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categories: Training, Reports
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title: Floating potential in glow discharge without toroidal magnetic field
...
\section{Introduction}
Goal of session conducted in 8th November 2012 at GOLEM tokamak was to study spatial floating potential dependence $U_f(z)$ of glow discharge induced
in tokamak chamber for various toroidal electric fields (current-drive) in H and He plasma for various
initial pressure values.
Floating potential sensor deployed on GOLEM consists
of linear array of 16 cylindrical tips, each of radius $R=0.35 \mathrm{mm}$ and length $b = 2 \mathrm{mm}$.
Array spacing is $a=2.5 \mathrm{mm}$. Every tip is grounded by 1/101 divider consisting of resistors 6 $\mathrm{k\Omega}$ and 600 $\mathrm{k\Omega}$.
Tips were oriented radially outward from torus center. Outermost 12 tips were connected through divider to 12 channel 16 bit Data Acquisition Systems (DAS)
referred as "Papuch\_Zacek" or "Papouch\_Za" in GOLEM documentation. Channel zero is the most outward
tip of the probe.
Parameters of Papuch\_Zacek are 1 MHz sampling frequency and range $\pm 10 \mathrm V$.
One shot consists of about 35 ms of data with 1 $\mu$s resolution.
We induced glow discharge in GOLEM tokamak chamber by electrode with potential +1000 V with respect to chamber vessel.
\begin{figure}
\includegraphics{/Diagnostics/ParticleFlux/RakeProbe/reports/1112FTTFpractice_RP/img/rake_in_golem_10262.jpg}\\
Rake probe immersed in He glow discharge (shot 10262)
\end{figure}
Automatized session summary is available at
\url{http://golem.fjfi.cvut.cz/tasks/Practica/PraktikaFyzikyPlazmatu/2012/PavelkaLangProbes/081112_1842/}
\section{Scenario}
First we have found non-invasive magnetic field during testing shots and use this value for rest of the session. Then we measured floating potential temporal and spatial dependence with different current-drive capacitor voltage at fixed pressure in He plasma. Second part is intended to scan through pressure dependence of H plasma with fixed firm current-drive. The last third part tried to create glow discharge in highly evacuated chamber with and without pre-ionization.
\subsection{Test shots sequence}
Test shots sequence was aimed to verify electrical connection and to found non-invasive magnitude
of magnetic field. No current drive is applied in Test shots sequence.
Channels number 7, 9 and 11 were found defective. (First channel is channel number 0, last channel number 11.)
We have found non-invasive toroidal magnetic field roughly
up to 200 $\mu\mathrm T$ by 2 V on capacitor bank. This toroidal field was used for the rest of the session.
\begin{tabular}{cccc}
Shot & Gas & Pressure & Notes \\
\hline
\hline
10250 & He & 725 mPa & Toroidal mag. field capacitor $U_b$ = 20 V,\\
& & & peak toroidal field $B_t$ ca 9 mT, too much,\\
& & & plasma destroyed by ca 6 mT\\
\hline
10251 & He & 725 mPa & $U_b$ = 10 V, $B_t$ peak ca 5 mT,\\
& & & plasma strongly altered by field\\
\hline
10252 & He & 725 mPa & $U_b$ = 5 V, $B_t$ peak ca 3 mT\\
& & & plasma perturbed from ca 1 mT \\
\hline
10253 & He & 725 mPa & $U_b$ = 2 V, $B_t$ peak ca 140 $\mu\mathrm T$\\
& & & no perturbation of floating potential observed \\
\hline
\end{tabular}
\subsection{Current driven He glow discharge}
We kept toroidal magnetic field peaking 140 $\mu\mathrm T$ (2 V on capacitor bank) and add variable toroidal
electric field current-drive.
\begin{tabular}{cccc}
Shot & Gas & Pressure & Notes \\
\hline
\hline
10254 & He & 731 mPa & Current drive capacitor bank $U_{\mathrm{CD}}=5\mathrm V$, \\
& & & current drive loop voltage $U_L$ ca 0.6 V peak \\
\hline
10255 & He & 728 mPa & $U_{\mathrm{CD}}=10\mathrm V$, $U_L$ ca 0.6 V peak \\
\hline
10256 & He & 728 mPa & $U_{\mathrm{CD}}=20\mathrm V$, $U_L$ = 1.2 V peak, \\
& & & No signal on two other channels\\
\hline
10257 & He & 734 mPa & $U_{\mathrm{CD}}=20\mathrm V$, $U_L$ = 1.2 V peak, \\
& & & repeated, previous signal loss did not appeared\\
\hline
10258 & He & 734 mPa & $U_{\mathrm{CD}}=40\mathrm V$, $U_L$ = 1.8 V peak \\
\hline
10259 & He & 731 mPa & $U_{\mathrm{CD}}=80\mathrm V$, $U_L$ = 3 V peak \\
\hline
10260 & He & 734 mPa & $U_{\mathrm{CD}}=160\mathrm V$, $U_L$ = 5 V peak \\
\hline
10261 & He & 731 mPa & $U_{\mathrm{CD}}=320\mathrm V$, $U_L$ = 9 V peak \\
\hline
10262 & He & 728 mPa & $U_{\mathrm{CD}}=320\mathrm V$, $U_L$ = 9 V peak, \\
& & & repeated for video *) record \\
\hline
\end{tabular}
*) \url{/Diagnostics/ParticleFlux/RakeProbe/videos/081112RakeProbeGlowDischargeCurrentDrive.MPG}
\subsection{Current driven H glow discharge: pressure dependence}
We kept toroidal magnetic field peaking 140 $\mu\mathrm T$ (2 V on capacitor bank) and
current drive capacitor 10 Volts. Working gas is switched to hydrogen and we study glow discharge
under different pressure in these weak fields.
\begin{tabular}{cccc}
Shot & Gas & Req. pressure & Notes \\
\hline
\hline
10263 & H & 2.5 mPa & no plasma detected \\
\hline
10264 & H & 1200 mPa & \\
\hline
10265 & H & 1575 mPa & no plasma detected \\
\hline
10266 & H & 13.1 Pa & \\
\hline
10267 & H & 9.8 Pa & \\
\hline
10268 & H & 4.5 Pa & \\
\hline
10269 & H & 2.02 Pa & \\
\hline
10270 & H & 862 mPa & \\
\hline
10271 & H & 1077 mPa & \\
\hline
10272 & H & 19.8 Pa & \\
\hline
\end{tabular}
\subsection{Low pressure and preionization experiments}
We kept toroidal magnetic field peaking 140 $\mu\mathrm T$ (2 V on capacitor bank) and
current drive capacitor voltage 10 Volts.
We tried to detect signal in highly evacuated chamber. Except two cases, no signal was detected,
Two successes are probably some contamination or adsorbed molecules.
\begin{tabular}{ccc}
Gas & Req. pressure & Notes \\
\hline
\hline
10273 & 2.6 mPa & no plasma control measurement \\
\hline
10274 & 2.5 mPa & only preionization\\
& & no plasma detected \\
\hline
10275 & 2.4 mPa & no plasma detected \\
\hline
10276 & 960 mPa & \\
\hline
10277 & 50 mPa & no plasma detected \\
\hline
10278 & 210 mPa & no plasma detected \\
\hline
10279 & 971 mPa & \\
\hline
10280 & 975 mPa & no plasma detected \\
\hline
\end{tabular}
Report on data analysis is available here
\url{http://golem.fjfi.cvut.cz:5001/Diagnostics/ParticleFlux/RakeProbe/reports/1112FTTFpractice_RP/index}
