Development of Microwave Pulse Compressors

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Yu.G. Yushkov, S.N. Artemenko, S.N. Novikov et al KORUS’2005838

Development of microwave pulse compressors

Yu.G.Yushkov, S.N.Artemenko, S.N.Novikov, P.Yu.Chumerin, R.V.Shpuntov

NPI634050, Tomsk, Lenina, 2-a, Russia

.

Abstract – The microwave pulse compression uses the

capability of a resonant cavity for amplification of the field

strength of an exciting falling wave about 20 dB and beyond.

Basics of microwave pulse compressor operation and main

parameters achieved are presented

I. INTRODUCTION

The microwave pulse compressor (MPC) operationconsists in excitation of resonant cavity during a

relatively long time and the subsequent rapid dumping ofthe stored electromagnetic energy to a load. The travelingwave power as the component of the standing wave in anexcited copper room temperature cavity can exceed aninput power by 15…20 dB [1].

Multimode cavities can provide 20…30 dB and takingaccount of electrical strength level a copper cavity andthe possible input power of megawatts the devices present themselves as pulse sources for high powermicrowaves (HPM) as well.

Some advantages of MPC are now recognized. It haslow cost as it uses conventional modulator and magnetronsource. Also one can use different vacuum tube – a

klystron or a triode. A modulator requires voltage about50 kV or less and higher voltages are constrained within acavity volume. There are no X-ray generation to claim forspecial protection. The compressor produces microwavesin a form of asymmetrical wave type which could beeasily radiated. It has reasonable energy efficiency andcan operate with high PRF.

II. EXCITATION OF A CAVITY

A microwave cavity can be characterized by following parameters: k – transition factor of an exciting element,

aT – attenuation constant attributed to double traveling

along a cavity, L – length of a cavity and Q0 – intrinsicquality factor, QL – loaded quality factor. It is use full tomention the relation QL = Q0/(1+b1)

-1, where b1 – inputcoupling coefficient b1= k

2/2aT. The rate at which the

electric field strength builds up in the cavity is found as a

solution of the linear non-uniform differential equation[2]. Also the report gives the schematic diagram of

experimental setup. The ratio of the field amplitude in thecavity to the amplitude of an input pulse gives the fieldamplification M. The square of the value is the poweramplification factor M2 which is expressed by

2

2 2 2 2

0

( ) 4 ( 2 ) 1 expT

t M t k k a

t

− ⎛ ⎞⎛ ⎞= + − −⎜ ⎟⎜ ⎟

⎜ ⎟⎝ ⎠⎝ ⎠

.

Here t – current time, t0 = 2QL/6.28f – time constant f –circular frequency. Corresponding efficiency of cavity

excitation is

20

0

( ) ( / )( )

2t/t

T L M t a Q Qef t

⋅=

0 2 40

50

100

150114.727

0

A1 to,( )

2 to,( )

2 .5 to,( )

4 to,( )

40 to

M 2 t

4

1

2 3

0 20

0.2

0.4

0.6

0.8

4

0.611

0.06

η A1 to,( )

η 2 to,( )

η 2.5 to,( )

η 4 to,( )

40.1 to

4

3

2

e f

a) t/t0 b) t/t0

1

Fig.1. Plots of the quantity M2 and ef as functions of normalized timet/t0 for different values t/t0. 1 – 1.5; 2 – 2; 3 – 2.5; 4 – 4.

Fig. 1 demonstrates the effect of the coupling

expressed by Q0/QL on the time dependence of the poweramplification and efficiency.

The pulsewidth tg may be less or above the timeconstant t0 and so it has an effect on compressor parameters. It is evident from Fig.1 that maximum valueof M2 does not necessarily coinside with the maximum

value of ef. At large values of tg the amplification M2 is

also high but if tg >> t0 a lot of energy is dissipated in

cavity walls. When tg < t0 the energy losses decrease andef is high.

The mode of compressor operation regarding ef can beoptimized by variations of b1 and tg. Values of Q0 and

stored energy WS are given. It is usefull to introduce theunitless quantity W1=WS/(t0Pg) where Pg – power of an

exciting generator, t0=Q0/2⋅6.282f – intrinsic timeconstant. Calculations for constant WS/(t0Pg) show thatef(b1) and tg/t0(b1) have extreme points at equal values of b1. Values of ef and tg corresponding the extreme pointare optimum as the exciting pulse energy is minimum atthat. Optimum values b1 and tg/t0 as functions of W1 are plotted in Fig.2.

