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.

image
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