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Inverse Free Electron Laser experiment at the Neptune Laboratory


Inverse Free Electron Laser schemes to accelerate particles have been proposed as advanced accelerators for many years [1]. Recent successful proof-of-principle IFEL experiments have shown that along with high gradient acceleration this scheme offers the possibility to strongly manipulate the longitudinal phase space of the output beam [2-4]. The Inverse Free Electron Laser is in fact, a strong candidate for microbunching and phase-locking electrons at the optical scales. Up to now, though, only modest energy gain has been achieved mostly because of the limitations in the peak radiation power available.

The purpose of the UCLA experiment is to achieve a substantial energy gain and to investigate the longitudinal structure of the electron beam. This experiment addresses problems common to other advanced accelerator schemes like the issue of increasing the interaction length of a laser-driven accelerator dealing with the limitations of radiation diffraction and to increase the final energy gain by tapering of the structure to maintain phase synchronism with the accelerating particles.

At the Neptune Laboratory at UCLA we fully take advantage of the unique opportunity of having a 10.6 micron high power laser and a relativistic high brightness electron beam in the same experimental facility. In the Neptune scheme, the 14.5 MeV electron beam from the split photoinjector linac system, interacts inside an undulator magnet with the high power CO2 laser focused by a lens (f/25) with focal distance of 2.6 m to a tight spot of few hundreds microns. Because the Rayleigh range of the laser is much shorter than the undulator length, the interaction is diffraction dominated [5].


The simulations performed with a three-dimensional IFEL code (TREDI IFEL version) predict a energy gain >300% with >25% of particles captured in the accelerating bucket.

IFEL experimental setup

The undulator is shown on the right. It has been designed and built at the Kurchatov Institute of Moscow[6]. In order to maintain phase synchronicity and preserve the accelerating bucket along the accelerator, the undulator is strongly (non adiabatically) tapered in period (from initial 1.5 cm to final 6 cm) and magnetic field amplitude (from 0.1 T to 0.6 T). It is 50 cm long and has a constant gap of 12 mm. An all aluminum vacuum box, with a plexiglass cover was built to house the undulator on the beamline. The vacuum in the box is on the order of high 10^-6 torr. The undulator was installed on the beamline on August 2003.

A TW CO2 laser system capable of generating 100 J in 100 ps pulse is used in the experiment. To match into the 0.5 m long 12 mm gap undulator with the 10.6 micron beam the f/25 geometry is used. The spot size at the focus has w = 350 micron so that the Rayleigh range is 3.5 cm is matching the tapering design.

To ensure clipping-free propagation of the focused beam in the vacuum pipe upstream of the undulator, we designed new quadrupole magnets with large aperture (second figure on the right). These magnets have a gap of 2.5 inches and tapered coils to maintain the field gradient 6.5 T required to focus the electron beam to 150 micron spot size. They have been designed with the help of the 3D magnetostatic code RADIA

The energy of the NEPTUNE LINAC has been upgraded to the design value 14.5 MeV replacing the old klystron. The available S-band RF power is now 22 MW. Also, the electric field gradient inside the 1.6 cell gun has been limited in the past by severe arcing inside the standing wave cavity. For this reason, a new 1.6 cell gun has been installed and conditioned up to 5 MW of RF power. After these improvements the energy of the beam injected in the IFEL experiment is up to the specifications.

Experimental results (on going)

The first runs of the IFEL experiments were held in the fall of 2003. The initial results are encouraging. Electrons up to 28 MeV were detected on the phosphorous screen at the exit slit of the high energy spectrometer (the initial energy was doubled !!!). The results agree well with the simulations if we take into account that available laser power was 3-4 times less than predicted. Work is underway to increase the power of the CO2 laser and hopefully increase the final energy of the accelerator and the fraction of particles captured.

In the figure, we report the experimental spectra of electron energy recorded by two different cameras looking at different point along the exit slit of the energy spectrometer for four different CO2 shots. The calibration of electron density between the two cameras can be done using the fact that in a region of energies (24-26 MeV) the two fields of view overlap. The input energy distribution is a narrowly peaked spectrum (0.5% energy spread) around 14.5 MeV. Note the reproducibility of the peaks of the spectrum at 25 and 27 MeV.


[1] P. Musumeci, ``Ph.D Dissertation'' final

[1] E.D.Courant, C. Pellegrini and W.Zakowicz, ``High-Energy Inverse Free Electron Laser Accelerator'' Phys. Rev. A 32, 2813 (1985).

[2] Liu Y. et al., ``Experimental observation of Femtosecond Electron Beam Microbunching by Inverse Free Electron Laser Accelerator'' Phys. Rev. Lett., 80 4418 (1998).

[3] Yoder R.B., ``Energy-Gain measurements from a Microwave Inverse Free Electron Laser Accelerator'' Phys. Rev. Lett., 86, 1765 (1985).

[4] W.D.Kimura et al. ''First staging of two laser accelerators'' Phys. Rev. Lett. 86, 4041 (2001)

[5] P. Musumeci and C. Pellegrini, ``IFEL experiment at the Neptune Laboratory'' Proc. Of Advanced Accelerator Concepts 2000 Santa Fe NM

[6] A.A. Varfolomeev et al., ``An undulator with non-adiabatic tapering for the IFEL project'' NIM 483 377 (2002)

[7] P. Musumeci, C.Pellegrini, ``Status of the Inverse Free Electron Laser Accelerator at the Neptune Laboratory'' Proc. of Particle Accelerator Conference 2003