Bekefi, G.

Freund, H.P.

Ganguly, A.K.

Kirkpatrick, D.A.

Rienzo, A.C.

DSpace@MIT

PFC/JA-88-26A High Power, 600 pm WavelengthFree-Electron LaserD.A. Kirkpatrick,t G. Bekefi, A.C. DiRienzo,H.P. Freund,t and A.K. GangulyttJune 1988Department of Physics and Research Laboratory of ElectronicsMassachusetts Institute of TechnologyCambridge, Massachusetts 02139 USAtPresent address: Science Applications International Corp.,McLean, VA. 22102 .ttPresent address: Naval Research Laboratory, Washington,DC 20375-5000.This work was supported by the Air Force Office of ScientificResearch and the National Science Foundation.Submitted for publication in: Physical Review Letters.A HIGH POWER, 600im WAVELENGTHFREE ELECTRON LASER*D.A. Kirkpatrickt, G. Bekefi and A.C. DiRienzoDepartment of Physics and Research Laboratory of ElectronicsMassachusetts Institute of TechnologyCambridge, Massachusetts 02139andH.P. Freundt and A.K. GangulyNaval Research LaboratoryWashington, DC 20375-5000AbstractWe report high power emission (18 MW) at a wavelength of A = 640 tim and abandwidth AA/A < 0.04 from a superradiant free electron laser (FEL) excited by a2 MeV, 1 kA electron beam. Comparison of the experiment with a nonlinear simu-lation yields good agreement. Theoretical extrapolation to a tapered wiggler exper-iment shows that power levels of - 140 MW could be achieved with an efficiency of, 7%.*Work supported by the Air Force Office of Scientific Research and the NationalScience Foundation.tPresent address: Science Applications International Corporation, McLean, VA22102.IThe outstanding capabilities of free electron lasers (FEL) are their high poweroutputs, reasonable efficiencies and wavelength tunability. In this paper we presentwhat we believe is the highest power output (18 MW) and highest efficiency (~ 1%)in a wavelength regime 3 (- 600 pm) that has not been studied extensively.The experimental arrangement is shown in Fig. 1. The Pulserad 110A accelerator(V = 2MV, -r ~ 30 ns) energizes a multielectrode field emission electron gun4 whichproduces a high current, low emittance electron beam (I - 900 A, AV/V < 0.005).A thin solenoid lens focusses the electron beam into a 0.79 cm radius stainless steelevacuated drift tube which also acts as an overmoded circular waveguide for theradiation. A bifilar helical wiggler magnet (with periodicity AW = 3.14 cm) is woundonto the outside of the stainless steel drift tube. An adiabatic entrance taper isprovided by a series of resistive termination loops5 which gradually tap currentfrom one winding to the other. A second thin solenoid lens focusses the electronbeam entering the drift tube to a beam waist such that the electron beam is matchedinto the helical wiggler field. The electron beam is then transported and focussedthrough the wiggler interaction region by means of the focussing provided by thestrong helical wiggler magnetic field. In contrast to many previous FELs operatedat relatively low beam energies and high currents,',2',,' there is no applied axialguiding magnetic field in the wiggler interaction region.8The radiated output power is measured with a combination of a Spectra-Physicsmodel 360001 Joule-calorimeter and a Laser Precision model 1540S pyroelectricpulse detector. The total output energy is measured with the calorimeter. Wave-length filters (conducting meshes used in reflection) limit the wavelength range ob-served by the calorimeter to 330pL m < A < 1000 pm. The pulse shape of the outputradiation is measured with the pyroelectric detector. The output power is calcu-2lated by dividing the measured output energy by the time (FWHM) of the radiationpulse. An output power of 18 MW at the FEL operating wavelength of 640 /Lm hasbeen measured under single pass, superradiant