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Geometrical optimization of cathode with direct heating for power magnetrons by using of linear thermal compensator.


The magnetron is a vacuum electronic tube of type-M, having a few kV potential between the cathode and anode, which generates an intense electric field, with radial directed uniform distribution field lines (Hinkel; Lewis; Maghiar, 2000).

The space between anode and cathode is under the action of intense constant and homogenous magnetic field, with field lines oriented in a parallel direction according with cathode axis, filling the interaction space. The cathode-emitted electron, which is part of the electronic cloud, nearby the cathode, in his way to the anode has influenced by the simultaneous action of electric field-E, and magnetic field-H, curving its path under the action of Lorentz's force. Sector equation of electrons movement around cathode, in their way toward the anode, has given by Lorenzo's force (Maghiar et al., 2002; Ungur, Pop et al., 2008):


Where: [??]--is electron speed, m--electron mass, [[mu].sub.0]--magnetic permeability in vacuum, [absolute value of e]--absolute value of electron's charge, [??]--electric field intensity between anode and cathode, [??]--magnetic field intensity in anode-cathode interaction space.

This paper presents a method of cathode achievement with direct heating for power magnetron of 800W with a least misalignment of filament to central core, with approximate equal of linear thermal dilatations. These same dilatations are correlation in an optimum ratio of them geometrical size such as hetero-junctions area of weld joints to bear diminish effects of linear thermal dilatation of elements components by temperature gradients


The solution presented in this paper removed the disadvantages presented before by that increased reliability of cathode with direct heating (Maghiar, 2003). The emissive filament from WTh is coaxial centered of central core from Mo.

This operation has done by in the shape of truncated cone frontal opposite grooves into lateral reflectors lids from Mo, on which supported the ends, coaxial with holes that include central core.

The connection between filament and lateral reflector lids has realized by heterogenic weld with Pt added, the weld made frontal in inner plan of shape of truncated cone grooves.

This solution, patented by authors (Maghiar et al., 2003) presents some advantages:

--has a great reliability,

--led at conservation of the physic-mechanical properties of components elements, of rest points and hetero-joints zones, and

--diminished the effects of internal stresses of constructive elements about of rest points and heterogenic weld.

The linear thermal dilatations of filament of WTh and core from Mo due the temperature gradient correlated with the ratio:

[sigma] = [DELTA][L.sub.f]/[DELTA][L.sub.4] = 1 (2)

Where: [DELTA][L.sub.f] = is linear thermal dilatation of filament of WTh, [DELTA][L.sub.4] = linear thermal dilatation of core of Mo. Considering [phi]-slop angle of helix arc and [d.sub.c]-cathode diameter for the condition:

tan [phi] > [L.sup.20[degrees].sub.7]/[d.sub.c] (3)

, the linear thermal dilatation of filament is:

[DELTA][L.sub.f] = [DELTA][L.sub.7] x sin [phi] = [L.sup.20[degrees].sub.7] x [[alpha].sub.WTh] x ([T.sub.7]-20[degrees]) x sin [alpha] (4)

, which is getting in Eq. (2) results the ratio:

[sigma] = [L.sup.20[degrees].sub.7] x [[alpha].sub.WTh] x ([T.sub.7] - 20[degrees]) x sin [alpha]/[L.sup.20[degrees].sub.4] x [[alpha].sub.Mo] x ([T.sub.4] - 20[degrees]) = 1 (5)

Where: [DELTA][L.sub.7]-is linear thermal dilatation of spiral filament from WTh; [DELTA][L.sup.20[degrees].sub.7]-constructive length of spiral filament from WTh wire at temperature of 20[degrees]C; [DELTA][L.sup.20[degrees].sub.4]-central core length from Mo at temperature of 20[degrees]C. [[alpha].sub.WTh-coefficient of linear average dilatation for WTh; [[alpha].sub.Mo]-coefficient of linear average dilatation for Mo; [T.sub.4]-temperature of cathodic core from Mo in working of 400[degrees]C; [T.sub.7]-temperature of cathodic core from WTh in charge of 1900[degrees]C.

The Eq. (5) determines the effective length of main constructive elements, respectively the filament of WTh and central core of Mo, assuring a correct behavior of weld joints during of magnetrons working. In addition, the difference between operational linear thermal dilatations of filament and central core is minim.

The possible differences between relative elongation of main constructive elements, spiral filament of WTh and central core of Mo due to different linear dilatations are getting by a thermal compensator, introduced in a back circuit of current after locking ring between central core and output current rod.

This solution assured a displacement of central core in length, thermal compensator could be a bimetallic lamella.


