We determine analytical expressions for the Risken-Nummedal-Graham-Haken mu
ltimode laser instability outside the uniform field limit in the case of ve
ry fast polarization decay (class-B laser). A new condition for the observa
bility of that instability, concerning the value of the cavity mirrors refl
ectivity, is predicted. (C) 1999 Published by Elsevier Science B.V. All rig
hts reserved.