Unique Multi-Physics Approach of Self Phase Locked Magnetron (SPLM) System with CST STUDIO SUITE™

SeungWon Baek*, Monika Balk**, KiHo Kim***, HyungJong Kim*** and JinJoo Choi**** *CST of America, San Mateo, USA

**CST AG, Darmstadt, Germany *** LIG NEX1, Gyeonggi-do, Korea

****Kwangwoon University, Department of Wireless Communications Engineering, Seoul, Korea

Abstract: In recent years, many types of Vacuum tubes have been successfully modeled using simulation codes. However, there wasn’t a complete solution which is suitable for multi-physics problem and is embedded in various industry standard workflows through integrated design environment which gives access to the entire range of numerical technology. CST STUDIO SUITE® provides engineers and researchers with a number of tools to tackle a wide range of challenging problems: CST EM STUDIO® for static and quasi-static simulations, CST MICROWAVE STUDIO® for transient and time-harmonic simulations, CST PARTICLE STUDIO® for the simulation of charged particle dynamics and CST MPHSICS STUDIO® offers a number of solvers to allow optimal performance for thermal and mechanical stress analysis. Each STUDIO for a given problem. Additional features such as GPU and MPI computing are available for accelerating the simulations.

Keywords: Magnetron, Self Phase Locked, Simulation, CST STUDIO SUITE®, CST MICROWAVE STUDIO®, GPU and MPI.

Introduction The strapped magnetron oscillator operating in crossed electric and magnetic fields is known to be the most successful vacuum electron device in generating high-power high-efficiency microwave radiation at decimeter and centimeter wavelengths [1,2]. The cooker magnetron used in a microwave oven has 10 cavities with double straps. A typical feature of the cooker magnetron operation is a relatively broad and noisy spectrum [3]. However, for many applications (microwave oven, radar, communication systems, plasma lighting system, charged-particle accelerators Wireless Power Transmission (WPT) for Solar Power Station (SPS), etc.), it is very necessary both to narrow the frequency spectrum and to reduce the noise. Improvements in the microwave spectrum can be achieved either by using a special magnetron design (Electric and Magnetic priming), which makes starting conditions more preferable for the operating mode, or by

feeding external electromagnetic radiation into the magnetron cavity. The advantage of the self-injection-locking technique is that no external source is used. A self-injection-locked technique has been used to reduce phase noise in solid-state oscillators [4]. In this paper, we have developed and tested the Phase Self Locked magnetrons using the unique multi-physics approach of CST STUDIO SUITE®, which consists of CST EM STUDIO® (CST EMS), CST MICROWAVE STUDIO (CST MWS), CST PARTICLE STUDIO® (CST PS) and CST MPHYSICS STUDIO® (CST MPS). The simulation model is a double-strapped 10-vanes magnetron with an output power of 1kW and an input coupler for a feedback signal through a dielectric resonator filter (DR filter).

Simulation/Experimental results In order to reduce the time to market and the cost of production, proven a simulation workflow is necessary. Therefore a complete solution is necessary. CST’s unique multi-physics approach for vacuum tubes is shown in Fig 1.

Figure 1. CST’s solution for vacuum tubes.

With this new work flow, we successfully developed a prototype SPLM by modifying a cooker magnetron as shown below in Fig. 2.

978-1-4673-5977-1/13/$31.00 ©2013 IEEE

Figure 2. A photo of a prototype of SPLM, a feedthrough for a feedback loop and DR-filter.

A. CST EMS – to design and optimize shape and size of a permanent magnet, which consists of two bar type Alnico magnets and Iron pole-pieces. The magnetostatic field solver can take the non-linear properties of the Iron material into account. The maximum B-field from the simulation agrees well to the design value of 0.19. Fig. 3 shows the numerical results from CST EMS.

Figure 3. CST EMS output : (a) isoline plot of a magnetic field, (b) magnitude of the magnetic field measured along an off-axis vertical line.

B. CST MWS – to design strapped 10 vane cavities, a DR-filter and the coupling section for self locking. In order to control the input power level of the feedback loop, we have simulated the coupling section by rotating a feedthrough. Fig. 4 (b) shows a comparison of measurement and simulation.

Figure 4. CST MWS output: (a) simulation model by rotating a coupler to control the input power, (b) comparison of measure and simulated insertion loss (S21) when varying the rotation angle from 0 to 90 degree, (c) electric field plot of the pi-mode plotted in the xy plane and (d) a photo of experiment to measure S-parameters with a network analyzer.

