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 The
Lexel plasma tube design is the result of a major to eliminate each
of the failure mechanisms that existed in other tube designs. The tube
design uniquely combines high vacuum technology and ceramic power tube construction methods for the ultimate in rugged, reliable high performance.
Consequently, the Lexel tube has a proven record of reliable, long-life
operation that is unmatched in the industry.
To view a
close-up picture of a Lexel plasma tube, click the photo at right.
Conduction cooled Be0 bore
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Low temperature operation with
unprecedented thermal stability
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Eliminates water boiling, frequency
jitter and thermal drift
The choice
of bore material is the most important aspect of any ion laser plasma tube
design. Beryllium Oxide (Be0) ceramic has proven to be the best material.
Its exceptionally good thermal conductivity allows a tube design that
transfers all of the thermal energy to the cooling water by direct
conduction at very low temperatures.
All other
bore materials must operate at extremely high temperatures and dissipate
their heat by radiation which may result in local water boiling and thermal
instabilities.
Conduction
cooled Be0 plasma tubes store little or no residual heat. Thus Lexel's
plasma tubes reach thermal equilibrium almost instantaneously after start-up
or changes in operating level. It is not necessary to wait for bore material
to heat up and realign to the new operating temperature, as with
radiation-cooled designs. Since there is virtually no heat stored in the Be0
plasma tube, the laser and cooling water can be shut off simultaneously
without any damage to the tube. With a radiation-cooled plasma tube, a water
shutdown can cause violent water boiling, and could even cause explosive
failure.
The Lexel
Be0 plasma tube is efficiently cooled by low velocity water flow. There is
no local water boiling or turbulent flow vibration such as is common to
other designs. The Lexel plasma tube cooling was designed to yield the best
frequency stability and lowest frequency jitter of all existing ion lasers.
Rugged ceramic and metal
construction
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Strongest, most compact
and streamlined structure
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Eliminates glass tube
and pin seal breakage
The rugged
and compact Lexel plasma tube structure is built using ceramic and metal
construction exclusively. Electrodes and feed-thru leads are rugged
brazed-and-welded assemblies.
Other
manufacturers' plasma tube designs, even those using Be0 bore material, use
a glass envelope construction with fragile electrode pin seals. The major
cause of failure of glass envelope tubes is glass and pin seal breakage.
The Lexel
ceramic and metal design has completely eliminated this failure mechanism
and has proven to be the strongest and most reliable plasma tube in the
industry.
High-conductance internal gas
return system
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Provides optimum
pressure equalization and dependable starting
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Eliminates fragile
external glass tubing
Lexel
plasma tubes use an innovative high conductivity gas pressure equalization
system that is completely isolated from the discharge bore yet entirely
contained within the ceramic tube structure (patent pending). This exclusive
Lexel design feature assures optimum pressure equalization through the
plasma tube and easy, reliable starting every time.
Other
laser designs require the use of external glass tubing for the gas return
path -- a another source of breakage.
Contoured bore profile
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Matches discharge with
magnetic field for long bore life
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Minimizes bore
enlargement and particle drop-out noise
The
discharge bore profile in Lexel plasma tubes is contoured to match the
discharge current density with the axial magnetic field from the surrounding
electromagnet. This important design step, coupled with the high erosion
resistance of Be0 ceramic, eliminates the problems of particle deposits and
dusting on Brewster windows and particle "drop-out noise" often reported in
other designs.
Ceramic-insulated electrodes
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Allows complete cooling
with ordinary tap water
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Eliminates electrolysis
tube failures
 In
the Lexel plasma tube, the heat from both electrodes (anode and cathode) is
transferred through high thermal conductivity ceramic into the surrounding
cooling water. Virtually all of the heat is removed from the electrodes to
the water by conduction without being allowed to transfer into the
surrounding laser head. Since the cooling water does not contact any metal
parts of the plasma tube, there is no possibility of electrolysis. The Lexel
laser can be cooled with ordinary city tap water. It is never necessary to
use special deionized water.
Other
conductive cooled plasma tube designs have the high voltage anode in direct
contact with the cooling water, a situation which requires special high
resistivity water and inevitably leads to electrolysis tube failure.
Radiation-cooled designs do not fully contain the electrode and discharge
heat, thus allowing substantial thermal losses to be radiated to the
surrounding optical structure.
