Numerical and Experimental Design Study of Quasi-Optical (QO) Multi-Gap Output Cavity for W-band Sheet Beam Klystron 10 th International Vacuum Electronics Conference (IVEC2009) April th th International Vacuum Electronics Conference (IVEC2009) April th 2009 (Presentation #: ) 15:20 Session 20 - Klystron II Thursday, 30 April 2009 Young-Min Shin, Larry R. Barnett, Jianxun Wang, and Neville C. Luhmann Jr. Department of Applied Science, University of California-Davis (UCD), CA 95616, USA

This work is supported by the Marine Corps Systems Command (MCSC), Grant No. M Acknowledgements We wish to acknowledge informative discussions with Dr. Glenn P. Scheitrum and Dr. Aaron Jensen on the SLAC WSBK We also wish to Dr. Ali Farvid at the Stanford Linear Accelerator Center (SLAC) for his generous help, advice, and assistance in setting up the electroforming system, and to acknowledge informative discussions with Dr. Frank Yaghmaie, Director of the Northern California Nanotechnology Center (NCNC) in the University of California – Davis (UCD)

Outline  Motivation and Objectives  Electron Gun and Focusing Magnet  QO Output Cavity Design and Analysis  Simulation Examination and Cold-Test  Full Tube Design (AJDISK) and PIC Simulation (MAGIC3D) Analysis  UV LIGA Microfabrication  Summary and Future Plans

Motivation Develop a transportable, modular system, employing four novel W-band Sheet Beam Klystron (SBK) devices (each capable of 2.5 kW of average power) and producing a minimum of 10 kW of 95 GHz radiation. Higher powers can be produced by adding more SBKs and combining their output powers either by waveguide multiplexing, or in space. Original SLAC MURI Concept

Original SLAC MURI WSBK Design Parameters Beam voltage: 74 kV Beam current:3.6 A Peak power:50 kW Average power: 2.5 kW Efficiency: 20% Gain:40 dB Brillouin Magnetic field: 1000 Gauss (RMS) Number of cavities:8 Circuit length (wg to wg):9 cm Beam size (elliptical):6 mm x 0.5 mm (12 : 1) Drift tube size (rectangular):8 mm x 0.72 mm

WSBK Problems Output Cavity: Incorrectly Designed Input Cavity: Correctly Designed/Incorrectly Fabricated Magnetic Anode Body Windows: High VSWR Need for Tuners PCM Sensitivity

Full Peak Power WSBK Demo Tube Seven gap velocity tapered output circuit with three gaps in other cavities MAGIC 3-D simulations predicted stability and 50 kW peak output Gun redesign: the original beam stick gun which produced high efficiency (91 %) transport was extremely sensitive to alignment ( 0.002” vertical misalignment would lose 40% of the beam in the gun). The anode aperture was therefore opened to reduce the anode hole effects Magnet stack redesigned.  Maximum beam transmission of 78 % at full design voltage and cathode current-attributed to magnetization of the gun weld ring resulting in beam rotation**  Maximum output power of > 11 kW and ~ 48 dB gain observed using sensitive external adjustments of the shunts and cavity tuning using retrofitted cavity tuners.  Low output is (as indicated by the test data) due to a combination of poor output coupling involving higher modes, field cancellation at the design frequency, beam and bunch formation, and cavity mistuning. *Summary of initial tests at SLAC and subsequent tests at UC Davis. **Magnetization problem eliminated by proper choice of stainless steel Summary of Prototype WSBK Test Results

Amelioration of Technical Issues (1) Anode Flange Magnetization Replace 304L S.S. with 310 S.S. which cannot be magnetized Carry out complete 3D Gun and Magnetic field (re-)simulations to verify/modify design using CST Particle-Studio, Advanced Charged-particle Design Suite from Field Precision, and Ansoft Maxwell 3D simulation packages. Independent modeling assessment by Stan Humphries of Field Precision (2) Incorrectly Machined Input Cavity Proper input cavity design/fabrication eliminates/ameliorates mode competition problem (3) Incorrectly Designed Output Cavity (7-Gap) Re-design the output cavity  Quasi-Optical (QO) cavity (4) Incorporation of cavity tuners (5) Extensive MAGIC3D simulations to determine optimum design

Quasi-Optical (QO) WSBK Output Concept

QO Cavity Design and Sensitivity Analysis Original 7-Gap Output Most critical dimension: cavity height ( dy )

Replacement of WSBK Output Cavity (7-Gap  QO) Original 7-Gap Output Pocket Machining QO Output Circuit QO Circuit Assembly Brazing Assembly Assembled Circuit

