provoques how much pressure in how big a volumehttp://www.jp-petit.org/Divers/Armes_sismiques/Armes_sismiques1.htm
but it's a wide-public webpage, so I should discuss it with him directly. I want to discuss more things anyway, these fully mainstream - or call it as you want.