Has this been attempted?
Been thinking about something for awhile now. Initially it was a 9 emitter convergence array (microwave, THZ would be difficult) in a spherical boundary chamber (such as a 3D geodesic array mounted inside a dielectric sphere) vacuum back filled with penning gas. 3 emitters on the equatorial plane spinning (mechanical or electronic phase rotation) around the center 120° apart, 3 stationary on top and bottom tight triad formation and a picosecond (maybe femto) aimed at the focal point. (That's the high level quick and dirty.)
If the sphere's internal radius is sized to a multiple of your drive wavelength, the spherical boundary should create a standing electromagnetic wave. The EF vectors from all 9 emitters should constructively interfere at the geometric origin, forming an ultra-intense central E-field node. That should create a stable (sustainable?) ball/toroid of dense plasma correct? In other iterations I thought about using 6 spinning emmiters. 3 on the polar axis and 3 equatorial or adding an x axis cut quartz (MgO coating?) at the focal point but I think I'm doing to much here. I feel like more is possible with less. I'm not a scientist but it's been a persistent thought for a few years. Has anyone tried something like this?
Apologies if this is not the place for this if there's a better one please let me know.