3-Gap 2  -Mode QO Output Cavity Being Installed in Modified Tube

Signal Response and Eigenmode Analysis Port-to-Port Transmission and Reflection Coefficients (S 21 and S 11 ) - FDTD simulation (CST MS) -  Total Q (Q t ) of the operation TE 10 mode (2  -mode) of 95.4 GHz is about Experimental measurement - f 0 (unloaded) = 95.3GHz f L (loaded) = GHz Q 0 = 1652 Q e = 621 Q tot = 451 Eigenmodes of Multi-Gap QO Cavities - 3-Gap QO Cavity - f 0 (unloaded) = GHz f L (loaded) = GHz Q 0 = 1665 Q e = 646 Q tot = Gap QO Cavity - f 0 (unloaded) = GHz f L (loaded) = GHz Q 0 = 1663 Q e = 655 Q tot = Gap QO Cavity - Young-Min Shin, Larry R. Barnett, and Neville C. Luhmann Jr. “Quasi-Optical Output Cavity Design for 50kW Multi-Cavity W-Band Sheet Beam Klystron”, IEEE Trans. Elec. Dev. (submitted, 2009)

QO WSBK Tube Design AJDISK Simulation Result of Optimized WSBK Tube Design

MAGIC3D Simulation of Full Circuit 1-Port Driving (~ 18.2 mW) - Input Power and Frequency Spectrum - - Output Power and Frequency Spectrum - Beam Voltage = 74 kV Beam Current = 3.6 A Input Frequency = 94.5 GHz Input Power = 18.2 mW Output Power = 53 kW ( = 2 × 26.5 kW) Efficiency = 20 % Gain = dB Multi-Cell (Export-Import) MAGIC3D Simulation Units: GHz 53 kW  2 × 26.5 kW = 53 kW

Parameter Sweep of QO-WSBK Models 3-Gap Output Cavity5-Gap Output Cavity7-Gap Output Cavity 3-Gap Intermediate Cavity (2 th – 4 th ) 1-Gap Intermediate Cavity (2 th – 4 th ) 2  -Mode (f = 94.5GHz) 2  -Mode (f = 94.5GHz) 2  -Mode (f = 94.5GHz) 2  -Mode (f = 94.5GHz) 2  -Mode (f = 94.5GHz) 2  -Mode (f = 94.5GHz) (Conductivity:  = 5.8 × 10 7 [  -1 m -1 ])

3-Gap Intermediate Cavities (MAGIC3D) Time History of Output Power - 3-Gap Output Cavity Gap Output Cavity Gap Output Cavity -

Single Gap Intermediate Cavities:MAGIC3D Time History of Output Power - 3-Gap Output Cavity Gap Output Cavity Gap Output Cavity -

Summary of MAGIC3D Simulations 3-Gap Output Cavity5-Gap Output Cavity7-Gap Output Cavity 3-Gap Intermediate Cavity - P in (Max. P out ) ~ mW - P out (Max.) ~ 40 kW - Gain (Max.) ~ 69 dB - P in (Max. P out ) ~ 20 mW - P out (Max.) ~ 53 kW - Gain (Max.) ~ 70 dB - P in (Max. P out ) ~ mW - P out (Max.) ~ kW - Gain (Max.) ~ dB 1-Gap Intermediate Cavity - P in (Max. P out ) ~ 300 mW - P out (Max.) ~ 39.5 kW - Gain (Max.) ~ 53.3 dB - P in (Max. P out ) ~ 300 mW - P out (Max.) ~ 52.5 kW - Gain (Max.) ~ 56 dB - P in (Max. P out ) ~ 500 mW - P out (Max.) ~ kW - Gain (Max.) ~ 50.5 dB

Demountable Tube Concept for Hot-Tests Assembly of the Demountable Tube Body of the Demountable Tube (currently being machined) Cover with a gold seal (machining in progress) Support for the cover

Demountable Tube

UV LIGA Fabrication of QO WSBK Circuit - UV lithography fabricated mold structure (thickness: ~ 450  m) - KMPR UV Lithography Patterning Electroplating and Mold Removal Lapping and Polishing

Summary and Future Plans 1. Summary 2. Future Plans - Electron gun and magnet being redesigned  Beam transport simulation is underway  PCM Magnetic field is being measured - QO output cavity has been designed  Dimension parameters optimized  Dimensional sensitivity analysis done  Cold-test and signal response simulation done - Full Power WSBK tube has been redesigned  Tube design finished using AJDISK  MAGIC3D simulation verification done  Parametric analysis done - UV LIGA Fabrication  Multi-Step UV LIGA development is underway - Full WSBK circuit engineering design - Demountable tube design - Hot-Test