Ultraviolet radiation source with envelope having pressure equalization region

A device for producing ultraviolet radiation of high spectral radiation intensity is disclosed in which the radiation is generated in a mercury-argon-filled discharge tube with a thermoemissive cathode by a wall-stabilized d.c. gas discharge at a mercury pressure p.sub.Hg between 5 .times. 10.sup.-.sup.3 and 5 .times. 10.sup.-.sup.1 torr, an argon pressure p.sub.Ar between 0.01 and 10 torr and a current density j.sub.o of the discharge current I between 1 and 25 A/cm.sup.2, and in which the two electrode spaces are connected to one another through a pressure-equalization region as well as through the discharge region wherein the two electrodes are arranged in tandem in a common envelope, the two arms of the discharge region are joined to the end of the envelope nearest the discharge region so that one arm projects coaxially into the envelope and passes through the toroidal electrode nearest the discharge region, and the inner wall of the envelope and the arm projecting into the envelope are each provided with a collar between the two electrodes, the collars being so arranged with respect to one another as to form a connecting passage acting as a pressure-equalizing region running between the cathode and anode spaces with a first portion running in the cathode/anode direction and a second portion running in the anode/cathode direction.

BACKGROUND OF THE INVENTION 
The invention concerns a device for generating ultraviolet radiation with 
high spectral radiation density, the radiation being produced in a 
mercury/argon-filled discharge tube with a thermoemissive cathode by a 
wall-stabilized d.c. gas discharge at a mercury pressure p.sub.Hg between 
5 .times. 10.sup.-.sup.3 and 5 .times. 10.sup.-.sup.1 torr, an argon 
pressure P.sub.Ar between 0.01 and 10 torr and a current density j.sub.o 
of the discharge current I between 1 and 25 A/cm.sup.2, and the two 
electrode spaces being connected to one another by a pressure equalization 
space as well as by the discharge region. 
Such a device -- in short: high-current-low-pressure-UV-radiation 
source-was described in its essentials in main German patent PA 24 12 
997.3 (copending U.S. Pat. application Ser. No. 551,425, filed Feb. 20, 
1975) where the value of the discharge current I or the current density 
j.sub.o is adjusted to suit the material to be irradiated, and the mercury 
pressure is controlled by the mercury temperature in such a way that the 
yield of the line of wavelength 253.7 .mu.m, i.e. the ratio of the 
radiation output at this wavelength to the electrical power input to the 
discharge, is at least 80% of the yield-maximum for the set current 
density j.sub.o. The quantitative relationship between the set 
dischargecurrent density j.sub.o and the mercury temperature to be 
selected is detailed in the main patent. 
In the practical form of embodiment given in the main patent the two 
electrodes, cathode and anode, are enclosed in separate envelopes, 
preferably of glass. This makes fabrication difficult and makes the device 
rather delicate for handling in industrial use. If the envelopes are 
located side-by-side, a more complicated, bulkier and more expensive 
holder becomes necessary. In certain installations, e.g. in packaging 
machines for sterile liquid filling in which filling is done through a 
tube, or in the sterilization of bottles, radiation sources with 
side-by-side electrode-space envelopes can be incorporated only with great 
difficulty. 
SUMMARY OF THE INVENTION 
It is an aim of the invention to overcome these disadvantages and to 
improve the radiation source of the main patent in such a way that it is, 
particularly in industrial use, resistant to handling and easy to install, 
while simultaneously being economical to produce in terms of cost and 
materials. 
The foregoing and other objects are attained in accordance with one aspect 
of the present invention through the provision of a device of the 
aforementioned type by arranging the two electrodes in tandem in a common 
envelope, joining the two arms of the discharge space to the end of this 
envelope, making one of these arms coaxial and inserted into the envelope 
so as to pass through the toroidal electrode on the discharge end of the 
envelope, and providing collars on the inner wall of the envelope and on 
the arm projecting into the envelope, the collars being situated between 
the two electrodes in such a way that a connecting passage acting as a 
pressure-equalization space is formed between the cathode and anode 
spaces, a first portion of this passage running in the cathode/anode 
direction and a second portion running in the anode/cathode direction. 
