Subcaption between figure and the actual caption itself

Question

I have a figure with some variable names in it. At the moment i use a floatfoot from floatrow to add the explanation for the variable names and i really like the looks of it, but it has multiple problems.

Here the code:

\documentclass[a4paper,12pt]{article}

\usepackage{floatrow}
\usepackage{graphicx}
\usepackage{subcaption}


\begin{document}

\begin{figure}[h]
    \centering
    \includegraphics[width=0.5\textwidth]{x-ray-tube}
    \floatfoot{
        \begin{center}
            $U_h$ heating potential, $U_a$ acceleration potential, $K$ cathode, $A$ anode,\\
            $W_{in}$ cooling water inlet, $W_{out}$ cooling water outlet, $C$ cooler
        \end{center}}
    \caption{Schematic x-ray tube.\\
    Source: \textit{Citation Source here}}
    \label{fig:x-ray-tube}
\end{figure}

\end{document}

1. it throws an error (besides working great?) whenever i add either \centering or as in the example a begin center.

2. i would love to have the float in between the figure and its caption since it is somehow part of the figure.

At the moment it is:

(Image)
Fig 1: Caption

Floating infos

And i want it to be:

(Image)
Floating infos    
Fig 1: Caption

3. i'd like to then reduce the spacing a little bit, at the moment it seems very clunky.

LaTeX Error: Something's wrong--perhaps a missing \item.

If you want to test it, here is the image. Image (from wikipedia):

Answer 1

So you want something like this?

Then you're overcomplicating things, just ditch the \floatfoot altogether.

\documentclass[a4paper,12pt]{article}

\usepackage{graphicx}
\usepackage{subcaption}


\begin{document}

\begin{figure}
    \centering
    \includegraphics[width=0.5\textwidth]{example-image}

\footnotesize
            $U_h$ heating potential, $U_a$ acceleration potential, $K$ cathode, $A$ anode,\\
            $W_{in}$ cooling water inlet, $W_{out}$ cooling water outlet, $C$ cooler
    \caption{Schematic x-ray tube.\\
    Source: \textit{Citation Source here}}
    \label{fig:x-ray-tube}
\end{figure}

\end{document}