The inviscid instability, with respect to supersonic
disturbances of a spatially growing plane mixing layer inside
parallel flow guide walls, is investigated using linear stability
analysis. For supersonic convective Mach numbers, it is found
that the maximum amplification rates of the shear layers approach
an asymptotic value and that this maximum amplification rate
increases to its maximum value and decreases again as the
distance between the walls decreases continuously. Contour plots
of the pressure perturbation fields indicate that there are waves
propagating outward from the shear layer along the Mach angle,
and that the walls provide a feedback mechanism between the
growing shear layer and this compression/expansion wave system.
The streak lines of the flow confirm that the spreading rate of
the shear layer is unusually small for supersonic disturbances.