Proceedings of IBIC2012, Tsukuba, Japan

TUPA47

MIDDLE-INFRARED PRISM SPECTROMETER FOR SINGLE-SHOT
BUNCH LENGTH DIAGNOSTICS AT THE LCLS∗

Abstract
Modern high-brightness accelerators such as laser
plasma wakeﬁeld and free-electron lasers continue thedrive to ever-shorter bunches. At low-charge (< 20 pC),
bunches as short as 10 fs are reported at the Linac Coherent Light Source (LCLS) [1]. Advanced time-resolved diagnostics approaching the fs-level have been proposed requiring the support of rf-deﬂectors [2–4], modern laser systems, or other complex systems. Though suﬀering from aloss of phase information, spectral diagnostics remain appealing by comparison as compact, low-cost systems suitable for deployment in beam dynamics studies and operations instrumentation. Progress in mid-IR imaging and detection of the corresponding micrometer-range power spectrum has led to the continuing development of a singleshot, 1.2 - 40 micrometer prism spectrometer for ultra-shortbunch length monitoring. In this paper we report further
analysis and experimental progress on the spectrometer installation at LCLS [5].

INTRODUCTION
In low-charge mode, the LCLS FEL linac produces
13.6 GeV electron bunches with lengths on the order of afew micrometers and transverse spot size of ∼50 μm [1] fordelivery to the undulator hall to generate x-ray FEL pulsesof comparable duration. Diagnosis of the longitudinal distribution at the undulator entrance is desired for FEL optimization studies and presents a unique challenge to the resolution of existing diagnostics, reaching into the 1-fs scale.
Time-domain measurements of the LCLS beam by use ofan x-band transverse deﬂecting-mode cavity are alreadybeing pursued [2–4]. Alternative frequency-domain diagnostics have advantages of economy, requiring only standalone optics capable of recording the spectrum of coherent
beam radiation (CxR).
Under a change in trajectory or medium, the beam willradiate. In the 1-D line-charge approximation and neglecting other transverse eﬀects, the radiation emitted will havea power spectrum that’s given by
I(k) ∝ Ie (k) N + N(N − 1)  f (k)|2 (1)where Ie (k) is the power spectrum for an individual electron in the given process, N is the number of electrons,∗ Work supported in part by US Department of Energy contract number
DE-AC02-76SF00515.
† tmaxwell@slac.stanford.edu

the “form factor” f (k) is the Fourier transform of unitnormalized longitudinal charge distribution ρ(z), and thewavenumber is deﬁned as k ≡ 1/λ (the spatial frequency).
The second term in the bracketed sum of Eq. (1) represents the CxR, having an additional factor N enhancement
over the incoherent radiation over wavelengths corresponding to a large form factor. With a measurement of the beam
spectrum in the region where the coherent spectrum falls
oﬀ, one can then in principle deduce the bunch length. As arule of thumb for the coherent cut-oﬀ wavelength, we consider a bunch with unit-normalized Gaussian distribution of
rms length σz . In this case the corresponding form factorsquared | f (k)|2 is also a Gaussian of frequency-independent
peak amplitude that falls to 50% at the cut-oﬀs√
ln 2 0.133
k50% =≈⇒ λ50% ≈ 7.55σz
(2)
2πσz
σz
Prior LCLS studies on low-charge operation suggest
bunch length scaling linearly with charge at a constant peakcurrent [1]. For Q = 10 − 200 pC, we have σz = 0.5 50 μm for typical bunch compression. This yields coherent
enhancement down to λ = 3.5 - 40 μm (up to k = 2900250 cm−1 ).
As the distribution of the LCLS beam can varyfrom shot-to-shot, traditional scanning THz interferometry methods are avoided in favor of a single-shot mid-IR
spectrometer inspired by that demonstrated in [6]. Here we
provide an update on a one-stage, Single-Shot THz Prism
Spectrometer (SSTPS) installation designed to cover thisspectral range [5], favoring compactness and simplicity.

SYSTEM OVERVIEW
Current LCLS relative bunch length monitors utilize thecoherent edge radiation (CER) emitted at the exit of upstream bending magnets [7]. For the present application in
the straight section of the LCLS beam transport hall, coherent transition radiation (CTR) from the beam impinging on an inserted foil [8] was chosen. Referring to theoverview for the SSTPS system in Fig. 1, light from thefoil (A) will be collected and focused onto the spectrometer slit (F). Light at the slit is collimated through a custom KRS-5 mid-IR prism to provide dispersion (H) before
being focused onto a linear pyroelectric detector array (J).
All-reﬂective imaging optics are used to avoid unwanted
chromatic abberations. The system is considered in two
parts: the light collection optics (A-F) and the spectrometer proper (F-J).
ISBN 978-3-95450-119-9

