Neutrons have zero charge but carry a magnetic dipole moment \(\mu\). As a result, they are subject to a magnetic force given by \(\mathbf{F}=(\mu \cdot abla) \mathbf{B}\). A beam of neutrons with \(\mu=\mu \hat{\mathbf{x}}\) is moving along the \(z\) direction into a region of magnetic field \(\mathbf{B}\). Find a simple form for \(\mathbf{B}\) capable of focusing the beam and prevent it from dispersing. This means that small disturbances in the beam profile would not grow. For any vector field \(\mathbf{V}\) satisfying \(abla \cdot \mathbf{V}=0\), we can write \(\mathbf{V}=abla f\) for a function \(f\) satisfying \(abla^{2} f=0\).