Numerical study of the knot structure in scaled protostellar jets by laboratory laser-driven plasmas

Knot structures exist ubiquitously in young stellar object (YSO) jets, which are a key tracer in astronomical observation to estimate the jet properties and eventually the YSO's parameters (age, size, mass and so on). Using 2D and 3D radiation magnetohydrodynamic simulations of the laser-produced plasma jets in external poloidal magnetic fields, we present a systematic analysis on the formation mechanism and characteristics of knot structures in collimated jets. The simulations demonstrate that the multi-knot pattern in jets can be formed by the oblique internal shocks in only single ejection. It is found that the distance L between different knots in jet is determined by the ratio of its thermal pressure to magnetic pressure beta as L alpha D beta(1/2), where D is the jet transverse diameter. There is a factor about 0.4-0.6 between the knot and jet velocities. And radiation cooling effect can alleviate the intensity of the external magnetic field required for collimating jets. These findings are scaled to the conditions of YSO jets, and can be applied to explore some characteristics of the astrophysical jets.