  Supersonic outflows from objects as varied as stellar jets, massive stars and
novae often exhibit multiple shock waves that overlap one another. When the
intersection angle between two shock waves exceeds a critical value, the system
reconfigures its geometry to create a normal shock known as a Mach stem where
the shocks meet. Mach stems are important for interpreting emission-line images
of shocked gas because a normal shock produces higher postshock temperatures
and therefore a higher-excitation spectrum than an oblique one does. In this
paper we summarize the results of a series of numerical simulations and
laboratory experiments designed to quantify how Mach stems behave in supersonic
plasmas that are the norm in astrophysical flows. The experiments test
analytical predictions for critical angles where Mach stems should form, and
quantify how Mach stems grow and decay as intersection angles between the
incident shock and a surface change. While small Mach stems are destroyed by
surface irregularities and subcritical angles, larger ones persist in these
situations, and can regrow if the intersection angle changes to become more
favorable. The experimental and numerical results show that although Mach stems
occur only over a limited range of intersection angles and size scales, within
these ranges they are relatively robust, and hence are a viable explanation for
variable bright knots observed in HST images at the intersections of some bow
shocks in stellar jets.
