8/3/2019 Kazuhisa Nakajima- Towards a table-top free-electron
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NEWS & VIEWS
92 nature physics| VOL 4 | FEBRUARY 2008
|www.nature.com/naturephysics
COMPACT X-RAY SOURCES
Towards a table-top free-electron laser
Kazuhisa Nakajimais at the High Energy Accelerator Research
Organization, 1-1 Oho, Tsukuba, Ibaraki
305-0081, Japan.
e-mail: [email protected]
S
ynchrotron radiation sources havebecome an indispensable tool in
a
wide range o disciplines, includingphysics, biology, materials
science,chemistry and medicine. Te reasonthey are so useul is the
high intensityo X-rays they produce generatedwhen the path o a beam
o electronsmoving at relativistic speeds is bent by aperiodic
magnetic eld in comparisonwith other X-ray sources. Such utility
isexpected to grow still urther with thedevelopment o X-ray
ree-electron lasers(FELs) devices that operate on a
similarprinciple to synchrotrons but whichproduce X-rays in
intense, coherent,
emtosecond bursts, enabling thedynamics o chemical reactions,
materialsand biomolecular systems to be studiedwith unprecedented
spatial and temporalresolution1. Several X-ray FEL acilitiesare
currently under construction24, ata cost o the order o US$1 billion
each;the particle accelerators that produce themulti-GeV electron
beams on which theiroperation relies span many kilometresin length.
But on page 130 o this issue 5,Schlenvoigt and colleagues describe
anapproach that could substantially reduceboth the size and cost o
synchrotron and
FEL X-ray sources, through the use oelectron beams produced by a
laser-drivenparticle accelerator6.
Although still in their inancy,great progress has been made in
thedevelopment o so-called laser-wakeieldparticle accelerators.
hese devices usethe immense electric ields producedat the ocus o
modern ultra-high-intensity lasers to accelerate electrons,protons
and ions over distances o justcentimetres thousands o times
shorterthan a conventional particle accelerator.In their set-up,
Schlenvoigt et al. ocusthe light rom a 5-W laser pulse into
a 2-mm-wide gas jet. he interaction othe laser with the jet
produces a beam oelectrons with a peak energy o between5575 MeV.
Directing this beam into a1-m-long undulator which consists o
a series o alternating magnets causesits electrons to wiggle
back and orth,transverse to the beam direction,producing light at
the red end o thevisible spectrum (with wavelength inthe range o
950550 nm). he authorsthereore provide the irst demonstrationo the
production o resonant-likesynchrotron radiation rom a
laser-generated electron beam. he results oseveral runs o their
experiment showthat the emission wavelength scales withbeam energy
just as theory predicts,suggesting that the generation o
muchshorter wavelengths by this approach
should be relatively straightorward. Byextending the length o
the undulator to3 m, and eeding it with a more energeticbeam such
as the 1-GeV, 30-pCbeams recently demonstrated in a 3-cm
capillary laser-plasma accelerator7
(seeFig. 1a) it should soon be possible toreach 3 nm in the
sot-X-ray range, ata peak brilliance comparable to that oeven the
largest modern synchrotronradiation sources.
Once the easibility o a laser-drivensot-X-ray source is
achieved, the nextstep will be to extend this approach tothe more
ambitious task o constructingan FEL. X-ray FELs rely on a
sel-ampliication o spontaneous emission(SASE)8, where coherent
radiationbuilds up in a single pass through theinteraction o
spontaneously emitted
Synchrotron radiation generated using an electron beam rom a
laser-driven accelerator
opens the possibility o building an X-ray ree-electron laser
hundreds o times smaller than
conventional acilities currently under construction.
102
Peakbrilliance[Phot./(sec.mrad2.
mm2.
0.1
%b
andw.)
]
1019
1021
1023
1025
1027
1029
1031
1033
1035
103 104 105 106
Photon energy (eV)
Large-scale
X-ray FELS
FEL
1-GeV beam
3-m undulatorRadiation source
Large-scale synchrotron
radiation source
40 TW, 30 fs
laser pulse
Focusing
magnets
Coherent X-ray
radiation
3 m
1GeV
electron
beam
Undulator
magnet
3-cm capillary
plasma
accelerator
Figure 1 The path to X-rays on a table top. a, By combining the
ability to produce 1-GeV electron beams rom a
3-cm-long capillary laser wakefeld accelerator recently
demonstrated by Leemans et al.7 with the synchrotron
radiation scheme demonstrated by Schlenvoigt et al.5, a compact,
high-brilliance source o coherent X-rays couldsoon become a
reality. b, Peak brilliance o undulator synchrotron radiation
sources as a unction o photon energy.
Data rom re. 3. Taking the beam parameters reported by Leemans
et al. (energy o 1 GeV, and an electron bunch
length and charge o 10 s and 30 pC), a high-brilliance source
comparable to existing large-scale synchrotron
radiation should be readily achievable. In the FEL regime, such
radiation could be amplifed by many orders o
magnitude, to levels o brilliance similar to kilometre-scale
FELs24 currently under construction.
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