Longitudinal Diagnostics and Synchronization1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○T. Maxwell† , Y. Ding, A.S. Fisher, J. Frisch, H. Loos
SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
C. Behrens, Deutsches Elektronen-Synchrotron, Hamburg, Germany

TUPA47

Proceedings of IBIC2012, Tsukuba, Japan275

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Design of the system is dictated by maintaining high,ﬂat light transmission over the one to tens of micron wavelength band. For coupling the light out of the vacuum
pipe, both CVD diamond and KRS-5 viewports are available with transmission spectra shown in Fig. 2. KRS-5
shows very ﬂat transmission over the full 1-40 μm (2508000 cm−1 ) range, but is brittle and slightly hygroscopic.
CVD diamond will be used for initial tests despite known
phonon absorption lines at 2 and 6 μm.
Also shown in Fig. 2 is the transmission spectrum
through 1 m of ambient air [9]. Several absorption bands
are seen from the presence of water vapor. These can be
improved to the better than 90% level with an 80% reduction in relative humidity. A dry-air purged enclosure has
therefore been designed with the added beneﬁt of protecting any slightly hygroscopic optics.

1
0.8
Transmission

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 1: Schematic of SSTPS to be installed at LCLS diagnostic cross OTR33, just prior to the undulator hall. CTR froma thin foil inserted into the beam (A) is collected and imaged to the spectrometer slit (B-F) and spectrally resolved on thedetector (F-J) using a custom KRS-5 mid-IR prism as the dispersive element (H).

0.6
0.4
Humid air
KRS−5
Diamond
Silicon

0.2
0
0

2000

4000
−1
k [cm ]

6000

8000

Figure 2: Transmission function of various substances inspectral range of interest.

Light Collection and Alignment
Collection optics require consideration of the CTR
source. Transition radiation is highly directional, concentrated on a cone with half-apex angle 1/γ in the far ﬁeld [8].
The light must pass the narrow aperture (100 μm) slit.
For these reasons, and to further discriminate against incoherent transition radiation by its larger inherent divergence, an odd number of “lenses” is used to provide wide
spatial acceptance and narrow angular acceptance. By ray
trace design, light from the foil within Δx = ±7.2 mm willclear the slit but only within Δθ x = ±0.2◦ .
The LCLS is a user-oriented facility with minimal downtime. Therefore the entire 2’×4’ breadboard has been de-signed for rapid installation and removal. The table mount(Fig. 3) is a quick-release, turn-buckle design so that theboard can be positioned and aligned within the required
tolerance. The initial turning mirror (Fig. 1, B) is used forﬁne angular control to position the image on the slit, andremotely as necessary. The board can be locked to a tablein an oﬄine optics lab for preparation and testing.
To get initial alignment, a reference alignment laser will
be added during installation. A new 6-way cross 7.3 m upstream of OTR33 will inject the laser into the beam line,
imaged to match the noted beam position on the existing OTR33 camera. With reference positioned to withinISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

TUPA47x(k) [mm]50−5Δk(k) [cm−1]

0600

The Spectrometer
A prism spectrometer conﬁguration was chosen as a matter of simplicity and economy, though order-sorted grating
conﬁgurations have gains in resolution [5, 6]. Chosen for
its broad mid-IR transmission (Fig. 2) and moderate dispersion, KRS-5 was chosen as the prism medium [5] withindex and dispersion curves shown in Fig. 4. Using the12.8 mm length of the detector array as a restriction to the
maximum spatial dispersion, the prism apex angle is 10◦ .
Figure 4 shows nonlinearity over this wide spectral
range. With a constant waist of 10-mm at the prism, thefrequency-to-position map x(k) and eﬀective spectral resolution Δk(k) is computed in Fig. 5. While the strong lowk dispersion and small diﬀraction-limited spots at high-k

2.45
dn/dk [μm]

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n(k)

2.35
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750
k [cm−1]

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2.25
2.2

10

0

2000 4000 6000 8000
k [cm−1]

0.1
02000 4000 6000 8000
k [cm−1]

Figure 4: Refractive index n(k) (left) and dispersion n (k)(right) for KRS-5.

6000

8000

2000

4000
−1
k [cm ]

6000

8000

200
0

Figure 5: Frequency dependence of spectral spot positions
x(k) (top) and spectral resolution Δk(k) (bottom) for prism
spectrometer (Fig. 1).
preserve good spectral resolution in these limits, the centerband about k = 2000 cm−1 suﬀers. The strong frequency
dependence of x(k) also suggests that excellent wavelength
calibration is required.

ABSOLUTE LEVELS
To compare expected light levels as compared to detector
sensitivity and to quantify the extent of information loss inthe system, additional modeling and testing was performed.
These include analysis of transverse imaging contributions,detector benchmarking and absolute power estimations.

