\begin{table}%t1 [htbp]
  %%\centering

\par
  \caption{\label{tab:DiocotronColonne}Eigenvalues~$\omega$ for the constant density 
    plasma annulus, for different modes~$m$ and different aspect ratios, 
    $d_1 = R_1 / W_2$, and $d_2 = R_2 / W_2$, obtained from 
    the analytical expression found in~\cite{Davidson1990}.}  \begin{tabular}{cccc}
    \hline\hline
    Mode $m$ & $d_1$ & $d_2$ & $\omega$ \\
    \hline
    2 & 0.4 & 0.5 & $-3.772$e-01 + 7.176e-02 ~ i \\
    3 & 0.4 & 0.5 & $-5.456$e-01 + 2.267e-01 ~ i \\
    4 & 0.4 & 0.5 & $-7.216$e-01 + 2.988e-01 ~ i \\
    5 & 0.7 & 0.9 & $-1.147$e+00 + 5.787e-02 ~ i \\
    7 & 0.6 & 0.7 & $-9.315$e-01 + 3.307e-01 ~ i \\
    \hline
  \end{tabular}
\end{table}