Floating potential in glow discharge without toroidal magnetic field
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.
Rake probe immersed in He glow discharge (shot 10262)
Automatized session summary is available at
http://golem.fjfi.cvut.cz/tasks/Practica/PraktikaFyzikyPlazmatu/2012/PavelkaLangProbes/081112_1842/
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.
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.
Shot | Gas | Pressure | Notes |
---|---|---|---|
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 | |||
10251 | He | 725 mPa | \(U_b\) = 10 V, \(B_t\) peak ca 5 mT, |
plasma strongly altered by field | |||
10252 | He | 725 mPa | \(U_b\) = 5 V, \(B_t\) peak ca 3 mT |
plasma perturbed from ca 1 mT | |||
10253 | He | 725 mPa | \(U_b\) = 2 V, \(B_t\) peak ca 140 \(\mu\mathrm T\) |
no perturbation of floating potential observed |
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.
Shot | Gas | Pressure | Notes |
---|---|---|---|
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 | |||
10255 | He | 728 mPa | \(U_{\mathrm{CD}}=10\mathrm V\), \(U_L\) ca 0.6 V peak |
10256 | He | 728 mPa | \(U_{\mathrm{CD}}=20\mathrm V\), \(U_L\) = 1.2 V peak, |
No signal on two other channels | |||
10257 | He | 734 mPa | \(U_{\mathrm{CD}}=20\mathrm V\), \(U_L\) = 1.2 V peak, |
repeated, previous signal loss did not appeared | |||
10258 | He | 734 mPa | \(U_{\mathrm{CD}}=40\mathrm V\), \(U_L\) = 1.8 V peak |
10259 | He | 731 mPa | \(U_{\mathrm{CD}}=80\mathrm V\), \(U_L\) = 3 V peak |
10260 | He | 734 mPa | \(U_{\mathrm{CD}}=160\mathrm V\), \(U_L\) = 5 V peak |
10261 | He | 731 mPa | \(U_{\mathrm{CD}}=320\mathrm V\), \(U_L\) = 9 V peak |
10262 | He | 728 mPa | \(U_{\mathrm{CD}}=320\mathrm V\), \(U_L\) = 9 V peak, |
repeated for video *) record |
*) /Diagnostics/ParticleFlux/RakeProbe/videos/081112RakeProbeGlowDischargeCurrentDrive.MPG
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.
Shot | Gas | Req. pressure | Notes |
---|---|---|---|
10263 | H | 2.5 mPa | no plasma detected |
10264 | H | 1200 mPa | |
10265 | H | 1575 mPa | no plasma detected |
10266 | H | 13.1 Pa | |
10267 | H | 9.8 Pa | |
10268 | H | 4.5 Pa | |
10269 | H | 2.02 Pa | |
10270 | H | 862 mPa | |
10271 | H | 1077 mPa | |
10272 | H | 19.8 Pa |
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.
Gas | Req. pressure | Notes |
---|---|---|
10273 | 2.6 mPa | no plasma control measurement |
10274 | 2.5 mPa | only preionization |
no plasma detected | ||
10275 | 2.4 mPa | no plasma detected |
10276 | 960 mPa | |
10277 | 50 mPa | no plasma detected |
10278 | 210 mPa | no plasma detected |
10279 | 971 mPa | |
10280 | 975 mPa | no plasma detected |
Report on data analysis is available here http://golem.fjfi.cvut.cz:5001/Diagnostics/ParticleFlux/RakeProbe/reports/1112FTTFpractice_RP/index