1. 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

2. 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

3. 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

4. ~12.7 cm Circular Waveguide ~7.4 cm Circular Waveguide Multi-Moded DLDS TE 01 Mode Extractor (Power is Extracted Evenly between Four Waveguides) TE 01 TE 12 (Vertically Polarized) TE 12 (Horizontally Polarized) TE 01 TE 12 (Vertically Polarized) TE 01 TE 01 Mode Extractor Mode Launcher (Fed by Four Rectangular Waveguides) TE 21 TE 21 -TE 01 Mode Converter Klystrons ~ 6 m TE 01 Mode Converter (Fed by Four Rectangular Waveguides) TE 12 to TE 01 Mode Converter ~53 m Tantawi 28/4/98 TE 01 Tap-Off

5. 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.

6. Input Output 1 Output 2 Simple SPDT

7.  Phase Shifter Input Output 2 Output 1 Input Output 2 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

8. The basic three-port network. The phase of the reflected signal from the third port depends on the status of the active element InputOutput Three Port Lossless Network that have a scattering matrix S Active Arm

10. The basic three-port network. Input Output Phase Shift/Period=3  n

11. Total Phase shift=3 

13. Number of Elements=6

14. Laser Light Silicon Wafer Sapphire Discs Short 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 10 19 /cm 3 silicon would have a conductivity of ~3.3x10 3 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 TE 11

15. 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 TE 11. 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 10 19 /cm 3 which corresponds to a conductivity of 3.3 x 10 2. 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.

16. CONCLUTION We 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.