\begin{table}%t2
\def~{\hphantom{0}}
  \caption{The loop's parameters (from Bray et al. \cite{Bray91}) and the bootstrap current calculating results.}\label{tab2}
  %\centering
\small
  \begin{tabular}{lccc}
  \hline \hline
      Parameter        & Cool~loop           & Hot~loop             & Flare~loop$^{*}$    \\
      \hline
\noalign{\smallskip}
       $R,L({\rm m})$           & $1.0\times {10^{7}}$  & $3.0\times{10^{7}}$   & $7.0\times{10^{6}}$\\
       $a_{0},({\rm m})$        & $1.0\times {10^{6}}$  & $5.0\times{10^{6}}$   & $2.8\times{10^{6}}$\\
       $d,({\rm m})$            & $7.85\times {10^{7}}$ & $4.0\times {10^{8}}$  & $1.0\times {10^{7}}$ \\
       $T_{\rm e},({\rm eV})$       & 10.0                  & 300.0                 & 900.0\\
       $B_{\varphi0},({\rm T})$ & 0.1                   & 0.1                   & 0.1 \\
       $n_{\rm e},({\rm m}^{-3})$   & $1.0\times{10^{16}}$  &$2.0\times{10^{15}}$   & $1.3\times{10^{15}}$\\
       $g$                & $1.2\times {10^{-5}}$ & $1.8\times {10^{-2}}$ & 1.05\\
       $I,({\rm A})$            & $4.0\times{10^{10}}$  &$1.0\times{10^{11}}$   & $1.0\times{10^{11}}$\\
       $I_{{\rm bs}},({\rm A})$       & $-$                  & $-$                  & $4.46\times{10^{10}}$\\
       $f_{{\rm bs}},($\%$)$    & $-$                  & $-$                  & 44.6\\
       \hline
  \end{tabular}
\end{table}