FELs, nice toys or efficient tools?

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Nuclear Instruments and Methods in Physics Research A 528 (2004) 8–14

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FELs, nice toys or efficient tools?

A.F.G. van der Meer

FOM Institute for Plasma Physics ‘Rijnhuizen’, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands

Abstract

An FEL is an intrinsically interesting device and pushing its performance presents a natural challenge to a physicist.

Nonetheless, the main justification for doing FEL research is of course its potential as a unique, versatile source of

radiation to be employed for something useful. After 25 years of FEL research, one may wonder how efficient these

tools have become. In this paper, I will reflect on this issue from the perspective of 10 years of operation of FELIX as a

user facility.

r 2004 Elsevier B.V. All rights reserved.

PACS: 41.60Cr; 42.62Fi; 82.80Gk; 82.50Bc

Keywords: Free-electron lasers; Infrared; Spectroscopy

1. Introduction

Whereas the (technological) challenges of a freeelectron laser probably present the main personalmotivation for the people active in the field, itspotential as a unique and versatile source ofradiation that could be used for a variety ofpurposes, has always been the main justificationfor the research on FELs. More than 25 years afterthe ‘invention’ and first demonstration of opera-tion of an FEL [1], it seems appropriate to askourselves how far we have come on the way torealizing this potential. In this paper, I will addressthis question with the experience gained in 10 yearsof operation of the IR User Facility FELIX. I willtherefore start with an evaluation of the efficiency

ddress: [email protected]

n der Meer).

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.nima.2004.04.008

of FELIX as a tool for scientific research, beforeattempting to generalize to other (types of) FELsand applications.

2. The IR user facility ‘FELIX’

FELIX consists of a normal conducting,12–45MeV linear accelerator that alternativelydrives a far-IR FEL with partial waveguide thatcovers the spectral range from 25 to 250 mm, or amid-IR FEL with a spectral range from 5 mm(3 mm on 3rd harmonic) to 40 mm. The generallayout of FELIX is shown in Fig. 1. As usual forthis type of linear accelerator, the output consistsof bursts (macropulses) of micropulses. Thespacing between the micropulses is either 1 ns(1GHz-mode) or 40 ns (25MHz-mode), the lattercorresponding to the roundtrip time of the 6m

d.

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Fig. 1. General layout of FELIX showing the two beamlines for far- and mid-infrared generation.

Table 1

Characteristic parameters of FELIX

Tuning range 3–250mm (3300–40 cm�1)

Rapid tuning >1 octave in a minute

Micropulse rep. rate 1 ns, 40 ns, single pulse

Macropulse rep. rate up to 10Hz

Micropulse energy 1–50mJMicropulse power up to 100MW

Macropulse duration 4–8msBandwidth (adjustable) 0.4–6% (transform limited)

Polarization >99%

Beam quality near diffraction limit

Beam hours >3000

Unscheduled downtime o5%

A.F.G. van der Meer / Nuclear Instruments and Methods in Physics Research A 528 (2004) 8–14 9

cavity. Using a transient optical switch, it ispossible to slice a single micropulse out of thepulse train with an efficiency of more than 50%.The main characteristics of the output are listed inTable 1. The facility is operated in two-shift mode,5 days a week, providing more than 3000 h ofbeam time for user experiments.

3. User experiments at FELIX

Presently, these experiments fall predominantlyin one of two classes: relaxation phenomena incondensed matter or spectroscopy of gas phase

species, (bio)molecules and clusters, either neutralor ionized. Experiments in the first class will usethe 25MHz or single-micropulse mode in view ofthe relaxation times involved, not only of theprimary process but also of the temperaturetransient due to the energy absorbed. Especiallyin the case of biological samples the latter isusually more limiting. The second class is char-acterized by (very) low absorption cross-sectionsand, because detection is based on dissociation orionization of the species, typically a stronglynonlinear dependence of the signal on the laserfluence, implying the use of the 1GHz-mode.For illustration, two examples of either class will

be discussed briefly. The first example of the firstclass concerns the relaxation of the stretch vibra-tion of hydrogen and deuterium in amorphoussilicon. Hydrogen is often used to passivate thedangling bonds, thereby enhancing the character-istics of this technologically important material,but the beneficial effect strongly reduces with time.Recently, it was found that this aging effect ismuch smaller when deuterium is used instead. Theexperimental result for hydrogen is given in Fig. 2[2] and for deuterium in Fig. 3 [3]. Whereas thedecay for deuterium is clearly single-exponential, itis not for hydrogen. This observation can berelated to the striking difference in energy decay

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Fig. 2. Relaxation of the Si–H stretch vibration in amorphous

Si: time dependence (upper panel) and temperature-dependent

rate (lower panel) [2].

