1. Microwave vacuum devices

1. Microwave M

J Smith,

EMI-Varian

vacuum Ltd.

Blythe

Road,

Hayes,

devices Middlesex,

UK

The paper is a survey of a few of the advances made in microwave tube design and technology during the past few years. Subjects covered are improvements in thermionic emitters, the attainment of good frequency bandwidth in klystrons, the achievement of high frequency agility rates in magnetrons, the latest techniques of gridded gun design in linear beam tubes, and a brief description of cyclotron wave devices for high power millimetre wave generation.

1. Introduction The author will not attempt to survey the whole area of microwave devices as this would be a very long and arduous task. Instead, recent technological developments will be described with reference to the requirements of potential users, and also to developments with which the author is most familiar. Apologies are made in advance for ignoring all the improvements in microwave tube design and technology which are not covered in this survey. The microwave tube is, of course, only a component in a large complex system. The development of new ideas in tubes is largely governed by the requirements of these systems, although often the system designer is constrained by what is available, or likely to be available after completion of a short development programme. It should be added that the tube is not normally the only constraining component in a complex system. All other microwave components, both passive and active, will have limited bandwidth and power handling capability or other limitations. The microwave tube was first developed principally for radar applications. Since then its uses have expanded into other forms of communication and into many forms of heating. Possible future uses such as power transmission from geostationary satellites and plasma heating are setting requirements for extremely high power levels and efficiencies. Radar and communication requirements have encouraged continual improvements in conventional microwave tubes such as magentrons, klystrons and travelling wave tubes. The more difficult requirements of very high powers at high frequencies have

sparked off some new exciting developments in cyclotron wave devices, which are the subject of three subsequent papers. Because of widely differing requirements, there is a wide range of devices on the market, each satisfying particular needs. Inevitably additional requirements come up and a technological advance has to be made to satisfy them. The requirements covered in this survey are summarized in Table 1, along with the purpose for which they are required. The third column summarizes the development solutions to achieve the aims.

2. Thermionic

emitters

Much progress has been made in recent years to develop thermionic emitters for long life (100,000 h) and real high current densities (4 A cm-’ and more) simultaneously. The first diagram (Figure 1) shows life figures for more conventional types of thermionic emitter taken from a number of different sources. It also shows the expected life figures for more recently developed cathodes based on more scant information, but about which there will be more detailed presentations later in the conference. One further comment should be made here. When lives of this magnitude are being considered, it has to be assumed that the material control in building the cathodes is very tight, and that poorly constructed cathodes are eliminated from any consideration, for instance there is no point in life testing cathodes with poor knee curves. In addition, where other materials are brought into close proximity to the cathode (such as in shadow gridded guns) the various diffusion processes will complicate these simple curves.

Table 1. Requirements covered in this survey Requirement

Purpose

Development

Upgrade existing tube technology

Improve life reliability and performance Reduce operating cost ECCM Agility/detection enhancement Improved resolution Provide several independent functions, e.g. in ECM Resolution Availability of spectrum Size/weight Plasma heating

Thermionic

Additional

band-width

Multi-function

radar

Higher frequency ranges up to 120 GHz

Vacuum/volume Pergamon Press

30/number LtdlPrinted

11/l 2. in Great

0042-207X/80/1 Britain

201-0427$02.00/O

emitters

Resonant coupled cavity klystron Spin tuned magnetron Channel tuned magnetron Gridded guns for long pulse and high average power Gyrotron amplifiers and oscillator

427

M J Smith:

Microwave

vacuum

devices I

I

I

Ill11

Tesl carried T -Thomson

I I I I WI by. Type of cathode CSF 0 OS/W

T f -Telefunken H -Hughes V - Vorian EV-EMI Vorian F - Phillps WJ-Watkins Johnson 1 Life

