2. Magnetically beamed triodes

2. Magnetically 1 G Thurlow

and

G H G Phipps,

beamed ITT Components

Group

triodes Europe,

STC Ltd.,

Brigham

Road,

Paignton, Devon

TQ4 7BE,

UK

When used as single valve oscillators in industrial heating equipment, presently available magnetically beamed triodes (MBTs) provide output powers from 3 to 50 kW, whilst output powers up to 100 kW are included in future developments. The robust structure of the MBT, made possible by the use of magnetic focusing, overcomes the inherent fragility of the filamentary cathode and fine wire grid of the conventional triode. The cool running gate (grid) of the MB T does not suffer from ‘grid emission’, thus allowing the use of oxide cathodes with significant savings in heater power. The construction, processing and use of MBTs is described. Low intercepted gate current, due to magnetic focusing, greatly reduces drive power requirements allowing the use of an electronic power control device which limits the swing of gate voltage. This can vary the output power of the MBT from 100% down to 5% by means of a change of only a few volts at the controller input. Various forms of control can be used ranging from simple manual to programmed, with feedback I loops to provide stabilization. The output power from an MBT oscillator can also be controlled by varying the focusing magnetic field. Systems range from mechanically adjustable permanent magnets to electromagnets fed from an electronic power supply providing feedback facilities similar to the gate swing limiter.

Introduction

Magnetically beamed triodes, with output powers ranging from 3 to 50 kW, have been designed for use in industrial heating equipment. Three typical MBTs are shown in Figure 1. This paper deals mainly with induction heating where output powers, for differing applications, vary from 8 to 50 kW and

Figure

frequencies range from 200 kHz to 3 MHz. Output powers up to 100 kW are included in future developments. Induction heating is the term applied to the heating of conductors, as opposed to insulators, and typical applications include crystal growing for the semiconductor industry and the melting, hardening and forging of metals. Before dealing with

1. Typical magnetically beamed triodes in magnets.

Vacuum/volume Pergamon Press

30/number &d/Printed

11 /12. in Great

0042-207X/80/1 Britain

201-0441~02.00/0

441

I G Thurlow

and G H G Phipps:

Magnetically beamed triodes

the details of the construction and use of MBTs, it is of interest to see why this type of tube, using the magnetic beaming principle, came to be developed. Early in the 195Os, triodes, which were designed for use in broadcast transmitters, were used as high power oscillators in industrial heating equipment. These tubes had short lives, in this application, particularly because of overheating of the

Figure 2. Conventionaltriode grid and filament.damaged by overheatingin service.

s

Cathode

Anode

Fine

grid wires

Electron flow in gridded tube whilst grid is positive to the cathode

control grid when adverseload conditions occurred. Varying loads are an essentialfeature of industrial heating, and new tubes had to be designedspecifically for these applications. These designswere mainly centred around optimization of electrode dimensionsand spacings,minimizing of grid intercepted current, and development of coatings to reduce grid primary emission. It is necessaryto drive the grid positive during part of its oscillatory cycle, and this gives rise to grid interceptedcurrent. To minimize this, fine grid wires are used,but this limits the rate at which heat can be conducted away to the supporting structure. In fact, conventional gridsare more usually radiation cooled, and as a result, grid temperaturesof 1000°Care commonly experienced.Several difficulties arise from high grid temperatures, suchas mechanicalinstability, and electronemissiondue to migration of emissivematerials from the cathode. This prohibits the useof oxide cathodes,due to the high level of primary emissioncausedwhen theseoxides are presenton a high temperaturegrid. Various forms of filamentary cathode are used,the material generally being carburized thoriated tungsten. The inherent fragility of wire grids and filamentary cathodescan be seenin Figure 2, which showsan assemblywith meltedwirescausedby mechanicaldistortion and localized grid emission. An alternative method of reducing intercepted current is to arrange a magneticfield betweenthe cathode and anode, of sufficient strength to ensure that electrons emitted from the cathode follow the field lines to the anode instead of being attracted to the grid during its positive swing. Figure 3 shows the electron flow in an MBT comparedto the flow in a conventional gridded tube. This magneticbeamingprinciple has lead to the designof electrode structures such as those shown in Figure 4. The former fine wire meshgrid hasbeenreplacedby a wide slotted electrode called the gate, and, for the convenienceof incorporating the magneticfield, multicathode structuresare made with an anode plate on both sidesof the slotted gate, so that emissionand beamingcan be usedin both directions. Actual electron trajectoriesin an MBT, with sinusoidallyvarying gate and anodevoltages are rather complex, but a simplifiedmodel can beobtained by consideringthe motion of a typical electron, asshown in Figure 5. Taking into account the componentsof velocity, perpendicular to and parallel with the magneticfield, it can be shown Mag field

