Report from the workshop

The Scientific Case of an Infrared
Free Electron Laser at MAX-lab

Lund 26-27 March, 1996

Sverker Werin 
Lund 1996

MAX-lab
Lund University
P.O. Box 118
S-221 00 Lund
E-mail: sverker.werin@maxlab.lu.se

Introduction and background  
Workshop layout  
Workshop summary 
	FEL workgroup 
		Basic specification 
		The accelerator 
		The optical cavity 
		Other topics 
	Applications workgroup 
The program 
List of participants 
The talks; copies of the overheads (not included)

During March 26-27 1996 a workshop to present and discuss the scientific case of an infrared free electron laser at MAX-lab was held. A number of foreign and Swedish speakers presented the FEL technique and facilities followed by overviews of IR research in chemistry, physics and medicine. Finaly two workgroups discussed both the FEL system and scientific applications. This is a report of the workshop, its background and actual results.

Introduction and background

As a national facility MAX-lab is charged with the responsibility to supply state-of-the-art radiation over a broad energy region to a diverse user community. In this context MAX-lab has started to investigate the case of an Infrared Free Electron Laser (IR FEL). Such a lightsource should be an expansion and a complement to the existing two storage rings for synchrotron radiation already in operation at the laboratory.

Initial studies on the possibilities and basic performance of an FEL built at MAX-lab demonstrated promising capacity in the range (approx.) 5 - 40 mm. The present workshop is the next step in the feasability study addressing the following questions:

The scientific case.

• What experiments can be done at an IR FEL?
• What wavelengths are interesting?
• Can this be done better by other techniques?
• What performance (spectral range/intensity/time structure/...) is needed for the source?

The user case:

• Are there users interested?
• Do potential users have the capability to explore the source?

The machine case:

• What are the key parameters for the project?
• Is MAX-lab and the MAX accelerators a realistic base for this project?
• Can a racetrack microtron (RTM) be successfully used as accelerator/injector?

With these questions in mind a workshop aimed mainly at interested and qualified users or potential users of a MAX-lab FEL was organised with invited speakers representing several categories:

• Users (several)
• User IR FEL facility/ies in operation abroad
• FEL experts
• Laser experts

The main objective of the workshop was to explore the scientific case which is regarded as the firm and absolutely necessary base for any continuing activities. It was also the purpose of the workshop to give the possibility for potential users and other interested to get an overview and introduction to the FEL field. At the same time MAX-lab would benefit from some expert input on the technical aspects of the project would be received.

Workshop layout

The workshop was organised to first give an introduction to the FEL technology followed by a presentation of a FEL user facility in operation (FELIX, Nieuwegein, Holland). To look ahead at colleagues/competitors a presentation of a planned IR FEL project (FERMI, Trieste, Italy) was invited and the first section finished by a presentation of an FEL facility in construction aimed at shorter wavelengths (TESLA, Hamburg, Germany).

The second section dealt with current applications and work in the IR range and outlooks into the future with a possible IR FEL at MAX-lab. Presentation of scientific opportunities in chemistry, physics and medicine were given.

Then an overview of the existing competing laser sources in the actual wavelength was given and, finally, a brief update was presented on the accelerators and FEL plans at MAX-lab.

The whole program is given below (chapter 4).

Workshop summary

The major time of the workshop was spent with the seminars (copies of the overheads from the presentations are found in chapter 0). In short the invited speakers made the following points:

Dr. Mike Poole started out with a history overview of the FEL telling that the first FEL operated in 1977. The theory of FELs started out in quantum mechanics (Madey), but later it was realised that classical descriptions are quite alright. After discussing the main principles, different operating regimes and pulse structures he made an overview of FELs in the world. Poole here showed around 25 existing facilities and another 25 proposed, most of these in the US. He mentioned two reviews made in the field "Levy" (US) and "TMR" (EC by M. Eriksson, M. Pooles et al.). They point onto a.o. a"niche philosophy" stating that the FELs have two niches at 10-1000 mm and 10-200 nm, areas not well covered by other sources. The research and development should fall in: compact IR sources, High average power IR and XUV technology. Poole made a quick overview of the user facilities in IR in operation: CLIO (Paris), FELIX (Holland), Vanderbilt, Stanford, Duke and UCSB (all US). He finished by giving the subjects of IR FEL applications (in order of number of applications): Solid state physics, Surface physics/chemistry, Molecular physics/chemistry, IR instrumentation, Biomedicine and Atomic physics.

