The acceleration is determined by force $\vec{a}=\frac{\vec{F}}{m}$.

The acceleration is in general not necerssary pendicular to the velocity.
The Lorentz's force is $\vec{F_b}=q\vec{v}\times\vec{B}$
For charged particle in uniform magnetic field.
The force $\vec{F_B}$ is always pendicular to both velocity $vec{v}$ and magnetic field $\vec{B}$.
So the uniform magnetic will only change direction of charged particle (it will not change the speed of charged particle).