The Design and Analysis of Multi-megawatt Distributed Single Pole Double Throw

(SPDT) Microwave Switches

Sami G. Tantawi, and Mikhail I. Petelin

Stanford Linear Accelerator Center, Stanford University

Outline

• Motivation• Different types of SPDT microwave switches• Distributed phase shifters• Microwave control through three port network• Periodic three-port networks, and the synthesis

process• Optically controlled SPDT

Eight 75-megawatt klystrons

RF

e+ or e- A Cluster of 9 Multi-Moded DLDS Sections

RF Power Sources

A Single Multi-Moded Delay Line RF Distribution System

Accelerator Structures

Delay Lines

~12.7 cm Circular Waveguide

~7.4 cm Circular Waveguide

Multi-Moded DLDS

TE01 Mode Extractor(Power is Extracted Evenly between Four Waveguides)

TE01

TE12 (Vertically Polarized)TE12 (Horizontally Polarized)

TE01

TE12 (Vertically Polarized)

TE01

TE01 Mode Extractor

Mode Launcher (Fed by Four Rectangular Waveguides)

TE21

TE21-TE01 Mode Converter

Klystrons

~ 6 m

TE01 Mode Converter (Fed by Four Rectangular Waveguides)

TE12 to TE01 Mode Converter

~53 m

Tantawi 28/4/98

TE01 Tap-Off

• The Current Next Linear Collider design have accelerator structure sections that requires a 200 MW, 375 nS pulses at 11.424 GHz. The available power supplies are 75 MW klystrons which produces more than 1.5 S pulses. Hence, pulse compression is needed.

• DLDS is an alternative to conventional pulse compression which enhances the peak power of an rf source while matching the long pulse of that source to the shorter filling time of the accelerator structure.

3/13/23/2

3/23/13/2

3/23/23/1

j

j

jj

S

Input

Output 1

Output 2

Simple SPDT

Phase Shifter

Input

Output 2

Output 1

Output 1Input

Output 2

Phase Shifting Active element

Phase Shifting Active element

Electric field polarization at input

Electric field polarization at output when the switch is off

Electric field polarization at output when the switch is on

. Schematic Diagram of a dual mode SPDT

The basic three-port network.

The phase of the reflected signal from the third port depends on the status of the active element

Input Output

Three Port Lossless Network

that have a scattering matrix S

Active Arm

cos2

sin

2

sin2

sin

2

cos

2

cos2

sin

2

cos

2

cos

jj

jj

ee

ee

S

jeVV 33

T he resultant, sym m etric, tw o-port netw ork, then, has the fo llow ing form :

2222

2222

2cos

2sin

2sin

2cos

jj

jj

porttwo

eej

ejeS ,

w here the angle is given by

1cos

cos

j

jj

e

ee ,

2

21

22

3

2

3 2coscoscos43

sin

VVVV

0

0.2

0.4

0.6

0.8

1

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

m=0m=1m=2m=3m=4

Rat

io b

etw

een

the

forw

ard

and

back

wor

d w

aves

of

an

eige

n so

luti

on

Number Of Switching Elements

The basic three-port network.

Input Output

Phase Shift/Period=3n

0

0.5

1

1.5

2

1 2 3 4 5 6 7 8 9 10 11 12

Number of Elements=6

Number of Elements=9

Number of Elements=12

|V1+

+V

2+|

Element Number

Total Phase shift=3

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2 4 6 8 10 12 14 16 18

Pea

k (|V

1++

V2+

|)

Number of Elements

2/1

33

3max /

GA

ZPEE in

n

3

/Z

RPP slnl

Number of Elements=6

Laser Light

Silicon Wafer

Sapphire DiscsShort Circuit

For the pulse compression system application associated with the NLC, the device should remain in one state for approximately 1.75µsec, and in the other state for 250 nsec. Since silicon has a carrier life time that can extend from 1 µsec to 1 msec it seems like a natural choice for this application. One can excite the plasma layer with a very short pulse from the external stimulus (~5nsec) and the device will stay in its new status long enough till all the rf signal is terminated. At a carrier density 1019/cm3 silicon would have a conductivity of ~3.3x103 mho/cm. This is two orders of magnitude smaller than that of copper. However, it is high enough to make an effective reflector. The skin depth of an rf signal at the NLC frequency at this conductivity level is ~8µm.

The active arm is made of a circular waveguide operating at the fundamental mode TE11

DESIGN EXAMPLE OF AN OPTICALLY CONTROLLED X- BAND SWITCH

One of the applications of this switch is the high power pulse compression system of the Next Linear Collider . This system operates at 11.424GHz. We can construct the phase shifter and, hence the switch from a series of six three-port networks. The three-port network may be composed of a WR90 rectangular waveguide with a circular waveguide coupled to it from the broad side. A propagation of 200 MW in waveguide junctions having similar dimensions has been demonstrated. If the switch is to operate at a 100 MW level, the phase shifter need to handle only 50 MW.

The third arm, in this case, is composed of a circular waveguide carrying the fundamental mode TE11. If the diameter of this waveguide is 2.54 cm, the peak field for a 50MW power level is 140 kV/cm. If the active element in these guides is a silicon wafer, which can be switched optically using a short pulse laser, the peak field need to be less than a 100 kV/cm at the wafer. Hence the normalized peak field need to be less than 0.714. If we assume to be 0; at the normalized peak field is 0.6, and the normalized losses is 0.914. Hence the peak electric field is 84 kV/cm. When the switch is on we assume a carrier density of about 1019/cm3 which corresponds to a conductivity of 3.3 x 102. Hence, the losses is 0.46% per element, i.e., a total of 230 kW is being wasted at the silicon wafer. The realizability of the cooling system to take out this power depends on the average power and the pulse length of the rf signal.

CONCLUTIONWe presented an abstract analysis and design methodology for a DTSP switch based on several distributed elements. We showed that such a switch, in principle, could be designed to handle a 100 MW at X-band.