Laser chamber insulator with sealed electrode feedthrough

Details Laser chamber insulator with sealed electrode feedthrough Laser chamber insulator with sealed electrode feedthrough

2002-06-07

2003-12-09

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Gas

C372S057000

Reexamination Certificate

active

06661826

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to laser chambers for excimer lasers and other electric discharge lasers and more particular to pulse high voltage feedthrough structures for such chambers.
BACKGROUND OF THE INVENTION
FIGS. 1A and 1B
are cross-sectional views showing the inner structure of a laser chamber
10
in a conventional transversely excited (TE) excimer laser (see Akins et al., U.S. Pat. No. 4,959,840, issued Sep. 25, 1990, and incorporated herein by reference in its entirety).
FIGS. 1A and 1B
are excerpts from the '840 patent.
FIG. 1C
is a cross section similar to
FIG. 1B
but showing the entire length of a prior art laser chamber. A laser enclosure
10
provides isolation between a laser chamber interior and the exterior. Typically enclosure
100
is formed by upper and lower enclosure members
12
and
14
, which are coupled together and sealed using an o-ring seal
16
, extending along a perimeter of enclosure
10
. The laser chamber interior is filled to a predetermined pressure with a lasing gas mixture including the hazardous gas fluorine, F
2
. A pulsed electric discharge is generated in the lasing gas mixture in a discharge region
22
by a high voltage pulse applied between a cathode assembly
18
and an anode assembly
20
. Since anode assembly
20
is generally electrically grounded to laser enclosure
10
, the entire pulse high voltage is applied between cathode assembly
18
and upper enclosure member
12
. The pulsed gas discharge typically produces excited fluorine, argon fluoride or krypton fluoride molecules, which generate laser pulse output energy. The pulse output energy propagates from discharge region
22
through an optical output window assembly (not shown in FIG.
1
A). Cathode assembly
18
and anode assembly
20
, defining discharge region
22
, and extend for about 28 inches substantially parallel to one another for most of the length of laser chamber
10
perpendicular to the plane of FIG.
1
A.
Recirculation of the lasing gas mixture is provided by a tangential fan
46
. As shown by arrows in
FIG. 1A
, the flow of lasing gas mixture is upward through tangential fan
46
and transversely across discharge region
22
as directed by a vane member
52
. The lasing gas mixture that has flowed through discharge region
22
becomes dissociated and heated considerably by the pulsed gas discharge. A gas-to-liquid heat exchanger
58
, extending substantially the length of laser chamber
10
perpendicular to the plane of
FIG. 1A
, is positioned in the gas recirculation path to cool the heated gas. Recirculation cools and recombines the lasing gas mixture, thereby allowing repetitively pulsed laser operation without replacing the lasing gas mixture.
In this prior art chamber high voltage pulses in the range of about 16 kv to 30 kv are applied to cathode
20
at repetition rates of about 1000 pulses per second from a high voltage bus
70
mounted on top of chamber
10
as shown in FIG.
1
C. Bus
70
consists of a thin copper plate mounted on a ½ thick aluminum plate with rounded surfaces. (This aluminum plate is referred to as a “corona plate” since its purpose is to reduce or minimize corona discharge from the high voltage bus.) The bus is energized by a peaking capacitor bank typically consisting of 28 individual capacitors (not shown) mounted in parallel and electrically connected between bus
70
and the metal enclosure
10
which functions as ground. The high voltage pulses are transmitted to cathode
18
through a feedthrough structure consisting primarily of 15 feedthrough conductor assemblies
72
as shown in FIGS
1
A, B and C.
Cathode
18
and each of the 15 feedthrough conductors carrying peak voltages in the range of 16 kv to 30 kv must be insulated from the metal surfaces of enclosure
10
which is at ground potential. Because of the corrosive F
2
environment inside the chamber only certain high purity ceramic insulators such as high purity A
2
lO
3
can be used for the portion of the feedthrough assemblies exposed to the gas environment.
With a design of the type shown in
FIGS. 1A
, B and C ceramic parts
28
are sandwiched in between a brass part
32
and an aluminum part
12
. The laser chamber is subject to temperature swings between normal ambient temperature of about 23° C. and temperature of about 120° C. The coefficients of thermal expansion of aluminum, brass and A
2
lO
3
are about 23×10
−6
/° C., 20×10
−6
/° C. and 8×10
−6
/° C. respectively. The distance between the two end feedthroughs is about 22 inches. Therefore, in this distance a 100° C. temperature increase would produce unrestrained expansions of about 0.052 inch, 0.045 inch and 0.017 inch respectively for aluminum, brass and AlO
2
. This makes a difference of about {fraction (1/32)} inch between the ceramic and metal parts. It is important that good seals be provided for the feedthrough assemblies to prevent hazardous fluorine from escaping into the working environment.
The issues discussed above have been dealt with in the design of the laser portrayed in
FIGS. 1A
, B and C. This laser utilizes three main insulators
28
A,
28
B and
28
C to insulate the cathode
18
from the chamber member
12
. In this prior art design as shown in
FIG. 1C
, fifteen feedthrough connectors are separated into three separate groups so that the effective length of the sealed region of each of the resulting metal-ceramic-metal sandwiches is only about 6 inches. This reduces the differential expansion by a factor of about 3.5 as compared to a single piece insulator covering the entire electrode length. Sealing at the feedthroughs is provided by tin-plated, nickel-copper alloy “C” seals
32
and
34
as shown in
FIGS. 1A and 1B
. Seal
32
are circular seals making a seal around each of the 15 feedthroughs at the insulator
28
, cathode support
26
interface. Each of three seals
34
make the seal between the bottom of upper chamber
12
and the top of one of the three insulator plates
28
, each seal
34
providing a single seal around five feedthroughs.
In this prior art design, cathode support bar
26
is bolted to cathode
18
. Threaded feedthrough rod
36
threads into cathode support bar
26
. Feedthrough insulator
41
insulates rod
36
and a feedthrough nut (not shown in
FIGS. 1A and 1B
) is threaded onto feedthrough rod
36
and holds insulator
41
in place. A holddown bolt with a Belleville washer is passed through an insulator cap called a “buttercup” is then screwed into the feedthrough nut to apply a compressive force clamping the electrode support to the top inside wall of the chamber with insulator plate
28
and seals
34
and
32
sandwiched in between.
The prior art feedthrough designs shown in
FIGS. 1A
, B and C has been commercially very successful and is utilized in hundreds of excimer lasers currently operating around the world. The design is basically trouble-free with extremely minimal problems with leakage or electrical failure despite the harsh F
2
environment and in many cases continuous round-the-clock operation for weeks and months at a time.
However, the very large number of parts of the above described prior art design make the fabrication expensive. Also, a need exists for a reduction in the electrical inductance associated with the feedthrough design. Therefore a need exists for a better electrical feedthrough system for electric discharge lasers.
SUMMARY OF THE INVENTION
A feedthrough structure of a gas discharge laser chamber conducts electric power through the wall of a sealed gas enclosure to a single piece elongated electrode inside the enclosure. The feedthrough structure includes a single piece integrated main insulator larger than the electrode. The main insulator is compressed between the electrode and the wall of the enclosure. The surfaces forming interfaces between the electrode and the single piece insulator are the insulator and the wall are all very smooth to permit the parts to expand and contract as the chamber temperature varies. The feedthro

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