X-band high-power variable RF splitter Hao Zha, Igor Syratchev (CERN) 18/06/2015 High Gradient work shop 2015 in Tsinghua University

Motivation We want to design an RF power splitter: Rectangle waveguide based, X-band (12GHz) With 1 input port, 2 output ports; Arbitrary split ratio and easy to adjust this ratio; Matched (reflection <-30dB) at any split ratio; High-power (~100 MW) use; Large band-width (>100 MHz); Compact size and easy machining.

Previous design ‘Classical’ movable short circuits. Do not work at high RF power! Input port 1 Output port 2 Output port 3 RF contact free short circuit piston Input port 1 Output port 2 Output port 3 Please see more details in CERN Indico page: Igor Syratchev, CLIC RF Structure Development Meeting. April 17, 2013

1-piston system Input port 1 Output port 2 Output port 3 Input port 1 Please see more details in CERN Indico page: Igor Syratchev, CLIC RF Structure Development Meeting. April 17, 2013 Input port 1 Output port 2 Output port 3 Input port 1 Output port 2 Output port 3

Network of the RF splitter Network of the splitter includes 4 port, 1 for the piston. The reflection phase ρ determines the splitter ratio.

4-port network system Impossible to achieve matching for symmetric 4-port networks. Asymmetric network (contain loss material) may be not good for ultra-high-power use. RF Circulator Ulta-High power?? Port 1 Port 2 Port 3 Port 4: To piston Lose port 4

5-port network system Port 1 Port 2 Port 3 5-port system : with two port connected to the short circuit piston.

5-port network system Such 5-port network is matched for any split ratio, according to the transmission line theory. Two short circuit piston should be adjusted simultaneously.  Port 1: input Port 2: output Port 3: Port 4 & 5: To piston       Phase = (N+0.5)*π          

RF polarizer RF polarizer can accommodate two orthogonal modes in the circular waveguide. It can emerge two short pistons → one!  Two port will be isolated, no coupling (-50dB) from each other. Inspired by the Dual Circular Polarizer (C. Chang, S. Tantawi, et al.) and Turnstile Orthomode Reducer (Giampaolo Pisano et al.)

Design of RF splitter Use WR90 waveguide. Maximum E-field @100MW = 48MV/m Dimensions: 16*10 cm

Design of RF splitter Maximum return reflection: -38 dB Bandwidth for: Port 2: < 180 MHz Port 3: < 270 MHz

Trapped-mode The trapped-mode will affect the power transmission as well as enhance the surface field. Frequency is determined by waveguide width. Gif from Web

Towards more compact and larger band-width The components however create some HOMs (mainly TE20) propagating in the waveguides. Enough distances between components are needed to suppress these HOMs.   Change waveguide dimensions: Waveguide width ↓: suppresses HOMs and trapped-mode Waveguide height ↑ : reduces field

New waveguide dimension a↓, b ↑ b <= 12.5 mm Final decision: a = 20 mm, b = 12 mm

Design of RF splitter (v2.0) Maximum E-field @100MW = 50 MV/m Dimensions: 10*8 cm

Design of RF splitter (v2.0) Maximum return reflection: -44 dB Bandwidth for: Port 2: 370 MHz Port 3: 280 MHz Trapped-mode moves further away (12.03 GHz->12.17 GHz)

Future Machining is undergoing. This power splitter will be future used in CLIC X-band test stands.

Thanks