4) Normal mode analysis. Assume a linear wave that varies as \( \exp [i(\mathbf{k} \cdot \mathbf{x}-\omega t)] \). This allows you to replace time derivatives with \( -i \omega \), spatial derivatives with \( i \mathbf{k} \), and first order quantities with \( a_{1} \longrightarrow \tilde{a} \). Rewrite the five equations of part 3 using these substitutions. (Hint: for a one dimensional wave, \( \mathbf{k}=k \).)
5) Dispersion relation. To solve these five equations, eliminate two of them to write the three remaining equations in the form
\[
\left(\begin{array}{ccc}
? & ? & ? \\
? & ? & ? \\
? & ? & ?
\end{array}\right)\left(\begin{array}{c}
\tilde{\phi} \\
\tilde{n}_{i} \\
\tilde{u}_{i}
\end{array}\right)=\left(\begin{array}{c}
0 \\
0 \\
0
\end{array}\right)
\]
The determinant of this equation (which you do not have to solve) yields the following
2
dispersion relation:
\[
\omega^{2}=k^{2}\left[\frac{Z_{i} T_{e}}{m_{i}\left(1+k^{2} \lambda_{D e}^{2}\right)}+\frac{\gamma T_{i}}{m_{i}}\right]
\]