Using the same variables the losses in walls WL andlosses due to reflection WR are expressed by following

equations

0-7803-8943-3/05/$20.00 2005IEEE Radiotechnics, Electronics, Communications



KORUS’2005 Yu.G. Yushkov, S.N. Artemenko, S.N. Novikov et al839

0/ / 201 1

21 1

4 ( ) 11 ((1 ) (1 ) )

2(1 ( ))

g g

L

g g

t t t t

g

W

t P

t b W e e

t b W

− −

=

⎡= − − + −⎢

+ ⎢⎣

0 ⎤

⎥

⎥⎦

0 0

2 2/ 201 1 1

11 1

( ) 11 (2 (1 ) (1 ))

( ) 1 2

g g

R

g g

t t t t

g

W

t P

t b W cc e e

b W t

− −

=

⎡ ⎤⎛ ⎞−= − − − −⎢ ⎥⎜ ⎟

+ ⎢ ⎥⎝ ⎠ ⎣ ⎦

/

0,01

0

50

100

150

200

Y Axi

0

2

b1

b1 tg/t0

W1

t g

t 0

Fig.2. Plots of optimum b1 and tg/t0 as functions of the unitless

parameter W1.

Here c1=2b1(W1)/(b1(W1)-1). Dependencies ofWL/(tgPg) and WR /(tgPg) on W1 are shown graphically in

Fig.3.

0,01 0,1 1

0,0

0,2

0,4

0,6

0,8

Y

A x i s T i t l e

X axis title

e f , W R / ( t g ⋅

P g

) , W L ( / t g ⋅

P g

) 1

32

W1

Fig.3. Plots of 1) ef, 2) WR /(tg⋅Pg), 3) WL(/tg⋅Pg) as functions of the

unitless parameter W1.

Fig. 2 and Fig.3 demonstrate that the energy loss due

reflections stays more or less steady within wide range ofW1 but the energy loss in cavity walls falls rapidly alongdecreasing of W1. The efficiency is practically constant

ef ≈0.8 at W1≤10-1. These data allow to choose the modeof operation with maximum ef value and required output pulse parameters. The efficiency increases due todecrease of WL when the cavity is excited by shorter

microwave pulses.

III. ENERGY EXTRACTION

A microwave nanosecond pulse is formed by energyextraction from the cavity having separate elements ofinput and output. During both processes of excitation and

extraction k stays steady but the output transition factor hincreases abrubtly for the process of extraction up to m

value, the relation being h<<m. The signal amplitudescan be derived as a solution of the uniform differentialequation with the new time constant

1 3

2

1

cm

t t

b b=

+ +,

where b3 =m2/2aT – output coupling factor duringextraction, tc – intrinsic time constant tc=Q0/6.28f and the

initial condition is determined by the wave amplitude inthe excited cavity. Amplitudes of interest are the value inthe cavity Ac and the amplitude of the output pulse A0.

Ac(t) = M ⋅ exp(-t(1+b1+b3)/2t c)

and

A0(t) = M ⋅ 1b (-t(1+b1+b1)/2t c).

The time dependences of this quantities are presentedin Fig,4a and Fig.4b correspondently.

4.2.106

4.3.106

4.4.106

4.5.106

4.6.106

4.7.106

0

5

10

12.0

0

10, t, )

20, t, )

40, t, )

4.7211×4.105106−

×t

t, c

b 4 / a 1 , a . u .

1 23

4

Fig.4. Time dependencies of Ac(t) and A0(t) plotted for the singlemode

S-band cavity of 1.2 m length, aL = 5.7⋅10-4 m-1 when the pulsewidth of

the input pulse is about 4⋅10-6 c.

Pashed line 1 in Fig.4a is for abrubt increase of the

output coupling factor from b2=h2/2aT = 0.6⋅10

-3 to b3 =

20; solid line 2 gives idea about absence of extraction.Output pulse envelopes in Fig.4b are for different b3 values 1 – 10; 2 – 20; 3 – 40; dash-dot line 4 showing theoutput pulse at complete matching at wave resistances ofthe cavity and output line.

