Confined Penrose process and black-hole bomb

We consider the decay of a particle with some energy $E_{0}>0$ inside ergosphere black hole. After first one particles $E_{1}<0$ falls towards hole while second $% E_{2}>E_{0}\,\ $moves in outward direction. It bounces back from reflecting shell and, afterwards, process repeats. For radial motion charged Reissner-Nordst\"{o}m metric, result depends strongly on concrete scenario. In particular, an indefinitely large growth is possible that gives rise to black-hole bomb. also similar multiple neutral background rotating axially symmetric stationary demonstrate that, if occurs turning point, bomb this case impossible at all. generic point ergoregion, there condition for exist. relates ratio masses before and after velocity fragment center mass frame.