I filament)

/ Anodes

Electron positive

Figure 3. Electrontrajectories in conventionaland magneticallybeamed triodes.

442

i

Cotliodes \

Go/es

flow in M.E.T. whilst to the cathode

Magnets gote Is

I G Thurlow

and G H G Phipps:

Magnetically beamed triodes

Figure 4. MBT gate/cathode assemblies and anode assemblies.

////,

,Gate, , ,,,,

J

r--

Figure 5. Path of typical electronin an MBT.

that the radius and pitch of the mainly helical paths are given by the equations

When B, the magneticfield strength, is in gaussand V, and V,, are in volts, theseequationsbecome Rk = 3.31&B-

and P=6.74

JVP B.

Also R, = 2:

J2(1

- cos wt),

where ER is the electric field component perpendicularto the

magneticfield, w is the cyclotron frequency eB/m and I is 0 at the cathodesurface. When viewed along a field line, the paths appear to be cycloidal, and the limiting caseoccurs for emissionfrom that part of the cathodewhich is nearestto the gate. The strengthof the magneticfield usedensuresthat electronsemitted from this region of the cathodejust graze the gate and return, when Vr is equal to the gate voltage at its maximum positive value. From other parts of the cathode,electronsturn back before reaching the gate, and make several orbits before leaving the influence of the gate potential. The table (Figure 6) showsthe performance obtained from some of the present range of MBTs. However, a detailedstudy of electron trajectoriesunder actual working conditions is beingmade,with the object of optimizing electrode shapesto give better electron interaction. This is expected to lead to improved cut off characteristics,higher efficiency and improved distribution of electric field within the MBT. As in aid to the designof tubesmadeso far, a set of largely empiricalequationshasbeenusedto relateelectrodegeometries 443

I G Thurlow

and G H G Phipps:

Magnetically

beamed

3RM /169G

Type Anode

voltage

Anode

current

kV

3RM/245S

3RM/266R

6.5

7.5

75

9.0

I6

3.6

6.5

8.6

50

70

66

7.5

9.2

current

mA

24

Gate

resistor

K&I

30

Gate

dissipation

6

W

15

25

50

40

Gate resistor dissipation

W

17.2

20

3675

43.6

Cathode power

heating W

72

Output power (anode)

KW

6. I

19

35

61

Drive

power

W

37

46

100

64

1

%

69

70

72

77

II4

200

440

MBT performance figures.

6. Typical

aFigure

3RM/204S

A

Gate

Efficiency (onode

Figure

triodes

7. MBT

dimensions.

to the voltages, current and magnetic field. Referring 7, these equations are:

to Figure

I = &mhk’ d

where I = instantaneous cathode current, L = emissive length of cathode, Vg, = peak positive gate voltage, Bmln = magnetic field required to give an acceptable minimum gate current, Vg2 = voltage on the gate measured from the bias level at any point on the load line, Vu = the corresponding value of Va, K,k,K,k, are constants, modified by test results, suited to particular gate and cathode configurations. The graph of gate current vs field shows that the magnetic field is not critical, once the knee of the curve has been passed

I ? : E z r ? s 0 . ;; 0

2-

1200 (Operating

1-

Gauss Flux.1 I I I

1 500

0

I 1000

w

I 1500

GSJUSS

INTERCEPTED GATE CURRENT Vs MAGNETIC FIELD (3RW245S) Figure 444

8. Intercepted

gate

current vs magnetic field.