Dr. Alexander van der Meer talked on Experiments and Experiences at FELIX. The FELIX facility is operated by 10 persons (1 scientific staff, 6 technicians and 3 station masters [post docs]) with at running kost of 2500 kf/year. Beamtime is allocated by a PAC. van der Meer welcomed swedish scientist to FELIX, though so far none hade come from Scandinavia. In beamtime Solid state physics takes more than 50% while Surface physics/chemistry, atomic and molecular physcics and biomedicine follow. The FELIX system is built around two LINACs and two undulators and thus covering 5 to 100 mm with peak powers of 0.5 - 20 MW. He continued to discus some experiments performed at FELIX: IR spectroscopy of PABA molecule, induced coalescence of C, Sum frequency generation on surfaces a.o. He finished by stating some important points, found in the experince from FELIX: beamstability is very important (amplitude, wavelength, poining), on-line chracterization is needed (wavelength, shape energy), user control of beam characteristics, flexible rep rate and in-house user teams.

Dr. Richard Walker gave a description of the FERMI project in Trieste, Italy. In the motivation for the project he mentioned that the demand for beamtime at European IR FEL facility is exceeded by 2. He pointed on that by building a FEL at a Synchrotron radiation source one can beneficially use the two sources in the same experiements (Pump-probe, surface physics). A future possiblity is to use the 1 GeV LINAC an building FELs in the VUV/Soft Xray region.

FERMI is a collaboration between Sincrotrone Trieste, ENEA-Frascati, INFN-Frascati/Naples and Univ. of Naples. In 1995 a concenptual design report was published. The projcet is divided into 3 steps, and right now awaits a "go ahead" for the first step. The FEL will in step 2 operate at 5 - 250 mm, using the LINAC already present at ELETTRA and undulator, optical cavity and beamline from an "old FEL" project at Frascati. After a technical description of the system Walker gave a review of the scientific case as investigated by the colaboration. Existing programs at other facilities: FEL-Internal Photo-emission techniques, Multiphoton spectroscopy, Near-field IR microscopy; Under implementation at other facilities: Surface photovoltage, surface photoluminiscence, two-photon photoemission; Other: surgical applications, cluster and ionic beam processes, dynamics of molecule-surface interaction, photostimulated chemistry (desorption, dissociation etc.).

Prof. Bernd Sonntag talked on the VUV FEL at the TESLA Test Facility in Hamburg, Germany. The principle of the FEL is different from the many other schemes as it is a SASE FEL (Self Amplified Stimulated Emission). A process where no optical cavity is needed but the spontaneous emission in the beginning of a long (30 m) is amplified in one passage. The FEL will use a 1 GeV electron beam with peak current of 2490 A reaching a 6.4 nm wavelength. In a first step though, only a 300 Mev, 600 A electron will be used to reach 70 nm. Sonntag continued, after a technical overview, into the areas of research: Transmission VUV-microscopy, VUV-scanning microscope, triple ionization, photoionisation of selected clusters, resonant multiphoton photoelectron-spectroscopy of mass-selected clusters, photochemistry of surfaces, photoelectron diffraction.

Prof. Villy Sundström gave an overview of Time resolved IR spectroscopy in chemistry and some steps into biology. Chemistry: Vibrational relaxation in small molecules in solution, Vibrational dynamics, Clusters, Vibrational energy transfer to surfaces, Photochemistry and biology: Cytochrome C oxidase, Protein - ligand dynamics. Sundström also gave some "FEL requirements from chemists": 0.5 -5 ps variable pulse width, 1-10 mJ pulse energy, tunable between 5 to at least 20 mm a.o.

Dr. Roger Ryberg talked on IR spectroscopy in physics, mainly surface physics. He elaborated on sum frequency generation, Direct lifetime measurements and excited state spectroscopy. The openings with an IR FEL should be in lifetime studies, fast spectroscopy and vibrational pumping.

CI. Christian Sturesson made an overview of the use of lasers in urology in a project in Lund. The important factor in "destroying" urnial stones is to find a proper wavelength window in which there is low absorption in water and good absorption in the material one wants to treat. Sturesson described the techniques used in the experiments.

