1. Improved Distributed - Loss Gyro-TWA Yi Sheng Yeh, Chi-Wen Su, Yu-Tsung Lo, Ting-Shu Wu, Department of Electrical Engineering, Southern Taiwan University of Technology, Tainan, Taiwan, ROC and Chien-Lun Hung Department of Electronic Communication, National Penghu Institute of Technology, Penghu, Taiwan, ROC

2. Outline  Introduction to gyro-TWA  Computer models of nonlinear simulation code  Self- excited oscillations  Severed gyro-TWT  Distributed-loss gyro-TWT  Improved distributed-loss gyro-TWT  Amplifier performance of the improved distributed-loss gyro-TWT  Conclusions  References

3. Introduction to Gyro-TWA output waveinput waveelectron beam [ NTHU ] rgrg rwrw rLrL  The gyrotron promises a new generation of high power, broadband, millimeter-wave amplifiers.  The electrons which are guided by an applied uniform magnetic field B 0 move along helical trajectories.

4. Computer Models of Nonlinear Simulation Code Boundary conditions (gyro-TWA) Boundary conditions (self-excitedoscillation) Fields of the circularly polarized TE mn mode Field equation The relativistic equation of montion waveguide

5. Self – Excited Oscillations  Gyrotron backward oscillation - the interaction in the backward wave region (point 3 and 4) are sources of the gyro-BWO.  Absolute instability – the interaction in the forward wave region (point 1 and 2) are sources of absolute instabilities.  Reflective oscillation – the oscillations take place in the sever-wall junction on the left and the output structure on the right.

6. Structure of the gyro-TWT (a) severed section conducting-wall section Severed gyro-TWT lossy section Distributed-loss gyro-TWT (b) (c) Improved distributed-loss gyro-TWT conducting-wall section conducting-wall section severed section lossy section Saturated bandwidth 3 dB(%) To publish, year Cyclotron harmonic no/mode (kV) Center frequency (GHz) Peak power (kW) Saturated gain (dB) Saturated efficiency (%) Leou, 1994 Chu, 1999 100 35 230 93 46 70 23 26.5 6 8.6

7. Severed Gyro-TWT Absolute instability (a) (b)

8. Severed Gyro-TWT Gyro-BWOReflective oscillation (a) (b) (a)

9. Distributed - Loss Gyro-TWT Absolute instability (a) (b)

10. Distributed - Loss Gyro-TWT Gyro-BWOReflective oscillation (a) (b)

11. Improved Distributed - Loss Gyro-TWT Absolute instability (a) (b)

12. Improved Distributed - Loss Gyro-TWT Absolute instability (a)(c) (b)(d)

13. Improved Distributed - Loss Gyro-TWT (a) (b) (c) (d) Gyro-BWO

14. Improved Distributed - Loss Gyro-TWT Reflective oscillation (d)(b) (c)(a)

15. Operating current : 10 A Center frequency : 35 GHz Peak power : 155 kW at 32.9GHz Saturated gain : 45 dB Saturated efficiency : 15 % Bandwidth : 2.2 GHz Velocity spread : Amplifier Performance of the Improved Distributed-Loss Gyro-TWT (a) (b)

16. Conclusions  A self- consistent simulation code is used to evaluate the amplifier’s nonlinear behavior and analysis of typical oscillations, including absolute instability, gyro-BWO and reflective oscillation, are presented.  In severed gyro-TWT was not completely suppressed self-excited oscillation. increasing the wall losses to suppress the gyro-BWO in the distributed-loss gyro-TWT degrades the out power and gain.  The improved gyro-TWT is predicted to yield a peak output power of 155 kW at 32.9GHz with an efficiency of 15 %, a saturated gain of 45 dB and a bandwidth of 2.2 GHz for a TE 01 mode 100 kV, 10 A electron beam with an axial velocity spread.  The oscillations have limited our maximum stable operating beam current 10 A. reducing the length of the conducting-wall section might be useful to increasing the start-oscillation current.

17. References Seftor, J. L., Granatstein, V. L., Chu, K. R., Sprangle, P., and Real, M. E., “The electron cyclotron maser as a high power traveling-wave amplifier of millimeter waves,” IEEE J. Quantum Electron, vol. 15, pp. 848-853 (1979). Chu, K. R., Barnett, L. R., Chen, H. Y., Wang, Ch., Yeh, Y. S., Tsai, Y. C., Yang, T. T., and Dawn, T. Y., “Stabilizing of absolute instabilities in gyrotron traveling-wave amplifier,” Phys. Rev. Lett., vol. 74, pp. 1103-1106 (1995). Leou, K. C., McDermott, D. B., Balkcum, A. J., and Luhmann, N. C., “Stable high power TE 01 gyro-TWT amplifiers,” IEEE Trans. Plasma Sci., vol. 22, pp. 585-592 (1994). Chu, K. R., Chen, H. Y., Hung, C. L., Chang, T. H., Barnett, L. R., Chen, S. H., Yang, T. T., and Dialetis, D. J., “Theory and experiment of ultrahigh-gain gyrotron traveling wave amplifier,” IEEE Trans. Microwave Theory Tech., vol. 27, no. 2, pp. 391-404 (1999). Yeh, Y. S., Chang, T. H., and Fan, C. T., “Beam characteristics of mechanically tunable magnetron injection guns,” Int. J. Inferared and Millimeter Waves, vol. 22, no. 7, pp. 983-997 (2001). Yeh, Y. S., Lo, Y. T., Wu, T. S., Su, C. W., and Wu, S. C., “Stability Analysis of TE 01 Gyrotron Traveling Wave Amplifier,” submitted to Int. J. Electron. Yeh, Y. S., Lo, Y. T., Wu, T. S., and Su, C. W., “Nonlinear Analysis of Absolute Instability in Gyrotron Traveling Wave Amplifier,” to be published the Fourth IEEE International Vacuum Electronics Conference.

18. Basic Mechanism of Gyro-TWA ※ Δω=ω-Ω c ≧ 0 where Ω c = eB 0 /γm, γ=(1-v 2 /c 2 ) -1/2 ※ Electron #1 initially gaining energy →γ increases →Δω increases →out of synchronism →weaker interaction ※ Electron #2 initially losing energy →γ decreases →Δω decreases →approaching synchronism →strong interaction output waveinput waveelectron beam

19. Computer Models of Nonlinear Simulation Code (a) (b) (c) (d)

20. Self – Excited Oscillations  Gyrotron backward oscillation - (point 3 and 4)  Absolute instability – (point 1 and 2)  Rreflective oscillation wave

21. Improved Distributed - Loss Gyro-TWT (a)(b) waveguide