Research Activities

Experimental Activity in the Beam-Wave Instability Group

For more information about this research area please contact Prof. Alan Phelps, Dr. Adrian Cross or Dr. Kevin Ronald

1. Beam-Wave interactions

It is well known that a truly 'free' electron in vacuum cannot radiate, because the laws of conservation of energy and momentum cannot be satisfied by the initial and final conditions of the electron and photon given the constraints of their respective dispersion relations. It is necessary to constrain the electron in some way to enable it to radiate, thereby modifying its dispersion characteristic, OR equally to modify the wave dispersion in some way. The radiation process takes three main forms, Cherenkov (where in place of a vacuum, one places some electric medium), Bremsstrahlung (where the presence of some static field constrains the electron) and scattering (where an incident photon is involved).(1)

The research conducted in the group concentrates on the first two processes, Cherenkov and Bremsstrahlung, between electrons in beams with energies in excess of 50keV and guided radiation with frequencies from 1 - 300GHz. That is to say that the total kinetic energy of the electron vastly exceeds the individual photon energy. Therefore the work is in the Raleigh-Jeans limit of the Planck radiation law, and the process may be described by the relativistic laws of motion and Maxwells equations with no reference to quantum theory required. On the other hand, the electron beam densities are usually large and therefore the interaction dynamics are complicated by the collective effects of large numbers of electrons. These interactions can be exploited to produce coherent electromagnetic radiation with high efficiency, (up to 50% in some cases) which makes the subject a matter of great interest for applications. Sources and amplifiers using these approaches have applications in non-linear material testing, broadcast telecommunications, radar, heating (both of ceramics and of fusion plasmas), plasma diagnostics and as the drivers for RF particle accelerators.

We have experimentally investigated Cherenkov interactions using both dielectric(2) and corrugation loading for the waveguides, Bremsstrahlung interactions have been investigated using both uniform magnetic fields (CRM instability, gyrotrons (3)) and periodic magnetic fields (Free Electron Lasers or FEL's, also called ubitrons). This has resulted in the production of Megawatt level radiation at frequencies from 1 to 100GHz. In FELs the interaction takes place between a beam oscillating in a fixed " wiggler" field, usually some variation of a magnetic undulator, and an electromagnetic wave.

In 1996 the RELD group achieved saturated FEL oscillator operation(4) and on the strength of this work we have received funding to experimentally study the physics of high gain FEL amplifiers. We have also investigated the Cyclotron AutoResonance Maser (CARM) instability (5) where the interaction occurs between an electron beam gyrating in a uniform magnetic field and an electromagnetic wave at a phase velocity of close to the speed of light, which leads to special enhancement of efficiency due to the mutual effects of the electric and magnetic components of the radiation field which must be considered in such relativistic conditions. Our oscillator experiments were the first (and thus far only) report of observation of the CARM instability at the second harmonic. Very recently we were the first to report on a new type of broadband Gyrotron amplifier, with a high efficiency and output power (28% and 1.1MW with a -3dB gain bandwidth from 8.4 to 10.4GHz) with a saturated gain of up to 37dB(6,7). Such performance was achieved by using a helical corrugation on the wall of a 'cylindrical' waveguide to synthesise an eigenmode having a finite and nearly constant group velocity in the region of near infinite phase velocity. This experiment offers exciting implications for a broad spectrum of applications as the potential is clear for this new concept to substantially improve on the current commercially available microwave amplifiers. We have also investigated the phenomenon of 'superradiance' in Cherenkov and Bremsstrahlung(8,9) interactions between a short electron pulse and an electromagnetic wave, resulting in the production of microwave radiation exhibiting an N² scaling of its peak amplitude (where N is the number of electrons participating in the interaction) with peak output powers ~60MW in 300ps duration pulses and were the first to report on Cherenkov and Cyclotron superradiance. We have an active research programme pursuing all these research lines, as well as some new ideas. These include investigation of the stability of microwave oscillators to chaotic automodulation (10) and the potential to use 2D-distributed feedback to improve the frequency and power scalability in future sources(11). Oscillating dynamics and the provision of spatial coherence of radiation from a large size active media together present a challenging problem in modern physics of major relevance to Lasers and Masers which produce electromagnetic radiation (microwave, infra-red, optical light). We are studying the use of multi-dimensional distributed feedback, which may be realised in multi-periodical Bragg structures. Transverse electromagnetic energy coupling in these novel Bragg structures may provide spatial coherence of radiation when the transverse size of the active media exceeds the radiation wavelength by more than 2 to 3 orders of magnitude. We are conducting a 2D Bragg FEL experiment which will employ an oversized relativistic annular electron beam as the active gain media. This electron beam will be passed through a spatially periodic magnetic field (wiggler) in the presence of a two dimensional Bragg cavity causing it to interact with an electromagnetic wave. The concept of two-dimensional feedback has wide implications for the cavities of many different types of laser. One such example is to use 2D Bragg resonators in high power solid state lasers to maintain single mode excitation as the size of the solid state crystal is increased.

