Note: Descriptions are shown in the official language in which they were submitted.
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HIGH-ENERGY LASER OF THE TEA TYPE WITH PRE-IONIZA~ION TUBES DISPOSED AXIALLY
PARP,L~ TO TH~3 I~:R
Specificatian:
The invention relates to a high~energy laser of the TE~ Type, especi-
ally a TEA Excimer laser with excitation carried out by a condenser discharge
being free of arcs and as homDgeneous as possible in the gas space betwe~n at
least two electrodes of a discharge tube extending parallel to the optical axis
of the laser and disposed opposite each other at a spacing, at least one of
which has a mushroomrshaped cross section.
U.S. Patent No. 4,365,337, issued December 21, 1982, which will be re-
ferred to as No. (1), proposes an excitation system for fast pulse discharge, andin particular a high-energy laser of the TE~-type which is distinguished by very
low self inductan oe and high switching capacity. The shape of the laser elec-
trodes, hcwover, can perhaps be called approximately mushroom-shaped. Electrodes
with mushroom~shaped cross sections are known fm m the journal "Electronics
Letters", March 25, 1971, vol. 7, pp. 141,142, referred to as (2); and further-
mDre through the journal "Applied Physics Letters", vol. 29 of De oe mber 1, 1976,
pp. 707 to 709, referred to herein as (3). It is also kncwn fro~ (2) to give the
laser electrodes a so-called Rogowski profile in order to obtain an improved
field distribution and homDgeneity of the discharge. The same purpose is served
by the so-called Chang profile, seen in the journal " m e Review of Scientific
Instruments", vol. 44, of April 1973, pp. 405 to 407, which will be given No. (4).
m e present invention is also con oe rned with the problem of obtaIning
the largest and m~st hcrlx~oeous discharge cross section
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perpe~ndicular to the optical axis and the longitudinal direction
of the electrodes that is possible in TEA lasers. This is ~Q be
accomplished by a suitable construction of the electrodes and
through the use of an effective pre-ionization unit. As has been
shown, the discharge in the TEA-Excimer lasers known so far still
has the tendency to be split into several individual regions
although the laser electrodes with mushroom cross sections have
already brought an improvement.
It is accordingly an object of the invention to provide a high-
energy laser of the TEA type with pre-ionization tubes disposed
axially parallel to the laser, which overcomes the hereinafore-
mentioned disadvantages of the heretofore-known devices of this
general type, and particularly a TEA-Excimer laser with respect
to a discharge cross section that is as large as possible and
homogeneous. This is of particular importance because the emphasis
of use of TEA lasers has shifted more and more into the industrial
area, and thus questions of econ~my and therefore of the efficiency
and the optical energy of a single'pulse are coming more and more
to the forefront~
With the foregoing and other objects in view there is provided,
in accordance with the invention, a high energy laser of the TEA
type, especially a TEA Excimer laser, comprising a discharge tube
having an optical axis of the laser and a gas discharge space
formed therein, at least two main electrodes being spaced apart
and oppositely disposed in the'discharge'tube'and extended parallel
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to t'he optical axis, excitation being carried out by condenser dis-
charge being free of arcs and as homogeneous as possible in the
gas discharge space between the electrodes, at least one of the
main electrodes having a mushroom-shaped cross section with a
mushroom-shaped stem for current conduction and a mushroom-shaped
hat for current distribution and being extended into the gas dis-
charge space,' and at least one rod-shaped auxiliary eIectrode
being disposed paralleI to the'optical axis between or adjacent
to the electrodes and at a given sparkover distance from one of
the'electrodes, for pre-ion'ization of the gas discharge space,the
at least one'auxiliary electrode having an inner~ conductor and a
dielectric surrounding the'inner conductor.
In accordance with another feature of thé invention, two of the
mushroom-shaped eIec~rodes are'disposed facing each other with
the'mushroom-shaped hats in mirror symmetry, at least one of the
mushroom-shaped hats of the at least two main electrodes having
two of th.e auxiliary el'ectrodes associated therewith.
In accordance with'a further feature of the invention, thé main
electrodes are'pierced by cooling canals formed therein being ex-
tended paralleI to the'optical axis.
In accordance with an added feature of the invention, the mush-
room-shaped hats are'in the'form of screen electrodes formed of
perforated she'et metal.
