DESIGN AND TECHNOLOGICAL ASPECTS OF KLYSTRON DEVELOPMENT

Transcription

1 DESIGN AND TECHNOLOGICAL ASPECTS OF KLYSTRON DEVELOPMENT Dr. L M Joshi Emeritus Scientist CSIR-CEERI, PILANI lmj1953@gmail.com 22 February 2017 IPR 1

2 Schemetic Diagram 22 February 2017 IPR 2

3 Basic Principle 22 February 2017 IPR 3

4 Schematic Diagram of a Multi-cavity Klystron RF Window Exhaust Port Interaction Structure Electron Gun Collector Anode Cathode IT-10

5 Design Considerations Bottom-up Design Approach Basic analytical design calculations Small signal modeling Large signal modeling: Mainly to design output RF section Realization of cavity or circuit properties through cavity simulation programs Design of electron gun and focusing section for desired beam optics. Thermo-mechanical design Lay out design of assemblies and piece parts

6 Challenges Improved performance and reliability Reduced lifetime cost of ownership Reduced environmental hazards Alternative materials Low voltage operation Spurious emissions Improved systems integration

7 APPROACH Improved computer modelling Improved understanding of device operation Improved understanding of breakdown phenomena Better characterisation of materials Dielectric properties Surface physics (secondary electron emission, cold cathodes) Novel tube types

8 Klystron problem areas Reliability (including rate of RF trips) Voltage breakdown in gun and output cavity Window failure Waveguide arcs Efficiency Electronic efficiency Solenoid power consumption Cost Industrial capacity

9 Design issues: High peak power High voltage and current High efficiency High voltage and low current Low solenoid power High reliability Low voltage to avoid gun and output cavity breakdown Low cathode loading for long cathode life Low peak power to avoid output window failure and wave guide arcs

10 High Voltage Breakdown Along the insulators Between electrodes of the gun Cavity gaps with high RF fields Window surface

11 Klystron Efficiency Perveance, P = I / V3/2 η (%)= X P (µperv.) For a given perveance improves with Shorter length of interaction region Use of second Harmonic cavity

12 BASIC RELATIONS FOR ANALYTICAL CALCULATIONS DC Electrons Velocity Angular Frequency Electron Wave Number ω= 2πf Drift Tube Radius Beam Radius Normalized Beam Radius

13 Charge Density Plasma Frequency ) ε0= farad/m, (e/m)= C/kg Reduced Plasma Frequency Reduced Plasma Wave Length Length of Drift Tube

14 Typical Tube Development Cycle Tube Specifications Electrical design Engineering design, Thermal/structrucal Design, Machining of Parts, Cold testing, Chemical Processing, Design of jigs, Brazing, Welding Leak testing Vacuum Processing Performance Evaluation Development of Key Technologies 22 February 2017 IPR 14

15 Synthesis of Gun Parameters S. No. Design parameters Synthesized data 1. Beam voltage (Volts) 130, Beam current (Amps) Beam perveance (µp) Emission current 5 A/cm 2 density (J c ) 5. Beam waist radius (r a ) 8.0 mm 6. Cathode disk radius mm (r c ) 7. BFE angle 67.5 Parameters Value 8. Beam voltage (V 0 ) 130 KV Half cone angle (θ) Beam current (I 0 ) 94 A 9. Cathode spherical radius (R c ) 50.0 mm Beam perveance 2 µp 10. Cathode anode distance (Z ca ) mm Beam waist radius 8.0 mm 11. Beam radius at anode plane r b mm (Z a ) 12. Anode aperture radius (r a ) mm 22 February 2017 IPR 13. Axial location of beam waist 70.8 mm w.r.t cathode (Z w ) 15

