\graphicspath{{figs/}}
%<*GOLEMSetup>
The basic tokamak features described in section \ref{sec:tokamakprinciple} can be implemented in a number of different ways, depending on the machine size and budget. The toroidal field coils, for example, can be superconducting (cooled by liquid helium), cryogenic (cooled by liquid nitrogen to reduce the resistance and losses via ohmic heating), water-cooled, or air-cooled (meaning not actively cooled). In this section we shall describe two particular properties of tokamak GOLEM: its capacitor power supply and its vacuum system.
The GOLEM discharge power is supplied by two \textit{capacitors banks} (one for the toroidal magnetic field circuit and one for the primary transformer winding circuit), which are stored in a separate room below the tokamak. During a discharge, significant currents must be driven through the 28 toroidal field coils and the transformer primary circuit in order to generate sufficient toroidal magnetic fields and plasma current. The necessary power is too large to be drawn directly from the electric network, and so GOLEM employs capacitors. Prior to the discharge, the capacitors are charged to the requested voltages $U_B$ and $U_{CD}$\footnote{CD = Current Drive.}, respectively. At the discharge beginning, computer-controlled thyristors connect them to their two respective circuits (see figure \ref{fig:expsetup}); either simultaneously, or with a variable time delay $T_{CD}$. This results in the typical sine-like time evolution of the toroidal field $B_t$ and the plasma current $I_p$ (see figure \ref{fig:discharge-evol}), as the capacitors discharge freely into the two respective circuits.
Tokamak operation always requires a reliable vacuum system. Fusion reactors will use a deuterium-tritium mixture, but most current tokamaks use the cheaper hydrogen, deuterium or helium as a similar-acting substitute. Plasma properties significantly depend on the plasma composition; even a small percentage of impurities (such as carbon, nitrogen, oxygen or water) can drastically change the tokamak performance. For instance, a pure hydrogen plasma glows pink, but GOLEM "hydrogen" plasma glows blue. The basic way to control the impurity content is to continuously evacuate the vacuum chamber to $\approx 0.2$ mPa and only fill it up with working gas shortly before the discharge. The GOLEM tokamak has two working gases: hydrogen and helium (deuterium is more expensive and we don't really need it). In this laboratory assignment, only hydrogen is used. Its initial pressure $p_0$ is one of the discharge parameters which must be set for every discharge.
%</GOLEMSetup>
%<*DischargeParameters>
\begin{figure}[ht]
\centering
\includegraphics[width=\linewidth]{ExperimentalScheme/drawing.pdf}
\caption{Experimental set-up: schematic of the GOLEM tokamak with its various sub-systems.}
\label{fig:expsetup}
\end{figure}
To set up a GOLEM discharge in the remote control room \cite{ControlRoom}, the user must specify the following parameters (in this order):
\begin{itemize}
\item \textbf{Initial neutral gas pressure $p_0$.} The recommended values are 15-40 mPa. The influence of initial gas pressure on plasma quality is complicated. Too low a pressure will result in a vacuum discharge (there is too little gas to make plasma from). Too high a pressure will also result in a vacuum discharge (the mean free path is too short for charged particles to gain sufficient energy in the accelerating loop voltage to ionise another particle, so the avalanche breakdown into a plasma fails; see Paschen's law). You can experiment with it a little.
\item \textbf{Working gas type.} Use hydrogen.
\item \textbf{Use/not use pre-ionisation.} Plasma is created in the vacuum chamber thanks to the loop voltage induced by the transformer. This voltage accelerates the existing charged particles in the chamber until they gain sufficient energy to ionise a neutral gas particle. The resulting ion-electron pair are also accelerated by the loop voltage, causing an avalanche ionisation which eventually turns the neutral gas into a plasma. Turning pre-ionisation on helps this process by providing a larger number of initial charged particles, namely electrons which are emitted from a heated tungsten filament inside the chamber. In effect, if you want a "vacuum discharge" (a discharge where plasma was not created despite the existing loop voltage and magnetic field), turn pre-ionisation off. Otherwise leave it on.
\item \textbf{Voltage $U_B$ on the capacitor bank powering the toroidal field coils.} The bank capacity is $C_B = 81$ mF. The recommended values of $U_B$ are 800-1000 V. The higher the voltage, the higher the toroidal magnetic field.
\item \textbf{Voltage $U_{CD}$ on the capacitor bank powering the transformer primary coils.} The bank capacity is $C_{CD} = 11.3$ mF. The recommended values of $U_{CD}$ are 400-600 V. Typically the higher the voltage, the higher the plasma current. However, plasma current depends strongly on other factors (plasma purity, current toroidal magnetic field, initial pressure etc.), so this rule isn't very reliable.
