Free-electron Maser Research Articles

Design of a high power CW free electron maser

A conceptual design is presented for efficient operation of a free electron maser amplifier for fusion applications. The system, which operates in the strong pump regime, is CW with output frequencies in the 100 GHz range or higher, output power levels of up to 5 MW and total efficiencies greater than 70%. The physical and technological data base for such a design is described.

'Plasma Emission' without Langmuir Waves

Recent observations have confirmed that the level of Langmuir waves associated with type III streams of electrons in the interplanetary medium is usually too low to account for the observed radio emission by the accepted 'plasma emission' processes, and it has been suggested that emission mechanisms which do not require Langmuir waves should be explored. Four such mechanisms are discussed. One is a parametric instability leading directly to second-harmonic emission; it is found inapplicable under conditions of interest here. The other three processes all involve ion-sound turbulence. One which is known in a different context is turbulent bremsstrahlung. Turbulent bremsstrahlung of transverse waves is found to compare unfavourably with the other two processes, which are scattering of an ion-sound (s) wave into a transverse (t) wave and double emission of both waves simultaneously. These latter two processes are related by a crossing symmetry and are treated together with the following results: (i) The processes become greatly enhanced when the beat (w�w', k�k') between the t wave and the s wave nearly satisfies the dispersion relation for Langmuir (I) waves. (ii) A bump-in-the-tail instability (due to electrons with dF(v)/dv > 0) can cause the transverse waves to grow due to double emission; this growth has been likened to a freeelectron maser. (iii) The familiar bump-in-the-tail instability for resonant I waves can be suppressed by the ion-sound waves, and the double-emission instability then takes over with about the same growth rate as the original I-wave instability. (iv) The conditions for the double-emission instability to occur are probably satisfied at least some of the time for type III streams. It is concluded that although 'plasma emission' without Langmuir waves may be possible in principle, it is unlikely to play any role in type III bursts.

Free-electron lasers and masers on curved paths

The possibility of building free-electron lasers and masers with quasi-circular undulators is considered. The two-mirror cavity is replaced by a suitable waveguide. The magnetic field along the orbit has a continuous component superimposed on the alternating one. Besides allowing large freedom for electron guiding and focusing, the structure allows effective negative mass conditions, which may enhance the electron beam modulation when space charge effect are important. Furthermore the radiation frequency can be higher than that in a linear undulator with the same electron energy, and long interaction times are possible in a compact structure.