Transverse Contributions
Considerable work has been done on modeling the generation and diﬀraction of near-ﬁeld, long-wavelength CTR
in recent years and is known to cause signiﬁcant lowfrequency losses. This was modeled for the SSTPS using [10] following the approach outlined in [7]. Diﬀraction of light through the system is accomplished by GaussLaguerre mode decomposition of the ﬁelds between apertures [11]. The ﬁnite TR source is estimated by the methodof virtual quanta [12] as the Fourier components of the relativistic single-electron ﬁelds diﬀracting through the OTR
screen.
The full system was optimized with results in Fig. 6. Signiﬁcant low-k losses are observed arising primarily at the
spectrometer slit (Fig. 1, F). The 3-lens imaging was chosen to prefer spatial acceptance and also improves peak system transmission shown here. However this choice gives
rise to the strong DC suppression and long rise in response
due to the frequency dependence of the divergence of thediﬀracted, near-ﬁeld TR.

Detector Sensitivity
The 128-element, linear pyroelectric array being used
is provided by Pyreos [13]. Pixels are 60 μm wide and500 μm tall with a 100 μm pitch. The manufacturer alsoISBN 978-3-95450-119-9

Longitudinal Diagnostics and Synchronization3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○±2 mm of the beam center at both crosses, the angle is restricted to within ±0.03◦ and well within the stated acceptance of the system.

4000

400

0

Figure 3: Rendering of the new OTR33 diagnostic station.
SSTPS optics and enclosure to be installed on the quickrelease breadboard.

2000

TUPA47

Proceedings of IBIC2012, Tsukuba, Japan35

3
Signal [V]30

T(k) [%]

25
20
15

2
1

10

0
0

5
2000

4000
−1
k [cm ]

6000

Figure 6: Simulated CTR diﬀraction losses for the SSTPS
system.

32

48

furnishes a compact data acquisition board for the sensor.
The kit generates the timing signals required by the sensor
and digitizes the analog burst with a sampling rate of ν samp
= 10 - 256 Hz. The sensor and kit are shown in Fig. 7.
As previously reported [5], the sensitivity is not provided. It has since also been noted that the sensor array
contains unpassivated silicon. To prevent disrupting photoelectron emission within the detector circuit, a low-pass
optical ﬁlter must block λ < 1.2 μm. A silicon window isbeing used along with the acquisition kit. Measured mid-IR
silicon transmission is shown in Fig. 2 [14], and is strongly
attenuating in the visible to near IR.
Illumination tests were repeated using a 2-mW, 3.4-μm
HeNe laser. The beam is ﬁrst combined with a visible
632 nm HeNe laser by transmission (reﬂection) of the IR(visible) beam through a germanium combiner. The beams
are then chopped and reﬂectively expanded. A silicon photodiode is used to check time of arrival of the laser pulse(s)and estimate the variable chopped IR pulse width τ pulse .
When τ pulse
1/(2ν samp ), the optimal array signal coincides with the kit triggered just after termination of the lightpulse.
The beam is focused to a tight spot on the array withan example image from a 100-Hz, 85-μs (fwhm) chopped
pulse shown in Fig. 8. From such data, and accounting
for 48% transmission of the 2-mW IR laser through the Ge
combiner, the sensitivity of the array (with window) is es-

96

112 128

timated to be 75 V/μJ/pixel. With saturation occurring at
4.5 V, maximum illumination is around 60 nJ/pixel. Averaging over many shots, rms electrical noise is 15 mV/pixel.
This corresponds to a lower limit of 0.2 nJ/pixel.

Simulated Signal
Finally we illustrate expected signals for low-charge operation of the LCLS linac. A simulated bunch distribution for the typical under- to fully-compressed, double-hornbunch [1] is used. Assuming still a constant peak current
and that bunch duration scales with charge, simulated initial distributions and spectra are shown in Fig. 9.
To simulate measured signals, the spectra are subjected
to losses (Fig. 6), mapped to detector position (Fig. 5,top), convolved with a resolution term (Fig. 5, bottom),integrated over the region subtended by each pixel, andsummed with psuedo-electrical noise of amplitude 0.5
nJ/pixel. The results are shown in Figs. 10 and 11 for the
200- and 20-pC cases, respectively.
By these estimates, the single-bunch light levels fallwithin the range of the sensor. For the 200-pC (10’s of μm)
case, there is a loss of amplitude deﬁnition as the edge ofthe spectrum falls within the low-resolution range (Fig. 5).
3
2
1
0
−30

Figure 7: The 128-element linear pyroelectric array (left)and data acquisition evaluation kit (right). Images courtesy
of Pyreos [13, 15].

