nohighlight">\(\dot p_r = F_r\)</span>, we use the paraxial approximation and consider <span class="math notranslate nohighlight">\(\gamma = const\)</span>:</p>
<div class="math notranslate nohighlight">
\[\gamma m\ddot r =\displaystyle\frac {eE_r} {\gamma ^ 2} =\displaystyle\frac {2 I e} {\gamma ^ 2 a ^ 2\upsilon} r,\]</div>
<p>we got a linear equation, but since all the particles are moving, we need to take into account that <span class="math notranslate nohighlight">\(a = a (t)\)</span>. The solution of 