THE HIGH POWER HYDROGEN ARC AS A PRECISE STANDARD SOURCE OF CONTINUUM RADIATION BETWEEN 53 AND 92 NM

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THE HIGH POWER HYDROGEN ARC AS A PRECISE STANDARD SOURCE OF CONTINUUM

RADIATION BETWEEN 53 AND 92 NM

K. Behringer, P. Thoma

To cite this version:

K. Behringer, P. Thoma. THE HIGH POWER HYDROGEN ARC AS A PRECISE STANDARD

SOURCE OF CONTINUUM RADIATION BETWEEN 53 AND 92 NM. Journal de Physique Collo-

ques, 1979, 40 (C7), pp.C7-829-C7-830. �10.1051/jphyscol:19797400�. �jpa-00219400�

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JOURK4L DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7- 829

THE

HIGH

POWER HYDROGEN ARC AS A PRECISE STANDARD SOURCE OF CONTINUUM RADIATION BETWEEN 53 AND 92 NM

K. Behringer and P. Thoma.

LehrstuhZ f. Techn. EZektrophysik, Techn. llniversitZit Miinchen, Fed. Rep. of Germany.

I. Introduction: In an earlier paper /I/ it has recorder after D/A conversion.

been demonstrated, how a stationary hydrogen arc Well-defined flow rates are adjusted for the gas se- discharge can be used as a true primary source of paration of H and He. A pure He input of 800 R/h at spectral radiance in the Balmer region, i.e. between the anode, the vuv end of the arc, covers mainly the

124 and 360 nm. The method is based on the fact that gas consumption of the differential pumping system, the H continuum emission coefficients are calculable a small fraction also streaming into the discharge within a few percent, since the theoretical cross channel. From the other end, a mixture of H and He sections are exactly known. For an optically thin is forced through the arc. Both gas flows leave the plasma layer, as realized in the Balmer range, the channel together at a port mid-way between the elec- spectral radiance can be predicted with similar ac- trodes. As a result the arc column consists of the curacy. In the following, an arc standard source

below 100 nm will be described, now involving the optically thin Lyman resonance continuum of hydrogen. While the basic idea is --ery similar to the one in /I/, self-absorption problems are now much more critical and can only be overcome by a helium gas separation in the arc.

11. Experiment: A stationary plgsma is generated, in a cascaded arc chamber /2/ (2mm 0, 11=54mm). The arc is operated between 80 and 100 A at atmospheric pressure, The plasma column is observed end-on from two.sides by two monochromators, a 2m instrument for the visible and uv range and a lm for the far vuv.

The general experimental set-up is very similar to the one in /I/, except for a three-stage differential pumping system, now connecting the arc chamber and the vacuum tank. The first capillary of this system is 0.8mm wide and located 2mm from the anodes.

In the vuv, a channeltron detector is used in photon counting mode. The counting rates can be stored di- gitally on magnetic tape or plotted on a stripchart

H-He plasma near the cathode, the radiation of which is the subject of the present investigation, and,a pure He layer near the anode. Fig. 1 shows the recorded vuv spectrum for 80 A. Below 100 nm the high- er Lyman lines merge into the advanced series limit.

Then follows the H resonance continuum, superimposed by some impurity lines (Ar has been used during ig- nition). The resonance lines of He are self-revers- ed in the cooler He layer at the anode. Below 50.4 nm, the He series limit, all radiation is completely reabsorbed. Thus, the hydrogen continuum radiation can be used for calibration purposes from 53 to 92 nm. The spectrum in Fig. 1 has been corrected for the vuv system response according to calculations of the H spectral radiance as described below.

111. Plasma Diagnostics: The recorded end-on spec- tra consist of line and continuum radiation, emited by the H as well as the He layers of the arc. In or- der to isolate the H continuum (only necessary in the uv region), measurements have also been made for pure He in the whole arc. From the ratio of the He1

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797400

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318.8 nm line intensities in pure He and in the above configuration, an effective He length has been derived and the respective continuum signals have been subtracted from the total. The electron density is measured from the half-widths of the lines H and

I3 H For the hydrogen concentration c =20%, the re-

Y' H

sult is 4x10'~ ~ m - ~ , varying only slightly with the arc current. The electron temperature Te is derived from the excited state population relative to the electron density (PLTE diagnostics). Again, for c =

H 20%, Te=19500 K for 80 A and 21500 K for 100 A, both values decreasing with increasing hydrogen frac- tions. Under these conditions, the H plasma will be very close to LTE.

cients must therefore be a constant, corresponding to line branching ratios. Though this simple consi- deration has to be corrected for the other mecha- nisms of continuum radiation (f ree-f ree, H-)

,

it can be shown that the total emission coefficients in the uv and in the far vuv are correlated by a calculable factor, which is virtually independent of temperatu- res and densities, if only the wavelength pairs are chosen in an appropriate way. If X is to remain in

B

the easily accessible range from 200 to 365 nm, A L lies between 73 and 87 nm. The above correlation must also be valid for the spectral radiances ofthe present arc plasma, provided that the radiation re- mains optically thin. A measurement of the optical thickness is therefore simply achieved by plotting

1 L y - I r _{1 y - b}

the recorded vuv vs. the uv signals. The result is

m-

indeed a straight line through the origin up to cHN

5.

25%. Above, an increasing influence of self-ab-

0-

.

^, ^, ^, ^,

. . .

sorption is observed, which corresponds approximate

6 ly to LTE estimates. Having thus excluded possible

b

boundary or non-LTE effects, the spectral radiance

2

1 0

H.1 H.1 A r l l

50 51 52 53 51 55 56 57 58 59 6 0 61 6 2

Fig. 1: Vuv arc spectrum, recorded for I=80 A.

1V.Calibrations below 100 nm: A recombinationofthe free electrons, populating the same energy interval, leads to the Balmer continuum radiation at XB, when the final state is n=2, and to the Lyman continuum at A=, when n=l. The two wavelengths are related by

~ c / A ~ = ~ c / x ~ + E ~ , E being the excitation energy for 2

in the mentioned vuv region can be calculated very accurately from a calibration of the H continuum in the uv range (uncertainty about 5%). For an appli- cation at other wavelengths the slope of the Lyman continuum must be derived from the temperature diag nostics, leading to error bars of about 15% at 53 nm. In the present experiment, this H vuv standard has already been used for measuring the continuum radiation of the pure He arc between 65 and 92 nm.

An excellent consistency of these results with respective theoretical calculations confirms the va- lidity of the described concept.

/I/ W.R. Ott, K. Behringer, and G. Gieres, Appl.

Opt.

14,

2121 (1975)

/2/ H. Maecker and S. Steinberger, 2. angew. Phys.

23, 456 (1967)

-

n=2. The ratio of the corresponding emission coeffi-