ITT

I G Thurlow

and

G H G Phipps:

Magnetically beamed triodes

(Figure 8). Although, as we shall see later, the output power from an oscillator using an MBT is dependent upon the magnetic field, a value of field which will give an acceptably low gate current is chosen. MBT

Construction

The main differencesbetweena magneticallybeamedtube and a conventional tube can be seenin Figure 4. This showsthe gate, which replacesthe grid, and the oxide cathode structure. Becausegate interception is now controlled by the magnetic field, there is no longer a needto usefine wires,and the low gate temperaturesachieved can be attributed both to the use of copper, and to the thick cross-sectionof the gate. As the gate temperaturesare now around 25O”C, there is no risk of gate emissionand oxide cathodescan be used.The fairly massive gatestructure also leadsto improved mechanicalstability. Sometypical gatesare shown in Figure 9. The one on the

left was milled from a solid pieceof OFHC copper, whilst the one on the right wasmadefrom electron beamweldedflat strip. Anodes also, have beenmadeby milling from singleblocks of OFHC copper, with flat plates brazed on to complete the water cooling channels.Later developmentsuseanodesmade by turning the water channel grooves in cylinders, which are then formed into flat sided anodes(Figure 10). The water jackets are similarly formed. Someof the more complex assembliesusedin MBT construction have beenreplacedwith onepiece vacuum cast parts. Typical of these is the top plate assemblyshown in Figure I I, which requiresonly a minimum of machiningto prepareit for use.All the ceramicsare of high alumina, and a high temperature metallizing processis used, followed by a nickel coating which allows the braze metal to flow. The copper sealingrings, shown in Figure 12, are butt brazedto the endsof the envelopeceramic.This wholeassembly is brazed in one operation. Compressivesealsare usedfor the heaterand cathode terminal ceramics. Various brazing temperaturesand materialsare usedto allow successiveassemblies to be made; theserangefrom 1030°Cto 780°C. All brazing is done in very dry hydrogen, which has a dewpoint of -80°C. and this assistsin the outgassingof the assemblyduring the pumpingschedule.A matrix type cathode is used,consistingof a nickel powder sinteredonto a tungsten nickel base.The cathodesare brush coated with a carbonate mixture beforebeingassembled onto the stemassemblies shown in Figure 4. The tungsten-rheniumheater has a coating which has been developedto withstand the vibration which occurswhen an ac supply is usedfor heatersoperating in a strong magneticfield. The ends of the heatersare joined to the nickel connector assemblies by touch brazing. Electron beamwelding is usedto join together the major sub-assemblies, and to complete the vacuum envelope. Pump schedule

Figure 9. Typical MBT gateassemblies.

The sealed-intube is joined to an all metal dry pump system, consistingof a turbo-molecularpump backedby a rotary pump (Figure 13 and 14).

Figure 10. MBT anodeand waterjacket. 445

I G Thurlow

and

G H G Phipps:

Figure

11. Vacuum

Figure

12. Anode/ceramic

446

cast copper

assembly.

MagnetIcally

parts.

beamed

triodes

1 G Thurlow

Figure

13.

and G H G Phipps:

MBT pump

Magnetically

station head.

beamed

triodes

I G Thurlow

Pigut

and G H G Phipps:

rIBT pump s

Magnetically

beamed

triodes

I G Thurlow

and

G H G Phipps:

Magnetically beamed triodes

Figure 15. MBT installed in 50 KW induction heating generator. A typical pump schedule consists of:

HT+

(1) A long bake-out at 500°C. (2) Cathode breakdown. (3) Cathode activation by drawing current to the gate and anode. (4) De-gassing of the gate and anode by bombardment, and finally a high voltage conditioning. The system pressure before bake-out is 5 x 10m6 torr, and this is allowed to rise to 10m5 torr during bake-out, and 5 x 10m4 torr during cathode breakdown. The final pressure at seal-off is usually better than 5 x lo-* torr. The MBT is sealed by pinching off the copper tubulation to form a leaktight cold weld. Figure

16.

