February 19-20, 2004

The DUV-FEL Workshop: Scientists Probe the Future of Free Electron Laser Physics

The Deep Ultra Violet Free Electron Laser (DUV-FEL), a laser linac facility at Brookhaven National Laboratory, is the world’s only facility dedicated to laser-seeded FEL research and development (R&D) and its applications. To explore opportunities for future experiments on free electron laser and beam physics at the DUV-FEL, the DUV-FEL Beam Physics and FEL Research and Development Workshop was held on February 19-20, 2004, at Brookhaven National Laboratory (BNL).

Representatives from seven countries and 14 institutions – including Lawrence Berkeley National Laboratory, the Massachusetts Institute of Technology, BESSY, and Pohang Accelerator Laboratory – participated in the workshop. More than 20 talks were presented, covering a broad spectrum of beam physics and FEL topics, such as cascaded high gain harmonic generation (HGHG) FEL, ultra-short FEL generation techniques, electron beam optimization, bunch compression, and femtosecond electron beam instrumentation. The presentations revealed that the seeded FEL, especially the HGHG type, will play a critical role in the proposed x-ray FEL facility.

Brookhaven’s Xijie Wang and Li Hua Yu began the workshop with presentations on the DUV-FEL facility and its current FEL R&D program. The facility’s main components are a high-brightness electron accelerator, a HGHG FEL, a chemical science endstation, and sophisticated electron and photon beam instrumentation.

The DUV-FEL accelerator system consists of a 1.6 cell photo injector driven by a Ti:Sapphire laser system, and a four-section 2856 MHz SLAC-type traveling wave linac that is capable of producing a 200 MeV electron beam. The facility’s magnetic chicane bunch compressor produces sub-picosecond-long electron bunches with a peak current of a few hundred amperes. The high-brightness electron beam travels down the 10 meter-long undulator to generate UV light with a fundamental wavelength of 266 nanometers (nm).

Past DUV-FEL Activity

One of the most important milestones at the DUV-FEL this last year is the initialization and completion of the first DUV-FEL user experiment by Arthur Suits and his collaborators from BNL’s Chemistry Department.

This first chemical science experiment – on ion pair imaging – used the HGHG’s third harmonic beam (89 nm) to study the super excited states of methyl fluoride, a highly flammable gas. Velocity-mapped ion images of the fluoride ion, obtained using intense, coherent, sub-picosecond pulses of 86-89 nm light, revealed a low translational energy, implying a very high internal excitation in the molecule’s methyl cation cofragment (W. Li et al, PRL 92, No. 8, 083002-1 [2004]).

In response to the requests of many users to study chemical science at the facility, the DUV-FEL linac is being upgraded from 200 to 300 MeV to enable the HGHG FEL to produce 100 μJ pulses of 100 nm light. This will help establish the DUV FEL as a premier user facility for ultraviolet radiation, and will enable state-of-the-art gas phase photochemistry research.

After successfully lasing at 266 nm with 800 nm laser seeding in late October 2002 (L.H. Yu et al, PRL 91, No. 7, 074801-1 [2003]), experiments were carried out at the DUV-FEL to further characterize the properties of the HGHG FEL and to demonstrate its stability and controllability. The narrower spectrum and better stability of the HGHG, compared to a Self Amplified Spontaneous Emission (SASE) FEL, were observed. Both the second and third harmonic HGHG FEL beams were experimentally characterized using a vacuum monochromator. The pulse energy for both harmonics (133 and 89 nm, respectively) was measured to be about 1 µJ, which is about one percent of the fundamental value at 266 nm.

A two-photon absorption auto-correlator with 100 femtosecond (fs) resolution was developed to characterize the HGHG output pulse length. It was experimentally demonstrated that the HGHG can produce output pulses with lengths from one picosecond (ps) down to 250 fs by varying the seed laser pulse length.

Experiments to investigate a chirped HGHG FEL were also initialized in 2003. The preliminary results are very promising, and the chirped FEL could lead to even shorter HGHS output pulses. The possibility of achieving HGHG output tuning via electron beam energy chirping was discussed by Timur Shaftan of BNL. Brian Sheehy, also of BNL, discussed various FEL output manipulation techniques and their limitations.

DUV-FEL in the Future

One of the main goals of FEL R&D at the DUV-FEL is to continue developing key technologies for a future x-ray FEL that will be based on the HGHG. For example, a cascaded HGHG and a higher harmonic (n>5) HGHG are critical for the realization of the HGHG x-ray FEL. Additionally, multi-laser electron beam synchronization and timing jitter reduction are very important technogies that will enable ultra-fast science at all future x-ray FEL facilties.

Paul Emma of SLAC discussed a technique that uses an emittance spoiler to achieve ultra-short FEL pulses. Computer simulations have shown that, by using a cleverly placed piece of slotted foil, the Linac Coherent Light Source (LCLS) at SLAC will be able to produce brilliant x-ray pulses that are extremely short – a few femtoseconds. This pulse length, which is more than 200 times shorter than the LCLS baseline design, will dramatically increase that facility’s x-ray time resolution, giving scientists the ability to study the movement of matter at atomic scales and observe the structural changes that occur when chemical bonds are made or broken. To test the feasibility of an emittance spoiler at the DUV-FEL, Emma proposed a demonstration experiment. This could reduce the FEL output pulse length by a factor of four.

Beam physics during the electron beam bunch compression was extensively discussed during the workshop. Zhirong Huang of SLAC gave an overview on microbunching instability during electron beam compression and its impact on the future x-ray FEL. The LCLS proposes using a laser heater or superconducting undulator to mitigate microbunching instability. Michael Borland of the Advanced Photon Source discussed the latest developments in electron beam bunch compression simulation. Experimental investigation of microbunching instability and validation of computer predictions will be the major part of beam physics R&D at the DUV-FEL.

To take advantage of the DUV-FEL’s unique capabilities, other possibilities in beam physics and FEL R&D will continue to be explored, such as coherent tera-hertz generation and a femtosecond electro-optical bunch detector.

The workshop ended with a lively discussion on the possible future FEL and beam physics experiments at the DUV-FEL. The workshop participants ranked possible future experiments according to the following criteria:

1. What are likely critical beam physics and FEL experiments for future linac-based light sources? Can the DUV-FEL make an impact?

2. What is the best way to carry out those experiments? How can we take advantage of the unique features of the DUV-FEL?

Acknowledgement
The submitted manuscript has been authored under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

The support from the Brookhaven National Laboratory Director’s Office and the National Synchrotron Light Source made the DUV-FEL Beam Physics and FEL R&D Workshop a great success. The encouragement and support from Steve Dierker, Jim Murphy, and Peter Paul are gratefully acknowledged. The professional support from Angela Bowden, Kathy Loverro, and Eileen Morello also made the workshop possible.

RELATED LINKS: Workshop Website

ARTICLE BY: Xijie Wang

Edited By: Laura Mgrdichian