64 80
Pixel #

Figure 8: Line array signal from focused 3.4-μm HeNe synchronously chopped to 85 μs pulse width at 100 Hz.

Q−1 dU/dk [μJ−cm/pC]

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○

16

8000

I(z) [kA]

0
0

4

x 10

−20

−10

−4

0
z [μm]

10

030

Q = 200 pC
Q = 100 pC
Q = 20 pC
Q = 10 pC

2
0

20

2000

4000
k [cm−1]

6000

8000

Figure 9: Simulated bunch proﬁles (top) and corresponding
emitted power spectra per unit charge (bottom).

ISBN 978-3-95450-119-9
4Longitudinal Diagnostics and Synchronization

0.1
0.05

E(k)

0
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4

−2.5

0
x [mm]

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Figure 10: Simulated detector array levels before (top) andafter (bottom) mapping to k axis for Q = 200 pC case.

0.05

−2.5

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E(k)

REFERENCES[1] Y. Ding et al., Phys. Rev. Lett. 102, 254801 (2009).

[3] Y. Ding et al., Phys. Rev. ST Accel. Beams 14,
120701 (2011).
[4] P. Krejcik et al., Proc. of IBIC2012, TUPA41 (2012).

64[5] C. Behrens et al., Proc. of DIPAC2011 pp. 386–388(2011).

20

We would like to thank Gene Kraft, Joe Stieber, Tom
Galetto, Sasah Gilevich, Mike Minitti and Jennifer Loos of
SLAC National Accelerator Laboratory for their valuable
contributions as well as Zhirong Huang, Rick Iverson andFranz-Josef Decker for fruitful discussions and support.

[2] P. Emma, J. Frisch, and P. Krejcik, A Transverse
RF Deﬂecting Structure for Bunch Length and Phase
Space Diagnostics, SLAC Technical Note LCLS-TN00-12 (2000).

0.1

0
−5
−5
x 10
8

Kramers-Kronig phase retrieval [16] after all expected distortions, demonstrating promise as an absolute diagnostic
in support of LCLS ultra-short FEL optimization studies
on track for ﬁrst beam measurements before end of calendar year.

ACKNOWLEDGMENT

20

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0

2000

4000
k [cm−1]

6000

8000

Figure 11: Simulated detector array levels before (top) andafter (bottom) mapping to k axis for Q = 20 pC case.
Simulations around 20 pC prove more promising with signiﬁcant structure of the spectrum preserved.

DISCUSSION
Progress continues on a Single-Shot, THz Prism Spectrometer for absolute, ultra-short bunch length diagnostics
at the LCLS. Rigorous simulations of the system have been
carried out with the novel detector’s behavior and control
now well understood. Practical installation issues havebeen addressed.
Staging of the ﬁnal optics and electronics is nearly complete. In parallel with the ongoing fabrication of various
necessary mounting components, plans for improved spectrometer characterization are underway. These will use the
LCLS Laser Group’s micron-range OPA and monochromator to provide a wavelength calibration measurement.
The projected instrument response suggests optimal operation is in the 10’s of pC range, with bunches of a few-μm
length. Though spectral methods do not directly deduce absolute bunch length due to loss of phase information, simulated signals in this range have also proven amenable to[6] S. Wesch et al., Nucl. Instr. Meth. A 665, 40 (2011).
[7] H. Loos et al., Proc. of PAC07 pp. 4189–4191 (2007).
[8] V. Ginzburg and I. Frank, J. Phys. USSR p. 353(1945).
[9] Harvard-Smithsonian Cen. for Astro. and V.E. Inst.
of Atmos. Opt., HITRAN Database, Available at
HITRAN on the Web: http://hitran.iao.ru(2012).
[10] H. Loos, CxR Simulation Tool, SLAC MATLAB code
spec_gui.m (2010).
[11] H. Kogelnik and T. Li, Proc. IEEE p. 1312 (1966).
[12] C. A. Brau, Modern Problems in Classical Electrodynamics (Oxford University Press, New York, 2004).
[13] Pyreos Ltd., PY-LA-S-128 Product Datasheet,
Available at http://www.pyreos.com/images/
docs/PY-LA-S-128_May2010finalrelease.pdf(2010).
[14] Tydex, Russia, private communication (2012).
[15] Pyreos Ltd., Line Sensor Evaluation Kit ProductDatasheet, Available at http://www.pyreos.
com/images/docs/PYDK_LAS_Demo_Kit_User_
Guide__JWrelease_Jan2010.pdf (2010).
[16] R. Lai and A. Sievers, Nucl. Instr. Meth. A 397, 221(1997).
ISBN 978-3-95450-119-9

Longitudinal Diagnostics and Synchronization5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○E [μJ/pix]

Proceedings of IBIC2012, Tsukuba, Japan