In accardance with an addi.tional feature of the invention, the
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auxi].iary electrodes are formed in niches between mushroom-shaped
hats and mushroom-shaped stems.
In accordance with again another feature of the invention, the
inner conductor of the auxiliary electrodes is cylindrical and
the dielectric surrounding the conductor is hollow!and cylindrical.
In accordance with again a further feature of the invention, the
mushroom-shaped electrodes have a Chang profile
In accordance with again an added feature of the invention, the
mushroom-shaped electrodes have a Rogowski profile.
In accordance with again an additional feature of the invention,
the dielectric is high voltage-proof and is formed of a material
from the group consisting of BaTiO3, SrTiO3, A1203 and BeO.
In accordance with a concomitant feature of the invention, there
is provided a Blumlein charge transer circuit, the inner conductor
of the pre-ionization rod-shaped auxiliary electrodes being con-
nected to and opposite one of the main electrodes in the circuit
in a conducting manner.
The advantages attainable with the invention are in particular that
through the orientation of the mushroom hat parallel to the laser
axis, ~he mushroom stem, and the rod-shaped auxiliary electrode,
a very uniform pre-ionization and therefore main discharge as well
can be achieved without splitting the discharge into the individual
regions.
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Other features which are considered as characteristic for ~he
inventlon are se~ forth in the appended claims.
Although the invention is illust~ated and described herein as
embodied in high energy laser of:the TEA-tyPe with pre-ionization
tubes disposed axially parallel to the 'laser, it i~ nevertheless
not intended to be limited to the details shown, since various
modifications and structural changes may be made therein without
departing from the'spirit of the invention and within the scope
and range of equivalents of the claims.
The construction and method of operation of the inventionJ however,
together with additional objects and advantages thereof will be
best understood from thé following description of specific mbodi-
ments when read in connection with the accompanying drawings, in
which:
Fig. 1 is a simplified diagrammatic cross-sectional view of a
la~er tube with two opposite mirror-symmetrical mushroom-
shaped laser eIectrodes, each including a pre-ionization
auxiliary pair of electrodes;
Fig. 2 is a cross-sectional view of a variant embodiment of the
laser electrode construction with a mushroom hat of screen
electrodes formed of perforated sheet metal;
Fig. 3 is a cross-sectional view of a rod-shaped auxiliary elec-
trode;
Fig. 4 iB a diagrammatic and schemat'ic pulse'circuît-diagram for
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generating the pre-ionization and main discharge pulses in the form of a
Blumlein-circuit; and
Fig. 5 is another diagrammatic and schematic pulse circuit diagram in the form
of a charge-transfer circuit for generating the firing pulses.
Referring now to the figures of the drawing and first particularly to
Fig. 1 thereof, it is seen that the high-energy laser of the ~EA type (TE~ laser
meaning "transversely excited atm~spheric pressure laser") is constructed in
such a way that, in a gas spa oe 1 between the two electrodes of the laser or dis-
charge tube 3 which extend p æ allel to the optical axis 2 of the laser and are
disposed opposite each other at a distan oe al, an æ c-free condenser discharge
which is as homogeneous as possible takes plaoe . The approximately rectangul æ
housing 3a of the laser which is designated as a whole with referenoe character
L, is assembled in a gas-tight manner from ceramic walls 3al, 3a2, 3a3 and 3a4,
which preferably are formed of high-purity A1203 ceramic. The laser gas has a
oomposition such as is described for instance in U.S. Patent No. 4,365,337,
issued Deoember 21, 1982. The electrcdes El, E2 each extend with a mushroo~
shaped stem 4 serving as the current lead and a m~shroomrshaped hat 5 serving
for the distribution of the current. The stem and hat extend into the discharge
or gas space 1 and have tabs 6 brought through the walls 3al, 3a2 to the outside.
