mu=1;
modell={r'[t]==(mu-r[t]^2)r[t],r[0]==r0};
r0=3;
loes:=NDSolve[modell,r,{t,0,30}];
abb1=ParametricPlot[Evaluate[{Sqrt[r[t]] Cos[0.3 t],
           Sqrt[r[t]] Sin[0.3 t]}/.loes],
        {t,0,30},PlotRange->All,AxesLabel->{"x","y"},
        ImageSize->{400,300},DisplayFunction->Identity];
r0=0.02;
abb2=ParametricPlot[Evaluate[{Sqrt[r[t]] Cos[0.3 t],
           Sqrt[r[t]] Sin[0.3 t]}/.loes],
        {t,0,30},PlotRange->All,AxesLabel->{"x","y"},
        ImageSize->{400,300},DisplayFunction->Identity];
abb=Show[abb1,abb2,DisplayFunction->$DisplayFunction];