Prof. Sune Svanberg started with a summary of different IR laser sources, diode lasers, optical parametrig generatiors(OPG), gas lasers, different frequency generation a.o.Svanberg showed that in principle one can cover most of the IR region up to 50 mm. But the systems do not carry the possiblities of an FEL: tunability in wavelength, pulse length, power at the same time. He envisaged that in certain wavelength regions an FEL would be very fruitful in finding the proper operation conditions, and by this knowledge a more traditional laser could be constructed with given data.

During the second day a few hours were set aside for workgroup discussions. Two work groups were formed. One dealing with the free electron laser itself, and one with the applications, mainly in physics. After lunch the groups joined to report and summarise their work.

FEL workgroup

Chairman: Mike Poole, Daresbury, UK

The focus of the discussion was to find a basic set of parameters covering the requirements of an FEL. It was pointed out that there might be additional requirements when the accelerators and systems have to operate in other modes (injection, pulse stretcher ) than the one given here.

Basic specification

The group started to discuss the wavelength region which of course will be dictated by the scientific requirements. As an illustrative working example an FEL covering 10 - 100 mm was chosen. In this wavelength region the FEL will be able to provide high power, easy tunability and a smooth power spectrum. Below 10 mm powerful conventional laser sources are available, but the tunability might still make the FEL useful below 10 mm. By choosing 100 mm as the longer wavelength one FEL will be able to cover the whole region and no waveguides will be needed inside the undulator structure.

The pulse energy should reach 10 microJ/pulse (extracted laser pulse).

The accelerator

This choice then led to an accelerator of 40-50 MeV, with capabilities to at least run down to 25 MeV. This would be enough to reach also 100 mm. A "conventional" but high quality LINAC will do the job. The very low emittance produced by photocathods is not needed in the IR-region, and thus there is no need for that kind of gun. The peak electron current should be 10-100 A, preferably at the high end.

The discussion continued into the domain of pulse structures. A macropulse of at least 10 ms will be needed, which will give an FEL pulse of ~5 ms. A longer macropulse can be useful in medical applications (e.g. ablation), but 10 ms was chosen as it is a "standard" value, and enough for other applications. This pulse should be generated at 10 Hz, but commissioning of the FEL will be much easier if the frequency has some tunability.

A short note on S-band (3 GHz) or L-band (1 GHz) system was taken. The S-band (3 GHz) system was chosen as this will give a shorter electron micropulse, around 5 ps, than the L-band and allow for an FEL pulse between 0.5 and 5 ps. (variation by detuning of the optical cavity).

Regarding the question of a Racetrack microtron v. a LINAC the immediate answer was that there is a need for peak currents approaching 100 A (FELIX achieves 50 A). The racetrack microtron can not achieve this, which leaves the LINAC as the only solution.

A 100 MeV LINAC will in this context fulfill the critera for the broad spectral region of the IR FEL. On the other hand a 500 MeV LINAC will provide: a very good IR FEL injector plus separate injection into MAX I and MAX II plus options for an UV FEL (following Prof. Sonntags lines).

The optical cavity

The FEL pulses should match the MAX-II pulses in time to allow for pump-probe experiments using both sources simultaneously which in turn gives limitations on the length of the cavity. The cavity should also be long enough for a fairly large pulse separation, if needed. The choice fell on a 7.5 m cavity giving maximum 50 ns pulse separation and a match with a 6 bunch fill pattern in MAX II.

Other topics

Regarding the electron beam quality the energy spread of the electron beam is very important and should stay at sE= 0.2% (~0.5 % FWHM)

The undulator system should not be particularly difficult with parameters in the neighbourhood of:

overall length           2 m          

undulator period         0.05 m       

gap                     20-30 mm      

The undulator should be split in two pieces, allowing for 2-colour operation.

In principle a variable polarising undulator could be used to provide any polarisation. It was recommended though, not to go for this option unless there is a very strong user demand, as there are good possibilities to create the necessary polarisation outside (after) the FEL.

A wavelength stability of dl/l ~0.1% is needed on a "pulse to pulse" basis (10 Hz) but

can be difficult to achieve. The main problem lies in peak current variations into the accelerator system. Due to the different beamloading this will create in the accelerating structure the electron energy will change and thus the output wavelength.

