1. Pseudospark - sourced electron beam for the generation of X-rays & THz radiation A.W. Cross 1, H. Yin 1, D. Bowes 1, W He 1, K. Ronald 1, A.D.R. Phelps 1, D. Li 2 and X. Chen 2 1 Department of Physics, SUPA, University of Strathclyde, Glasgow, G4 0NG, UK 2 EECS, Queen Mary University of London, UK UNIVERSITY OF STRATHCLYDE ABP Atoms, Beams & Plasmas Compact Accelerator Workshop,18 th April 2012, Cockcroft Institute, Warrington, UK

2. Contents Description of pseudospark discharge (PSD) Pseudospark e-beam generation - X-rays - Coherent millimetre wave radiation Numerical simulation of PSD Experiments - X-ray imaging - Backward wave oscillator - Millimetre wave klystron amplifier Conclusion & future work

3. Spark, operating pressure (p) is large (~500 torr) - mean free path is very small and so there is very frequent electron – neutral collisions - ionisation growth and large current density is due to electron multiplication by electron-neutral collisions - gas breakdown is fast (ns) & discharge current is large - phenomena observed at large pressure on the RHS of the Paschen curve Pseudospark operating pressure is (typ 50 - 500 mtorr) - mean free path is larger than d eff - hole in centre results in spark like phenomena at lower (p) Pseudospark is a low (p) gas discharge that operates in a spark mode with hollow electrodes J. Christiansen and C. Schultheiss, Z.. Phys., vol. A290, p. 35, 1979. Generalised discharge characteristics d eff

4. Introduction 1. What is a pseudospark?  low pressure, 50 mtorr – 500 mtorr (for a gap separation of several mm) self-sustained, transient hollow cathode discharge  occurs in special confining geometry  in various gases such as nitrogen, argon, hydrogen, xenon

5. SparkPseudospark Pressure range~ 500 torr< 500 mtorr Structureparallel plateparallel plate with axial hole (mm) e-fielduniformnon-uniform and focused Mean-free-pathvery smallgreater than d Ionizatione–neutral collisionsfield-enhanced thermionic emission, vacuum arcs Breakdown lawf(pd)f(p 2 d) Discharge occurRHS of Paschen curve LHS of Paschen curve Gas breakdownfast (ns) and large current rise (ns)

6. 3 stages during a pseudospark discharge: a) Townsend discharge b) Hollow cathode discharge c) Superdense glow discharge (conductive phase) M. Stetter, P. Felsner, J. Christiansen, K. Frank, A. Gortler, G. Hunts et al, IEEE Trans Plasma Sci., vol. 23, no. 3, Special Issue on Pseudospark Physics and Applications, pp283-293, 2004

7. 3. PS discharge can be scaled down in size (mm to  m)  Intense electron beam – point-like X-ray source – generation of coherent high power mm-wave radiation 1. Characteristics of a pseudospark  Pseudospark is a low pressure gas discharge that operates in a spark mode with hollow electrodes  High quality electron beam extraction before and during the conductive phase  High current rise rate (dI/dt ~ 10 11 Am -2 ) 2. Applications  Pulsed-power switching, can operate at high PRFs  Material processing

8. A.W. Cross, H. Yin, W. He, K. Ronald, A.D.R. Phelps, L.C. Pitchford, Journal of Applied Physics, pp.1953-1956, 2007.

9. PSD Numerical Simulation MAGIC: Particle-In-Cell and Monte-Carlo Collision (PIC-MCC) C.K. Birdsall et al, Computer Phy. Comm 87, 1995.

10. PSD-2D Computational Model MAGIC Model: Constant A-K voltage 10kV, gap d=6mm Radius of hollow cathode = 25mm Room temperature Insulator: 6mm thick Perspex Anode aperture: 0.5mm radius Anode thickness: 12mm Cathode aperture: 1.5mm radius Argon 100mTorr

11. Plasma formation at 30ns Plasma expansion at 50ns Plasma expansion and emission at 80ns

12. Simulation results Observed voltage between the anode and the cathode Observed current at the anode aperture

13. PS beam experimental results Cathode, anode aperture diameter = 1 mm Separation = 6 mm V = 10 kV P = 100 mTorr I = 4 A Plasma density 5 x10 12 cm -3

14. X-ray generation from a 4-gap PS discharge Schematic of experimental setup Experimental setup

15. Pseudospark e-beam and X-ray image Object for X-ray image (100 micron diameter wire) X-ray image of the object Molybdenum target for X-ray generation showing the beam spot

16. 4-gap pseudospark e-beam experiments

17. The cross-section image of the beam 3mm diameter anode aperture 500  m diameter anode aperture 500  m Quality of pseudospark-sourced electron beam pulses High quality electron beam extraction before & during the conductive phase - electron beam quality is decided by emittance, PS normalised rms emittance of 18 mm mrad - brightness 1 x10 11 A m -2 rad -2

18. Pseudospark e-beam post-acceleration experiment

19. Trigger system for the pseudospark powered by a cable pulser

20. A typical record of the time-correlated pseudospark discharge voltage, beam current and the acceleration voltage pulse

21. Experimental setup of the PS powered by a cable pulser and beam-wave interaction investigation

22. BWO Interaction W-band (75 to 110)GHz Ka-band (26.5 to 40)GHz Advantages: a) e-beam source for THz radiation; b) simplicity (no B-field); c) compactness (table-top size); d) high power, high PRF operation W-band Aluminium positive former - Under construction at the Univ of Strathclyde - Copper is deposited - Aluminium dissolved in alkali solution G-band (140 to 220)GHz H. Yin, A.W. Cross, W. He, A.D.R. Phelps, K. Ronald, D. Bowes and C.W. Robertson Physics of Plasmas, 16, 063105, 2009.

