  The pulsar magnetic inclination angle is a key parameter for pulsar physics.
It influences the observable properties of pulsars, such as the pulse beam
width, braking index, polarisation, and emission geometry. In this study, we
give a brief overview of the current state of knowledge and research on this
parameter and its implications for the internal physics of pulsars. We use the
observed pulsar data of magnetic inclination angle and braking index to
constrain the star's number of precession cycles, $\xi$, which reflects the
interaction between superfluid neutrons and other particles inside a neutron
star\,(NS). We apply the method proposed by Cheng et al. (2019) to analyse the
data of PSR J2013+3845 and obtain the constraints for $\xi$ ranging from
$2.393\times 10^{5}$ to $1.268\times10^{6}$. And further analysis suggests that
the internal magnetic field structure of PSR J2013+3845 is likely dominated by
toroidal component. This study may help us understand the process of internal
viscous dissipation and the related evolution of the inclination angles of
pulsars, and may have important implications for the study of continuous
gravitational wave emissions from NS.
