y u no work

(*Adjustables*)
B = 1.2;
rfvoltage = 18000;


(*constants and useful functions*)
c = 2.998*10^8;
q = 1.60*10^-19 ;
m0 = 1.672622*10^-27;

lorentz[r_] := Sqrt[1 + 0.1466032475398677* r^2];
timearound[r_] := Pi*r/(c* Sqrt[1 - 1/((lorentz[r])^2)]);
kinetic[r_] := (lorentz[r] - 1)*m0*c^2;
radiusfromkinetic[
   ke_] := (r /. (Solve[(lorentz[r] - 1)*m0*c^2 == ke, r][[2]]));
freqr[r_] := q*B/(lorentz[r]* m0*2*Pi);


rad = radiusfromkinetic[q*rfvoltage];
tbegin = 0;
pieces = {};
While[rad < .16, tfin = timearound[rad] + tbegin;
 AppendTo[pieces, {rad, t >= tbegin && t < tfin}]; tbegin = tfin;
 rad = radiusfromkinetic[kinetic[rad] + q*rfvoltage]]
Plot[Piecewise[pieces], {x, 0, 3*10^-6}]