operation.The growth rate F = (1/P)dP/dz of the electromagnetic wave is obtained6 bymeasuring the output power as a function of the wiggler interaction length z. Thislength is varied by moving a transverse kicker magnet along the length of the wiggler,thereby turning the electron beam into the drift tube wall and terminating theinteraction. The data, shown in Fig. 2, for two different wiggler strengths B.,clearly show the regions of exponential gain and, in the case of the stronger wigglerfield, the region of saturation9 of the FEL.The observations are compared with a theoretical analysis of the experimentalconfiguration based on the nonlinear simulation code ARACHNE"0 . This simulationnumerically integrates a set of coupled nonlinear differential equations which governthe evolution of an arbitrary number of transverse electric (TE) and/or transversemagnetic (TM) waveguide modes as well as an ensemble of electrons. The analysisis fully three-dimensional and self-consistently includes the overlap of the electronbeam and the radiation field of each mode. The electron trajectories are integratedusing the complete Lorentz force equations in the presence of a three-dimensionalrepresentation of the helical wiggler field, and includes the effects of betatron os-cillations and velocity shear on the interaction. Neither self-field nor space-chargeeffects are included in the simulation, which is valid in the strong-pump Comptonregime of operation. The collective dielectric effect of the beam upon the waveguidemodes is included self-consistently in the analysis. For the data shown in Fig. 2,the measured power gains of 56 ±3 dB/m for B. = 1275 G and 70 ±6 dB/m forB,= 1510 G are consistent with the results of the ARACHNE code for a beam3temperature of AV/V - 0.0025. This is the effective temperature at full width(and 1/e point) due to nonzero beam emittance, and is in good agreement with themeasured beam emittance which gives AzV/I T < 0.005 (our resolution limit).The spectrum of the output radiation is measured with a grating spectrometer".The measured spectrum for a 2.3 MV, 930 A electron beam, at a wiggler fieldstrength of 1275 G is shown in Fig. 3., along with the calculated spectrum fromARACHNE. The measured spectral width is approximately AA/A ~ 0.04. The ob-served wavelength corresponds predominantly to the excitation of the lowest (TE11)mode of the circular waveguide, with a small TMI, component (< 5%).In conclusion, the measured output power of 18 MW at the measured frequencyof 470 GHz represents an order of magnitude increase in the available power forany FEL source operating in the - 600 pm wavelength regime. The correspondingefficiency of 0.8% represents a two order of magnitude increase in efficiency for anFEL operating in this wavelength regime. Increasing the wiggler field amplitudefrom 1275 G to 1510 G yields an output power of 25 MW for the same wiggler length.Inspection of Fig. 2 shows that this output power might be increased even furtherin the 1510 G case by terminating the beam earlier.Ve note that simple one-dimensional criteria' 2 suggest that the experiments de-scribed above are in the so-called collective (Raman) regime where RF space chargeeffects cannot be neglected in general. Nonetheless, the ARACHNE simulation(which neglects space charge effects) is in very close agreement with the experimen-tal observations. This suggests that for out experiment three-dimensional effectslead to a substantial reduction in the magnitude of the space charge fields and theireffect on the interaction.4Previous experiments have noted substantial increases in efficiency when the wig-gler field amplitude is tapered' 3" . An example of the possible effect of tapering thewiggler field amplitude in this experiment is shown in Fig. 4, where the ARACHNEsimulation is extended in the axial direction, for both a tapered and an untaperedwiggler. The parameters for the simulation shown in Fig. 4 are identical to those inFig. 