Numerical and Experimental Design Study of Quasi-Optical (QO) Multi-Gap Output

Cavity for W-band Sheet Beam Klystron

Numerical and Experimental Design Study of Quasi-Optical (QO) Multi-Gap Output

Cavity for W-band Sheet Beam Klystron

10th International Vacuum Electronics Conference (IVEC2009)

April 28 - 30th 2009

10th International Vacuum Electronics Conference (IVEC2009)

April 28 - 30th 2009

(Presentation #: 1558349)

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. M67854-06-1-5118.

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 kVBeam current: 3.6 A Peak power: 50 kWAverage power: 2.5 kW Efficiency: 20%Gain: 40 dBBrillouin Magnetic field: 1000 Gauss (RMS)Number of cavities: 8 Circuit length (wg to wg): 9 cmBeam 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

PCMSensitivity

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)

s

z

W

dzE

Q

R

2

2

dzE

dzeE

M

z

ziz

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 (S21 and S11)

- FDTD simulation (CST MS) -

Total Q (Qt) of the operation TE10 mode (2-mode) of 95.4 GHz is about 450 - Experimental measurement -

f0 (unloaded) = 95.3GHz

fL (loaded) = 95.3504GHz

Q0 = 1652

Qe = 621

Qtot = 451

• Eigenmodes of Multi-Gap QO Cavities

- 3-Gap QO Cavity -

f0 (unloaded) = 95.3527GHz

fL (loaded) = 95.4074GHz

Q0 = 1665

Qe = 646

Qtot = 465

- 4-Gap QO Cavity -

f0 (unloaded) = 95.3731GHz

fL (loaded) = 95.385GHz

Q0 = 1663

Qe = 655

Qtot = 467- 5-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 = 64.64 dB

• Multi-Cell (Export-Import) MAGIC3D Simulation

94.5 94.64 94.5 94.64 94.82 95.6 94.85 94.5Units: GHz

53 kW 2 × 26.5 kW = 53 kW

Parameter Sweep of QO-WSBK Models

3-Gap Output Cavity 5-Gap Output Cavity 7-Gap Output Cavity

3-Gap

Intermediate

Cavity

(2th – 4th)

1-Gap

Intermediate

Cavity

(2th – 4th)

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 × 107 [-1m-1])

3-Gap Intermediate Cavities (MAGIC3D)

• Time History of Output Power

- 3-Gap Output Cavity - - 5-Gap Output Cavity - - 7-Gap Output Cavity -

Single Gap Intermediate Cavities:MAGIC3D

• Time History of Output Power

- 3-Gap Output Cavity - - 5-Gap Output Cavity - - 7-Gap Output Cavity -

Summary of MAGIC3D Simulations

3-Gap Output Cavity 5-Gap Output Cavity 7-Gap Output Cavity

3-Gap Intermediate

Cavity

- Pin (Max. Pout) ~ 12.13 mW

- Pout (Max.) ~ 40 kW

- Gain (Max.) ~ 69 dB

- Pin (Max. Pout) ~ 20 mW

- Pout (Max.) ~ 53 kW

- Gain (Max.) ~ 70 dB

- Pin (Max. Pout) ~ 24.27 mW

- Pout (Max.) ~ 54.76 kW

- Gain (Max.) ~ 63.54 dB

1-Gap Intermediate

Cavity

- Pin (Max. Pout) ~ 300 mW

- Pout (Max.) ~ 39.5 kW

- Gain (Max.) ~ 53.3 dB

- Pin (Max. Pout) ~ 300 mW

- Pout (Max.) ~ 52.5 kW

- Gain (Max.) ~ 56 dB

- Pin (Max. Pout) ~ 500 mW

- Pout (Max.) ~ 56.14 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