Results of the calculation show that MPC with a singlecavity can provide the amplification M2 about ~20 dB

and output pulsewidths close to tout=2L/vg, where L –cavity length and vg – group velocity. The parameters areachieved provided MPC has the output element capablefor step-like increase of the output coupling.

Usually a T-junction with one short-circuited arm isused as an output element. In this arm the power in the

cavity is commuted. During the excitation process waves

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in the output arm are mutually canceled but duringextraction are added in phase. As a matter of factcancellation is not perfect and some power leaks intoa load. This formed signal is called as a prepulse [3].The prepulse power is totally determined by power

attenuation in the short-circuited arm and so depends onits length. The output pulse amplitude normalized to theamplitude in the cavity is demonstrated by Fig.5. Thelength of the short-circuited arm was taken into account.

0 1 . 1062 . 10

63 . 10

64 . 10

60

0.002

0.004

0.006

0.008

t c

A po,a.u.

Fig.5. Time dependencies of the leakage amplitude for

the cavity having aT=4.5⋅10-3, T = 9.5⋅10-9 and and

critical coupling b1 = 1 at different lengths of the tee

arm 1) 0.07 m; 2) 0.139 m; 3) 0.208 m; 4) 0.278 m.

The effect of the tee arm loss exerted on the totalamplification factor can be observed in Fig.6. The results

show the importance of maintaining low losses and proper matching of the tee.

0.01 0.1 10

50

100

150104.349

1.582

d1, 1, )

d2, 1, )

d3, 1, )

d4, 1, )

0.60.01 k 1

M2

k

1

2

3

4

Fig.6. Plots of power amplification as functions of the

input k for different losses values uT=uB+uP in the

switching tee arm at uB= 6.25⋅10-4 and uP=4.5⋅10-3 1) uP

= 0; 2) uP = uB; 3) uP = 3uB. Here uP – attenuationconstant in the switch; uB – attenuation constant in the

tee arm both values correspond to double traveling

time.

It is obvious that during excitation the losses in theswitch should be an order of magnetron smaller thanlosses during double traveling time along the cavity.

Fig.7. Plots of the energy transfer efficiency (1) and the timeoftransfer (2) as functions of the transition factor h1.

IV. SERIES COMPRESSION

The series compression is used when there is need toincrease the pulse power of a single cavity MPC and toshorten the output pulsewidth further [4].

The energy extraction comes in sequence: first theswitching increases the output transition factor of the first

cavity h →h1 (h1>>h) and after the second cavity gets

excited its output factor increases in turn m →m1

(m1>>m).The total transient process performed by calculating

the system of differential equations showed the necessityof discoupling between cavities during excitation of the

first cavity. If h>2⋅10-3 than the energy is resonantlytransferred to the second cavity without switching and the

efficiency goes down. The values below h<5⋅10-4 do notgive rise to the energy in the first cavity and the transientdepends totally by losses in walls and input couplingcoefficient. Here we assume that h i.e. is the integrated

transition factor of the output of the first cavity and thesecond cavity input.

Calculated values of the transfer are shown in Fig.7 as

functions of the factor h1 making the transfer possible.The dependencies are ploted for the input pulsewidth

tg=6⋅10-6 c, working frequency f=2.8⋅109 Hz and

following cavity parameters. First cavity – T1=5.23⋅10-9 c –

double traveling time, Q01=7⋅104 – intrinsic quality factor,

k = 0.5; second cavity T2=3.43⋅10-9 c, Qor =1.9⋅104 c,m=10-2.

The envelope of the output pulse is shown in Fig.8.Parameters of cavities were the same. For the sake ofillegibility the prepulse envelope is presented with

smaller scale of different axes.




KORUS’2005 Yu.G. Yushkov, S.N. Artemenko, S.N. Novikov et al841

Fig.8. Output pulse envelope of MPC with two series connected cavities

for h=104; h1=5⋅10-2; m1=0.5; 1) envelope of the total pulse; 2) prepulseenvelope.

0 2 4 6 8 f, GHz

М2,dB

20

10

4

12

3

Fig.9. Dependence of power amplification on frequency for different

single mode cross section types. 1 – square waveguide; 2 – rectangular

waveguide; 3 – rectangular waveguide taking losses in a switch intoaccount; 4 – coax line.

