Evaluation of the TE 12 Mode in Circular Waveguides for Low-Loss High Power Transportation Sami G. Tantawi, C. Nantista K. Fant, G. Bowden, N. Kroll, and A. Vlieks SLAC Yong Ho Chin, H. Hayano, and Vladimir Vogel KEK J. Neilson Calabazas Creek, Inc.

The NLC RF pulse compression and RF Distribution System Sami G. Tantawi SLAC

Outline Introduction Multi-Moded DLDS Mode Analyzer TE 12 mode launchers TE 01 mode launchers Waveguide Tapers Transport line measurements Conclusion

The high power rf pulse compression techniques suggested for the future linear colliders involves long runs of low loss transportation lines. These runs range from 1000 km to 240 km depending on the system. These lines are suppose to carry rf pulses with power levels up to 600 MW for 1.5 micro-seconds at GHz. These transportation lines were envisioned to be a circular waveguides with smooth walls using the low loss TE 01 mode. Several experimental pulse compression systems based on these lines were built and operated at power levels up to 500 MW[7-8]. The usage of HE 11 mode in corrugated guides were deemed impractical because the corrugation depth required at X-band is large and that made the cost of the waveguide high. To reduce the length of the waveguide and consequently the cost, a multi-moded rf system was suggested. The reduction in cost using this technique was analyzed and shown to be considerable.

Delay Lines Accelerator Structures Bank of n k of klystrons A set of hybrids that switches the combined rf to different outputs Not all the output need to be used. The unused outputs are terminated by an rf load Accelerator Structures Multi-Moded Delay Lines. The total number of these lines is n p Bank of klystrons A set of hybrids that switches the combined rf to different outputs A mode launcher which takes n m inputs and produces n m modes into a single waveguide delay line A Unit of a Single-Moded DLDS A Unit of a Multi-Moded DLDS

Single-Moded Delay Lines 3 dB 90 Degree Hybrid Accelerator Structure Two banks of power sources each has an n k /2 klystrons 3 dB 90 Degree Hybrid Accelerator Structure Two banks of power sources each has an n k /2 klystrons Single-moded Binary Pulse Compression Single or Multi-Moded Delay Lines Circulator Short Circuit Binary pulse compression can have several improvements including the use of a circulator and several modes to reduce the delay line length.

Relative attenuation of different modes per unit time in circular waveguide versus the normalized diameter of the waveguide.

Multi-Moded DLDS System

Circular Guide modesSquare Guide Modes TE11 (Polarization #1)TE10 TE11(Ploarization #2)TE01 TM01TM11 TE21 (Polarization #1)TE20 and TE02 (In Phase) TE21 (Polarization #2)TE11 TE01 TE20 and TE02 (out of Phase by 180 degrees) TM11 (Polarization#1)TM12 TM11 (Polarization#2)TM21 TE31 (Polarization #1)TE12 TE31 (Polarization #2)TE21 TM21 (Polarization #1)TM22 TM21 (Polarization#2)TM13 and TM13 (In phase) TE41 (Polarization#1)TE22 TE41 (Polarization#2)TE31 and TE13 TE12(Polarization#1)TE30 TE12(Polarization#1)TE03 TM02 TM31 and TM13 (out of phase by 180 degrees) Modal Connection Between Circular and Square Waveguides.

(a) (b) (a) The Circular-to-Rectangular-Tapers TE 12 Mode Transducer. (b) A cut away view of the structure.

Rectangular port (TE 10 ) Circular Port TE 12 This plane is simulated as a perfect magnetic wall This plane is simulated as a perfect electric wall Simulated electric field distribution inside the TE 12 mode transducer. The colors represent the electric field strength.

Simulated performance of the TE 10 (rectangular) to TE 12 (circular) mode converter. Simulations are done using HP-HFSS.

Measured frequency response of two TE 12 mode tranceducers connected back to back.

The Wrap-Around Mode Converter. The physical model shown in the picture does not have the back wall shorting plate, this is done for illustration purposes only.

HFSS simulation results for the wrap around mode converter. The color shades represents the magnitude of the electrical field. (a) is a cut plane through the slots, (b) is a cut plane in the circular guide 2.5 cm away from the slots.

Measured Transmission coefficient for two wrap-around mode converters back to back. The device is optimized at GHz.

