2D Periodic Surface Lattice Cherenkov maser experiment

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1 2D Periodic Surface Lattice Cherenkov maser experiment Alan R. Phipps Department of Physics, SUPA, University of Strathclyde, Glasgow, G4 0NG ABP Atoms, Beams & Plasmas

2 Table of Contents Introduction Theory Dispersion CST Microwave Studio Simulations W-band (75GHz 110GHz) 2D PSL Design and Construction 2D PSL Vector Network Analyser mm-wave measurements 3D MAGIC Beam/Wave interaction simulations W-Band 2D PSL Cherenkov Maser Experiments Conclusion Future Work 2

3 Introduction Aim of project: to find a route to higher power, high efficiency sources that can scale and can be used at frequencies from RF to THz. One of the strategies we are using is to increase the transverse dimensions of the interaction region so that the diameter to wavelength ratio is larger than in traditional devices, while avoiding the decrease in efficiency that usually comes with multi-mode operation. This approach is particularly attractive for the shorter wavelengths in the mm-wave and THz ranges. To avoid multi-mode operation we are using a two dimensional periodic surface lattice (PSL) that sustains a surface mode which couples to a volume mode resulting in eigenmode formation that can be efficiently driven by an electron beam. 3

Theory The Two-Dimensional Periodic Surface Lattice (2D PSL) provides a mechanism for inducing coupling of an incident near cut-off (TM 0,n ) Volume Field (VF) and the HE m,1 Surface Field (SF) that

4 Theory The Two-Dimensional Periodic Surface Lattice (2D PSL) provides a mechanism for inducing coupling of an incident near cut-off (TM 0,n ) Volume Field (VF) and the HE m,1 Surface Field (SF) that is formed around the perturbations when the Bragg conditions are satisfied. VF k ± = k i k s m = ± m 1 m 2 SF The perturbations are defined analytically as: r = r 0 + rcos k z z cos m azi ϕ Where r 0 is the unperturbed waveguide radius, r is the perturbation amplitude and k z and m azi are the longitudinal wavevector and the azimuthal variation respectively. Coupled mode theory describes the VF/SF interaction: da V dz + iκa S = 0 da S dz iκa V = 0 A V, A S = Amplitudes of the volume and scattered waves. The amplitude of the incident volume wave is dependent on the amplitude of the scattered wave through a coupling coefficient κ. I. V. Konoplev, A. R. Phipps, et al, Appl. Phys. Lett. 102, (2013) 4

5 Dispersion of Coupled Fields Inside Cylindrical Periodic Surface Lattice e e e e α is the normalised coupling coefficient Λ is the normalised wave vector ω e is a variable angular frequency The detuning parameter Γ is a function of the geometry of the structure. c d z where c is the cut-off wavelength of the volume field inside the cylindrical waveguide and d is the longitudinal lattice period. z I.V. Konoplev, A.J. MacLachlan, C.W. Robertson, et al., Cylindrical, periodic surface lattice - Theory, dispersion analysis, and experiment, Appl. Phys. Lett., 101, ,

6 Dispersion Analysis of Cylindrical Periodic Surface Lattice: Γ= Uncoupled surface field = Uncoupled volume field = Coupled dispersion = The position of maxima and minima points f k of the dispersion indicate z 0 the positions of the cavity eigenmodes. The f k z sign variation illustrates that slow forward or backward waves may be observed. I.V. Konoplev, A.J. MacLachlan, C.W. Robertson, et al., Cylindrical, periodic surface lattice as a metadielectric: Concept of a surface-field Cherenkov source of coherent radiation, Phys. Rev. A., 84, ,

7 CST Microwave Studio Simulations PSL Parameters (Cherenkov Maser structure): o Frequency = ~100GHz: o Azimuthal variations = 7 o Inner radius = 4 mm o Perturbation amplitude dr = 0.8 mm o Prominent modes = TM 0,1,TM 0,2 and TM 0,3 7

8 Construction of W-band 2D PSL 2D PSL o 3D printed wax former (high resolution) o Molten silver (92.5%) chromium (7.5%) alloy deposited into mold o +/- 125 micron resolution Parameters Unperturbed Radius 4 mm Azimuthal Variations 7 Azimuthal Period Longitudinal Period ~ 3.6 mm 1.6 mm Number of Periods 16 Perturbation Amplitude 0.8 mm 8