16 Optimization using Analytical codes Sl. No. Design parameters Synthesized data Simulated data TRAK MAGIC 1. Beam voltage (V 0 ) 130, , , Beam current (Amps) Beam perveance (µp) Emission current 5 A/cm 2 5A/cm 2 5A/cm 2 density (J c ) 5. Beam waist radius (r a ) 8.0 mm 8.1 mm 8.0 mm 6. Cathode disk radius mm (r c ) 7. BFE angle Half cone angle (θ) Cathode spherical 50.0 mm 50.0 mm 50.0mm radius (R c ) 10. Cathode anode mm 30.0mm 30.0mm distance (Z ca ) 11. Beam radius at anode plane r b (Z a ) mm 12.89m m 12.76m m 12. Anode aperture radius (r a ) mm m m m m 13. Axial location of 70.8 mm 74.8mm 75.0mm 22 February 2017 beam waist w.r.t IPR cathode (Z w ) 16

17 Focussing structure design Linear collider installations use thousands of klystrons The power consumed by solenoids is of the same order as is needed for running rest of the system Major design emphasis is on Super conducting magnets Permanent Magnets PPM

18 Magnetic Focusing Field 22 February 2017 IPR 18

19 Electron Gun Simulation Beam Radius = 8.0mm Perveance = 2 micro Beam Current = 94A Beam Voltage =130KV 22 February 2017 IPR 19

20 Klystron with Electromagnet Magnetic field (Gauss) Magnetic field measurement with pole pieces magnetic field at main current=100a,aux.current=150a magnetic field at main current=120a,aux.current=150a Axial distance(cm) 22 February 2017 IPR 20

21 Cavity Design 22 February 2017 IPR 21

22 22 February 2017 IPR 22

23 When electrons are passed through the modulating field, some electrons have their velocities increased and some will have their velocities decreases when the voltage is reversed. As the electrons leave the gap, those with increased velocities overtake the slower electrons, as a result electron bunching

24 Reentrant Cavity

25 Cavity Parameters

26 Power Coupling

27 Cavity Tuning

28 Cavity Simulation 22 February 2017 IPR 28

29 Computation of Input cavity Parameters 22 February 2017 IPR 29

30 Simulated Cavity Parameters S.No. Resonant frequency f o (MHz) Q o (unloaded quality factor) Q e (external quality factor) Q L (loaded quality factor) R/Q , , , February 2017 IPR 30

31 RF Window It is most susceptible organ of the tube More than 70% klystron failures are attributed to window failure Reasons for Window Failure Thermal stresses caused by heat generation due to dielectric losses. Tensile stresses at the peripheral surface cause window fracturing

32 Increase in window ceramic thickness can reduce this Problem

33 Fractures due to Thermal Stresses

34 RF Window(cont.) Multipactoring causes high localized heating. Suppression of secondary emission form window surface, for example, coating of TiN film, reduces this problem substantially RF electrical breakdown at the surface of window is also responsible for window failure Threshold is 8 kv/mm for highly purified ceramic Remedies: Larger diameter of window ceramic Mixed mode window design

35 Window Simulation 22 February 2017 IPR 35

36 Simulation of beam-wave interaction using AJDISK code

37 Electron Bunching 22 February 2017 IPR 37

38 Particle Energy along Axial Direction 22 February 2017 IPR 38

39 Output Power Computation Parameters Desired Simulated values Values Efficiency (%) 45(nominal) 41 Gain (db) 45(nominal) 49 Output Power (MW) February 2017 IPR 39

40 Collector Design 22 February 2017 IPR 40

41 Thermal Design of Collector Simulation for a smooth collector with full geometry Heat dissipation = 52KW Water inlet = 15 deg. 22 February 2017 IPR 41

42 Engineering Design Design drawing Selection of Material of individual parts Electrical Selection of Magnetic brazing materials Thermal and chemical Vacuum compatibility processes 22 February 2017 IPR Jig fabrication 42

43 Development of Sub-assemblies Chemical Processing Parts Assembly & alignment verification Brazing/ Welding Leak testing Cold testing 22 February 2017 IPR 43

44 Processes Involved All components must be hermetically sealed. Joining techniques must be suitable for high temperature processing All materials must be ultra-pure and clean. Brazing, up to 1100C in H2, N2 Vacuum firing up to 1100C, 10-7 torr TIG welding Laser beam welding Acid and alkali cleaning Clean room assembly Klystron evacuation to 10-9 torr