\item \textbf{Time delay $T_{CD}$ between discharging the two capacitor banks.} Typically $B_t$ takes a longer time to reach its maximum value than $I_p$. Therefore, it may be desirable to switch the transformer circuit on \textit{after} the toroidal field coil circuit. $T_{CD}$ gives the time delay in microseconds. The recommended value is 0-10000 $\mu$s.
\item \textbf{Discharge comment.} An eloquent comment is important for data processing, as it allows quick retrospective identification of the discharges. Examples:
\end{itemize}
\begin{center}
\begin{tabular}{l|l}
bad comment & good comment \\
\hline
\texttt{(empty)} & \texttt{PRA2 group6 test} \\
\texttt{test} & \texttt{PRA2 sk6 UB=800, UCD=1000, TCD=0, p=20} \\
\texttt{UB=800, UCD=1000, TCD=0, p=20} & \texttt{PRA2 group6 vacuum discharge} \\
\end{tabular}
\end{center}
Finally, note that we recommend some values because they maximise the chance of getting a nice plasma discharge, not because values outside these intervals aren't safe for the machine. The only downside of a failed discharge is that it takes 2-3 minutes to repeat it, which might be a lot during the laboratory.
%</DischargeParameters>
%<*RemoteControl>
The GOLEM tokamak has one feature which makes it unique among the dozens of today's tokamaks --- it can be controlled remotely, via internet, from any place on the Earth. In a typical research tokamak such as COMPASS (Czech Academy of Sciences, Prague) or ASDEX-Upgrade (Max-Planck Institute for Plasma Physics, Garching, Germany), the discharges are set up, reviewed and controlled by a group of schooled workers in a so-called control room, located next to the tokamak. On GOLEM, a literal child can make a plasma discharge using their smartphone. In this lab assignment, you will operate GOLEM using the "remote control room" as well.
The URL address of the web interface is \url{http://golem.fjfi.cvut.cz/remote/control_room}. To log in, one must specify their identification and a so-called access token. Identify yourself as "PRAxx" where "xx" is your group number. The access token is a strong of characters which provides access to a specific level of tokamak control, and it will be provided to you by the lab assistant. Alternatively, the access token can be specified directly in the URL (see \cite{gw:KFpraktdopr} for more information).
\begin{figure}[t]
\centering
\includegraphics[width=\linewidth]{RemoteControl/drawing.pdf}
\caption{Remote control interface of the GOLEM tokamak.}
\label{fig:remotecontrol}
\end{figure}
The control room is shown in figure \ref{fig:remotecontrol}. The left part of the top navigation bar contains links to 4 pages of the remote control
interface. These are in order:
\begin{itemize}
\item \textbf{Introduction:} a video and other materials introducing the user to the GOLEM tokamak and its handling.
\item \textbf{Control room:} set up discharge requests and submit them to the discharge queue.
\item \textbf{Live:} a live, real-time view of the vacuum chamber, the tokamak room, the current machine state (capacitor voltages, working gas pressure, discharge procedure progress) and the discharge queue.
\item \textbf{Results:} a table of executed discharges for the currently logged-in user, including links to discharge results.
\end{itemize}
To submit a discharge, go to the Control Room tab and follow the instructions. \textbf{An eloquent, information-filled discharge comment is mandatory!} One day in the near future, you will be forced to search through thousands of GOLEM discharges and figure out which ones you made and what they were intended for. Proper comments are invaluable to this end. Examples:
\begin{center}
\begin{tabular}{l|l}
bad comment & good comment \\
\hline
\texttt{(empty)} & \texttt{PRA2 group6 test discharge} \\
\texttt{test} & \texttt{PRA2 group6 UB=800, UCD=1000, TCD=0, p=20} \\
\texttt{UB=800, UCD=1000, TCD=0, p=20} & \texttt{PRA2 group6 vacuum discharge} \\
\end{tabular}
\end{center}
%Furthermore, the \textbf{Control room} tab consists of two panels. The left panel contains a panel with 6 tabs at the top. Each tab corresponds to a step in the discharge setup procedure configuring a given tokamak sub-system. It is possible to arbitrarily switch between tabs, for instance when one wishes to go back to a previous setup step. The right panel contains a rendering of the 3D tokamak model which dynamically changes according to the currently selected tab.
%</RemoteControl>
%\subsection{Discharge setup procedure}
%\label{sec:remote-discharge-setup}
%The discharge setup procedure consists of several steps/tabs corresponding to the subsystems described in~\autoref{sec:discharge-procedure}. The control page serves as a walk-through which guides the user through the setup procedure. \textbf{All settable parameters are perfectly safe.} At each step the user selects a value of some numeric parameter and/or a given option with a checkbox. The \textbf{Set recommended value} will select a predefined setting. Then the user moves to the next step by clicking the \textbf{Next} button.