TE 12 incident on 2" D Sum of Reflected power : dB Transmitted power results: Mode Output Power (dB) TE TM TE TE 01 incident on 2" D Sum of Reflected power : -70 dB Transmitted power results : Mode Output Power (dB) TE TE TE TE Simulation Arc-taper profile, distances are in meters. Vertical axis is radius and horizontal axis is axial distance

Two TE 12 mode converters back to back including up tapers to 4.75” diameter

Two TE 01 mode converters back to back including up tapers to 4.75” diameter

Calculated mode amplitude profiles along the mode rotator, or polarization converter. The asterisks here indicate cross-polarized modes.

MAFIA graphic showing electric field arrows for the WC475 choke resonance. The horizontal axis is r and the vertical axis is z, both in meters. The bottom edge of the plot is the symmetry plane at the gap center.

The Mode Analyzer Linear Stage Azimuthal Stage Outer Pipe (Middle Waveguide) Inner Pipe (Transport Line Waveguide) Spring Ring ( to guarantee electrical contact) The orientation of the rectangular waveguide determine the component of the surface magnetic field being measured The Middle waveguide is connected to the moving stages using a ball joint

The scattering of modes due to the step discontinuity when an incident mode is the TE 01 mode The scattering of modes due to the step discontinuity when an incident mode is the TE 12 mode

HP 8510C Display/Processor HP 8510C IF Detector HP 8350 Sweep Oscillator 8514A S-Parameter Test Set Transport Line, 55 meter of circular waveguide that has a diameter of cm diameter Mode Analyzer Multi-mode Load 1-Watt Amplifier 20-dB Directional Coupler 54 meter of WR90 Rectangular waveguide All Connections are made with a phase and amplitude stable cables HP 8510 System Bus Sweep In Stop Sweep Low Noise Amplifier PC (Pentium based) GPIB Mode Launcher This PC controls both the network analyzer and the mode analyzer. It is also used for data acquisition Test Set RF Input Typical Measurement Setup

Stability of measurements over time

Rectangular waveguide calibration measurements

SLAC’s TE 12 mode launcher

Measured Mode spectrum of the TE 01 mode transducer.

Mode Spectrum of the KEK Mode Launcher

U of Maryland

Mode Spectrum after the 55 meter of Waveguide. The mode is Launched using SLAC’s TE 12 mode converter

Mode Spectrum after the 55 meter of Waveguide. The mode is Launched using SLAC’s TE 01 mode converter

Transmission Measurement through a TE 12 mode launcher 55- meter of WC475 Waveguide and a receiving TE12 Mode Converter. The TE 12 was Launched and received with horizontal polarization

Time domain response of the transport line plus the mode launchers (two mode transducers plus two arc-tapers). In this figure the two mode transducers were always aligned with respect to each other

The effect of rotating one of the mode TE 12 mode transducer with respect to the other.

Time domain response of the transport line plus the mode launchers (two TE 01 mode transducers plus two arc-tapers).

Losses Of The TE 01 Mode is 1.08%; Theory is 1.1% Losses of the TE 12 Mode is 4.5% to 5.1% (Polarization dependant); Theory is 2.8% No mode rotation was observed None of the mode TE 12 converters performed satisfactory.

Conclusion We have demonstrated the possibility of using the TE 12 mode in highly over-moded circular waveguides as a means of low-loss transport of rf signals. The over all losses were small and compared relatively well with theory. The waveguide used in the experiments were extruded oxygen-free high-conductivity copper. It was shown that these waveguides could be manufactured good enough to eliminate all cross polarization mode mixing. Nonetheless, we observed some conversion to the virtually degenerate mode, TE 41. However, the conversion levels were small. We also compared our results for TE 12 with those of the low loss TE 01. In this process we showed that connecting flanges and waveguides could be used to propagate either modes. This paves the way to developing a multi-moded system were different signals could be loaded over different modes. We reported a novel technique for measuring the modal content of a highly over- moded waveguides. We also, reported a technique for efficiently exiting the TE 12 mode and the TE 01 mode. Finally, we showed how to design and implement a polarization rotator for the TE 12 mode. Over the 55 meter of WC475 losses of The TE 01 Mode is 1.08%; Theory is 1.1%. Losses of the TE 12 Mode is 4.5% to 5.1%; theory is 2.8%

The Mode analyzer being Aligned

The Mode Analyzer System

TE 12 Mode Launcher, a spacer for the mode rotator, nonlinear taper and, transport line

The end of the mode analyzer and transport line is terminated by a multi-moded load

The Wrap-Around Mode Converter, The Arc-Taper, and the Mode Analyzer