\GWinput{root/GlobalParameters.page} \GWinput{Tokamak/BasicCharacteristics/commons.page} \def\itemBtcircuit{\item Circuit for generation of a toroidal magnetic field consisting of a capacitor bank ($C_B = 24.3$ mF) charged up to $U_{C_B}=2$ kV, which is triggered by PC controlled thyristor into a set of 28 magnetic field coils to generate a toroidal magnetic field up to $B_t\approx 0.8$ T.} \def\itemEcdcircuit{\item Circuit for generation of a toroidal electric field for ohmic current drive and heating ($C_{CD} = 10.8$ mF , $U_{C_{CD}} \le 400$ V). Discharge of capacitor bank is triggered by PC controlled thyristors into primary winding of the transformer. The time delay with respect to the magnetic field $\tau_{CD}$) can be independently selected. } \def\itemEbdcircuit{\item Circuit for generation of a toroidal electric field for breakdown ($C_{BD} = 10.8$ mF , $U_{C_{BD}} \le 400$ V). Discharge of capacitor bank is triggered by PC controlled thyristors into primary winding of the transformer. The time delay with respect to the magnetic field $\tau_{BD}$) can be independently selected. } \def\itemEqcircuit{\item Circuit for generation of an equilibrium magnetic field, consisting of a set of capacitors charged up to $U_{C_B}=1$ kV, which is triggered by a PC controlled thyristor into a dynamic stabilization coil with time delayed pulse with respect to a magnetic field generation $\tau_{DS}$.} \def\itemPreionElGun{\item Pre-ionization of the working gas is performed by an electron gun.} \def\itemVacuum{\item Vacuum system, which allows reaching the background pressure $\approx 0.5$ mPa. } \def\itemGasHandling{\item Gas handling system (again computer controlled) to control the pressure of the working gas (hydrogen) in the vessel in the range of $p_{H_2}$ $\approx$ 10 - 200 mPa.} \def\itemChamberConditioning{\item System for baking of the tokamak vessel and for glow discharge cleaning.} \def\GOLEMbasics{ GOLEM is a limiter tokamak with circular poloidal cross-section and with an iron transformer core. It has 18 diagnostic ports and it is equipped with basic controls and diagnostics. Main parameters are as follows: \begin{itemize} \GolemCharacteristicBase \end{itemize} It is a device with full remote control capability and being operated mainly for an educational purpose. \begin{figure}[ht] \GWincludegraphics{width=0.48\textwidth}{/Tokamak/ExperimentalSetup/EngScheme/expsetup_L3} \GWincludegraphics{width=0.48\textwidth}{Tokamak/TriggerSystem/kresba.pdf} \centering \caption{Left: Engineering setup of the experiment. Right:Time delay parameter $\tau_{CD}$.} \label{fig:EngSetupL3} \end{figure} The experiment is composed from the following principal parts (see Fig. {\ref{fig:EngSetupL3}}): \begin{itemize} \itemBtcircuit \itemEcdcircuit \itemEbdcircuit \itemEqcircuit \itemPreionElGun \itemGasHandling \itemVacuum \itemChamberConditioning \end{itemize} } \def\GOLEMbasicsL1{ GOLEM is a limiter tokamak with circular poloidal cross-section and with an iron transformer core. It has 18 diagnostic ports and it is equipped with basic controls and diagnostics. Main parameters are as follows: \begin{minipage}{0.45\textwidth} \begin{itemize} \GOLEMCharacteristicBase \end{itemize} \end{minipage} \begin{minipage}{0.37\textwidth} \GWincludegraphics{width=\textwidth}{ShowRooms/PhotoGallery/0912GolemDischargeBD.jpg} \end{minipage}\\~\\ It is a device with full remote control capability and being operated mainly for an educational purpose. \begin{figure}[ht] \GWincludegraphics{width=0.5\textwidth}{Tokamak/EngineeringScheme/FirstLevel/expsetup_L1.pdf} \GWincludegraphics{width=0.48\textwidth}{Tokamak/TriggerSystem/kresba.pdf} \centering \caption{Left: Engineering setup of the experiment. Right:Time delay parameter $\tau_{CD}$.