%The final \textit{Submit} step is where the selected discharge configuration is submitted into the discharge requests queue along with a comment describing the configuration (i.e. the scientific aim). The comment is mandatory and must be put into the input field above the \textbf{Submit} button. Once the comment filed is filled, clicking the \textbf{Submit} button will send the discharge request into the queue. The panel also features button links to the Live real-time view of the experiment and the Introduction step to restart the walk-through. It is also possible to just go back to a specific step via the tabs at the top and change some parameters and then go back to the Submit step and submit the modified configuration.
Figure \ref{fig:delays} shows the effect of time delay parameter.
\begin{figure}[ht]
\centerline{\resizebox{120mm}{!}{\rotatebox{0}{\includegraphics{TimeDelay/kresba.pdf}}}}
\caption{Time delay parameters.}
\label{fig:delays}
\end{figure}
\subsection{Plasma discharge procedure}
\label{sec:discharge-procedure}
%<*dischargeProcedure>
The discharge procedure is controlled by a computer according to the parameters
requested by the user in the remote control room described
in~\autoref{part:remote-interface}. First the capacitor banks are charged up to
the requested voltage. This takes up to 1 minute. Meanwhile the vacuum vessel is
filled with the selected working gas type up to the requested pressure above the
background pressure. This stationary pressure balance is then maintained by the
vacuum and working gas filling system.
The working gas pressure must be high
enough for any plasma to form, but low enough for the neutral gas to breakdown
into plasma according to Pachen's law.
Once the capacitors are charged the filament (electron gun) is heated and emits
electrons which locally ionize the neutral working gas.
Without any pre-ionization, no plasma can form a a so called ``vacuum
discharge'' (useful for calibration) would be executed. The data acquisition
systems are armed (made ready) and after a 10 second countdown the capacitor
banks are discharged into the coils. This generates a pulse of strong
magnetic field $B_t$ in the chamber. The higher the voltage, the
larger the magnetic field confining the plasma. The generated electric field
$E_t$ first through and avalanche process (ionized particles are accelerated
and collide with neutrals, ionizing them, etc.) almost fully ionizes the gas
and ``breaks it down'' into a plasma. The $E_t$ field then continues to induce
a current in the plasma which heats and further ionizes the plasma. The plasma
continues to exist while the confinement is sufficient or until it touches the
vessel wall.
%</dischargeProcedure>
\subsection{Remote control interface}
\label{sec:remote-interface}
%<*GOLEMhistoryBriefly>
The device was originally called TM1. Designed and constructed in Kurchatov Institute of Nuclear Research (Soviet Union), it was one of the first operational tokamaks in the world. The original concept of the device did not include poloidal field coils of stabilization however, it was believed that having a multiple layer metallic chamber enclosing layers of vacuum would help to achieve better stability of plasma column. This resulted in the realization of a liner inside the vacuum vessel that also acts as a plasma limiter. The capacitor battery for toroidal field coils and transformer filled several rooms.
Some time later, there was a microwave heating system integrated and the device was renamed to TM1-MH. The microwave heating, in addition to ohmic heating had to heat the plasma further. After the device was moved to Institute of Plasma Physics, Czech Academy of Sciences (IPP CAS) in September 1977, thanks to cooperation between the Kurchatov Institute and IPP, some changes in the engineering took place. The microwave heating system was left in Russia, alongside with the most of the oil capacitors of the toroidal field generation, since there was not enough room in the new tokamak hall. A few years later, the device went under major reconstruction. The vacuum vessel was replaced for a new one, the layer of vacuum between the liner and coating were fully removed and a feedback stabilization system was integrated instead. The power supply was substituted by a stronger one, and the ignition was replaced by a glow discharge. Between the years 1977 and 2007 there were several small changes over the device, such as the use of new diagnostics sensors.
In the end of 2007 the device was transferred to the Faculty of Physical and Nuclear Engineering of the Czech Technical University (CTU). Work on the reoperation started on the 14th of July 2008 with limited capabilities, and improvements are still underway. Further upgrade of GOLEM is envisaged in a near future - an increase of $B_t$, $I_p$ and the discharge duration. Dynamic plasma position stabilization is under present consideration and investigation. Basic diagnostics will be enriched with the plasma density measurement (microwave interferometer), $H_{\alpha}$ and X-ray radiation measurement will be installed in a near future. Investigation of plasma edge physics with the help of the various probe measurements is planned, as the previous version of the GOLEM tokamak, the CASTOR had a very good inspiring tradition in this field of interest.
More information can be reached via the home site of the GOLEM tokamak at \url{http://golem.fjfi.cvut.cz}
%</GOLEMhistoryBriefly>
%<*GOLEMactualphoto>
\begin{figure}[ht]
\centerline{\resizebox{100mm}{!}{\rotatebox{0}{\includegraphics{Photo/0917GolemWithPlasma24620.jpg}}}}
\caption{Photo of the GOLEM tokamak}
\label{fig:golemphoto}
\end{figure}
%</GOLEMactualphoto>
