POSITIVE IN EXTRACTION BY AN ANODIC ORIFICE PROBE

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POSITIVE IN EXTRACTION BY AN ANODIC ORIFICE PROBE

H. Lergon, K. Müller

To cite this version:

H. Lergon, K. Müller. POSITIVE IN EXTRACTION BY AN ANODIC ORIFICE PROBE. Journal

de Physique Colloques, 1979, 40 (C7), pp.C7-821-C7-822. �10.1051/jphyscol:19797396�. �jpa-00219395�

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JOURNAL' DE PHYSIQUE Colloque C7, suppZ6ment au n07, Tome 40, J u i l l e t 1979, page C7- 821

POSITIVE ION EXTRACTION BY AN ANODIC ORlFlCE PROBE

H.G. Lergon and K.G. Muller.

Fachbereich Physik, Universitiit Essen-GesamthoehschuZe, 43 Essen, Fed. Rep. of Germany.

Abstract

From a nonthermal plasma ions are extracted by .an orif ice probe being anodic, i .e.

positive with respect to plasma potential.

In front of the orifice a secondary plasma is observed providing an intense source for ion extraction. The extraction mechanism can be explained by a potential of the secondary plasma above probe potential. The thermal energies of the ions from the secondary plasma are of the order of 1/10 eV.

Introduction

The extraction of positive ions from a plasma by an orifice probe is strongly in- fluenced by the potential of the probe.

Increasing it and thus changing from cathodic to anodic probe the extracted ion current drops to small or even negli- gible values, until it finally rises again

(see e.g. Pahl, Lindinger, Howorka )

.

This current increase has been explained by ionization in the orifice or the ad- jacent region behind. A corresponding mass analysis of the ions has been recom- mended for the investigation of the neu- tral gas. In own experiments we find an increase of the collected ion current up to values an order of magnitude larger

than those for cathodic probes. parallel to this effect the formation of an intense secondary plasma in front of the anodic probe can be observed. Apparently the secondary plasma provides an ion source. In the following the ion extraction from this ion source will be investigated.

Experiment

The norlthermal plasma of a gositive column in Argon is analyzed by a combined mass spectrometer

-

probe diagnostic (see Ler- gon and ~iiller 2). By an orifice probe ions are extracted into a mass spectrometer allowing a retarding field analysis of ion energy. The experimental para- meters are: discharge diameter 2,8 cm;

pressure 0,056 mbar; discharge current 30 mA; orifice probe diameter 0,62 cm;

orifice diameter 50pm; orifice depth 50vm.

By this diagnostic the probe characteristic, probe current I versus probe voltage

P

and the 40~r+-characteristic, ion current I? versus probe voltage U

P, are measured (see Fig.1). By a retarding voltage of a few volts the low energy component, attributed to the secondary plasma (subscript: sec) can be suppressed and thus separated from the high energy component, attributed to the main plasma (subscript:

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797396

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Discussion

Fig. 1 : Probe characteristic I (U ) , ion P P

characteristics I ~ ( u ~ ) of high energy component (subscript: main) and low energy component (subscript: sec)

--

Uret

Fig. 2: Retarding field characteristics 1?(uret) for different probe currents I c : Uret = const (reference voltage).

P;

main). For a set of probe voltages, in Fig. 1 indicated by arrows, retarding field characteristics, i.e. current I+ 4 0 versus retarding voltage Uret, are taken

(see Fig. 2 ) .

The high energy component I 40 of Pig. 1 +main

is attributed to ions stemming from the main plasma, being heated up in the pre- sheath in front of the wall 2. Their tem- perature agree with the electron tempera- ture at the wall (see Table 1). The low energy component, attributed to the secondary plasma, shows temperatures in the range of 1 / 1 0 eV/k, falling with increasing U Assuming a potential distribution

P'

possessing a flat maximum in the secondary plasma and a positive sheath in front of the probe the usual extraction mechanism exists. Thus for an anodic orifice probe the existence of an additional ion source, provided by the secondary plasma, has t o b e taken into account.

a

method

I

^probecharacteristic Te* ev ion

I

characteristic = '2 eV

retarding field1

+ 1 1

characteristic T+= 1.3 eV at - I ~ = l r 6 d

= 3,4 mA T+= 0,24 eV

at -Ip= 5,O mA

I

^T+=^{0.16 eV}

1

^secondary

1

at -I = 7,O mA T+= 0.13 eV P

Table 1: Ion and electron temperatures', measured by different methods ²

Acknowledgements

This work has been supported by Ministerium fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen.

References

1 M. Pahl, W. Lindinger, F. Howorka,

2. Naturforsch.

e,

678 (1 972) 2 H.G. Lergon, K.G. &1uller, 2. Natur-

forsch. 1093 (1977)