Emission

Figure 1. Expected

life of thermionic

cathodes

as a function

of current

3a. The resonant coupled cavity klystron For wide bandwidth and high gain, the conventional answer is to use a travelling wave tube (TWT) of a type suited to the power level required. Such a tube has relatively low electronic conversion efficiency, and to keep the overall efficiency up to a reasonable level, depressed collector voltages have to be employed. For high electronic efficiency and hence lower operating voltage, the klystron has advantages over the TWT, but because it is composed of resonant circuits there is a problem in achieving the same bandwidth. The bandwidth of the output circuit can be improved considerably by the use of a resonant coupled cavity circuit. An example of such a circuit is shown in Figure 2. Each cavity is tuned to a different frequency and the mutual and external couplings set to give the optimum interaction impedances in the gaps. Three gaps (and cavities), or the addition of a third idler or filter cavity to the externally coupled cavity of a two gap circuit, can also be used to obtain stable operation and a still greater bandwidth, but this has not yet been tried in practice. The bandwidth and efficiency obtainable in theory are shown in Figure 3 for various power levels (assuming a perveance of 428

current

density.

mixed motrix-

Ir/W M-Type

0

B-Type

o

~~$~“ed

-

feel continuing

All cathodes emssion

v 0

IIll_

fully

space

exceptO&Ozem

charge

limited field

emission

A cmz

loading.

2 x lOme). These theoretical curves agree very well in practice but assume that optimum drive is obtainable from the buncher section of the tube right across the whole frequency band. The problem then becomes one of designing a suitable buncher section to match the broadband output without getting discontinuities (zeros) within the band, due to interactions between non-adjacent cavities. The theory gets extremely complicated when dealing with six or more buncher cavities, but computer techniques have been devised to calculate accurately the position of the zeros. The cavity loading and disposition along the axis of the tube are then set to either push the zeros outside the operating band, or cancel them out by a pole. In this way, broadband klystrons using seven and eight cavity buncher sections have been designed. Examples are a I MW S-band klystron with 30 dB minimum gain and 7% bandwidth, and a 100 kW L-band klystron with 25 dB gain and 8% bandwidth. The operating voltages are 75 kV and 33 kV respectively. Figure 4 shows the 100 kW tube with its eight driver cavities, and Figure 5 is a typical performance curve for constant input drive power. Figure 6 is a typical tube performance for a particular I MW broadband klystron.

Couplihg

iris

“4” a,:t,$ Output

Figure

2. n and

2~ modes

in

a two-cavity I

I

resonant

Section

AA

iris

waveguide

coupled I

Iil”l’

A4

waveguide

Coupling

Output

Section

output I

circuit. I

Ill””

I

Effmency,

I

Perveance

Figure

3. Band-width/efficiency

relationship

for

RCCO.

power,

I

Illllr

%

Two-cavity

Output

I

11111~

RCCO 20

x 10-6AV-'5

W

429

Figure 4. Broadband 8 driver cavity hand I

I

I

klystron.

I

I

I

I -30

I

I

I

I

I

I

I

I

I

I

I

-10

0

1 lo

I 20

I 30

I 40

I 50’

I 60

33 kV 13 A 300W drive

I

I

-60

-50

-40

I -20

Frequency,

Figure 5. Performance of broadband 100 kW L&and klystron (PT6006). 430

MHZ

M J Smith:

Microwave

vacuum

devices I

I

I

PT 1120

I

S

I

I

I

No 8 01 78 kV beorn

I

39

wItage,

A beom

I

I

I

I

current

14

I I

200

MHz

.

04

0.2

0

MHz,

Frequency, Figure

S-bond

Il-

-1

6. Performance of broadband IMW S-band klystron (PTI 120).

3b. Agile

magnetrons

The magnetron remains as a small, compact and highly efficiency oscillator for non-coherent radars. It requires a considerably lower HT voltage than a linear beam tube of comparable power level. To improve radar performance some sort of rapid frequency agility is very beneficial. For instance, a fading target at one frequency, due to phase cancellation from different parts of the target, will be enhanced at another frequency; and errors due to glint at one frequency will be offset at other frequencies for similar reasons. A number of methods of achieving frequency agility have been developed. One method is to couple one or more trans-

