  We study the large scale dynamo process in a system forced by helical
magnetic energy. The dynamo process is basically nonlinear, but can be
linearized with pseudo scholars $\alpha$ & $\beta$ and large scale magnetic
field ${\overline{\bf B}}$. A coupled semi-analytic equations based on
statistical mechanics are used to investigate the exact evolution of
$\alpha$\&$\beta$. This equation set requires only magnetic helicity and
magnetic energy. They are fundamental physics quantities that can be obtained
from the dynamo simulation or observation without any artificial modification
or assumption. $\alpha$ effect is thought to be related to magnetic field
amplification. However, in reality it converges to $zero$ very quickly without
a significant contribution to ${\overline{\bf B}}$ field amplification.
Conversely, $\beta$ effect for the magnetic diffusion maintains a negative
value, which plays a key role in the amplification with Laplacian
$\nabla^2\rightarrow -k^2$. In addition, negative magnetic diffusion accounts
for the attenuation of plasma kinetic energy when the system is saturated. The
negative magnetic diffusion is from the interaction of advective term $-{\bf
U}\cdot\nabla {\bf B}$ and the strongly helical field. When plasma velocity
field $\bf U$ is divided into the poloidal component ${\bf U}_{pol}$ and
toroidal one ${\bf U}_{tor}$ in the absence of reflection symmetry, they
interact with ${\bf B}\cdot\nabla {\bf U}$ and $-{\bf U}\cdot\nabla {\bf B}$ to
produce $\alpha$ effect and (negative) $\beta$ effect, respectively. We
discussed this process using the theoretical method and intuitive field
structure model.
