Two-Chicane Compressed Harmonic Generation of Soft X-Rays Page: 1 of 4
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TWO-CHICANE COMPRESSED HARMONIC GENERATION OF SOFT
D. Ratner t (Stanford University, Stanford, California)
Z. Huang, A. Chao (SLAC, Menlo Park, California)
We propose a simple single-stage scheme to produce
fully coherent 3nm radiation. Seeding an electron bunch
prior to compression simultaneously shortens the laser
wavelength and duration, and increases the modulation am-
plitude. The final X-ray wavelength is tunable by control-
ling the compression factor with the RF phase. We pro-
pose a two chicane scheme that allows for nearly arbitrary
modulation amplitudes, extending the method to photo-
cathode beams. We also show that transportation of fine
compressed modulation structure is feasible due to a can-
celing effect of the second chicane.
A Free Electron Laser (FEL)  can theoretically pro-
duce fully coherent X-rays, a promising tool for the fields
of physics, chemistry and biology. Current X-ray FELs
in use or under construction rely on self-amplified spon-
taneous emission (SASE) [2, 3]. SASE FELs can reach
the hard X-ray region, but are saddled by long saturation
lengths and poor longitudinal coherence. In contrast, FELs
'seeded' by optical or UV lasers promise full coherence and
shorter FEL lengths. At present, the leading seeded FEL
scheme is high gain harmonic generation (HGHG) [4, 5].
However, single-stage HGHG suffers from noise during the
high-energy modulation, requires high laser power (expen-
sive and degrading to the FEL), and is limited to around
20nm . Multiple-stage HGHG can reach shorter wave-
lengths, but is expensive and technically challenging.
Seeding the electron beam prior to bunch compression
is an alternative approach [7, 8, 9]. Dispersion from the
compression smears out longitudinal modulation, but the
modulated structure remains imprinted in phase space and
can be revived later. Echo Enhanced Harmonic Generation
(EEHG) is a promising proposal to manipulate such hidden
structure to produce high harmonics . We propose an
alternative simple one-stage seeded FEL that exploits this
beam memory to recover harmonics down to 3 nm, well
within the water window.
Our scheme uses two dispersive sections to first com-
press and then bunch the modulation. Starting from the
gun, an accelerator section brings the beam to energy Ea,
while adding a linear chirp, h. A laser then modulates the
beam energy by AL cos(kLz). A dispersive section, Ra6,
* Work supported by Department of Energy contract DE-AC02-
simultaneously compresses the bunch length by a factor of
a 1/(1 + R "2lh), while also strongly over-bunching
the laser modulation. Another accelerator section flattens
the beam with a second chirp, -ahz, while increasing the
energy to Eb gEa. Finally, a second dispersive sec-
tion with effective opposite sign, R56 -R56 g/a
g/(La2kL), unwinds the over-bunched laser modula-
tion, recovering a maximally bunched beam at wavevector
akL while simultaneously suppressing second order effects
in the accelerator and first chicane. The entire process,
which we will refer to as Compressed Harmonic Genera-
tion (CHG), is summarized in Fig. 1.
e Gun RF E Laser Modulator Rs5 RF2, E2
e-Gun RF1, E Laser Modulator Rsb RF2,E R
Figure 1: Diagram of CHG scheme. a) The first accelera-
tor section gives a linear chirp, h. After modulating with
a laser, the first dispersive section, Ra6), compresses the
beam and over-bunches the modulation. A second RF sec-
tion accelerates and cancels the chirp of the first section.
The final dispersive section unwinds the over-bunching. b)
Operating R(" in over-compression rotates the electron
beam head-to-tail, allowing us to use a chicane for R56
CHG offers several advantages over alternative seeding
methods. First, the bunch compressor reduces the laser
wavelength by a factor of a. Seeding from a 157 nm F2
laser with a 10 gives 3 nm radiation at the 5th har-
monic (50th harmonic of the original seed laser). Second,
the RF phase controls a, so changing the phase tunes the
final wavelength. Third, the bunch compressor amplifies
the laser modulation by the compression factor, reducing
the required laser power by a2.
CHG also requires relatively few components. The first
chicane doubles as the bunch compressor (required to in-
crease current for all FELs), so we need only one addi-
tional modulator and chicane, the same as for single-stage
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Ratner, Daniel; Huang, Z. & Chao, A. Two-Chicane Compressed Harmonic Generation of Soft X-Rays, article, July 30, 2010; [California]. (digital.library.unt.edu/ark:/67531/metadc1014085/m1/1/: accessed November 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.