unitflux = 2.07*10^-15

BI[a_] := (2*unitflux)/(Sqrt[3]*(a)^2)

NU[m_, n_] := m*m + m*n + n*n

BK[m_, n_, a_] := (2 - BI[a])*((-1)^

NU[m, n])*(3^0.75/(2*\[Pi]))/(NU[m, n])^1.5

K[m_, n_,

a_] := ((2*\[Pi])/(a*(Sqrt[3]*a)/2)) {m*(Sqrt[3]*a)/2,

n*a - m*a*0.5}

B[r_,a_] := BI[a] + (2 - BI[a])*

Sum[Sum[BK[m, n, a]*Cos[K[m, n, a]*r], {m, 8}], {n, 8}]

Manipulate[Plot[B[{r, a}], {r, 0, 3*10^-9}], {a, 5*10^-10, 15*10^-10}]

Haha, thanks. Unfortunately my real code is a bit more complicated and for some reason that method doesn't work. It works when I define a =1*10^-9 at the beginning but I can't seem to get it to work where 'a' can vary. Either like above or with ContourPlot[B[{x,y}],{x, 0, 6*10^-9}, {y, 0, 6*10^-9}]. Any ideas?