mission lines into the magnetron cavities, and vary the reactance of the line by means of a multipactor discharge, or some solid state device. Alternatively a mechanical tuning method can be adopted provided suitable vacuum bearings can be found. Figure 7 shows a cross-section of a tube employing a combination of these ideas. A regular agility rate is supplied by a spinner coupled to each of the magnetron cavities. With 16 cavities and a spin rate of 50 Hz, an agility rate of 800 Hz can be achieved. The centre frequency of the tube can then be shifted by means of a PIN diode coupled to one cavity. Figure 8 shows a complete tube with spinner only. Figure 9 shows a similar idea using a coaxial magnetron. This Anode

Block \

Pin‘ Diode

Figure

7.

Spin tuned magnetron with PIN diode tuning. 431

M J Smith:

Microwave vacuum devices

F&gum8. Spin tuned magnetron (pT5022).

Fbtating

Fixed

Element

Element

‘Thin was caramlc

Cylinder

p&+cam

Envelope)

Figure 9. Shuttertunedmagnetronschematic.

device is known as a ‘shutter tuned’ magnetronasthe rotating vanesin the outer cavity act like a shutter. When the castellations line up, the resonantfrequency is at its lowestexcursion; when completely out of line the frequency is at the highestend of the range. This type is in development, but has several advantagesover the more primitive type of spin-tunedmagnetron; not the leastis the greater easewith which a read-out of tuner positioncan beachieveddueto the much greaterdiameter of the rotor and easeof accessto it. 4. Gridded guns

Control of the pulse length and pulse repetition frequency is clearly best accomplishedby the useof a grid placed in front of the cathode of a linear beamtube. High meanpowersand long pulselengthswill causethe interceptedcurrent on the grid to raise the grid temperatureto unacceptablelevels. The grid interception can be considerably reduced by the use of a shadowgrid betweenthe cathodeand control grid (Figure 10). Ideally the shadowgrid shouldbe operatedat a much lower temperaturethan the cathodesothat it doesnot emit. However, 432

Figure10. Shadowgriddedgun.

for smallgridded gunsthis getstoo impracticableas the clearancesget very small. The latest developmentrelieson coating the shadowgrid with a non-emissivecoating and attaching the grid firmly to the cathodesurface(Figure 11). This appearsto work well for at least a few thousand hours, but full life capabilities have not been established.Life will depend on other factors besidesemissioncurrent density. A high p grid for instanceinvolves placing the grid closerto the cathodeand this increasesthe probability of early failure. A still more recent developmentis the mounting of the control grid onto an intergrid insulator; this gives a very robust structure, but the engineeringdifficulties involved in preventing leakageand grid emissionare large. Sometypical practical characteristicsare shownin Figure 12.

MJ

Microwave vacuum

Smith:

devices

Impregnated Tungsten cathode

5. High

Figure 11. Shadow gridded gun with shadow grid embedded in cathode.

GRIDDED

power

tubes

As the wavelength becomes smaller, so do the structures associated with the microwave circuits. For instance, a klystron cavity to operate at 20 GHz (1.5 cm) would only be about 7 mm dia if working in the fundamental mode. For wavelengths shorter than this, it is easily seen that there are both fabrication difficulties and power handling problems. Some years ago it was suggested that the solution to these problems lay in using the electron beam itself to act as the vehicle for the microwave field/electron interaction. A number of methods of achieving this were suggested, but it is only recently, as the pressures to make more use of millimetre wavelengths have become greater, that serious development of practical devices has begun. There are two principal types of device. (a) Those in which the velocity modulation takes place transversely to the direction of the electron beam, such as in the gyrotron. (b) Those in which the velocity modulation takes place in the same direction as the electron beam, such as the ripple beam amplifier and ubitron. Interaction in the first of these types of tube naturally occurs between the beam and TE waveguide modes; the second between the beam and TM modes. 6. Gyrotron

GUNS

millimetre

type

tubes

The basic interaction is shown in Figure 13. An electron beam is formed in a magnetic field in which the cyclotron frequency corresponds to the frequency of operation: o,d-B= Beam voltage

Vo

Beam current

I,

Pmplification Cut-off Area

10.5

factor factor

(Vo/V

(Vo/Vg) cut-off)

convergence

41 kV

75 kV

1A

kV

16 A

40 A

80

ED

58

100

100

1BC

20

22

60

Figure 12. Characteristics of some gridded guns.