} \label{fig:EngSetupL1} \end{figure} The experiment is composed from the following principal parts (see Fig. {\ref{fig:EngSetupL1}}): \begin{itemize} \itemBtcircuit \itemEcdcircuit \itemEbdcircuit \itemPreionElGun \itemGasHandling \itemVacuum \end{itemize} } \def\dassystem{ Currently, the GOLEM tokamak is equipped by a limited set of basic plasma diagnostics for measurement of the loop voltage, the plasma current, the toroidal magnetic field, the plasma position with the set of Mirnov coils. Furthermore, the radial profile of the visible and soft X-ray radiation is measured by the array 20 bolometers. %\begin{figure}[ht] %\centerline{\resizebox{120mm}{!}{\rotatebox{0}{\GWincludegraphics{width=0.3\textwidth}{Diagnostics/Basic/BasicScheme/das.pdf}}}} % \caption{Basic diagnostic system of the tokamak GOLEM measuring Loop voltage $U_{l}$, Magnetic field $B_t$, Plasma current $I_{pl}$ and plasma radiation in the visible part of spectrum.} % \label{fig:BasicDas} %\end{figure} As an example, a typical evolution of a discharge with an antiparallel orientation of the toroidal magnetic field and the plasma current is shown in Fig. \ref{fig:Shot3494}. %\begin{figure}[h] %\centering %\GWincludegraphics{width=0.7\textwidth}{Presentations/10SOFTPorto/private/figs/3494/graph.pdf} %\caption{Evolution of a "typical" Golem discharge. From top to bottom - the loop voltage, toroidal magnetic field, plasma current, and the signal of a photodiode.} \label{discharge} %\label{fig:Shot3494} %\end{figure} \begin{figure}[h] \centering \begin{minipage}{0.48\textwidth} \GWincludegraphics{width=0.95\textwidth}{Diagnostics/Basic/BasicScheme/das.pdf} \end{minipage} \begin{minipage}{0.48\textwidth} \GWincludegraphics{width=0.95\textwidth}{Presentations/10SOFTPorto/private/figs/3494/graph.pdf} \end{minipage} \caption{Left: basic diagnostic system of the tokamak GOLEM measuring Loop voltage $U_{l}$, Magnetic field $B_t$, Plasma current $I_{pl}$ and plasma radiation in the visible part of spectrum. Right: evolution of a "typical" Golem discharge. From top to bottom - the loop voltage, toroidal magnetic field, plasma current, and the signal of a photodiode.} \label{discharge} \label{fig:Shot3494} \end{figure} It is seen that that the breakdown occurs at the toroidal magnetic field $B_t \approx 0.4$ T. The loop voltage at the breakdown is $ \sim 10$ V and it decreases to $\approx 5$ V. The plasma current reaches the value $I_p \approx 4$ kA. The central electron temperature can be estimated from the plasma resistivity as $T_e\approx 80$ eV. The safety factor at the plasma edge is about $q(a)\approx 15$. The integral visible radiation is monitored by a photodiode (see the bottom panel in Fig. 2. It is interesting to note that this relatively long and stable discharge is achieved without any external vertical magnetic field. } \def\spool{ A unique feature of this experimental arrangement is a possibility of a complete remote handling operation through the Internet access . From the client side the tokamak is operated via putty or ssh connection with the help of a command line, where remote operator set all the discharge parameters and trigger charging process followed with the plasma discharge itself. Consequently all data in graphical/raw form are accessible via the special discharge web page. \textbf{The parameters to be set remotely:} \begin{itemize} \item Toroidal magnetic field ($B_t$) through the voltage of the toroidal field capacitor bank ($U_B=U_{C\_Bt}$), range: $400-1400 \;\rm V$. \item Toroidal electric field ($E_t$) through the capacitor bank for the current drive ($U_E=U_{CD}$), range: $100-600 \;\rm V$. \item The time delay between the triggers of the toroidal magnetic field and the current drive ($T_{CD}=\tau_{OH}$), range: $0-20000 \;\rm \mu s$. \item Hydrogen gas pressure ($p_{H2}$), range: $0-100 \;\rm mPa$. \item Preionization ON/OFF \end{itemize} Figure \ref{fig:delays} shows the effect of time delay parameter. \begin{figure}[ht] \centerline{\resizebox{120mm}{!