23. 96 GHz Klystron Pulse duration 50 ns V beam 8 kV I beam 15mA Freq96.8GHz P IN 200mW P OUT 8.86 W Gain45 Efficiency7.4 % 500  m 100  m

24. Design of the 96 GHz PS driven klystron

25. Construction of 96GHz Klystron 500  m

26. Conclusion: Electron beam generation and diagnostics from a 3-gap PS discharge powered by a DC power supply. A beam was measured up to 300 A at 50kV and propagated as far as 20 cm away from the anode with no external guiding magnetic field – Point-like X-ray source for imaging Beam-wave interactions were simulated with BWO structures in the W-band (75 to 110GHz) frequency range and with dielectric slow-wave structure in Ka-band (26.5 to 40GHz) – mm wave radiation was successfully generated in the Ka and W band High current conductive phase pseudospark beam from a 3-gap DC powered pseudospark was post accelerated Small-size beam (100  m) has been measured from both a 4-gap and single- gap DC powered pseudospark, to be used as an electron beam source for – 200GHz BWO – 96GHz Klystron

27. Future Work: Re-entrant cavities for the klystron operating with a higher order mode will enable the diameter of electron beam drift section to be increased Klystron multiplier operation at a higher harmonic will enable a lower frequency driver to be used to power the amplifier – both these concepts will be of great benefit to industry – result in scaling of the klystron to THz frequencies Any questions? Thank you for listening!

28. The authors would like to thank EPSRC for supporting this work

29. R r dr L d W Normalised beam brightness

31. Beam propagation with a plasma filling the channel Plasma density 6 x10 12 cm -3 CP beam 200A, 200V Beam propagation, plasma filling the region up to the end of the cathode aperture Plasma density 1x10 12 cm -3 HC beam 50A 22kV yellow - plasma red electrons

34. Pseudospark e-beam further investigations Further study of pseudospark physics and its plasma process will enable Potential future applications: 1) high power coherent sources of millimetre and sub-millimetre wave radiation 2) high brightness electron sources for post acceleration in the next generation of accelerators.

35. Electron beam current pulse vs the applied voltage pulse from a cable pulser Current / A Voltage / kV

36. Dispersion diagram of BWO interaction

37. Particle-in-Cell code simulations of beam wave interaction

38. Particle-in-Cell code simulation of the beam wave interaction

39. Time-correlated electron beam pulse (green), microwave pulse (red) and applied voltage pulse (blue)

40. 1mm aperture single gap pseudospark beam measurements

42. Four cavity klystron

47. Comparison of 2, 3 and 4 cavity klystron simulation Q Cav.1 Q Cav.2 Q Cav.3 Q Cav.4 Gain dB %% 2 cavity P in/rf = 100mW 726 1600 gap 12 = 10mm ---- 7.3dB P o = 0.55W 2.2 3 cavity P in/rf = 100mW 726 1600 gap 12 = 6mm 1600 gap 23 = 3.3mm ---- 10.5 P o = 1.14W 4.6 4 cavity P in/rf = 25mW 726 1600 gap 12 = 3.15mm 1600 gap 23 = 3.15mm 1600 gap 34 = 3.15mm 23dB P o = 5W 20

48. Four cavity klystron energy recovery system Klystron interaction efficiency: 20% Recovery efficiency: 62% Total efficiency: ~40%

49. Work in Progress: More simulations Puffed gas feeding experiment Construction 200GHz microklystron driven by a pseudospark electron beam using – Micro-electromechanical (MEMS) systems

50. Acknowledgements The authors would like to thank the EPSRC for financial support of this work and the UK Faraday Partnership in high power microwaves for providing PIC MAGIC simulation package

52. conductive phase beam hollow cathode phase beam

54. Beam current versus propagation distance Electron beam can propagate as far as 20 cm with no guiding B-field H.Yin, IEEE Trans.Plasma Sci.,32, Special Issue on Pseudospark Physics and Applications, 2004

55. Experimental results The propagation of the electron beam from a three-gap pseudospark discharge chamber was studied as a function of the length of a collimator of 3mm internal diameter. The beam was measured at 150 mm away from the anode of the pseudospark chamber. The results are shown in the following table. Beam measured 150 mm away from the PS anode Collimator length / mm Beam current / A Percentage of beam transported No collimator 240  35 - 30 168  20 70 % 60 118  10 49 % 90 36  5 15 %

56. Pseudospark-based Cherenkov maser experimental configuration Pseudospark e-beam in Cherenkov interaction Operating voltage can be increased by using multi-gaps

57. A typical beam pulse from a 8-gap pseudospark discharge Voltage / kV Current / Amps Time / ns

58. Impedance / 

59. Frequency [GHz] Axial wavenumber [1/m]

61. Typical beam current, voltage and microwave traces from the 8-gap pseudospark-based Cherenkov maser experiments

62. Dispersion diagram of TM 01 mode calculated (dotted line) and simulated by MAGIC code (squares) and slow space charge mode of the electron beams (75 kV, 25 A-top thin line)

63. Measured e-beam voltage, current and mm-wave pulse

66. The Cherenkov maser results :  Beam parameters: 10 A, 70– 80 kV  Mode: TM 01  Frequency: 25.5– 28.6 GHz  Output Power: 2 0.2 kW  Gain: 29 ± 3 dB ±

67. Typical beam current, voltage and microwave traces from the 8-gap pseudospark-based Cherenkov maser experiments