\documentclass[tikz,border=7pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}[font=\sffamily]
\begin{axis}[width=11.5cm,height=6.1cm,axis lines=left,xmin=0,xmax=12.7,ymin=0,ymax=2.25,
xlabel={cycle parameter $\eta$},ylabel={scale $a(\eta)$},
xtick={0,3.1416,6.2832,9.4248,12.5664},xticklabels={$0$,$\pi$,$2\pi$,$3\pi$,$4\pi$},ytick={.3,2.0},yticklabels={$a_0$,$a_0+2A$}]
\addplot[teal!75!black,very thick,domain=0:12.5664,samples=240]{.3+.85*(1-cos(deg(x)))};
\end{axis}
\end{tikzpicture}
\end{document}
