Answer
$i_{d}=2.3 \times 10^{12} \frac{\mathrm{V}}{\mathrm{m} \cdot \mathrm{s}} .
$
Work Step by Step
$i_d=J_d \pi r^2=\varepsilon_0 \frac{d \Phi_E}{d t}=\varepsilon_0 \pi r^2 \frac{d E}{d t}$, we get
$
\frac{d E}{d t}=\frac{J_d}{\varepsilon_0}=\frac{20 \mathrm{~A} / \mathrm{m}^2}{8.85 \times 10^{-12} \mathrm{~F} / \mathrm{m}}\\=2.3 \times 10^{12} \frac{\mathrm{V}}{\mathrm{m} \cdot \mathrm{s}} .
$
