G.R.M. Robb

Theory of free-electron maser with two-dimensional distributed feedback driven by an annular electron beam

The use of two-dimensional (2D) distributed feedback is considered as a method of providing spatially coherent radiation from an oversized annular electron beam. To realize the feedback mechanism, 2D Bragg structures formed from doubly-corrugated waveguide sections of coaxial geometry are suggested. The properties of two types of coaxial cavities formed using such structures are compared: a single-section 2D Bragg cavity and a two-mirror cavity. The eigenmodes of both cavities are found and their high selectivity over both azimuthal and longitudinal indices was demonstrated. Time-domain analyses of the excitation of the cavities by an annular electron beam were carried out. The influence of the cavity parameters on the oscillation regime is analyzed and discussed. It was shown that for a specific set of 2D Bragg cavity parameters it is possible to obtain a regime of steady-state oscillations when the transverse size of the beam exceeds the wavelength by a few orders of magnitude, while outside this parame...

Mode competition and control in free electron lasers with one- and two-dimensional Bragg resonators

We present a time domain analysis of the longitudinal and azimuthal mode dynamics which occur in relativistic free electron lasers and cyclotron autoresonance masers with one-dimensional (1-D) and two-dimensional (2-D) Bragg resonators. It is shown that when the 1-D oscillator is moderately above threshold, a single longitudinal mode generation regime is established due to nonlinear mode competition. The process of longitudinal mode selection is more effective if the region of synchronous interaction between the electron beam and the forward propagating wave is extended inside the Bragg reflectors. Further above the oscillation threshold, multimode chaotic behavior occurs. In the second part of this paper, it is shown that both traditional 1-D and novel 2-D Bragg resonators can produce radiation at a single frequency with a one-mode azimuthal distribution which corresponds to spatial synchronization of the electromagnetic radiation. In 1-D systems, electronic mode selection occurs via nonlinear mode competition. In contrast, electrodynamic mode selection occurs in 2-D Bragg resonators, resulting in the production of a single azimuthally symmetric mode after the linear stage of evolution. The 2-D Bragg resonator is shown to retain its selectivity when its radius greatly exceeds the radiation wavelength.

The Strathclyde terahertz to optical pulse source (TOPS)

We describe the newly created free-electron laser facility situated at the University of Strathclyde in Scotland, which will produce ultra-short pulses of high-power electromagnetic radiation in the terahertz frequency range. The FEL will be based on a 4 MeV photoinjector producing picosecond 1 nC electron pulses and driven by a frequency tripled Ti:sapphire laser thus ensuring synchronism with conventional laser based tuneable sources. A synchronised multi-terawatt Ti:sapphire laser amplifier will be used in the study of laser/plasma/electron beam interactions and as a plasma based X-ray source. A substantial user commitment has already been made in support of the programme.

Quantum theory of SASE FEL

We describe a free-electron laser (FEL) in the Self-Amplified Spontaneous Emission (SASE) regime quantizing the electron motion and taking into account propagation effects. We demonstrate quantum purification of the SASE spectrum, i.e. in a properly defined quantum regime the spiking behavior disappears and the SASE power spectrum becomes very narrow.

Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing

Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process.

Pseudospark-based electron beam and Cherenkov maser experiments

Detailed experimental results from the first free-electron maser experiment to use a pseudospark-based electron beam are presented in this paper. These include the design and realization of a pseudospark-based electron beam source and Cherenkov maser experiment. A pulsed, 70–80 kV, 10 A electron beam was obtained from the hollow cathode discharge phase of an 8-gap pseudospark (PS) discharge. The beam was used to produce coherent microwave radiation via a Cherenkov interaction between the electron beam and the TM01 mode of a 60-cm long alumina-lined waveguide. A gain of 29±3 dB was measured and an output power of 2±0.2 kW in the frequency range 25.5–28.6 GHz. Results from numerical simulations of the Cherenkov amplification are also presented and found to be consistent with the experimental results.

Superradiant light scattering and grating formation in cold atomic vapours

A semi-classical theory of coherent light scattering from an elongated sample of cold atoms exposed to an off-resonant laser beam is presented. The model, which is a direct extension of that of the collective atomic recoil laser, describes the emission of two superradiant pulses along the samples major axis simultaneous with the formation of a bidimensional atomic grating inside the sample. It provides a simple physical picture of the recent observation of collective light scattering from a Bose-Einstein condensate [Science 285 (1999) 571]. In addition, the model provides an analytical description of the temporal evolution of the scattered light intensity which shows good quantitative agreement with the experimental results of Inouye et al.

Experimental and theoretical study of 2D Bragg structures for a coaxial free electron maser

Abstract Experimental results are presented of the first observation of two-dimensional distributed feedback in a coaxial 2D-Bragg structure. The use of two-dimensional distributed feedback has been proposed as a method of producing spatially coherent radiation from extremely powerful large sized relativistic electron beams. To obtain coherent high-power (GW) microwave and millimetre wave radiation, 2D Bragg resonators are needed to overcome the problems of mode selection and synchronisation of radiation from different parts of an oversized beam. The design and cold microwave measurements of a 2D Bragg coaxial structure for use in a Free Electron Maser (FEM) driven by an annular electron beam of circumference 25 times larger than the radiation wavelength is presented. The cavity for the 2D Bragg FEM consists of two 2D Bragg coaxial reflectors separated by a regular coaxial waveguide. The formation of pure two-dimensional distributed feedback without a mixture of one-dimensional parasitic feedback was demonstrated for the first time in this series of experiments. The first experimental comparison of 2D and 1D Bragg structures was also conducted and good agreement between experimental results and theoretical predictions was observed. The eigenmodes of the two-mirror cavity were calculated and it was shown that a single-mode steady-state operation regime could be obtained in a FEM based on such a novel cavity.

Progress of the Strathclyde Free Electron Maser experiment using a 2D Bragg structure

Abstract The use of two-dimensional (2D) Bragg resonators of coaxial geometry, realising two-dimensional distributed feedback, is considered as a method of providing additional mode selection and producing spatially coherent radiation from oversized annular electron beams. The main goals of the project are to prove operability of a FEM using 2D Bragg structures and compare the performance of the FEM using the novel 2D Bragg resonators with conventional 1D Bragg resonators.

A quantum model for collective recoil lasing

Free Electron Laser (FEL) and Collective Atomic Recoil Laser (CARL) are described by the same model of classical equations for properly defined scaled variables. These equations are extended to the quantum domain describing the particles motion by a Schrodinger equation coupled to a self-consistent radiation field. The model depends on a single collective parameter which represents the maximum number of photons emitted per particle. We demonstrate that the classical model is recovered in the limit >> 1, in which the Wigner function associated to the Schrodinger equation obeys to the classical Vlasov equation. On the contrary, for ≤ 1, a new quantum regime is obtained in which both FELs and CARLs behave as a two-state system coupled to the self-consistent radiation field and described by Maxwell-Bloch equations.