}{\rotatebox{0}{\GWincludegraphics{width=0.9\textwidth}{\path/kresba.pdf}}}} \caption{Time delay parameters.} \label{fig:delays} \end{figure} \textbf{The diagnostics used during the session to be accessed online:} \begin{itemize} \item Time resolved measurement of loop voltage ($U_l$). \item Time resolved measurement of total toroidal current by Rogowski coil ($I_t$). \item Time resolved toroidal magnetic field by coil measurement ($B_t$). \item Time resolved measurement of plasma radiation by photodiode. \item Vacuum chamber pressure ($p_{ch}$). \item The temperature of the vacuum chamber ($T_{ch}$). \end{itemize} } \def\VirtualControlRoom{ Students have the opportunity to learn the basics of operating the GOLEM tokamak in advance through the virtual interface (see Fig. \ref{fig:VirtualControlRoomLI}) where they can set up the parameters in the same way as in the real operation (see Fig. \ref{fig:remote_control}). The only difference is that virtual operation is inspired and results are generated from the real discharge database of the previous GOLEM tokamak operation (a discharge from the database is selected to have setup parameters as close as possible to the parameters chosen by the student). \begin{figure}[ht] \GWincludegraphics{width=0.9\textwidth}{Tokamak/ExperimentalSetup/VirtualControlRoom/VirtualControlRoomLI.jpg} \caption{Virtual control room} \label{fig:VirtualControlRoomLI} \end{figure} } \def\RemoteControlRoom{ Since the GOLEM tokamak is an educational device, it is neccessary to uncover the complexity of the experiment "step by step". This is demonstrated in the Fig. \ref{fig:SetupLevels} where from left (the most simple setup to produce plasma) to right (the setup with breakdown and equilibrium fields) additional components are added to the system. \begin{figure}[ht] \def\size{60mm} \resizebox{\size}{!}{\rotatebox{0}{\GWincludegraphics{width=0.3\textwidth}{Tokamak/ExperimentalSetup/EngScheme/expsetup_L1.pdf}}} \resizebox{\size}{!}{\rotatebox{0}{\GWincludegraphics{width=0.3\textwidth}{Tokamak/ExperimentalSetup/EngScheme/expsetup_L2.pdf}}} \resizebox{\size}{!}{\rotatebox{0}{\GWincludegraphics{width=0.3\textwidth}{Tokamak/ExperimentalSetup/EngScheme/expsetup_L3.pdf}}} \caption{Experimental setup from level I to level III.} \label{fig:SetupLevels} \end{figure} } \def\RemoteControl{ Measurements are to be set up and discharges (often referred to as ``shots'') initiated using the web interface of GOLEM tokamak which can be seen on figure \ref{fig:remote_control}. The exact url address of it is provided by a tokamak operator at the beginning of a session. \begin{figure}[ht] \centerline{\resizebox{100mm}{!}{\rotatebox{0}{\GWincludegraphics{width=0.9\textwidth}{Tokamak/ExperimentalSetup/RemoteControlRoom/RealControlRoom.jpg}}}} \caption{Remote control interface of the GOLEM tokamak - level I.} \label{fig:remote_control} \end{figure} } \def\RemoteDataAccess{ All the recorded data and the parameters of each discharge are available via a shot homepage (see Fig. \ref{fig:ShotHomepage}) at the GOLEM website. The root directory for the files is: \begin{figure}[ht] \centerline{\resizebox{150mm}{!}{\rotatebox{0}{\GWincludegraphics{width=0.9\textwidth}{Tokamak/ExperimentalSetup/RemoteControlRoom/wwwshot2012.jpg}}}} \caption{An example of a shot homepage.