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\section{Introduction}
When the right tail of the Maxwell's distribution is popullated by electrons exceedeng certain crtical velocity, this electrons are less affected by Coulomb collision friction, therefore they are accelerated up to relativistic velocities. The upper limit for the velocity if given by the synchrotron radiation friction. In a collisional plasma, Dreicer field is the lower electric $E$ limit for which REs can be created. It is given by
\begin{equation}
E_{D}=\frac{n_{e}e^{3}}{4 \pi \epsilon_{0}^{2} k T_{e}}Ln\Lambda \\
\end{equation}
Where $Ln\Lambda=Ln\frac{\lambda_{D}}{b_{0}}$ is the Coulomb logarithm, $\lambda_{D}=\sqrt{\frac{\epsilon_{0}kT_{e}}{n_{e}e^{2}}}$ the Debye length and $b_{0}=\frac{Ze^{2}}{4\pi\epsilon_{0}}\frac{1}{m_{e}v^{2}}$ the lower limit for Coulomb impact parameter. In general, electrons with velocity greater than $v_{c}$ along the electric field $E$, will be accelerated to relativistic velocities.
\begin{equation}
v_{c}=\sqrt{\frac{n_{e}e^{2}(1+Z_{eff}/2)}{4\pi\epsilon_{0}^{2}m_{e}E}Ln{\Lambda}}
\end{equation}
Hot tail mechanism occurs when the plasma is rapidly cooled so that the critical velocity is decreased (look at the Coulomb logarithm). Therefore a bigger population of electrons become relativistic. Avalanche production occurs when energetically enough electrons collide with thermal ones accelerating them to the RE region.

\begin{figure}
 \includegraphics[width=0.5\textwidth]{https://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Runaway_electron_dynamic_friction_in_air.svg/1200px-Runaway_electron_dynamic_friction_in_air.svg.png}
 \caption{Electron friction force as a function of it's energy. RE regime is in between collisional and radiative friction zones.}
\end{figure}

In the scope of isotopic studies in H2 and Ar, break dawn voltage is to be considered. In principle, when working with Ar, it should be possible to go to lower pressures than those for H2.

\begin{figure}
 \includegraphics[width=0.5\textwidth]{https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Paschen_curves.svg/1200px-Paschen_curves.svg.png}
 \caption{Paschen's curve: Break down voltage as a function of gas pressure.}
\end{figure}

\href{https://en.wikipedia.org/wiki/Relativistic_runaway_electron_avalanche}{Relativistic runaway electron avalanche}\\
\href{http://golem.fjfi.cvut.cz/wiki/Experiments/RunAwayElectronStudies/Library/CASTOR_GOLEM/Svoboda2019_Article_OperationalDomainInHydrogenPla.pdf}{RE Studies}\\
\href{https://en.wikipedia.org/wiki/Dreicer_field}{Dreicer field}\\
\href{https://en.wikipedia.org/wiki/Coulomb_collision#Coulomb_logarithm}{Coulomb collision}\\
\href{https://commons.wikimedia.org/wiki/File:Paschen_curves.svg#/media/File:Paschen_curves.svg}{Paschen Law}

\subsection{Aims}

Investigation of REs' on H2 and Ar.
\begin{itemize}
 \item Characterize RE and background plasma.
 \item Estimate REs distribution.
 \item Involve colleagues on experimentation.
\end{itemize}

\section{Aparatus}

Possible diagnostics:
\begin{enumerate}
 \item Scintillation detector plus photo-multiplier (NaI(Tl), YAP(Ce)). Hits / deposited energy.
 \item PH32-based detector (Strip detector).
 \item Timepix (SXR + HXR? Timepix(3?) CdTe?)
 \item Dosimeters
 \item Cherenkov Detector?
 \item Spectrometer(s)
\end{enumerate}

Possible parameters:
\begin{enumerate}
 \item Pressure, keep low ($5\ mPa < P \leq 10\ mPa$, $14\ mPa$).
 \item Electric field, keep low for low Plasma current ($440 \geq E\ V$, $325\ V$).
 \item B field ($\sim 1300\ V$, $1200\ V$).
 \item Working gas (H2, Ar or He).
 \item Stabilization System.
\end{enumerate}

Reference shots: \href{http://golem.fjfi.cvut.cz/shots/29398/}{29398} \href{http://golem.fjfi.cvut.cz/shots/27762/}{27762} \href{http://golem.fjfi.cvut.cz/shots/27738/}{27738} \href{http://golem.fjfi.cvut.cz/shots/27745/}{27745} \href{http://golem.fjfi.cvut.cz/shots/27751/}{27751} \href{http://golem.fjfi.cvut.cz/shots/27756/}{27756} \href{http://golem.fjfi.cvut.cz/shots/27757/}{27757} \href{http://golem.fjfi.cvut.cz/shots/29364/}{29364} \href{http://golem.fjfi.cvut.cz/shots/31052/}{31052}

\section{Methodology}

Functional areas:
\begin{itemize}
 \item Background plasma characterization by means of electrostatic probes.
 \item Plasma column stabilization system.
 \item RE count and energy distribution measurements.
 \item Working gas manipulation.
 \item Background plasma characterization by means of electrostatic probes (César and Arturo). Use ball-pen and Langmuir probes to estimate $T_{e}$.
 \item Plasma column stabilization system (Daniil). Revise parameters of the shot's command.
 \item RE count and energy distribution measurements (Andrés). Revise spectrometers, strip silicon, Timepix3 data.
 \item Working gas manipulation (César, glow discharge).
\end{itemize}


Session schedule:
\begin{enumerate}
 \item Reference shot on H2 ($Bt=1200\, V,\ E=325\, V,\ P\leq 10\, mPa,\ \delta t\leq 5\, ms$).
 \item Several shots with different conditions, still on H2.
 \item Chamber conditioning from H2 to Ar.
 \item Several shots with similar conditions as 2 but for Ar.
\end{enumerate}