Fig. 3. Relaxation of the Si–D stretch vibration in amorphous

Si: time dependence (upper panel) and temperature-dependent

rate (lower panel) [3].

A.F.G. van der Meer / Nuclear Instruments and Methods in Physics Research A 528 (2004) 8–1410

channel for both cases: whereas the relaxation forH occurs mainly via an almost resonant energytransfer to the bending mode at the same site andis therefore a local event, the relaxation of Dprimarily involves phonons, so modes of the bulk.This non-localized energy release is now believedto be the main reason for the strongly reduced‘aging’ effect.In the second example, far-IR radiation of

FELIX was used to probe the time-dependentexciton density in a GaAs quantum well afterexcitation across the bandgap with a synchronizedpump laser. The result was compared to aconventional measurement, i.e. by monitoring theluminescence of the sample in the spectral intervalassociated with the presence of excitons (Fig. 4) [4].

Whereas there is a prompt absorption signal,resulting from 1s–2p transitions of the boundelectron–hole pairs, when the sample is excitedvery close to the bandgap, there is no signal if theexcitation is well above the bandgap. In thelatter case, the conventional method does showa rapid signal though. This result seems toprovide experimental evidence for the possibilityof exciton-like emission without excitons beingpresent. Recently, this behaviour was predictedtheoretically and is attributed to a correlationof the unbound electron and hole population.The rise time of the photo-luminescence reflectsthe time for the electrons and holes to relaxinto the low-lying k-states at the bottom of theband.

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Fig. 4. The transient transmission change of the FIR probe

pulse is shown in the upper panel for three cases: exciting at

resonance (middle), 36meV below resonance (bottom) and

62meV above resonance (top). In the lower panel the transient

photoluminescence at the ‘exciton peak’ is plotted while exciting

62meV above resonance [4].

Fig. 5. The infrared spectra of Ti8C12, Ti8C11 and Ti13C14clusters measured using IR-REMPI as a function of the FELIX

wavelength (three upper traces) [5]. The insets show the

previously reported structures. As a comparison the lowest

trace shows the EELS spectrum of bulk TiC(1 0 0).


As a first example of an experiment of thesecond class, a measurement of the IR spectrum oftitanium-carbide clusters is shown in Fig. 5. AYAG-laser is used to ablate material from atitanium rod and subsequently a gas-puff, in this

case methane, is applied, resulting in a molecularbeam containing titanium-carbide clusters.Previous work, using UV-lasers to ionize these

clusters, showed that clusters consisting of 8titanium and 12 carbon atoms were particularlystable, as well as clusters with, respectively, 14 and13 atoms. Due its high fluence, it proved to bepossible to ionize the clusters with the IR beam ofFELIX and record an infrared spectrum [5]. Avery strong signal around 8 mm, characteristic for aC–C stretch vibration, is present for the Ti8C12cluster and absent for the Ti14C13 cluster, in

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Fig. 6. IR-spectra of protonated water dimers [7] and p-methyl

ether dimer, p-ethyl ether dimer and p-diglyme, respectively [8].


accordance with the ordering of the C-atoms in thestructures that had been proposed (see insets inFig. 5). The signal around 20 mm for the Ti14C13cluster shows a great similarity with a spectrumrecorded with EELS on bulk TiC, suggesting thatit is indeed a nano-crystal. Moreover, this spectralfeature is now believed to be the source of ahitherto unexplained strong emission around postasymptotic giant-branch stars [6]. In a similarmanner, by looking for dissociation rather thanionization, the IR spectrum of ions can berecorded. Usually the ions are confined in a trapin order to increase the density. Recently, the IRspectrum of two water molecules bridged by aproton was measured using a tandem ion trapseparated by a mass selector (Fig. 6) [7]. Using anFTICR high-resolution mass spectrometer, protonbridging of larger molecules was investigated [8].The similarity of the main features of the spectrashows that these are really characteristic forproton bridging.

4. Performance assessment

From the above, I hope it is obvious thatFELIX can be used for high-quality research, butwhat about productivity? A typical 3rd generationsynchrotron has some 40 beam lines and an annualoutput of some 500 papers, whereas the number ofuser papers for the FELIX facility is only 20–25per year. Given the difference in investment andrunning costs, typically a factor of 15, the relativeoutput of FELIX however comes close to that of asynchrotron. So it seems justified to conclude thatan FEL can be an efficient tool for scientificresearch. On the other hand, the total number ofuser papers that have been produced at FELfacilities is substantially less than twice the annualproduction of a single synchrotron! So, on thewhole, the impact of FELs on scientific researchhas until now been almost negligible and does notyet justify the efforts put into their development.

