  Linear global instability of the three-dimensional (3-D),
spanwise-homogeneous laminar separation bubble (LSB) induced by
shock-wave/boundary-layer interaction (SBLI) in a Mach 7 flow of nitrogen over
a $30^{\circ}-55^{\circ}$ double wedge is studied. At these conditions
corresponding to a freestream unit Reynolds number, $Re_1=5.2\times 10^{4}$
m$^{-1}$, the flow exhibits rarefaction effects and comparable
shock-thicknesses to the size of the boundary-layer at separation. This, in
turn, requires the use of the high-fidelity Direct Simulation Monte Carlo
(DSMC) method to accurately resolve unsteady flow features.
  We show for the first time that the LSB sustains self-excited,
small-amplitude, 3-D perturbations that lead to spanwise-periodic flow
structures not only in and downstream of the separated region, as seen in a
multitude of experiments and numerical simulations, but also in the internal
structure of the separation and detached shock layers. The spanwise-periodicity
length and growth rate of the structures in the two zones are found to be
identical. It is shown that the linear global instability leads to
low-frequency unsteadiness of the triple point formed by the intersection of
separation and detached shocks, corresponding to a Strouhal number of
$St\sim0.02$. Linear superposition of the spanwise-homogeneous base flow and
the leading 3-D flow eigenmode provides further evidence of the strong coupling
between linear instability in the LSB and the shock layer.
