Status of FEL Physics Research Worldwide  Claudio Pellegrini, UCLA April 23, 2002 Review of Basic FEL physical properties and definition of important parameters: gain length, pulse length, spiking and intensity fluctuations, saturation and peak power. Data from the SASE-FEL experiments, comparison with theory, and benchmarking the numerical models.

SASE-FEL Physics A SASE-FEL is based on a phenomenon of self-organization of an electron beam interacting with an electromagnetic wave in an undulator magnet. The interaction generates a transition from a beam with a random longitudinal electron distribution to one where the electrons are ordered in a series of microbunches, separated by the undulator radiation fundamental wavelength similar to a one dimensional crystal structure. The transition is characterized by an exponential growth rate, and a saturation level.

SASE-FEL Physics A SASE-FEL is completely characterized by one universal FEL parameter, r (Bonifacio, Pellegrini, Narducci, 1984). FEL parameter: r={(aW/4g)(WP/wW)JJ}2/3 where wW=2pc/lW ,, WP=beam plasma frequency Exponential growth rate (Power Gain Length): LG=lW/2pr Saturation power: P~r Ibeam E Undulator saturation length: Lsat~10LG ~lW/r Line width: ~1/NW~r Cooperation length LC=lr/4pr Number of spikes M=Lbunch/2pLc

SASE-FEL Physics SASE-FEL Power and spikes along the undulator

SASE-FEL experiments UCLA/Kurchatov M. Hogan et al. Phys. Rev. Lett. 80, 289 (1998).

SASE-FEL experiments UCLA/Kurchatov/LANL/SSRL: Gain of 3x105 at 12 m. Demonstration of fluctuations and spikes, in agreement with theory. M. Hogan et al. Phys. Rev. Lett. 81, 4897 (1998).

SASE-FEL experiments LEUTL exponential gain and saturation at 530 nm, A &B, and 385 nm, C. The gain reduction for case B was obtained by reducing the peak current. Milton et al., Sciencexpress, May 17, 2001. LEUTL has saturated also at 130nm. The solid lines are Theoretical predictions.

SASE-FEL experiments TESLA-TTF SASE-FEL. Saturation at 98.1nm. Ayvazyan et al., Ph. Rev. Lett. 88, 104802 (2002).

SASE-FEL experiments VISA: a BNL-LLNL-SLAC-UCLA collaboration (A. Murokh et al., PAC 2002, Int. FEL Conf. 2002) Saturation results Wavelength: 830nm Average Charge:170 pC Gain Length 18.5 cm SASE Energy:10 mJ Total Gain: 2×108

SASE-FEL experiments VISA Operates in 2 set of conditions: a) with no compression, emittance of 1.5 to 2.3 mm mrad, peak current of 55A, charge ~0.2nC. Gain of about 103. b) with strong compression, obtaining a peak current of 250A, Q~0.2nC; the compression is nonlinear and produces a strongly correlated beam phase-space, with a larger emittance; both the current and the emittance change along the bunch; in this case one obtains a power gain length ~18cm, and saturation.

SASE-FEL experiments Case b). Strong nonlinear bunch compression, peak current about 250A. Complicated beam phase space distribution. Bunch length measurements, and simulation results Longitudinal distribution before and after compression

SASE-FEL experiments Case b measured SASE intensity and intensity fluctuations during exponential growth and saturation. Results show single spike, and change in fluctuations after saturation.

SASE-FEL experiments VISA spectral and angular distributions in cases a and b. G ~ 103 G ~ 107

SASE-FEL experiments We have fully simulated the experiment, starting from the photocathode and linac (Parmela), the transport and compression (Elegant), and the FEL (Genesis), and comparing with measured charge, bunch length, FEL intensity, spectral and angular distribution. Genesis simulation of angular distribution in case b.

SASE-FEL experiments Other SASE-FEL and HGHG generation experiments have been done at the ATF and SDL at Brookhaven. There is a very good agreement between the experimental data and our simulations. We have a good and detailed understanding of the physics of a SASE-FEL.

SASE-FEL quantum theory A recent paper by C. Schroeder, C. Pellegrini and P. Chen , (Phys. Rev. E , 64, 1063 (2001)) has evaluated the quantum theory of a SASE-FELs, showing that also at short, X-ray wavelength the results of the classical theory are valid to a very good approximation.

Conclusions Many experiments at several laboratories and over wavelengths from the IR to the UV have demonstrated the validity of the SASE-FEL theory. Theory, simulation codes, and experimental data are in good agreement, also for cases of strong compression and unusual beam phase-space distribution. Data and simulations include details of spectral and angular distributions and intensity fluctuations, important for future use of SASE-FELs as a IV generation radiation source. The LCLS design is based on a good and detailed understanding of the physics of a SASE-FEL.