A UV-radiation source of such a design optimally satisfies the 
aforementioned requirements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1 thereof, in FIG. 1 is shown an envelope 1 
essentially of heat-resistant glass in which the two electrodes, the 
cathode 9 and the anode 10, are arranged in tandem. As is also 
particularly apparent from FIG. 2, the discharge region 5 consists of two 
arms 6 and 7, which are both joined to the end of the envelope 1. Here the 
arm 6 projects coaxially into the envelope 1 and passes through the 
toroidal anode 10. 
The arms 6 and 7 are of fused quartz or the like, so that the UV-radiation 
can get out. Because of the different thermal expansion coefficients of 
pyrex and fused quartz, the arm 7 is joined to the envelope 1 by a graded 
seal 18. For this a number of glass rings are fused together, the rings 
having thermal expansion coefficients graded between those of the pyrex 
and fused quartz. The arm 6 can then, according to the concept of the 
invention, be fused directly to the arm 7, so that a second graded seal to 
envelope 1 is unnecessary. 
At their ends away from the electrodes the arms 6 and 7 are fused together 
so as to produce a closed, tubular, flat-meander form of discharge region 
5 (FIG. 2). 
Between the cathode 9 and anode 10 there is on the inner wall of the 
envelope 1 a collar 12 which engages with a collar 11 on the bottom end of 
arm 6 so as to form a connecting passage 4 which acts as a 
pressure-equalization region. This connecting passage 4 runs in one 
portion of it in the cathode 9/anode 10 direction, and in its other 
portion in the anode 10/cathode 9 direction. In this way there are 
prevented instabilities, e.g. plasma oscillations, on the one hand, and on 
the other, backfiring through the pressure-equalization region (cf. here 
Swiss patent Ges. Nr. 8456/74 or German patent P24 33 557.7). 
The collar connection 11/12 divides the envelope 1 into two electrode 
spaces, the cathode space 2 and the anode space 3. 
Between the graded seal 18 and the collar 12 the wall of envelope 1 has a 
Kovar-metal ring 14. (Kovar is an alloy of approximately 27% nickel, 19% 
cobalt and the rest iron, which has the same thermal expansion coefficient 
as pyrex up to the latter's softening point and therefore can make a seal 
with the glass). Inside on the Kovar-metal ring 14 there is attached a 
metallic support for the anode 10. The electrical connection for anode 10 
can be provided on the outside. 
The cathode 9 is situated in a hollow cylinder 13 of nickel, for example, 
to reduce heat radiation from the cathode. Cathode 9 itself consists, for 
example, of a coiled nickel lattice-ribbon with a coating of BaO, the 
anode 10, for example, consisting of solid molybdenum with a zirconium 
coating. 
The basic construction of the envelope 1 of FIG. 3 is like that of FIG. 1, 
but here the electrodes are interchanged, the connecting passage formed by 
the collars is somewhat different in design and the envelope is 
constructed for a cooling-trap shaped discharge region 5, as is apparent 
from FIG. 4. 
In FIG. 3 the toroidal cathode surrounds the arm 6 projecting coaxially 
into the envelope 1. Since the cathode 9 requires two electrical 
connections for heating, two Kovar-metal rings 14 and 15 are provided, one 
for each connection. 
The connecting passage formed by the collars 11/12 is self-explanatory as 
represented in the drawing. 
Since in the configuration of FIGS. 3 and 4 the tubular arm 6 is located 
coaxially inside the dead-ended tubular arm 7 of the discharge space 5, 
the arms 6 and 7 are joined by only one or a few supports 8 spaced over 
the length of the arms 6, 7 and, since there is never more than one 
support anywhere along this length, the gas discharge in the discharge 
space 5 is not obstructed. 
The configuration of FIG. 1 with a discharge region 5 in the form of a 
double spiral with 3m of coiled tube of 10mm inner diameter was 
successfully operated with a 10 A discharge current. The diameter of the 
envelope 1 was 50 mm, its length 200 mm. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described herein.