3 used in comparison with our experiments. As can be seen in the figure, forthe case of the tapered amplitude wiggler, the output power reaches a maximumof 142 MW, corresponding to a 6.6% efficiency. Maintaining the parameters of theelectron beam constant, but changing the wiggler period to A, = 2.2 cm and the ini-tial wiggler field amplitude to B, = 3100 G(to provide more latitude in the taperingprocess), results in a maximum possible output power of 263 MW at a frequency of498 GHz.5ReferencesI.L. Granatstein, S.P. Schlesinger, M. Herndon, and R.K. Parker, Appl. Phys.Lett. 30, 384 (1977).2D.B. McDermott, T.C. Marshall, S.P. Schlesinger, R.K. Parker, and V.L. Granat-stein, Phys. Rev. Lett. 41, 1368(1978).3L.R. Elias, R.J. Hu, and G.J. Ramian, Nucl. Instrum. Methods Phys. Res. A237, 203(1985).4D.A. Kirkpatrick, R.E. Shefer, and G. Bekefi, J. Appl. Phys. 57, 5011(1985).'J. Fajans, J. Appl. Phys. 55, 43(1984).6J. Fajans and G. Bekefi, Phys. Fluids 29, 3461 (1986).7S.H. Gold, D.L. Hardesty, A.K. Kinkead, L.R. Barnett and V.L. Granatstein,Phys. Rev. Lett. 52, 1218 (1984)."To our knowledge, the only other experiment to operate without an axial guidefield is J.A. Pasour, R.F. Lucey, and C.A. Kapetanakos, Phys. Rev. Lett 53,1728 (1984).9J. Fajans, J.S. Wurtele, G. Bekefi, D.S. Knowles, and K. Xu, Phys. Rev. Lett. 57,579 (1986).'0 A.K. Ganguly and H.P. Freund, Phys. Rev. A 32, 2275(1985)."D.A. Boyd (private communication).2P. Sprangle, R.A. Smith and V.L. Granatstein in Infrared and Millimeter Wavesedited by K.J. Button (Academic, New York, 1979), Vol. 1."T. J. Orzechowski, B. Anderson, J.C. Clark, W. M. Fawley, A.C. Paul, D. Pros-nitz, E. T. Scharlemann, S.M. Yarema, D. Hopkins, A. M. Sessler and J.Wurtele, Phys. Rev. Lett. 57, 2172 (1986)."H. Boehmer, M. Z. Caponi, J. Edighoffer, S. Fornaca, J. Munch, G. R. Neil, B.Saur and C. Shih, Phys. Rev. Lett. 48, 141 (1982).6List of FiguresFigure 1. The schematic layout of this experiment, and typical voltage, beam cur-rent, and microwave output waveforms.Figure 2. Observed output power as a function of wiggler length, for two differentwiggler pump strengths, B. = 1275 G, and B, = 1510 G. V = 2.0 MV, I =780 A.Figure 3. The measured output spectrum for a 2.3 MV, 930 A electron beam; wig-gler pump strength B, = 1275 G. The measured output power is PA = 18 MW.Figure 4. The output power as a function of wiggler length for a tapered and unta-pered wiggler as calculated with ARACHNE. The parameters of the simulationare: V = 2.3MV, AV/V = 0.25%, Ib = 930 A, B. = 1275G, A. = 3.14 cm,beam radius = 0.4 cm. The output frequency is 470 GHz. The on-axis am-plitude of the wiggler field used in the simulation is shown at the top of thefigure.7PULSERAD IIOAACCELERATORTHIN SOLENOIDFOCUSSINGLENSESBIFILAR HELICALWIGGLERDRIFT TUBEMULTI- ELECTRODE GUN SUBMILLIMETER-WAVEAND EXIT BEAM2 DIAGNOSTICSwI-0zUea:J0a.aLIjC0I0. I0.80.60.40.206420I a I i k i a k i I iS a I I I I I I I I I I I I0 40 80 120 160TIME (ns)Fig. 1Kirkpatrick et al(N10I0*'Bw= 1510 Gr= 70 dB/~40 60 80 100 120 140DISTANCE z (cm)Fig. 2Kirkpatrick et alm*-2100z3:wi~I~~~~~1~ IIIBw= 127P= 56 dI I I I5 GB/mI I II IS4dSa a i0I p pICO6(su!un -iej) U3Md G1~lVVC-,4-,(Y) 'uo.~ s.~*r- *r-C0ICOICO0ON0UzwLL0ICT00IH3MOdQ? 0 t- - 0I * I S I I0 0 0N~ 00 0 0N~(MWA) JGMOd PGIDIpD80 0ww cwz0Ni0)UC0(I)000

A high-power, 600um wavelength free-electron laser

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A high-power, 600um wavelength free-electron laser

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