5. Outlook

May we expect this balance to improve in thefuture, either by more and high-impact scientificresearch applications or applications in differentareas, for example industrial processing? Follow-ing the successful demonstration of a 1 kW averagepower IR FEL, based on superconducting accel-erator technology combined with energy recoveryat Jefferson labs, very high average power(>100 kW) units are now under study. At thesepower levels, military applications once againappear at the horizon, but history-based skepti-cism still seems justified. High average powerwould in principle also greatly increase the numberof applications in industrial processing, and asuccessful application in a billion dollar marketcould already affect the balance. An industrialapplication that is clearly economically sound stillneeds to be demonstrated though. Returning tothe use of FELs for scientific research, it should benoted that to my opinion the future of IR FELs isnot very bright. Primarily because of the rapiddevelopment of alternative sources over the past10 years, especially those based on today’s workhorse, the Ti:Sapphire laser. Table-top sources

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Fig. 7. General layout of the FELICE cavity including two intra-cavity setups: for IR-REMPI and IR-MPD experiments a high-

resolution FTICR mass spectrometer and a molecular beam machine.


based on parametric generation are very competi-tive in the mid-IR, especially in the field ofrelaxation studies for which the pulse structure isoften better suited for the time scales involved. Inthe far-IR, the micropulse energies available fromFELs are still unchallenged and applicationsrequiring high peak powers to pump the systemfar from equilibrium will also in the foreseeablefuture require the availability of an FEL. If thefar-IR radiation is just used for probing, it ishowever not power that counts but sensitivity.Broad-band, ‘single-cycle’ THz pulses can begenerated by focussing a short-pulse laser, againusually a Ti:Sapphire laser, onto a semiconductorsuch as GaAs. Typically these pulses have ratherlow energy, at the pJ level, but by using detectionschemes that are based on (e.g. electro-optic)sampling with part of the pulse used for thegeneration, a very high sensitivity has beenobtained: about 1 part in 108 for a 1 s integrationtime [9]. By varying the delay between the THzpulse and the sampling beam, very high timeresolution can also be obtained. As the detection isoften sensitive to the electric field of the THz pulserather than its intensity, it has the added advan-tage that also phase information of the differentfrequency components within the bandwidth isobtained. As a matter of fact, the use of an FELfor the experiments given above as examples of the

first class can no longer be justified. FELs willmost likely continue to have an advantage forexperiments of the second class. This field ofresearch is rapidly expanding at our facility andrecently funding was obtained for the constructionof a third beam line [10] that will allow this class ofexperiments to benefit from the much higherpowers present within the FEL cavity. A schematicof FELICE, the Free Electron Laser for Intra-Cavity Experiments, that should cover the wave-length range from 3 to 100 mm, is shown in Fig. 7.Nonetheless, the niche for IR FELs has decreasedquite significantly compared to when FELIXstarted operation and even though there willcertainly be a need for a number of FEL-basedIR facilities in the future, it is not realistic toexpect an increase of the impact of FELs in the IR.Also in this respect the recent progress madetowards very short-wavelength operation of FELsis of course very encouraging for the FELcommunity. But based on the experience in theinfrared it should be realized that the success ofX-FELs is not necessarily guaranteed merely bytheir realization.

References

[1] L.R. Elias, et al., Phys. Rev. Lett. 36 (1976) 717.

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[2] M. van der Voort, et al., Phys. Rev. Lett. 84 (2000) 1236.

[3] J-P.R. Wells, et al., Phys. Rev. Lett. 89 (2002) 125504.

[4] R. Chari, et al., Phys. Rev. Lett., submitted for

publication.

[5] D. van Heijnsbergen, et al., Phys. Rev. Lett. 83 (1999)

4983.

[6] G. von Helden, et al., Science 288 (2000) 313–316.

[7] K.R. Asmis, et al., Science 299 (2003) 1375.

[8] D.T. Moore, et al., Chem. Phys. Chem., (2004) in press.

[9] A. Leitenstorfer, et al., Physica B 314 (2002) 248.

[10] B.L. Militsyn, et al., Nucl. Instr. and Meth. A 507 (2003)

494.

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FELs, nice toys or efficient tools?