Only the
Lexel plasma tube eliminates both the electrolysis failures and thermal
radiation problems of other designs.
Large capacity free flow gas supply
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Provides optimum gas
supply throughout tube life
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Eliminates gas fill
valves and fill system failures
 A
large capacity stainless steel gas supply tank is an integral part of the
Lexel plasma tube. The gas supply tank holds up to 1,600 milliliters of
argon or krypton, enough for 8,000 to 10,000 hours of full-power operation.
This large amount of reserve gas, combined with Lexel's Be0 bore design,
maintains the gas pressure in the optimum range throughout the tube life.
Lexel's
Free Flow design eliminates the need for gas valves, refill buttons, and
pressure monitoring required in other plasma tube designs. A Lexel laser
will never fail because of a sticking or leaking fill valve, nor will it
ever run out of gas because gas pressure monitoring was neglected.
High-temperature Brewster window
seals
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Strain free non-organic
quartz to quartz window seal
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Eliminates epoxy window
leaks and contamination
Lexel
Brewster's Angle windows are sealed using a fused quartz-to-quartz glass
seal which is bakeable to over 1,000°C. This proprietary Lexel seal
completely eliminates window leaks, water vapor diffusion, birefringent
strain, and window contamination commonly observed on epoxy-bonded tubes.
The
combination of the Lexel quartz window seal, 1,000°C window bake, and
proprietary window material selection yields the best low-loss,
distortion-free and color-center-controlled Brewster windows available for
long lifetime operation.
Ultra-high vacuum processing
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Uses only high vacuum
materials and techniques
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Eliminates window,
cathode and gas contamination
The
materials used in Lexel plasma tubes are completely compatible with high
temperature ultra-high vacuum processing. There is absolutely no epoxy,
rubber O-rings or any other organic material used in Lexel plasma tubes.
Other ion laser manufacturers appear to compromise their plasma tube vacuum
integrity by using either epoxy seals, rubber O-rings or organic fill valve
seals. The use of any organic material in a plasma tube means that the tube
cannot be completely out-gassed by a high temperature bake-out, and there
will be some residual contamination gases to slowly evolve and contaminate
the windows, cathode or discharge plasma.
One of
Lexel's major contributions to ion laser reliability is the completely
vacuumcompatible plasma tube. The entire tube is processed at high
temperatures and at an ultra-high vacuum of better than 10-9 torr.
The result is the ultimate in long lifetime, contamination-free laser
operation.
Contamination-free copper pinch-off
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The ultimate seal-off
method
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Eliminates O-ring leaks
and glass stress cracks
After a
plasma tube is completely processed it must be sealed off and separated from
the vacuum processing station without affecting the vacuum integrity of the
tube or the gas purity. We use a copper pinch-off, the most advanced,
most reliable and truly contamination free method.
Other
designs use either an O-ring seal or a glass "tip-off," neither of which is
completely satisfactory. The O-ring seal method has tended to introduce air
into the tube at the time of sealoff and can slowly leak or outgas
contaminants into the tube during life. The glass tip-off requires melting
of a glass exhaust tubing which sometimes results in leaks or glass stresses
which can crack at a later time.
The Lexel
copper pinch-off is the method used on virtually all high power vacuum tubes
for over 25 years. It has proven to be 100% reliable and absolutely
contamination-free.
Continuously active flash-type
barium getter
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Assures long shelf life
and sure starting
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Eliminates tube
failures during storage or inactive periods
The final
requirement of any tube is the inclusion of a getter to absorb any
contaminant gases such as CO, CO2 or H20 that may
slowly evolve during its lifetime. We use a flash type getter to deposit
a thin film of barium over the inner surface of the stainless steel gas
supply tank. This type of getter is the same as is flashed inside of
virtually all TV picture tubes. lt is continuously active, absorbing any
possible contaminants even when the tube is not operating.
Other
laser manufacturers use a bulk-type getter material which is active only
when the tube is on and hot. Such bulk-type getters are ineffective when the
laser tube is cold. The results are plasma tubes that often fail to start
after periods of storage or inactivity.
We have eliminated tube storage failure through its unique vacuum techniques and the
use of the continuously active getter. Lexel plasma tubes start reliably,
even after storage periods of well over a year.
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