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Active Components for Microwave Vacuum Tubes

time2008-08-27 02:44:38  View:        

Active Components for Microwave Vacuum TubesMany microwave systems rely on vacuum tubes (Traveling Wave Tubes,Klystrons, Magnetrons) to generate or amplify the required RF energy. This is especially true where high power bandwidths are required. Satellite uplinks and downlinks, some cellular applications, and direct broadcast
applications typically use vacuum tubes for the final amplifier. Additionally, many military systems rely on vacuum tube
technology to achieve the required frequency agility and extremely high power bandwidth required for these systems.
Dispenser Cathodes
In order to achieve the high efficiency and high power bandwidths demanded by many applications, a very high
brightness and precisely focused electron beam is required in the tube.The electron source for these tubes must therefore be capable of sustained high current density and long life. The dispenser cathode was developed to fill
this need. Emission current densities up to 8 A/cm2 and lifetimes up to 120,000 hours are achievable with todays
dispenser cathodes. Figure 1 shows typical dispenser cathode specifications.Figure 1. Typical Dispenser Cathode SpecificationsCommercial, Military, or Space QualificationSize Range 0.010 - 8.00 (0.25 mm - 200 mm)
Tungsten Density Range 74% - 84%Impregnant Types 4:1:1 (S), 5:3:2 (B), 6:1:2, 3:1:1, or any other required typeSputter Coatings Osmium Ruthenium (M), Iridium. Other coatings possible.
Materials
Tungsten, molybdenum, molybdenum-ruthenium,rhenium, tungsten-rhenium, Kovar, nickel,
stainless steel, Monel and others.Machined Tolerances To .0002" (0.005 mm)
Brazes
Ranging from molybdenum-ruthenium (mp1980C) to low temperature alloys such as coppergold(mp 910C)
Operating Temperature From 910 CB to 1200CB
Emission Density
Continuous, as high as 20 A/cm2, typically 2-5A/cm2; pulsed, as high as 120 A/cm2, typically 30-
70 A/cm2Life Expectancy From 3,000 hours to 150,000 hours
A dispenser cathode consists of a porous tungsten matrix infiltrated with a barium electron emission-enhancing
compound. The microstructure of the tungsten sponge is carefully controlled to produce predictable performance. An integral vibration hardened heater and mounting structure is a part of the cathode design. In many cases the
entire tube including the cathode structure must be capable of withstanding the high shock and vibration encountered during satellite launch. Mechanical properties are precisely controlled which is necessary for consistent operation of the vacuumdevice. Strength and stiffness arebalanced against thermal efficiency for various applications.
Semicon Associates, A CeradyneCompany, is a major supplier of dispenser cathodes for the microwave tube industry for over 40 years.Complete design and manufacturing capabilities allow Semicon to satisfy any requirement for virtually all of the worlds microwave tube manufacturers.Recently Semicon Associates has embarked on a program to produce higher level assemblies incorporating the cathode and the gun structure. This is a natural addition to the manufacture of dispenser cathodes since the cathode must be integrated into the gun structure in order to function. By performing some or all of this integration process,Semicon Associates assumes the risk of handling the environmentally sensitive cathode and can provide the tube manufacturer with a value added assembly ready to install in the tube.The tube manufacturer benefits from reduced total costs and improved allocation of technical resources. Figure
2 shows a variety of Semicon
Associates components for microwave vacuum tubes.Figure 2. Semicon AssociatesComponents for Microwave Vacuum TubesSamarium Cobalt MagnetsHigh-energy magnets are used to focus the electron beam in microwave vacuum tubes.
Samarium Cobalt magnets have replaced Platinum Cobalt, Alnico, and ferrite magnets in microwave devices requiring volumetric efficiency, thermal stability, and high temperature operation.The high coercivity of SmCo5 allows magnetic circuit geometries that are not possible with Alnico, and enables much smaller magnet structures to focus electron beams or radial flows.Significant weight and space reduction is possible.With four times the energy density of ferrites, SmCo5 can produce higher focusing fields, or allow size reduction to achieve the same field levels. With thermal stability maintained to 300oC,SmCo5 devices can survive operating conditions that would permanently damage ferrite magnets. Modem miniature traveling wave tubes would not be practical without the PeriodicPermanent Magnet (PPM) focusing structures made possible with SmCo5 materials.Partial substitution of Samarium withGadolinium allows tuning the intrinsic strength of the magnets from 5000 to8400 Gauss. Temperature coefficients can be varied from 0.0 %/oC at 6000Gauss to -.045 %/oC at 8400 Gauss.The Sm2Co17 family of materials allows slightly higher strengths, ranging from7000 Gauss to 9600 Gauss, with the highest temperature coefficient being-0.035 %/oC. Semicon Associates, A CeradyneCompany, specializes in supplying magnets to the microwave tube industry. Our focus is precision ground magnets, calibrated to close tolerance strength levels, in sets that are ready to assemble in traveling wave tubes. Figure 3 shows typical Semicore®samarium cobalt magnets produced bySemicon Associates for use in coupled cavity tubes and klystrons.Figure 3. Semicore® Samarium Cobalt MagnetsThe requirements for the slightly higher energy Sm2Co17 magnets are expected to increase as microwave tubes are placed in more demanding applications, with both decreased size and higher temperature operation. SemiconAssociates has recently formed a partnership with the University ofKentucky to study ways to improve the performance of these materials.Lossy CeramicsThe efficient operation of many microwave vacuum tubes (TWT,Klystrons, Magnetrons) rely on the absorption of specific higher order mode(HOM) frequencies and/or on the absorption of microwaves over a wide range of frequencies at terminal ends of the tubes. Components such as absorbers, slot mode absorbers, severs,pellets and terminations are used in various parts of the tube to absorb the unwanted frequencies. These components are manufactured from ceramic materials with tailored dielectric properties based on beryllium oxide(BeO), magnesium oxide (MgO), aluminum oxide (Al2O3), or aluminumnitride (AlN). The choice of ceramic formulation is dependent on the tubes power requirements and the specific frequencies that are to be absorbed.Ceradyne, Inc. offers lossy components from all of the above material families.Examples of components manufactured by Ceradyne, Inc. are shown in Figure.
The microwave industry is becoming aware of the potential health andenvironmental issues during BeO and
BeO based lossy component manufacture. The inhalation of fine dustparticles created during theceramicmanufacturing and the final machining process can result in chronic beryllium disease, a severe respiratory illness.This has caused Ceradyne and others to discontinue the manufacture of BeO based lossy components and has prompted Ceradyne to investigate alternatives to the BeO materials.Ceradyne is working on a new generation of AlN based lossy materials that have tailored dielectric properties designed to replace existing BeO based lossy materials. Figure 5 shows the
output of an S-Band tube with BeO based lossy terminations and AlN based lossy terminations is equivalent. Figure
6 shows the AlN terminations. The AlNbased lossy compositions have equivalent thermal conductivities at
operating temperatures. These materials are designed to retain their dielectric loss at cryogenic temperatures(a requirement for some accelerators).AlN based lossy components are vacuum compatible, thermally stable and can be bonded to metals by appropriate techniques.AlN can be metallized using either thick film or thin film techniques and then coated with nickel for brazing to metal structures. Active metal brazing techniques can also be employed. List of Figures
  

 
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