Amplitude variations in the FEL-pulse are of less importance, and fairly large numbers (~1%) are accepted.

Linewidth narrowing schemes can be applied but were judged to be of no immediate interest.

Some points were given as "important to think about":

• The technology requirements discussed above are well known, and no technical risks can be foreseen. Still a very careful design of the system is needed and is a challenge in accelerator technology.
• The layout to fit the machines into MAX-lab is not trivial and should be thought about.
• Electron beam transport can be complicated and the transport should be isochronous, or with "variable isochronity".
• The IR-radiation, though, can easily be transported over fairly large distances (several 10 m).

Questions and comments:

• What are the power levels? This system should provide multi-MW peak power and from 10 mW to 1 W average power, depending on the number of pulses in the optical cavity. At least 1/2 of these values should be retained over the whole wavelength region. The photon spectrum is more or less flat over the whole tuning range.
• Tuning; complexity and speed? The tuning of wavelength is done by changing the undulator gap. A large part of the wavelength region should thus be covered in a couple of seconds, more or less like "turning a knob".

Applications workgroup

Chairman: Roger Ryberg, Chalmers, Göteborg.

The workgroup dealt with applications of FEL. Both physicists and chemists had joined the group, though the stress came to lie on participants from the "physics field".

First a few areas of interest were defined:

1. Quantum dots and wires.
2. Clusters (nuclear, metal)
3. Phonons (High Tc, polymers)
4. Surface physics

The demands on an FEL were discussed, but more in general terms than specific technical demands on the FEL and accelerator system.

• There should be possibilities to operate a microscope (area 1,2,3 Htc).
• A frequency range of 5 - 50 mm is the primary interest. The 5 mm limit might possibly be a little too high energy (short wavelength), but some people would ask for a wider range: 2- 300 mm.
• Lifetime studies asks for pulse lengths >= 1 ps (up to 10 ps).

The group pointed out some specific projects that should be interesting to perform at an IR FEL facility.

• Study of molecules on small water clusters, and bridge over to molecules in solutions.
• Lifetime studies
• Broad bandwidth FEL as a lightsource for FTIR (Fourier Transform InfraRed studies) above 100 mm.
• Phonons in small High Tc-crystals and clusters.
• Different types of vibrationally excited states.
• Combined FEL + SR studies. Spectroscopy where you get the synchronisation for free.

As an important point the group took up the question on "where to go from now", resulting in some clear ideas and proposed actions:

• Identify areas of interest and contact specific researchers.
• Organise a meeting with these + active scientists from abroad.
• The project must have some researchers who are willing to push specific projects.

A few questions and comments followed, and among them were:

• Frequency stability of several 0.1% could be a problem, as that is "far from a fixed frequency".
• If an upper limit is put at 50 mm the phonon applications fall out, but there will not be much impact on surface physics.
• To meet all the scientific requirements more than one IR FEL has to be built (the FEL working group treated 10-100 mm in detail). Thus spectral regions have to be defined and priorities set.

The program

Tuesday 26 March

Introduction

THE FEL

The FEL-lightsource and the future; Dr. Mike Poole, Daresbury, GB

Experiments and Experiences at the User Facility FELIX; Dr. Alexander van der Meer, FELIX, Nieuwegein, Holland

The FERMI Project; Dr. Richard Walker, ELETTRA, Trieste, Italy

A VUV FEL, the TESLA project at DESY; Professor Bernd Sonntag., DESY, Hamburg, Germany

SCIENCE AND USERS

Time resolved infrared spectroscopy in chemistry; Professor Villy Sundström, Lund,

Scientific use of IR radiation in physics; Dr. Roger Ryberg, Chalmers, Göteborg

IR thermo therapy of benign prostatic hyperplasia and related medical applications of IR radiation; CI & MK. Christian Sturesson, Lund.

Wednesday 27 March

The alternative IR laser sources; Professor Sune Svanberg, Lund

MAX-lab

The accelerators at MAX; professor Mikael Eriksson, MAX-lab, Lund

A MAX-lab FEL; Dr. Sverker Werin, MAX-lab, Lund

WORKING GROUPS
A) A MAX-lab FEL facility B) Science and demands in physics and chemistry

DISCUSSION and SUMMARY