9 Vector Network Analyser Measurements of High Frequency 2D PSL 9

10 o Electron beam-em wave interaction k z vz + 2π d z v z o k z is the wave s longitudinal wave number o 100kV electron beam, g ~ 1.2 o v z is the electron beam longitudinal velocity, 0.55c o d z = 1.6mm Electrons interact with localized surface field f = c d z 1 γ 2 = 103.6GHz o The number of lattice azimuthal variations should be larger than the number of wavelengths along the unperturbed circumference of the waveguide 2πr 0 g > m I.V. Konoplev, A.J. MacLachlan, C.W. Robertson, et al., Cylindrical, periodic surface lattice as a metadielectric: Concept 10 of a surface-field Cherenkov source of coherent radiation, Phys. Rev. A., 84, ,

11 CST Beam Wave Interaction Dispersion Beam Wave Interaction Dispersion Diagram 11

12 MAGIC 3D Simulations 12 o Square perturbation to optimize simulation time o 16 Longitudinal periods, (1.6mm) o 7 azimuthal periods, (3.2mm) o 1.5 T B z 12

13 MAGIC 3D Simulation Results 2 nd Harmonic: 3 kw at ~210 GHz o Electron Beam Accelerating voltage 100kV Beam current, 100A o Millimetre waves Output power, ~300 kw Frequency GHz Efficiency, ~3 % 13

14 Experimental Design 14

15 Electron beam source Designed using CST Particle Studio 15

$Cherenkov source: diode\ electron gun\beam-wave interaction region Interaction Region Output Horn Diode insulator, perspex Electron beam source,$

16 Cherenkov source: diode\ electron gun\beam-wave interaction region Interaction Region Output Horn Diode insulator, perspex Electron beam source, velvet Cherenkov beam/wave interaction region, silver 2D PSL Output horn and mylar window Vacuum Pressure 5.0 x10-6 mbar (16 th December 2014) 16

17 Solenoid & B Field Profile DC conventional coil, designed, constructed and tested B-field up to 2 T B z Profile 17

18 Cherenkov maser experimental setup Diode insulator, perspex Electron beam source velvet Cherenkov beam/wave interaction region silver 2D PSL Output horn and mylar window 5 x 10-6 Vacuum Pressure 18

19 Beam Current Measurements HV power supply, cable Blumlein generator Cold field emission from velvet cathode Beam current ~ 100 A Beam voltage ~ 100 kv Pulse duration 100 ns 19

20 Beam Profile Measurements Beam profile measured using electron sensitive film placed on the end of the Faraday cup Beam profile displayed as red ring on electron sensitive film 20

21 Millimetre Wave Measurements Measured using W-band (75GHz-110GHz) rotary vane attenuator and W-band rectifying crystal detector Peak power measured at 60 cm from output horn and window Multiple shots have been measured demonstrating pulse to pulse reproducibility 21

22 Conclusion Successful cavity formation within W-band 2D PSL structure observed Agreement between cavity measurements and numerical analysis Operating frequency and bandwidth dependent on perturbation amplitude and period of 2D PSL Determined range of design parameters for experimental W-band Cherenkov maser Beam/wave interaction demonstrated using MAGIC 3D GHz, 300 kw, 3% efficient Cherenkov maser oscillator Construction of W-band Cherenkov Maser incorporating a 2D Periodic Surface Lattice completed Electron gun Solenoid Vacuum envelope (5 x 10-6 mbar) High voltage power supply tested Experimental measurements as of the 31/03/ kv, 100 A, 4 mm diameter electron beam mm-wave pulses measured 22

23 Future Work Measure the frequency using an in-band mixer (Millitech MXP-10-R) and a local oscillator signal produced by a 95GHz Gunn diode (Millitech GDM IR) with the resultant IF measured using a 20GHz deep memory digitising oscilloscope (Agilent DSX-X 92004A) Calibrate the power as a function of voltage from the Flann W-band millimetre wave detector taking into account the attenuation used in the rotary van attenuator Integrate the power measured at the detector over the mode pattern to ascertain the output power generated by the 2D PSL Cherenkov maser. Identify the cavity eigenmode is due to the coupling of the volume mode and the surface wave from accurate measurement of the frequency using the W-band mixer combined with measurements of the output mode pattern. 23

24 Acknowledgments The authors would like to thank the Engineering and Physical Sciences Research Council (EPSRC) for providing the PhD studentship for Alan Phipps and EOARD for supporting this work. 24

25 Acknowledgments University of Strathclyde Satellite MURI Research Team Collaborating in the Transformational Electromagnetics MURI Alan Phelps Amy MacLachlan Alan Phipps Adrian Cross 25

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