\begin{table}%t1
\def~{\hphantom{0}}
\caption{Experimental results of bootstrap current in tokamak devices.}
\label{tab1}
%\centering
\small
 \begin{tabular}{lccc}
 \hline \hline
Tokamak~Devices ~ & time ~ & $I_{\rm b}(MA)$ ~& $f_{\rm bt}($\%$)$ \\
\hline 
 TFTR            & $1988$   & $0.52$       & $58$\\
 JET             & $1989$   & $0.85$       & $28$\\
 $\rm JT-60$           & $1990$   & $0.37$       & $77$\\
 ${\rm JT}-60U$          & $1992$   & $0.85$       & $58$\\
 \hline
  \end{tabular}
\end{table}