The new cathode, in optimized variant has presented in Fig.1 and Fig.2 (Maghiar, Ungur et al., 2003). It has the following components: 1,2-opposite reflectors lids from Mo; 3-guide ring from Mo; 4-central core from Mo; 5,6-ceramics bushes; 7-spiral filament from WTh; 8-two locking rods from Mo; 9-feed rod from Mo; 10-guide rod; 11-insulation ring from ceramic. 12-rod of output current; 13-thermal compensator; 14-clamped plate; 15-locking ring; 16-rod for connection of clamp filament; 17-glass bushing. (a)-central hole of lids; (b)-frustum of cone grooves; (c)-side surfaces; (d)-frontal surface, and (e)joint between filament and reflectors lids.

The cathode with direct heating used at power magnetrons of 800W (Fig. 1-2) consists of cathodic circuit by a non-homogenous structure formed of two opposite reflectors lids-1,2 from Mo and a direct ring-3 of Mo, which has a central cylindrical hole incorporated a central core-4 from Mo.

The insulation of central core between lids is doing by two ceramic bushes-5,6. The spiral filament-7 from WTh is positioning between lids, has a emissive role that covered without contact the central core and it's rest at ends on a frustum of cone grooves-b, getting in the side surfaces-c of opposite lids, coaxial with central holes-a. These holes have the role of filament alignment in center. The filament is connecting with lids by heterogenic weld on frontal surfaces-d from frustum cone grooves-b.

Between the reflector lid-2 and guide ring-3 are mounting symmetrical by heterogenic weld two locking rods-8 from Mo, which have a double role: one to lead the current at filament and second to stiffness the aggregate.

In right side of ring-3 is symmetrical mounting on its frontal by heterogenic weld the feed rod-8 from Mo and a guide rod-10, embraced of insulation ceramic ring-11, which included the output current rod-12. The rods-10 and 12 are insulation between them by insulation ring-11. The link between central core-4 and exit rod-12 is doing by get in a thermal compensator-13 from a bimetallic lamella.

The thermal compensator has a double role: one to lead electric current and second to assure of central core an axial displacement, such as different thermal dilatations of component elements




Into the output of current circuit has added by heterogenic weld the clamped plate-74 and a locking ring-75, and in input circuit a rod-16 for connection o filament clamps, jointed by heterogenic weld a feed rod-9, the tight doing from a glass bushing-17.

The curves of simple quality dilatometer of WTh and Mo materials, from which are making spiral filament and central core of cathode with direct heating is presenting in Fig.3.


The cathode with direct heating for power magnetrons of 800W has a thermal compensator get in back circuit of cathodic current.

The optimization method of cathode consists in realizing by heterogenic weld with Pt added a joint between spiral filament and reflectors opposite lids.

This method assures a growing of cathode reliability by correlation of linear thermal dilatation coefficients of spiral filament from WTh and central core from Mo, such as the difference between operational linear thermal dilatation of filament and core to be minim.

The thermal compensator has the role of avoid possible thermal dilatation between filament and core, which have different average thermal dilatations and gradients temperature, and taken by compensator.


Hinkel, R. (1963). Les Magnetrons, Dunod Ed., Paris

Lewis, F.P. (1993). Magnetrons Muni Dun Blingage, Patent France, Nr.2680912

Maghiar, T.; Ungur, P. et al. (2000). Magnetron, Theory Elements, Construction and Technology, Oradea Univ. Editor, Romania

Maghiar, T.; Ungur, P. et al. (2003). Power Magnetron Direct Heating Welded Cathode Includes a Spiral Filament with a Molybdenum Core and Correlation of Linear Thermal Expansion Coefficients, Patent No.RO118500-B, DPA No. 2003-655414

Ungur, P. et al. (2002). Theoretical Contribution Regarding Electron Path Modification in Cathode-Anode Interaction Space of the Magnetron, Annals of DAAAM for 2002 & Proceeding of International DAAAM Vienna, pp.579-580

Ungur, P.; Pop, P.A.; Gordan, M. & Gordan, C. (2008). Theoretical and Practical Aspects Regarding of Electronic Efficiency Improving of Power Magnetrons with Continuous Working and Bimetal Anode, Proceedings of MSEC/ICMP2008, pp.1-9, ISBN-0-7918-3836-6, Conference of ASME MSEC_ICMP 2008, Oct. 7-10, 2008, Evanston, IL, USA
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Author:Pop, Adrian Petru; Ungur, Ana Patricia; Gordan, Mircea; Lazar, Liviu; Marcu, Florin
Publication:Annals of DAAAM & Proceedings
Article Type:Report
Geographic Code:4EUAU
Date:Jan 1, 2009
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