Nuclear Instruments and Methods m Physics Research A304 (1991) 825-826North-Holland

Panel discussion on Raman FELsH.J . DoucetCentre d'Études de Bruyères-le- Châtel, Seance de Physique et Techniques Nucléaires, BP 12, 91680 Bruyères-le- Châtel, France

This article presents a short report on the panel discussion on Raman FELs, held at the 12th Int . FEL Conf., Pans, France, 1990

This panel on Raman effects followed the first panelon the general theory of FELs in which some theoreticalaspects of Raman FELs, such as the comments onfrequency spectrum stability in the Raman regime byDr . Kishimoto of JAERI, had already been discussed.

In a short introduction by Dr. Henri Doucet of theFrench Commissariat a l'Energie Atomique, we werereminded that the pure Compton and pure Ramanregimes in cold electron beams appear to be dust twoextreme cases of a single mixed regime, as described inthe early 1980's by the group of P. Sprangle of NRL.This model allows us to clearly define the limits of theRaman regime .

The first part of the panel was devoted to discussionof the theoretical aspects of the Raman regime . Dr .Henri Freund of the Naval Research Laboratory inWashington, DC, presented the main characteristics ofthe effect of an axial B field in the Raman regime . Thisis especially important for experimentalists, as the Ra-man regime can be obtained only with moderate-to-lowenergy electrons, and the propagation of electron beamsis greatly improved by an axial B field . Freund pointedout the importance of the negative mass instability forgroup I electrons when the axial B field is smaller thansome critical value, as well as the importance of spacecharge instability above the critical axial B field for thegroup II electrons. This model is clearly supported byexperiments done at NRL and other centres, whichdemonstrate large power emission in the mm spectralregion .

Continuing on other aspects of Raman FEL theory,Dr. Tulupov of the Kurchatov Institute in Moscowmade a comparison between Compton and Raman regi-mes concerning the gain as a function of current den-sity . He distinguished two different degrees of nonlin-earities : regime A, a case m which the amplificationsaturates, and regime B, where a number of nonlinearmechanisms can be involved : i) the tapered wiggler, asdescribed by Sprangle et al . and Kroll et al . (called

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SFEL for synchronous FELs); ii) the effects of high rfpower, as described by Pantell ; iii) the long electricfield, as studied by Gover; and iv) transverse magneticfield effects, as described by the Kurchatov group.

Tulupov showed that the small gains of the short-wavelength Compton regime make it difficult forCompton FELs to leave the linear aspects or the nonlin-ear saturation of regime A and enter directly into theregime B characterized by various nonlinear mecha-nisms.

The situation is different for the Raman regime dueto the high gain values possible for long wavelengths, sothat Raman FEL experiments appear to be a veryexciting field for demonstrating strong nonlinear behav-ior of FELs .

The second part of the panel was devoted to RamanFEL experiments. Dr . Battacharjee of Columbia Uni-versity reported on three experimental aspects:1) Refractive optical guiding in the saturated regime of

an FEL. Even if this refractive guiding is expected tobe much weaker than the gain guiding, it could bevery useful in tapered systems. Apparently. however,no experiments have yet clearly validated refractiveguiding in the saturation regime . The importance ofwaveguide effects in optical guiding studies for Ra-man FELs was also pointed out.

2) Spike formation. This intriguing phenomenon hasbeen described in a few papers at this conferenceand probably merits more attention in the future .

3) The importance of space charge, which is presentlyused to investigate the concept of an inverse FELbeat wave accelerator working in the Raman regime,was noted.Following Dr . Battacharjee, Dr. Saito of the Institute

of Space and Aeronautical Science in Japan made somecomments on the interest of Raman FELs and onJapanese efforts to reach shorter wavelengths .

Following Dr. Saito, Dr . Caplan of LLNL discussedthe interest of CARMs and Raman FELs for mm and

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sub-mm wave generation . The Livermore CARM ex-periment looks quite promising, but CARMs still haveto prove their ability to reach high frequencies, in orderto make a clear comparison of interest with respect toRaman FELs .

Finally, a general consensus was that short-period

HJ Doucet / Panel discussion on Raman FELS

wigglers could be of considerable interest for RamanFELs, since they increase the space charge effect versusthe purely direct magnetic pumping effect . More experi-ments, some of which are already on line, are needed inthis field .