\begin{table}%t1
\caption{\label{para}The parameters of two dimensional simulation.}
\par
%%\centering

\small
\begin{tabular}{lccr}
\hline \hline
Parameters &Symbols &Simulation &Realistic scale\\
\hline
Number of time steps& &4500 &\\
Time step & $\Delta t$ &0.01 &\\
Timescale & $1/\omega_{\rm pe}$ &0.25 &$1.6\times 10^{-5}~{\rm s}$\\
Number of grid points &$N_{x}\times N_{y}$   &$128 \times64$\\
Grid spacing&$\triangle_{x} \triangle_{y}$ &1\\
Spatial scale & $c/\omega_{\rm pe}$&25 &$4.8\times 10^3~\rm m$\\
Ambient Number of particles   & &16\\
Beam Number of particles  & &4\\
Ambient electron density & $n$ &0.005 &$1.24\times 10^6 m^{-3}$\\
Electron plasma frequency& $\omega_{\rm pe}$ &4.0&10 kHz\\
Electron gyrofrequency&$\Omega_{\rm e}$ &0.5 \\
Electron beam density&$n_{\rm b}$ &0.025n \\
Electron thermal velocity&$v_{\rm the}$ &1& 50 eV\\
Ion thermal velocity&$v_{\rm thi}$ &0.005&2.3 eV\\
Ion mass&$m_i$ &1836$m_{\rm e}$\\
Speed of beam&$v_{\rm b}$ &10 & 5 keV\\
Speed of light&$c$&100& 3$\times 10^8\;\rm ms^{-1}$\\
\hline
\end{tabular}
\end{table}