So MPC with series cavities has M2≈25 dB and thatexceeds M2 value of MPC with a single cavity. If M2 isgiven the MPC with series compression has lower energy

of the prepulse of ≈0.6 factor compared with the singlecavity M

2value.

V. DEVELOPED MPC

Dimension of single mode waveguide cross section are

different for different frequency bands and for lower

frequencies the increase of linear dimensions is

undesirable as it causes falling down of the commuting

efficiency. The electron density and switching plasma

area dimensions do not vary at constant electric field

strength and the reflection factor drops if a waveguide

cross section increases. So high values of Q0 inherent to

volume cavities at lower frequencies can not be

practically implemented for the increase of M2

.Fig.9 shoes M2 for f ranging up to 10 GHz for different

cross section types of a cavity structure.

It was assumed for calculations that L=7lw, lw –

waveguide wavelength, and ratio a/l is kept steady for

each point of lines 1, 2, 3. Here a – broad wall dimension,

l – wavelength. Line 2 is for b/a=0.47, b – narrow wall

dimension, line 3 is plotted for –3 dB loss in a switch. At

low frequencies, p.ex. L-band, M2 can reach 30 dB but

for cavities with corresponding big cross section

dimensions. Taking everything into account the coax

resonant lines could be advantageous in lower frequency

bands.The common experimental stand including MPC is

presented below in Fig.10.

G 1 st Cavity 2nd Cavity

Phase shifter

DirectionalCoupler

Circulator

Phase shifter

DirectionalCoupler

Matched Load

HV SwitchHV Switch

DirectionalCoupler

Matched

Load

gasInlet

gasOutlet

gasInlet

gasOutlet

Window Window Window

Fig.10 Schematic diagram of the common experimental setup.

Principal results in development of MPCs latterly are:- MPCs were designed with singlemode or

multimode cavities and tested in 0.15 – 36 GHz range.Output pulse width ranged – 0.1 to 100 ns.

- Output radiation is locked in phase with one of anexciting generator. Two cavities connected in parallel

were tested while they operate synchronously for acommon load. The system benefits from the distribution





of high microwave input power between cavities and provides more stable low field operation of each cavity.Input power was up to 10 MW

- The series compression is necessary when therequired power amplification is beyond the possibility of

a single cavity. Two – and three stage MPC were tested.Higher amplification is attributed to lower excitationtimes of each consecutive cavity. Most short pulses0.1 – 2 ns were obtained along with 26 – 27 dB poweramplification.

- Pulse repetition frequency is determined by anexciting source and the highest tested value was 20 kHzat output pulse power 50 kW. The gaseous plasma switchlimits PRF at output pulse energy about 1 J and above.

The development of designs and manufacturing MPCin different frequency band proved estimations and

assumptions noted above. The Table 1 presents parameters of MPC developed in Nuclear Physics

Institute, Tomsk.

TABLE 1

MAIN PARAMETERS ACHIEVED IN MPCS

BandPulse power,

MWPulsewidth,

nsPRF, Hz

(max)Amplification,

dB (max)

P 1.6 15 100 22

L 1 - 20 1.5 - 15 1000 20

S 0.05 - 1000 0.5 - 250 400 28

X 0.3 - 5 2 - 20 500 21

K 0.3 - 10 1 - 10 1500 23

Q 1.6 5 1000 15

REFERENCES

[1] A.N. Didenko and Yu.G.Yushkov, High Power Microwave Pulses

of Nanosecond Pulsewidth, Moscow: Energoatomizdat, 1984.[2] J.L.Altman, Microwave Circuits, Prinston: D. Van Nostrand Co.,

1964.

[3] Yu.G. Yushkov et al., “Powerful microwave compressors of RF

pulses, Proceedings of the International Workshop “Strongmicrowave in plasma”, Nizhny Novgorod: Institute of Applied

Physics, pp.911-925.

[4] R.A. Alvarez and D.P.Byrne, “Prepulse suppression in microwave pulse – compression cavities” in Review of Scientific Institutes,

vol.57, pp.2475-2480, October 1986.


Development of Microwave Pulse Compressors

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