EXTERNAL AND INTERNAL PROBE MEASUREMENTS IN DISCHARGES OF LUMINESCENCE LAMP TYPE

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EXTERNAL AND INTERNAL PROBE MEASUREMENTS IN DISCHARGES OF

LUMINESCENCE LAMP TYPE

G. Schaal, J. Zabel

To cite this version:

G. Schaal, J. Zabel. EXTERNAL AND INTERNAL PROBE MEASUREMENTS IN DISCHARGES OF LUMINESCENCE LAMP TYPE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-849-C7- 850. �10.1051/jphyscol:19797410�. �jpa-00219411�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppldment au n07, Tome 40, J u i l Z e t 1979, page C7- 849

EXTERNAL AND INTERNAL PROBE MEASUREMENTS IN DISCHARGES OF LUMlhlESCENCE LAMP TYPE

G. Schaal, J. Zabel.

Z e n t r a l i n s t i t x t 1'ii.r EZektronenphysik, V Akademid der Wissensehaf t e n d e r DDR, DDR 22 Grei fmaZd.

Introduction: Vlilliams /? / and l,Villiams and Turner /2/ made experiments to determine the wall potential, the axial elec-

tric field and the electrode falls by external probes in luminescent lamp discharges. Systematic comparisons of the results of internal and external probe measurements in the same type of discharges were given in /3/ which also descri- bed statistical methods to determine the reliability of external probes. In the present paper some data are given on the mechanism of coupling the external probe to the plasma and about the accuracy with which data are obtained from axial field and floating potential measurements by this method, time resolved and with rms.

Method: In a cylindrical discharge veasel of the geometry of a 40-watt lminescent lamp without luminescent material three internal probes were introduced at a distance of 10 cm, 55 cm and 95 cm from the electrode. The tube was heated, for- mation of electrodes was done and it was filled with mercury and argon ( 3 Torr).

External probes with a width of 10 cm, 4 cm, 2.4 cm and 1.2 cm were installed near the internal probe which was at a distance of 55 cm from the electrode, In another arrangement up to ten external probes were used in different regions of the tube. The measuring probes were coup- led vla a white cathode follower to an oscilloscope or to quadrant electrometer.

In this way one could reach an input re- sistance of 10 ohms and an input capaci- 9 tance of 1.6 pf.

Reeults: Different resistors and capaci- tors were connected parallel to the input of the cathode follower. The output of the cathode follower was given fo the ordinate

of the oscilloscope. The signal of the internal probe was handled in the same way and given to the x-axis. Prom this kind of Lissajous figure it is clearly to be seen that the coupling agent to the external probe is not a reaistive one but a capacitive one (Fig. 1).

Pig.?: Response to different input impedances

Fronr the input voltage of the cathode follower used with different input capa- citors we determined the coupling capacity of the external probe (method 1).

If the input of the oacilloscope with an input resistor of l o 6 ohms is connected directly to the external probe, it is possible to determine the same capacity from measuring the decay time of the probe voltage (method 2). Applying the formula of a cylindrical capacitor the value of the coupling capacity can be calculated.' Table 1 shows the values obtained.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797410

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Probe width

4 cm '=I ~OIUmeasured 2,4 cm '=I 02Umeasured 1,2 cm ,035Umea~~red Table 1

Vidth of Method

probe Method 2 Calculated 10 cm 330t20pf 360 pf 360 pf

4 cm 145'10pf 170 pf 144 pf 2,4 cm 93'10pf 140 pf 86 pf 1,2cm 50'8pf 80 pf 43 pf Taking the above mentioned second arrangement and determining rms values of the axial electrical field by internal and external probes the data obtained showed good agreement.

Considering these corrections a good

U

agreement of the potentials measured by both kinds of probes (Pig.3) is obtained.

External probes with a width of

consideration of thecapacitive

voltage divider

1

distance from electrode

I I

I

0 20 40 xEm] 60

Fig.2: Floatingpotential determined by external and internal probes

Fig.2 shows a diagram of the measurements of floating potentials by internal and external probes against electrode distance which makes evident that the measured values of external probes are smaller than those of the internal ones. Taking into account the impact of the voltage divider, which consists of coupling 'capacity and input capacitance, the following corrections of the measured values have proved suitable.

I

^I

distance from electrode

I I I I I I

0 20 40 60 x Fm)

Fig.3: Influence of capacitive voltage divider

If in the system the quadrant electrometer is replaced by an oscilloscope one gets the time resolved potentials of probes. It was found that the shape of internal and

external probe voltage is similar, however the amplitude of the external one is in- fluenced by cathode follower transmission and input capacitor, whose impact can be taken into account.

Conclusions: The coupling mechanism of external probes at 50 cps is definitely of a capacitive nature. The application of high input impedance cathode followers affords a simple method to reduce devia- tions between the data taken from internal and external probes.

References:

/I/ C.E. Williams, 2nd 1nt.Conf.Gas Dischargee, London, 1972, 85-87.

/2/ C.E. l?lilliams, J.C. Turner, 3rd Int.

Conf. Gas Discharges, London 1974, 49.

/3/ U, Puchert, AbschluBarbeit, Ingenieur- hochschule MBttweida 1976.