MBT oscillatorcircuit for inductionheating.

Uses of MBTs

Figure 15showsan MBT installedin a 50 kW induction heating generator.In industrial heating applications,a commonly used circuit is the one shown in Figure 16. This is a tuned anode, inductively coupled gate, self biasedoscillator. In this circuit the rf power is transferredto the work pieceby the induction or work coil Lw, which is in serieswith the anode or tank coil Lt. If we refer to the constantcurrent curveson the Vg, Va graph (Figure 17), it can be shown that the locus of corresponding valuesof Vg and Vu for a purely resistiveload is a straight line whoseslopeisdeterminedby the load resistance.The threelines shownon this graph are for different classesof operation. The mostefficient is classC, which is obtained at optimum loading, together with the correspondingvalue of biasvoltage. Control of output power is an important requirement in industrial heating, and various methodshave been used,with both conventional tubesand MBTs. Typical methodsare:

(1) Variation of ht potential. (2) Variation of load coupling to the tank coil. (3) Switching anodeor grid voltage on and off rapidly, with a variable on/off ratio. All of thesemethodshave disadvantages,in that they are either expensive,slow to adjust, operateover only a very smallpower range or, in the caseof the on/off switching, give undesirable high voltagesat low frequenciesat the work coil. The improved method of controlling output power, made feasibleby the low gate current requirementof the MBT, is to connecta smallhigh-voltagediode to the MBT gate(Figure 18). This diodeconductsonly when the positive swingof Vg reaches the diode cathode potential, effectively preventing Vg from risingabove this level. Thus, the swingof Vg is limited, and this in turn limits the swing of Vu acrossthe tank coil. This then 449

I G Thurlow

and

G H G Phipps:

Magnetically beamed triodes

I

.i ’

I __

. ! I I3 = Yh.PO.., 1 I

VdkV)

TYPICAL SHOWING

’ Vbupply)

CONSTANT CURRENT CURVES LOAD LINES 3RM/244S

ITT

Figure 17. Typical constantcurrentcurvesshowingloadlines.

OSCILLATOR

RF.

EMT.

OUTPUT

POWER CONTROLLER

AND LIMITING

DIODE

ITT

Figure 18. Powercontrollerand limitingdiode.

reducesthe feedback voltage to the gate, so that the circuit re-adjuststo a new, lower level of output power. As the mean gate current is now smaller,the bias voltage developedacross Rg becomessmaller, and the MBT operation shifts towards class‘B’ or ‘AB2’ (Figure 17). It is important to note that the diodedoesnot clip the rf waveform, asthe gateswingre-adjusts. to just reachthe limiting voltage level. It may alsobe noted that asthe power output is reducedby this method, the meananode current, and therefore the dc input power, alsofalls. The diode cathode potential is controlled by the transistor Tr, which is part of the potentiometer chain formed with R, 450

(Figure 18). The basevoltage of tr is controlled via the amplifier A, from a low voltage input and the variable potentiometer. A variation in the control voltage from 0 to 10volts varies the output power from 100% down to 5 %. If a secondamplifier A2 is added, two inputs can be used(Figure 19). The variable potentiometer then becomesa pre-setcontrol, whilst the other input can be derived from a feedbacksourcesuch as the tank coil voltage. By adjusting the sensitivity in the feedback path, the tank coil voltage can be stabilized at the pre-set level. Thus the feedback can be used, for example, to remove the 300 cycle

I G Thurlow

and G H G Phipps:

Magnetically beamed triodes

OsCluAfOR

ITT

POWER CONTROLLER WITH FEEDBACK AND PRESET INPUTS. FIgwe

19.