2. Numerical Simulations

The investigations of the interaction between electromagnetic radiation and electron beams involves the complex problem of multifrequency fields coupled to imperfect, high density, beams. The experimental work is supported by the Particle In Cell (PIC) code KARAT which solves Maxwells equations and the ballistic equations of motion to predict the evolution of complex systems. This programme, written by Vladimir Tarakanov and Vladimir Simonov from the High Energy Research Centre in Moscow, has been applied with considerable success to the simulation of the superradiance experiments(9).

Within the research group a large requirement exists to model the electron beam formation in regions of complex magnetic and electric field configurations. To aid this work a suite of simulation programmes have been created which solve in an iterative and self-consistent manner for the electron beam trajectories taking into account not only the static electric and magnetic fields themselves but also the fields due to the electron beam. This suite of programmes allows us to model complex electron guns and beam transport solenoids in 2.5 Dimensions, and it has proven very accurate in comparisons with experimental observations. This set of programmes is a great asset in our research as it provides accurate predictions of the electron beam velocity and spread(5-7,12).

3. Cathode and beam formation physics

Experimental work in the group has investigated and used thermionic* emission in both Pierce and MIG electron beam sources. We were the first to report on the use of both cold field enhanced emission in a quasi MIG like arrangement(12) and Pseudospark discharges (2) as electron beam sources for high power microwave generators. The results obtained from this research are very exciting since the pseudospark-sourced electron beam has the highest simultaneous electron beam current density (~1.5kAcm-2) and brightness (10¹¹ to 10¹² A m-2 rad-2 ) when compared to all other sources including thermionic, explosive emission, field emission and photo-cathodes. Electron emission from velvet surfaces in Pierce configurations (5,6) and Explosive Electron Emission in co-axial diodes(13) are also very active research topics. The MIG type sources produced a hollow cylindrical electron beam with gyrating electrons describing small orbit trajectories with a diameter much less that the transverse beam size and were used as the beam source for gyrotron oscillator experiments, as were the co-axial explosive emission diodes. The Pierce type sources produce a solid, cylindrical, rectilinear electron beam to which a twist in the guiding magnetic field was applied (either through single or multi-turn bifilar wigglers) to produce a solid electron beam describing a helical trajectory with a diameter equal to the total beam transverse diameter. This arrangement was used in CARM, FEL and Gyrotron amplifier arrangements.

* In conjunction with e2v Technologies, Chelmsford

4. Summary

The experimental Beam-Wave Instability group has a wide spectrum of research activities, ranging from CARM, FEL and CRM experiments through to ultra-short pulse superradiance research. In addition there is considerable activity in the area of cathode physics. We use beams with energies in the range 50keV to 1MeV with currents up to 1kA.This work is supported by sophisticated numerical modelling capabilities.

We also interact extensively with the theoretical group members with regard to the novel work they are pursuing in the field of electron beam-wave interactions. This efficient internal collaboration is fruitful in bringing new ideas into the experimental area for confirmation and in the provision of interesting experimental results requiring theoretical analysis and understanding.

Also indicated above, the research group has collaborated successfully with researchers at other institutes resulting in a number of important experimental and theoretical publications. S.V. Samsonov, N.Yu. Peskov and A.V. Savilov, Researchers at the Institute of Applied Physics, Nizhny Novgorod, Russia recently jointly won the Russian Academy of Science's Gold Medal for the most outstanding work performed by young Russian scientists, partly with respect to work conducted in collaboration with the RELD group at Strathclyde. We have also enjoyed fruitful collaborations with industrial organisations, and the group is always keen to seek industrial contact. This is commensurate with our aim to ensure that the research we conduct is not only of the highest quality, but also to ensure that where appropriate, industrial exploitation of the research output is facilitated.

