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defpath{Tokamak/Introduction} defGOLEMbasics{ The go GOLEM tokamak is a tokamak with full remote control capability and educational purpose. It is a small sized tokamak device equipped with basic controls and diagnostics having main parameters begin{itemize} item Major radius GolemMajorRadiusGolemMajorRadiusUnit item Minor radius (vacuum vessel) GolemMinorRadiusGolemMinorRadiusUnit item Plasma radius (limiter radius) GolemPlasmaRadiusGolemPlasmaRadiusUnit item Toroidal magnetic field $0.1-0.5$T item Plasma current $2-8$kA item Length of the discharge $<50$ms item Central electron density $0.5-3times 10 ^{19}{rm m}^{-3}$ item Central electron temperature $100-300$eV item Edge electron density 1$times 10^{18}{rm m}^{-3}$ item Edge electron temperature $10-40$eV item Liner material: Bellows Stainless Steel item Diagnostic Ports: 6x3 end{itemize}
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 integrating one more layer of vacuum into the chamber would help to achieve better stability of plasma column. 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.
begin{figure}[ht] centerline{resizebox{120mm}{!}{rotatebox{0}{GWincludegraphics{width=textwidth}{Tokamak/ExperimentalSetup/EngScheme/expsetup_L3.pdf}}}} caption{Engineering setup of the experiment.} label{fig:EngSetup} end{figure}
The experiment is composed from the following principal parts (see Fig. {ref{fig:EngSetup}}):
begin{itemize} item Circuit for generation of the toroidal magnetic field consisting of the 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 the toroidal magnetic field up to $B_tapprox 0.8$ T. item Circuit for generation of the toroidal electric field is composed of two capacitor banks. The first one is used for breakdown of the working gas $C{BD} = 2.7 mF$, $U{C{BD}}=400$ V). The second bank is used for ohmic current drive and heating $C{OH} = 10.8$ mF , $U{C{OH}}=400$ V). Both banks are triggered by PC controlled thyristors into two primary windings of the transformer. The time delay with respect to a magnetic field ($tau{BD}$ and $tau{OH}$) can be independently selected. The commutation switch can change the mutual orientation of the toroidal magnetic field and the plasma current. An inductance ($L = 5.9$ mH) can be included in the OH circuit to modify the plasma current ramp-up. item Circuit for generation of the equilibrium magnetic field, consisting of a set of capacitors charged up to $U{C_B}=1$ kV, which is triggered by PC controlled thyristor into a dynamic stabilization coil with time delayed pulse with respect to a magnetic field generation $tau{DS}$ item Pre-ionization of the working gas is performed by an electron gun. item Vacuum system, which allows reaching the background pressure $approx 0.5$ mPa. 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. item System for baking of the tokamak vessel and for the glow discharge cleaning. end{itemize}
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=0.3textwidth}{Diagnostics/Basic/Overview/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=0.7textwidth}{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}
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 $T_eapprox 80$ 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.
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=0.9textwidth}{path/figs/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}
}
defVirtualControlRoom{ Students have the possibility learn the basics of the GOLEM operation in advance through the virtual interface (see Fig. ref{fig:VirtualControlRoomLI}) where they have the opportunity set up the parameters in the same way as in the real operation (see Fig. ref{fig:remote_control}). The only difference is that operation is inspired and results are generated from the real shot database of the previous tokamak GOLEM operation (shot from the database is selected to have setup parameters as close as possible to parameters chosen by the student).
begin{figure}[ht] GWincludegraphics{width=0.9textwidth}{Tokamak/ExperimentalSetup/VirtualControlRoom/VirtualControlRoomLI.jpg} caption{Virtual control room} label{fig:VirtualControlRoomLI} end{figure} }
defRemoteControlRoom{ Since the tokamak GOLEM is 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 a plasma) to right (the setup with breakdown and equilibrium fields) additional components are placed to the system.
begin{figure}[ht] defsize{60mm} resizebox{size}{!}{rotatebox{0}{GWincludegraphics{width=0.3textwidth}{Tokamak/ExperimentalSetup/EngScheme/expsetup_L1.pdf}}} resizebox{size}{!}{rotatebox{0}{GWincludegraphics{width=0.3textwidth}{Tokamak/ExperimentalSetup/EngScheme/expsetup_L2.pdf}}} resizebox{size}{!}{rotatebox{0}{GWincludegraphics{width=0.3textwidth}{Tokamak/ExperimentalSetup/EngScheme/expsetup_L3.pdf}}} caption{Experimental setup from level I to level III.} label{fig:SetupLevels} end{figure} }
defRemoteControl{ Measurements are to be set up and 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 tokamak operator just at the beginning of the session.
begin{figure}[ht] centerline{resizebox{100mm}{!}{rotatebox{0}{GWincludegraphics{width=0.9textwidth}{Tokamak/ExperimentalSetup/RemoteControlRoom/RealControlRoom.jpg}}}} caption{Remote control interface of GOLEM tokamak - level I.} label{fig:remote_control} end{figure} }
defRemoteDataAccess{
All the recorded data and the settings for each shot are available via 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=0.9textwidth}{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/<shotnumber>/} end{center}
Basic data of the present shot series are collected at a page to be reached at: begin{center} url{http://golem.fjfi.cvut.cz/operation/currentsession/} end{center}}
- defRemoteDataAccessMatlab{
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}{ll} File name&Input parameters&Description\ hline GOLEM_get_data.m & shot_nr&begin{tabular}[x]{@{}c@{}}Loads raw data from database\into the MATLAB workspaceend{tabular} \ GOLEM_plot_rawdata.m & shot_nr&begin{tabular}[x]{@{}l@{}}Makes plots of the time varying\raw dataend{tabular} \ GOLEM_offset_correction.m & begin{tabular}[x]{@{}l@{}}raw_signal, time_vector, \t1,t2end{tabular} &begin{tabular}[x]{@{}l@{}}Makes offset correction\for raw dataend{tabular} \ GOLEM_cut_data.m &begin{tabular}[x]{@{}l@{}}raw_signal, time_vector, \t1,t2end{tabular}&Crop the given signal \ GOLEM_integrate.m &time_vec, signal&begin{tabular}[x]{@{}l@{}}Integrates the given signalend{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 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. }
defGolemWiki{ There have been developed documentation project inspired by the Wikipedia project. Screenshot of the one particular www page describing special GOLEM diagnostics tool - 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} }