Accelerating

Figure

13.

field

m

eB rn,(l + u2/c2y

ino = rest mass u = electron velocity

and the radius of gyration is given by

An electron starting in phase with the electric field will remain in phase since during the time required for one half orbit, the

Decelerating

field

Individual electron orbit in a cyclotron wave device. 433

M

J

Smith:

Microwave vacuum

devices

field will reverse in phase and the electron will therefore be continuously accelerated (Figure 13). The velocity acquired by the electron will increase and hence the angular frequency will be reduced. Conversely, electrons starting out of phase will receive tangential deceleration with increased angular frequency. However, the period of time taken for one revolution of an electron is independent of the velocity: I=-

2nm eB ’

Hence accelerated electrons move to a larger orbit, decelerated electrons to a smaller orbit. Angular bunching will take place, however, when relativity is taken into account. Accelerated

Applied

magnetic

Circular

wave-

electron

orbits

110~

electron

Figure 14. A section through the gyrotron.

Accelerate

Figures 15-18. Electron

position

as a function

electrons will increase in mass and their period be slowed down. Figure I4 shows how a hollow electron beam interacts with the circular waveguide TEol mode. This angular velocity modulation produces electron bunching in angle as the beam is allowed to drift. However, no net transfer of energy to a microwave field can take place unless the bunching takes place in a region of decelerating field. If the rf frequency and cyclotron frequency are the same, then the phase slip of electrons after each completed rf cycle for electrons starting off at O”, m/2, n and 3n/2 out of phase with the field is shown in Figures 15 to 18. Reference to Figures IS to I8 shows that after about 12 rf cycles, the electrons tend to bunch at the r/2 phase, i.e. when the electric field is zero, so that no transfer of the rf field energy takes place. However, by using an rf frequency slightly higher than the cyclotron frequency, bunching can be produced in the correct phase for maximum energy transfer. Figures I9 to 22 show the corresponding phase lags for an rf frequency equal to 1.029 times the cyclotron frequency. Figure 23 shows graphically how electrons starting off at different phases tend to bunch after about 12 rf cycles. By getting as much transverse energy as possible into the beam compared to the longitudinal energy (where no interaction takes place), the efficiency can be made quite high. Varian have built oscillator tubes with greater than 40% efficiency at Q-band. Very high continuous power levels (over 200 kW) have also been generated at Q-band. There are other ideas and developments coming out of this work on cyclotron wave devices and some of these are covered in subsequent papers.

Decelerate ofNc.ycles,

w,, =

wcl

M J Smith:

Figure 16.

Figure 17.

Microwave

vacuum

devices

M J Smith:

Plgure

Figures 436

Microwave

vacuum

devices

position

as a function

18.

19-22.

Electron

of N cycles

W0 = 1.029

WC.

M J

Smith:

Figure

20.

Plgure

21.

Microwave

vacuum

devices

M J Smith:

Microwave

vacuum

devices

Figure 22. -90 \ \\

\

0

4

a

12

16

20

24

N cycles

Fipe I 23. Electron 438

\

phase as a function

of N cycles showing

bunching.

26

32

36

40

44

48

M J Smith:

Microwave

vacuum devices

Conclusions

Acknowledgements

This paper has covered just some of the new developments in the field of microwave devices. It shows that there are many interesting areas in the field of vacuum tubes, especially at the millimetre end of the frequency range.

The author is indebted to many colleagues at EMI-Varian and at Varian Associates for useful discussions in the preparation of this paper, in particular to Mr D Perring who developed the gyrotron theory presented. This work has been carried out with the support of Procurement Executive Ministry of Defence, sponsored by DCVD.

439