45 22 February 2017 IPR 45

46 Thermal characterization of the cathode 22 February 2017 IPR 46

47 Sub-assembly Fabrication 22 February 2017 IPR 47

48 Cold testing of RF Section 22 February 2017 IPR 48

49 Integration, Vacuum Processing & Testing Integration of Sub-assemblies through Brazing/welding Leak testing of full tube Vacuum Processing Cathode breakdown Pinch-off Hot Testing 22 February 2017 IPR 49

50 Vertical Retort Furnace Brazing 22 February 2017 IPR 50

51 22 February 2017 IPR 51

52 TIG Welding 22 February 2017 IPR 52

53 Problems still to be solved Bringing down the manufacturing cost Necessity of high voltages for high power operations. Higher beam voltage causes not only higher costs for the devices themselves, but also for many things in their periphery like power supplies and modulators, high voltage insulation, X-ray shielding, size of building and environmental protection measures.

54 Multi Beam Klystron The Multibeam Klystron uses several electron beams and each beam propagates along its own individual transit time path through a resonator unit. The current and perveance of the individual beam are not high but the total current and perveance of the entire multibeam stream can be high. The operating voltage is significantly reduced (2 to 10 times) with a consequent reduction in the dimensions and weight of the devices and their power supplies. At the same time, the individual low perveance beams are better focused and bunched and give up their energies to the field of the resonator in an efficient manner resulting in an excellent performance

55 Design Issues The major challenge is to focus the off-axis electron beams. This requires substantial threading of the focusing magnetic field to each cathode with flux tubes shaped in a manner so as to have symmetrical convergence about each beam centerline. The transverse field should be negligible. One of the critical issues for gun design using off-axis electron beams is actual dimensions of the gun at operating temperature. The transverse field affects the beam transmission and some times is responsible for parasitic oscillations also. Magnetic screens are employed near the beam trajectories to suppress the transverse field and to restore the local cylindrical symmetry of the field. Values of B1/Bn< have been practically achieved.

56 5: Multi beam Klystrons Electron trajectories through square aperture Electron trajectories through circular aperture Electron trajectories from OPERA 3-D simulations of a four beam electron gun with different apertures

57 Multi Beam Gun Simulation using MAGIC 22 February 2017 IPR 57

58 Different Parts of MBG 22 February 2017 IPR 58

59 Parts of Multibeam RF Cavity 22 February 2017 IPR 59

60 6: Design Studies on Sheet Beam Klystrons MODE NO: CST MWS simulated results Experime ntally measured results (GHz) (GHz) February 2017 IPR 60

61 Klystron continues to be main work horse for RF accelerators. It is most reliable device for applications involving high peak and average microwave power. Its capabilities are being continuously improved by suitably addressing most of the design and technological issues responsible for limiting power output, bandwidth efficiency gain and life. Efficiency figures have reached beyond 65%, Beam transmission close to 99% and MTBF, ranging from 25,000 to 35,000 hours.

62 REFERENCES: [1] Synthesis of the Pierce gun, Vaughan, IEEE Trans. Electronics Devices, Vol 2.8 no. 1, PP , [2] High efficiency klystron amplifier Erling, L. Lien, Varian Associates, Palo Alto, California. [3] Design consideration for high power multicavity klystron Paul J Tallerico, IEEE Trans. Electron Devices, vol. ED-18, no. 6, June [4 ] Computer aided study of some re-entrant cavity structures for klystrons M.V. Karthikeyan, L.M. Joshi, A.K. Sinha & H.N. Bandopadhayay, Journal of IETE, vol. 39, no. 6, Nov-Dec-1993, PP [5] Numerical studies of the shapes of drift tubes and linac cavities Hary C Hoyt, IEEE Tans., Vol. NS-12, PP , [6] RF section design of a high power klystron M Tech. thesis by Indrajit Banerjee. [7] Study on RF section design for a high power klystron M.Tech. thesis by Shiv Narain. 22 February 2017 IPR 62