  We consider Alexander spirals with $M\geq 3$ branches, that is symmetric
logarithmic spiral vortex sheets. We show that such vortex sheets are linearly
unstable in the $L^\infty$ (Kelvin-Helmholtz) sense, as solutions to the
Birkhoff-Rott equation. To this end we consider Fourier modes in a logarithmic
variable to identify unstable solutions with polynomial growth in time.
