High power pre-TR switch

A high power pre-TR switch utilizes hot pressed boron nitride to form a vial. The vial contains a halogen gas such as chlorine.

BACKGROUND OF THE INVENTION 
Modern day, high power radars require receiver protection (RP) that can 
handle hundreds of watts overage and still provide recovery times in the 
fractional microsecond range. These requirements are usually met by the RF 
by incorporating a pre-TR stage in the overall design. This stage is 
typically a sealed quartz vial, containing a halogen gas, such as chlorine 
installed in an aluminum mount that incorporates an appropriate iris. One 
serious limitation for high power systems is that these types of pre-TR 
designs are power limited due to the quartz temperature in the area of the 
plasma discharge. The poor thermal conductivity of quartz prevents thermal 
equilibrium from being achieved. The quartz temperature in the discharge 
area is considerably higher than that measured on the end of the vial. 
These extreme temperatures eventually result in the vial losing its vacuum 
integrity, and therefore, result in a catastrophic failure of the receiver 
protector. The receiver protector failure then results in a serious damage 
to the sensitive radar receiver, in particular, the expensive 
pre-amplifier stage. A second failure mechanism of these quartz type 
pre-TR's is the longer term gas cleanup phenomenon. This results in a more 
gradual type of receiver protector failure. The gas cleanup is basically a 
result of the energetic ions, generated in the RF induced plasma, striking 
the quartz surface and diffusing into the material. Some of these ions 
will diffuse deep enough to be permanently trapped. This activated 
diffusion then results in gas cleanup. The cleanup rate is proportional to 
the square root of the diffusion coefficient of chlorine into quartz. A 
different vial material would result in a different diffusion rate for the 
same gas. The present invention solves both the thermal problem and the 
gas cleanup. 
SUMMARY OF THE INVENTION 
A higher power TR switch is provided. The TR includes a vial housing a 
halogen gas such as chlorine. The vial is comprised of hot pressed boron 
nitride which eliminates the associated thermal problem and also the gas 
cleanup problem.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the single FIGURE of the preferred embodiment there is shown vial 1 
consisting of hot pressed boron nitride material. This material is easily 
machinable and is fabricated into a brazeable, hard vacuum assembly. The 
dielectric constant is essentially identical to quartz therefore requiring 
no significant microwave circuit re-design. The Thermal conductivity is 
approximately forty times better than that of quartz, and therefore 
eliminates the thermal run-away problems that occur in quartz vials in the 
area of plasma discharge. For example, the dielectric constant of quartz 
is 4 and that of hot, pressed boron nitride is 4.1. Further, the thermal 
conductivity of quartz is 0.0033 and that of hot, pressed boron nitride is 
0.115 (cal.cm/sec cm.sup.2 .degree.C.). The diffusion coefficient, of 
chlorine gas into boron nitride, is less than that of chlorine into 
quartz. This helps to extend the life of the pre-TR stage since the gas 
cleanup rate is less. 
Boron nitride vial 1 contains a halogen such as chlorine gas 2. It is noted 
as hereinbefore mentioned that the boron nitride is machined. The single 
FIGURE indicates some of the dimensions. It is also noted that sharp 
corners for vial 1 are avoided. The overall length is 12.40 inches. It is 
also helpful if the indicated 0.4000 inch surface is substantially 
parallel to the indicated 3.38 inches surface. 
Two piece kovar heat sink 3 is brazed and fitted into a machined groove in 
boron nitride vial 1. Heat sink 3 may be placed against a water-cooled 
plate during operation of the pre-TR. 
Surrounding boron nitride vial 1 at outer extremity is compression type 
metal container 4 which is in ceramic brazed-mode with heat sink braze. 
There is then kovar spinning-gold plating after the ceramic braze. Quartz 
reservoir 5 is positioned and fitted into compression type metal container 
4. As completed vial 1, compression type metal container 4 are vacuum 
assemblies. 
The single FIGURE illustrates the preferred design and replaces the quartz 
pre-TR stage for a high power receiver protector such as the Westinghouse 
WD 260. The kovar spinning and heat sink is brazed onto the machined boron 
nitride, and then glassing of the quartz reservoir to the spinning is 
performed. 
During operation, the plasma generated heat is conducted axially through 
the boron nitride cylinder and then removed via the kovar heat sinks. 
These heat sinks are seated against a water cooled plate. 
This pre-TR design allows the receiver protector to handle RF power in the 
range above 2 kW average at 50% duty.