References

1) "Generation and Application of High Power Microwaves" Eds. R.A. Cairns and A.D.R. Phelps, Proceedings of the 48th Scottish Universities Summer School in Physics, IoP Publishing, Bristol)

2) H. Yin, W. He, G.R.M. Robb, A.D.R. Phelps, K. Ronald, and A.W. Cross, 1999, "Coherent microwave generation from a pseudospark cathode Cherenkov maser", Phys. Rev. ST Accel. Beams, 020701

3) A.W. Cross, S.N. Spark and A.D.R. Phelps, 1995, "Gyrotron experiments using cavities of different ohmic-Q", Int. J. Electronics, 79, pp481-493

4) N.S. Ginzburg, N.Yu. Peskov, A.D.R. Phelps, A.W. Cross, W. He and P. Winning, 1996, "Theoretical and experimental studies of a Ka-band free electron laser with a guide magnetic field", Digest of the 23rd Annual Plasma Physics Conference (IoP), Creiff (Scotland), p17

5) S.J. Cooke, A.W. Cross, W. He and A.D.R. Phelps, 1996, "Experimental operation of a cyclotron autoresonance maser oscillator at the second harmonic", Phys. Rev. Lett., 77, pp4836-4839

6) G.G.Denisov, V.L. Bratman, A.W. Cross, W. He, A.D.R. Phelps, K. Ronald, S.V. Samsonov and C.G. Whyte, 1998, "Gyrotron traveling wave amplifier with a helical interaction waveguide", Phys. Rev. Lett., 81, pp5680-5683

7) V.L. Bratman, A.W. Cross, G.G. Denisov, W. He, A.D.R. Phelps, K. Ronald, S.V. Samsonov, C.G. Whyte and A.R. Young, 2000, "High-gain wide-band gyrotron traveling wave amplifier with a helically corrugated waveguide", Phys. Rev. Lett., 84, pp2393-2396

8) N.S. Ginzburg, I.V. Zotova, A.S. Sergeev, I.V. Konoplev, A.D.R. Phelps, A.W. Cross, S.J. Cooke, V.G. Shpak, M.I. Yalandin, S.A. Shunailov and M.R. Ulmaskulov, 1997, "Experimental observation of cyclotron superradiance under group synchronism conditions", Phys. Rev. Lett., 78, pp2365-2368

9) N.S. Ginzburg, N.Yu. Novozhilova, I.V. Zotova, A.S. Sergeev, N.Yu. Peskov, A.D.R. Phelps, S.M. Wiggins, A.W. Cross, K. Ronald, W. He, V.G. Shpak, M.I. Yalandin, S.A. Shunailov, M.R. Ulmaskulov and V.P. Tarakanov, 1999, "Generation of powerful subnanosecond microwave pulses by intense electron bunches moving in a periodic backward wave structure in the superradiative regime", Phys. Rev. E, 60, 3297-3304

10) K. Ronald, A.W. Cross, A.D.R. Phelps and W. He, 2001, "Observations of dynamic behaviour in an electron cyclotron maser oscillator", J.Phys.D:Appl. Phys. (Rapid Communications), 34, L17-L22

11) N.S. Ginzburg, N.Yu. Peskov, A.S. Sergeev, A.D.R. Phelps, I.V. Konoplev, G.R.M. Robb, A.W. Cross, A.V. Arzhannikov and S.L. Sinitsky, 1999, "Theory and design of a free-electron maser with two-dimensional feedback driven by a sheet electron beam", Phys. Rev. E, 60, pp935-945

12) M. Garven, S.N. Spark, A.W. Cross, S.J. Cooke and A.D.R. Phelps, 1996, "Gyrotron experiments employing a field emission array cathode", Phys. Rev. Lett., 77, pp2320-2323

13) K. Ronald, A.W. Cross, A.D.R. Phelps, W. He, H. Yin, S.N. Spark, 1998, "Explosive cathode gyrotron experiments", IEEE Trans. on Plasma Science, PS-26, pp375-382

This information on Experimental Activity in the Beam-Wave Instability group was written by Dr Kevin Ronald