We build them all the time. The difficulty is measuring the right regions of the plasma with the right precision. The plasma is pretty sensitive to perturbation

We build them all the time. The difficulty is measuring the right regions of the plasma with the right precision. The plasma is pretty sensitive to perturbations so you can't always just stick a probe in them. This often restricts us to using noninvasive measurements like laser diagnostics, but because the plasma is quite rarified, it can be difficult to get a good signal to noise ratio. Additionally, the laser diagnostics can be restricted in what frequencies (spatially or temporally) they can measure, depending on the technique employed. Lastly, even if we fully understood the process, modeling it would remain a challenge (for all the same reasons that turbulence modeling is, despite the fact that turbulence is much better understood)