\begin{table}%t1
\caption{\label{tab1}Model parameters.}
%\centerline
 { 
\begin{tabular}{ccccc}
\hline\hline
Model & $m_{\rm i}$/$m_{\rm e}$  & $n_{\rm b}$/$n_{\rm e}$  & $v_{\rm b}/c$ & $\omega_{\rm ce}/\omega_{\rm pe}$ \\
\hline
A & 16 & 1/8 & 0.666 & 0.0  \\
B & 16 & 1/8 & 0.666 & 0.1 \\
C & 16 & 1/8 & 0.666 & 0.5 \\
D & 16 & 1/8 & 0.666 & 0.7 \\
E & 16 & 1/8 & 0.666 & 1.0 \\
F & 16 & 1/8 & 0.666 & 1.3 \\
G &  1 & 1/8 & 0.666 & 0.0 \\
H &  1 & 1/8 & 0.666 & 1.3 \\
I & 100 & 1/8 & 0.666 & 0.0 \\
J & 100 & 1/8 & 0.666 & 1.3 \\
K & 16 & 1/8 & 0.333 & 0.0 \\
L & 16 & 1/8 & 0.333 & 1.3 \\
M & 16 & 1/40 & 0.666 & 0.0 \\
N & 16 & 1/8 & 0.234$^a$
 & 0.0 \\
O & 16 & 1/8 & 0.234$^b$  & 1.3 \\
\hline
\end{tabular}}
\par
\smallskip
$^a$ Mean velocity of the power-law
beam distribution;
$^b$ mean velocity of the power-law
beam distribution.
\end{table}