C. CST PS – to evaluate the output power and frequency shift of an externally excited magnetron with respect to a free oscillation. Fig 5 shows that SLPM is able to fix the operating frequency and essentially decrease the noise of the radiation. The frequency of SPLM is measured at 2.453GHz. This means a downshift of 3MHz compared to the free oscillation frequency of 2.456GHz of the cooker magnetron without feedback loop. In order to consider the feedback signal from the DR-filter, the simulation model is excited with a 2.456GHz signal at the input port. Thus the DR filter doesn’t need to be physically included in the model.

Figure 5. CST PS output: (a) Spokes, (b) Spectrum of free oscillation (pink colored line) and self-injection locked oscillation (blue colored line) with a matched load.

D. CST MPS – to evaluate the temperature distribution and mechanical stress on the structure. It is important that the temperature stays below the curie temperature of the Alnico magnet. In addition the maximum stress has to be analyzed, because it may cause a crack on the magnet. An optimization of the heat plate can be done by means of the built-in optimizers. This optimization is fully parametric and couples thermal and mechanical analysis. Fig 6 shows the maximum temperature of 116 °C and the Von Mises stress of 0.0153 GPa, which is much lower than the maximum stress from the specification of a magnet manufacture.

Figure 6. CST MPS output: (a) temperature plot, (b) Von Mises stress plot and (c) temperature measurement setup.

Conclusion In this paper, a SPLM was theoretically designed using CST’s simulation technology and experimentally fabricated by modifying a cooker magnetron. One of the major advantages of simulation is replicating measurements. For this, an accurate result is necessary as well as a fast simulation time while the latter can be achieved by using GPU computing [6].

CST Solver Part Quantity Simulate

d value Measured

value Estatic solver and Mstatic solver

Magnet

Cathode

B-field [T]

E-field [kV]

0.19

- 4.3

0.19

- 4.3

Transient solver and Eigenmode solver

Feedthrough

Cavity

S21 [dB]

Mode

-10.1

Pi

-9.6

Pi

Particle in cell Solver

Resonator Spokes [#]

Frequency [GHz]

5

2.456

5

2.456

Stationary Thermal solver and Structural Mechanics solver

Anode

Magnet

Heat plate

Magnet

Temp. [°C]

Stress [GPa]

116

95

65

0.0153

120

90

62

0.158*

*(Max. spec.)

Table 1. Comparison of measurement and simulation

Table 1 shows a comparison for all aspects excellent correlation was achieved.

Last but not least, CST STUDIO SUITE® offers a fully parametric approach for coupled simulations. In combination with the built in optimizers a convenient design of complex tasks is possible. Furthermore, the Transient, Frequency Domain and Eigenmode solver feature sensitivity analysis to aid the optimization engine. This analysis yields the sensitivity of either S-parameters or resonance frequency with respect to a given geometry parameter. With that information much fewer 3D EM simulations are needed during an optimization process.

Acknowledgements The authors wish to thank Martin Schauer at CST of America for his valuable comments and the technical support of CST MPS and CST EMS as well Yong-Soo Lee at LG Electronics for his help with prototype construction and measurement. Special thanks to Frank Hamme for developing the PIC solver on GPU. That reduces the simulation time to a couple of hours instead of days.

References 1. G. B. Collins, Microwave Magnetrons (McGraw-

Hill,New York, 1948). Hofelich, T. C., D. J. Frurip, and J. B. Powers, “The Determination of Compatibility,” Process Safety Progress, vol. 13, no. 4, pp. 227-233, 1994.

2. A. S. Gilmour, Jr., Microwave Tubes (Artech House, 1986), Chap. 13.

3. K. Kanamoto, H. Kuronuma, T. Koinuma, and N. Teshiro, “A study of magnetron noise,” IEEE Trans. Electron Devices, vol. ED-34, no.5, pp.1223-1226, May 1987.

4. T. Ohta and K. Murakami, “Reducing negative resistance oscillator noise by self-injection,” Electron. Commun. Jpn., vol. 51-B, pp. 80–82, Oct. 1968.

5. Gil Wong Choi, Hae Jin Kim, Hyoung Jong Kim, and Jin Joo Choi, “The Self-Injection-Locked magnetron” Vacuum Electronics Conference, 2008. IVEC 2008. IEEE International.

6. Monika Balk, “3D magnetron simulation by means of graphical processing units with CST STUDIO SUITE™“, Vacuum Electronics Conference, 2012, IEEE International.