Without going into the details, it is not easy to understand the physical phenomenon.

Fiztekh instructed many generations of physicists: "Physics needs to be understood!"

Let me see...

Bremsstrahlung is a process where you put a charged particle into an electromagnetic field and it looses kinetic energy through emission of photons. Inverse Bremsstrahlung is the, well, inverse process: You put a charged particle into an electromagnetic field and it gains kinetic energy through absorption of photons. Since you _absorb_ photons, you need a rather strong electromagnetic field for this process to be relevant. For example the high-power lasers used to create plasmas . AFAIR you also needed to be close to an atomic nucleus to fulfill momentum conservation.

An alternative viewpoint: Inverse Bremsstrahlung is just run-of-the-mill excitation of electrons, but from/to Rydberg states. In a plasma, you have many electrons that are weakly bound to atoms, similar to Rydberg states in atoms. If you shine a sufficiently powerful laser on the plasma, it can (among other things) excite these electrons. As a rule of thumb, for typical optical lasers, this excitation is more efficient if the electron is stronger bound, because the energy difference matches photon energy better and the initial and final wave functions tend to overlap more. Hence, this process is more efficient for electrons that are stronger bound, i.e., close to a nucleus.

hi , see 

http://iopscience.iop.org/article/10.1088/1742-6596/810/1/012059/pdf