For pre-ionizating of the discharge spa oe 1, rod-shaped auxiliary elec~rodes Hll,
H12, are disposed between the electrodes El,
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E2 immediately adjacent to the mushroom-shaped hat 5 of the
electrode El, and auxiliar~ electrodes H21, H22 are disposed
immediately adjacent to the mushroom-shaped hat 5 of the electrode
E2. Each of the auxiliary electrodes`are located at a sparkover
distance a2 from the electrodes. I~mediately prior to the firing
of the laser, a sparkover between the surrounding dielectric of
the auxiliary electrodes Hll, H12, H21, H22 and the associated
electrodes El and E2, respectively, takes place. The ~V light emit-
ted by the sparks ionizes the gas enclosed by the electrodes El
and E2. The electrodes El, E2 are disposed opposite each other,
with their mushroom-shaped hats 5 in mirror symmetry as can be
seen. The two auxiliary electrode pairs Hll, H12 and H21, H22
are disposed opposite each other in mirror symmetry. This arrange-
ment is particularly advantageous for obtaining a discharge cross
section which is as large and as homogeneous as possible.
According to Fig. 3, the auxiliary electrodes designated with
reference character H have rod-shaped inner conductor 7,which
is made of copper, for instance, and a dielectric 8 formed as a
circular ring enclosing the inner conductor 7, which is preferably
a high purity BaTiO3, SrTiO3~ A12Q3 or BeO ceramic as well~. These
auxiliary electrodes H are fastened at the ends thereof in a suit-
able manner to the non-illustrated end walls of the laser chamber 3
of Fig. 1. Cooling canals 9 pass through the laser electrodes El,
E2, i.e. the mushroom-shaped hats 5 and the mushroom-shaped stems 4.
The cooling canals 9 preferably extend parallel to the optical axis
2 of the laser, through which a coolant such as water is pumped.
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The inner conductor 7 of the auxiliary electrode H can also be a
liquid conductor pumped by a circulating system through the ceramic
tu~e 8 for simultaneous cooling.
Fig. 2 shows the construction of perforated mushroom-shaped hats
5' as screen eIectrodes w~ich for this purpose are fonmed of per-
forated sheet metal. The auxiliary electrodes H are therefore dis-
posed within niches 10 formed ~y the mushroom-shaped hat 5' and
the'mushroom-shaped stem ~. In the firing process, the W -radia-
tion generated~by the sparkover between the'auxiliary electrodes H
and the mushroom-shaped hat 5' can come in through the screen
openings 11 into the'di'sc~arge space between the main electrodes
El, E2 and ionize thé'latter.
In the'fired gas di'scharge'or pumping of the laser, the current pulse
must flo~ through'thé stem 4 of the mushroom towards the center of
the mushroom-shaped hat and can only then be distributed in the
hat 5. Preferably, the hat 5 of the electrodes El, E2 is developed
as a Chan~ or Ro~owski profile as is known from the literature on
electrodes of constant homo~eneous surface field intensity. The
dual arran~ement of the mushroom and the auxiliary electrodes is
particularly advanta~eous even thoup~h'it is possible in principle
to construct only one'of the electrodes El, E2 as a mushroom elec-
trode. The pulse'circuit according to Fig. 4 for generating the
firing and discharge'pulses is a so-called Blumlein circuit. A
pulse'generator P shown therein includes a hi~h-voltage power supply
HV, the voltage'of w~i'ch'can be'applied through'a low inductance
switch S to a first charging caPacity Cl. A series circuit including
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a second charging capacity C2 and the laser electrode sections El-E2 `
is shunted across this capacity Cl. A low resistance resistor R is
shunted across the latter and connected to ground, which is a high
pontential as compared to the resistance value of the fired plasma.
As is seen in Fig. 4, the second poles of the switch S and of the
switching capacity Cl are also connected to ground potential, as
are the laser electrode E2 and the auxiliary electrodes Hll, H12
associated with the opposite laser electrode. The auxiliary elec-
trodes H21,` H22 on the other hand are connected to the oppositely
disposed laser electrode El in a conducting manner.
The charge-transfer-circuit according to Fig.5 in which like
switching elements carry the same reference symbols as in Fig.4,
differs from the circuit according to Fig. 4 particularly by the
feature that the second switching capacity is connected in parallel
with the laser discharge path El-E2, and that the first switching
capacity Cl is connected in series with the low-inductance switch S
and therefore to the high voltage power supply.
In both circuits, a shock-like high voltage discharge is released,
by first closing the high voltage switch S which may be a spark gap
or a thyratron, through the auxiliary electrodes and then through
the laser electrodes, the pulse width, height and phase of which
results from the inductive, capacitive and ohmic values of the
switching elements and leads.
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