THE EXTRACTION OF THERMAL ENERGY POSITIVE IONS FROM CORONA DISCHARGES IN AIR

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THE EXTRACTION OF THERMAL ENERGY POSITIVE IONS FROM CORONA DISCHARGES IN

AIR

R. Sigmond, D. Linhjell

To cite this version:

R. Sigmond, D. Linhjell. THE EXTRACTION OF THERMAL ENERGY POSITIVE IONS FROM CORONA DISCHARGES IN AIR. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-323-C7-324.

�10.1051/jphyscol:19797159�. �jpa-00219134�

JOURNAL DE PHYSIQUE Colloque C 7 , suppl6ment au n07, Tome 40, JuiZZet 1979, page C7- 323

THE EXTRACTION OF THERMAL ENERGY POSITIVE IONS FROM CORONA DISCHARGES IN AIR

R.S. Sigmond and 0. Linhjell.

The Norwegian I n s t i t u t e o f TechnoZogy, Electron and Ion Physics Research Group, N-7034, Trondheim-NTH.

Abstract T h e m a l e n e r g y p o s i t i ~ e ions i n a i r positive ions was expected from a 20pA pos- have been found t o be very d i f f i c u l t t o e x t r a c t itive corona in air at pressures up to about through holes o f 1 ~ m diameters i n A2 f o i l s , and lOkPa, according to the extraction hole area impossible through such holes i n Au f o i l s . ExpZana- and the j a cos59 current density distribu-

t i o n s are proposed and disproved. tion law of Warburg [11. For the main ion

Introduction This is not a normal type species this is at least lo4 times our de- paper, it is rather a warning and an adver- tection limit'

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tisement for help. We understand very little Experimental results: Gold extraction of the effects which we report upon here. foil. For the first ten happy minutes of

the many months of trials, weak currents of In our experimental work on positive co-

ronas we have chosen positive ion extrac- ⁰⁺ and NO+ were extracted from our 20pA pos.

2

tion with subseauent enersv and mass analv-

- -

We expect this to give information about the gas purity (hydrated ions etc)

,

cathode field (through the 0$/03 ratio), and about possible ion bunching in the space charge dominated field in the positive glow type coronas.

It was deemed essential that the extrac- ted ions should give a true picture of the ion population arriving at the cathode plane. Thus, the extraction holes had to have diameters and lenaths of the ord&s of some gas kinetic mean free paths or less, to avoid the formation of undercooled gas jets where ion-molecule clustering processes could occur. The mean free path in NTP air is.about .lpm, and a large and quite suc- cessful effort was made to produce holes of suitable size and form in Au and A1 foils of sufficient thickness to stand the pres- sure differential. This will be reported elsewhere. In the present work matrices of 4 x 4 conical holes of about 5pm smallest diameter were used, as illustrated in Figs. la and b.

Using such a gold foil, combined with the

It req2eared for an~khsr 1 C minutes sor,?

months later, then disappeared for good.

Of course various remedies were tried, like increasing the extraction field and varying the gas pressure, but to no avail.

It is noteworthy that whenever the foil was made cathode in a negative glow dis- charge in the same gas, positive ions were extracted readily enough. They were the ex- pected primary ions from ionization in air:

N;, N+, o;, o+, all having energies above 100 eV.

Aluminum extraction foil Because of its insulating oxide layer, an A1 foil should behave very differently from Au. As seen from Fig.lb the A1 foil holes did not get quite the desired conical shape, and, to make matters worse, we mounted the foil with

the conical openings facing the discharge.

However, this time the expected positive ions did come through, though with far less intensity than expected from geometrical considerations.

The following properties have been found:

1) A negative glow discharge to the foil gold and gold plated, differentially pumped gives primary positive ions, as with AU

.

But, after the glow, or even after only one ion focussing and analyzing system of Fig. - -

-

Ic, an ion current of 10-"A of unharmed positive streamer crossing to the foil sur-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797159

face, all thermal energy ions were blocked for some tens of minutes. Then they gradu- ally reappeared, as exemplified by Fig.2.

(We have the feeling that this also happens with newly installed Au foils, with the difference that these foils do not recover).

2) Decreasing the focus electrode potential from -800 to -2200V did not change the ex- tracted ion currents, not even at times when the ions were partly blocked due to a previous negative glow or positive streamer.

Increasing the focus electrode voltage to -400V gave some 20% reduction in the ex- tracted current. All other electrode poten- tials were'kept ajusted for maximum extrac- tion currents.

3) A central disk, 5mm dia., of the extrac- tion foil including the holes was insulated electrically and the ion current to it was monitored, to check whether the disappea- ring ions were deflected away from the extraction hole area by surface charges accumulated during negative glows. No effect whatsoever was found.

Discussion. Our experiments have shown:

A) The positive ion current to the part of the cathode foil containing the extraction holes is not disturbed by the effect that blocks the extraction.

B) The extraction field is sufficient to collect a fair and field-independent part of the ions passing through the holes.

C) A negative glow or a positive streamer to the extraction foil send >lo0 eV ions through the holes, but also blocks the holes for thermal energy ions for 10-20 minutes afterwards (A1 foils)

,

itely (Au foils)

The most straight-forward conclusion from these facts is that the A1 surface oxide layer must be benificial for thermal energy positive ion extraction. This means that a positive surface charge must prevent positive ions in being destroyed on the hole wall, but not prevent them in entering the holes. The higher energy ions in a ne- gative glow must then be supposed to destroy the insulating layer, which after- wards, on Al, slowly will reform in air.

difficulties:

a) At 2kPa air pressure the mean free path is about equal to the hole diameters. The only obvious effect that could block the holes is the mirror potential attraction of positive ions to the wall. Computations show, however, that this only will affect thermal velocity ions less than .3vm from the hole wall. Thus, geometrically, an easi- ly detectable ion current should pass at these low pressures. It does n ~ t .

b) At high pressures, *20kPa, the ions must diffuse in an air stream through the hole.

Stationary solutions for diffusion in air streaming through a cylindrical hole of dia.

and lenght 5prn show an ion concentration decrease by a factor of about 20, which again should leave ample ion current for our detection apparatus. It does not appear.

References [ 1 1 W a r b u r g , E . "Handbuch d e r P h y s i k " VoZ.14, pp154-155. S p r i n g e r 1 9 2 7 .

Acknowledgements We g r a t e f u l l y a c k n o w l e d g e

-

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C o r o n a

a ) Au hole p r o f i l e s

a ) and b ) : Typical

e x t r a c t i o n holes and quadrupo Ze

c ) Ion e x t r a c t i o n system

F i g Electrode system and foils for ion extraction

However, this explanation has serious %@ ^{- e r ~} of ion -action through Al foil after negative glow discharge

1 ^Ion 15pA p o s i t i v e glow corona current 5. I @a a i r

#

o 1

/-

I . I 5b min: a f t e r n i g . g l m +