Answer
$E = 0$
Work Step by Step
We can find the enclosed charge in a sphere if radius $r$ when $r \leq a$:
$q_{enc} = \frac{r^3}{a^3}~q_1$
We can find an expression for the electric field at radius $r$ where $r \leq a$:
$\epsilon_0~\Phi = q_{enc}$
$(\epsilon_0)(E)(4\pi~r^2) = \frac{r^3~q_1}{a^3}$
$E = \frac{r~q_1}{4\pi ~\epsilon_0 ~a^3}$
When $r = 0$, then $E = 0$