} \label{fig:ShotHomepage} \end{figure} \begin{center} \url{http://golem.fjfi.cvut.cz/operation/shots//} \end{center} Basic data of the present shot series is collected at a page to be reached at: \begin{center} \url{http://golem.fjfi.cvut.cz/operation/currentsession/} \end{center}} \def\RemoteDataAccessMatlab{ In order to facilitate the procedure of data analysis, a MATLAB package is available for basic data processing (this package is also compatible with the OCTAVE freeware software). The task is to build a proper work flow using these building blocks. It should be noted that these routines do not cover the whole procedure, some additional programs are supposed to be written by the students. The routines are listed in the table below: \begin{tabular}{l|l|l} File name&Input parameters&Description\\ \hline GOLEM\_get\_data.m & shot\_nr&\begin{tabular}[x]{@{}c@{}}Loads raw data from database\\into the MATLAB workspace\end{tabular} \\ GOLEM\_plot\_rawdata.m & shot\_nr&\begin{tabular}[x]{@{}l@{}}Makes plots of the time varying\\raw data\end{tabular} \\ GOLEM\_offset\_correction.m & \begin{tabular}[x]{@{}l@{}}raw\_signal, time\_vector, \\t1,t2\end{tabular} &\begin{tabular}[x]{@{}l@{}}Makes offset correction\\for raw data\end{tabular} \\ GOLEM\_cut\_data.m &\begin{tabular}[x]{@{}l@{}}raw\_signal, time\_vector, \\t1,t2\end{tabular}&Crop the given signal \\ GOLEM\_integrate.m &time\_vec, signal&\begin{tabular}[x]{@{}l@{}}Integrates the given signal\end{tabular} \\ GOLEM\_chamber\_current.m &\begin{tabular}[x]{@{}l@{}} time\_vec, \\$I_t$, $U_l$, $R_{ch}$, $L_{ch}$\end{tabular}&\begin{tabular}[x]{@{}l@{}}Calculates chamber current\\integrating equation \eqref{eq:vac} \end{tabular} \\ GOLEM\_diff.m &$x$, $y$& Calculates $dx/dy$ \\ \end{tabular} \paragraph{GOLEM\_get\_data.m} The return value of GOLEM\_get\_data.m contains then a \textbf{rawdata} structure with the following elements: \begin{itemize} \item \textbf{nr:} shotnumber \item \textbf{timedata:} structure, contains vectors of time signals \begin{itemize} \item \textbf{U\_l:} loop voltage measurement raw signal vector in [V] \item \textbf{dB\_t:} toroidal filed coil raw signal vector in [V] \item \textbf{dI\_t:} Rogowski coil raw signal vector in [V] \item \textbf{Photo:} photodiode raw signal vector in [V] \end{itemize} \item \textbf{N:} number of data points \item \textbf{samplerate:} samplerate of the measurements in [Hz] \item \textbf{pressure:} pressure of vacuum chamber in [mPa] \item \textbf{T\_ch:} temperature of the chamber in [K] \item \textbf{trigger:} time delay between starting diagnostics and toroidal magnetic field drive in [s] \item \textbf{time\_delay:} time delay between toroidal field and inductive current drive in [s] \item \textbf{Bt\_calibration:} calibration factor of toroidal magnetic field diagnostic in [T/Vs] \item \textbf{Rogowski\_calibration:} calibration factor of plasma current diagnostics in [A/Vs] \item \textbf{U\_loop\_calibration:} calibration factor of loop voltage diagnostic [V/V] \end{itemize} Elements of structures can be referenced as e.g. \textbf{rawdata.timedata.U\_l}. \emph{Measured signals are saved in the timedata structure, but these are raw signals needing further processing to produce the physical quantities measured!} Signal processing steps are described in the next section. } \def\GolemWiki{ A documentation project inspired by the Wikipedia project has been developed. Screenshot of the one particular page describing a special GOLEM diagnostics tool - the rake probe as an example is at the Fig. \ref{fig:golemwiki:rakeprobe}. \begin{figure}[ht] \GWincludegraphics{width=\textwidth}{Education/Wiki/WikiRakeProbe.jpg} \caption{GOLEM wiki} \label{fig:golemwiki:rakeprobe} \end{figure} } \def\PlasmaGeneration{ Temporal evolution of a typical discharge of the CASTOR tokamak is shown in figure \ref{main-param}. Before discharge, the vacuum vessel is evacuated down to the pressure of $10^{-4}$~Pa and filled by working gas (hydrogen). After that, the power supplies are connected to the toroidal magnetic field coils. Since now, the toroidal magnetic field $B_{tor}$ starts increasing (detailed behaviour is described in appendix \ref{ch_appendix_Btor}). When $B_{tor}$ reaches the range of 0.8 -- 1 T (which is usually 10 -- 25 ms after switching it on), the primary transformer winding is automatically connected to its power supplies (capacitor banks) \cite{start-up-CJP, valovic-start-up} and the toroidal electric field $E_{tor}$ is induced within the vacuum vessel. The $B_{tor}$ is measured by an open loop fixed at the top of the vessel. The measured voltage is termed a loop voltage $U_{loop}$, its temporal evolution is shown in the second panel of figure \ref{main-param}. The $E_{tor}=U_{loop}/2\pi R$ starts to accelerate free electrons, which are produced by an electron gun placed in the limiter shadow. Some free electrons are always present due to the cosmic radiation, \begin{figure}[htb] \centering \GWincludegraphics{width=0.6\textwidth}{ShowRooms/TextBookPlasma/ShotNo4665.png} \caption{Temporal evolution of a typical CASTOR discharge, with parameters: toroidal magnetic field ($B_{tor}$), loop voltage ($U_{loop}$), plasma current ($I_{pl}$), line averaged electron density ($n_e$). The whole discharge is plotted in left panels, the zoomed in start-up phase is shown in the right ones. Blue vertical lines (left panels) indicate the beginning and the end of the discharge. Green vertical lines (right panels) denote the breakdown.} \label{main-param} \end{figure} but their amount is not sufficient for a fast and reproducible breakdown (the moment of ignition of the discharge). After breakdown, the electron density $n_e$ increases exponentially, as shown in figure \ref{main-param}, right bottom panel. After $\sim1$~ms from breakdown, the working gas is completely ionized. Simultaneously, the plasma current $I_{pl}$ increases to the rate of $2~\rm{MA}/\rm{s}$, which is determined by the primary circuit parameters. The slope $dI_{pl}/d{\rm t}$ has to be kept relatively low to negate the skin effect, which could drive the current only on the surface of the plasma column. In this case, the radial profile of plasma current and plasma current density gets a hollow shape, plasma gets unstable and consequently disrupts. After the plasma current reaches values of $\sim10$~kA, it tends to remain constant for the next $20-30$~ms. During this quasistationary phase of discharge, the loop voltage is $2-3$~V, as shown in figure \ref{main-param}. It is interesting to realize that the plasma current in the range of 10~kA is driven by toroidal electric field $1~\rm{V}/\rm{m}$ only. The quasistationary phase is exploited for physical measurements. After the $20-30$~ms, the primary winding of the transformer is set to be short circuited. The plasma current exponentially decays. This is called a soft termination of discharge or a ``soft landing". } \def\RabiLoewText{ The new location of the tokamak is just next to the old Prague Jewish cemetery where Rabi Loew (Golem builder) is burried, and that is why it was renamed GOLEM (and also for the symbol of potential power you get if you know the magic). Interestingly, here in Prague, where the Golem legend originated, Golem is not perceived as a symbol of evil, but rather as a symbol of power which might be useful but is very challenging to handle. To learn more of the Golem legend, see e.g. wikipedia. } \def\GOLEM_RabiLoew{\begin{frame} \frametitle{GOLEM} \begin{columns}[c] \column{0.3\tw} \GWincludegraphics{width=1.4\textwidth, angle=90}{Tokamak/Introduction/figs/GolemekLT.JPG} \column{0.7\tw} \RabiLoewText \end{columns} \end{frame}} \def\GOLEMintro{\begin{frame} \frametitle{Let us start with the tokamak GOLEM \\ \large the smallest \& oldest tokamak with the biggest control room} \GWincludegraphics{width=0.9\textwidth}{ShowRooms/PhotoGallery/0912GolemDischargeBD.jpg} \end{frame}}