Can a free electron laser operate with a broad momentum spread?
The gain in a usual free electron laser (FEL) vanishes when the energy spread of the electron beam is too broad because electrons with positive detuning parameter {ital v} are bunched around a certain phase contributing to a positive gain, while those with negative {ital v} are bunched around another phase contributing to a negative gain. The net gain therefore vanishes for a beam of electrons distributed over a broad range of positive and negative values of {ital v}. To avoid this problem, it was proposed recently to use two undulators and to insert a device between them which separates the electrons of negative v and to displace them in phase so that they contribute to an overall positive gain after passing through the second undulator. The device will be referred to as the ``redistributor``. The scheme, was an effort to extend the idea of the inversionless atomic laser to the FEL, and, if true, would have an important consequence for future FEL development. The purpose of this paper is to show that the scheme does not work as proposed. The reason is simple: consider an initial electron distribution which is uniform extending to a large positive and negative detuning where the FEL interaction vanishes. The distribution, because of Liouville`s theorem, will remain the same after the first undulator. The redistributor would have no effect on this distribution, and the gain of the total system vanishes. The reasoning above is wrong because we did not take into account the contributions of {ital all electrons}. The first group contributes to a positive net gain. However, this is canceled by the negative net gain due to the second group, irrespective of how the phase of electrons in the region of negative detuning is displaced relative to those in the positive detuning.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research, Washington, DC (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 415196
- Report Number(s):
- LBNL-38863; CBP-Note-181; CONF-9608133-10; ON: DE97001497; TRN: 97:001264
- Resource Relation:
- Conference: 18. international free electron laser conference and 3. FEL Users` workshop, Rome (Italy), 25-30 Aug 1996; Other Information: PBD: Aug 1996
- Country of Publication:
- United States
- Language:
- English
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