Power controller with feedback and preset inputs.

.... .... .... .... .... H-....

0 I.... i 40

. . I.... I.... I.... I...., . . . . 1 . 00 SO 100 YegIld

I. . . 120

se0

I . . . . (. 140

. . .... 100

mm

Figure20. Poweroutput and etliciencyvs magnetgap. ripple introduced by an unsmoothed ht supply. Other parameters can be stabilized if the feedback voltage is derived insteadfrom one of theseparameters.For example, the temperature of the work piececan be maintainedat a pre-setlevel. Alternatively, or additionally, the power control voltage can be obtained from a pre-programmedsource, so that set power output profiles can be repeatedautomatically. The output power of an MEIT can also be controlled by varying the magneticfield (Figure 20). As the field is reduced, gate current, which increases asthe gate voltage swingstowards its peak positive value, is allowed to flow. This has a similar limiting effect on the gate voltage swing, to that of the diode describedearlier, resulting in a reduction in output power to suit the lower field. At the sametime, the meangateand anode

currents both fall, so that the power input is lower than at full output. A smooth changein output from 100% down to 5% can alsobe obtained with this system.Various waysof varying the field are possible.One simplesystemis to vary the gap of a pair of permanentferrite magnetsby somemechanicalmeans (Figure 21). This offers only a manualcontrol of output power, but it has the advantage over the variation of ht voltage of being cheaperand more simple in appropriate circumstances. Alternatively, an electromagnetcan be usedto provide the field. The graph of Figure 22 showsthe wide variation and smooth changeof output power possiblewith an electromagnet.The electromagnetcan besuppliedfrom a variac and rectifier system to provide a manual control of output power from a point remote from the MBT, but a more versatile methodis to usea 451

1300 1200 1100 : g u

1000 900

P 8 0 Q

800 700 .500 500

L

50

50

70

80

100

00

Olxtmeohnwn

A@tnP

Figure 21. Magnetic field vs magnet gap.

100

00

@ 50 .I

.

E

.

i 40

.

f

20

.

0

0

.2

.4

.-6 Floltl

.5

l

Currant

1.0

1.2

mpr

Figure 22. Power output and efficiency vs electromagnet field current. 0 TO*?V.FEEDIACK

CONTROL.LOW

INTERNAL

RESl5TANCE

wo,EI: Iufi!Lp$y---

YAI)UALCOWTKOLCANDK2EtTO .Of FILL0 FKOY lOI T O lOOS

TNE FEEDDACK 20s

Figure 2!3. Thy& 452

T O 401

ANYVALUK OF YAXlYUY

VOLTAQE VARlK2 TNE OF IT8 ?KEIET VALU2.

ontrolled power supply for electroma~et.

FIKLD

VALUE. 2V

I C Thurlow

and G H G Phipps:

Magnetically

beamed

triodes

.

AC

et Trensformer

Thyrlrtor

Gate

Voltage

across ( wlthout

Current

Figure 24. Waveform1.5of electromagnet power

through

Trigger

Magnet CS )

Pulses

Coil

Magnet

supply.

thy&or-controlled supply (Figure 23). The output from the thyristors is a partial sine wave with a variable on/off period (Figure 24). After rectification and smoothing, this provides a mainly dc current in the magnet coils, the amplitude of which can be varied, by varying the time of switch-on of the thyristors. The switch-on time of the thyristors can be controlled electronically with either a simple manual control or from a feed-

back source, together with a pre-set control such as the one used with the limiting diode system. At present the electromagnet system is limited to a response time of 0.1 s, but this may be acceptable for some of the more severe industrial situations. The overall package of electromagnet and thyristor supply can also be cheaper than the permanent magnet and controller plus limiter package.

453