The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

SPring-S Compact SASE Source (SCSS) Soft X-ray FEL ProjectTsumoru Shintake*a, Hiroshi Matsumotoa, Tetsuya Ishikawab and Hideo Kitamuraba KEK, Tsukuba, Ibaraki 305-0801, Japan~arima Institute; RIKEN, SPring-S
Abstract
The SPring-8 Compact SASE Source (SCSS) is a highpeak-brilliance soft X-ray free electron laser project. It is
the linac-based FEL dedicated machine. Combination of
the high-gradient C-hand accelerator and the in-vacuum
short-period undulator realizes a SASE-FEL facility forsoft X-ray within 100 m machine length. It has been
funded in April 200 1, aiming to generate first light in2003 at VUV region, and ultimately 3.6 nm in waterwindow in 2005. This paper report design work SCSS.

1 SCSS PROJECT OVERVIEW
1.1 Site and Schedule
The SCSS has been funded in April 200 I. It will be
constructed at SPring-8 site supported by multi-laboratory
collaboration including JASRI, JAERI, KEK and RIKEN.
SCSS construction is scheduled to complete in 2004 andobtain FEL radiation at VUV region in 2005.

1.2 Why Do We Need Compact Machine andHow?
One big problem in X-ray laser based on the SASEPEL is its high machine cost. This is the reason whypeople design the X-ray FEL as a parasitic facility
attached to an existing electron accelerator (LCLS uses
the Two-mile Linac at SLAC), or as an integrated facility
into a future large-scale machine such as TESLA linear

collider. Therefore, chance to have X-ray FEL facility isquite limited and regionally localized.
However, if we can make construction cost of the Xray FEL much lower than existing synchrotron lightsources based on the storage ring, each country or region
can possess their machines in each local sides, and it will
accelerate research in material science. To do this, themost important point is to make the machine size compact.
The building cost will be lowered, as well as the machine
itselfbecomes cheaper.
In the SCSS project, the following three key
technologies realize the compact machine.
(1) In-vacuum short-period undulator.
This makes the undulator period shorter, thus the beamenergy lower, as a result smaller the accelerator size. It
also contributes to shorten the FEL gain length.
(2) High gradient C-hand accelerator.
The accelerating gradient of 35 MV/m enables themain linac length being only 30m to reach 1 GeV.
(3) Low emittance beam injector.
Low emittance beam injector based on thermionicsingle crystal CeB6 cathode makes the FEL gain lengthand saturation shorter.

2 SCSS MACHINE
2.1 Machine Layout
Figure 1 shows the machine layout and Table-1
summarizes the machine parameters. SCSS consists of
the low emittance electron injector, the C-hand main linac,

SPring-8 Compact SASE Source
E=0.5MeV
q= 1 nClpk=3A
al = 0.33 nsec

20MeV

20MeV
1nC250A

1 nC30A

33 psec

4psec

~10mm

1.2mm

E = 300MeV>..,=

E = 1 GeV
q=1 nC
lpk=2kA

q= 1 nC
lpk=2kA
al = 0.5 psec.FWHM~ = 0.15 mm.FWHM

al = 0.5 pseC.FWHM
AZ = 0.15 mm.""""'

3.6nm
p = 8.9x 10-4
Lg =0.94m
Lut=20m
Put=1.9GW

oEIE = 2x1tr
en • 2 xmm.mrad

c~u-L·band

C-bandU118C

Unduletor

LinK

Au=15mm

Kma.< = 1.3gap.nin = 3. 7 mm
<jlx,y>= 10m
L =4.5x 5 = 22.5m

C-INood~Cavlly-111¥

Fig.l SCSS beam line layout. Total system length is about 80 m.

-114-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Table-1 SCSS parameters at 3.6 nm radiation wavelength.
Note that the bunch length is denoted by FWHM value.
nC1
Qbunch charge
21tmm.mrad
emittance
normalizedl;,x,yGeV
1
E
final electron energy%
0.02
final rms energy spread
CSomm
0.15!!.z
final FWHM bunch length
psec0.5!!.tkA2
peak current/pkundulator period
minimum gap
maximum K-parameter
undulator unit length
total undulator length
beta function

A,

g15
3.7

K

1.3

L14.5
22.5
10mm
mm

for 500 GeV ·e+e-mmmchopper cut the rising and falling parts of the pulse, thenforms 2 nsec pulse. The 476 MHz pre-buncher adopts
energy modulation of 400 kV peak-to-peak. The
f3
following energy filter cut the energy tail (top and410X
8.9bottom). After drifting 800 mm beam pipe, due to theFEL parameterpm velocity difference, electrons form a short bunch at the0.94
Lggain lengthm centre energy.
20saturation length
Lsat
The step-up cavity and the L-band accelerator capture
GW
Psat 2.0
saturation power
the single bunch and accelerate to 20 MeV, followed by
the undulator section for FEL interaction and the bunch
the first bunch compressor (BC 1).
compressors.
In order to saturate FEL at shortest wavelength in
2.3 Bunch Compressor BCJ
SCSS: A,. =3.6 nm, we need a short bunch of 0.5
In BC1, the bunch is compressed to 4 psec.FWHM• andpsec.FWHM, 2 kA, and 1 nC beam.
sent to C-band main linac. The short section of C-bandNote that, in this paper, we describe the bunch length
accelerator right before the bunch compressor is preparedin FWHM value rather than rms vale (crz). This is because
compensate the curvature in L-band accelerator field totoin our system the longitudinal current profile is close to alinear energy slope on the bunch.
form
square pulse.

2.2 Electron Injector

2.4 C-hand Main Linac {2]In the SCSS project, we chose a high-voltage pulsegun with thermionic-cathode, instead of RF-Gun. As
kwon in SASE-FEL theory, the quality of "internal"
structure of bunched beam is important, such as the sliced
emittance dominates the FEL gain. We believe a single
crystal LaB6 or CeB6 cathode, whose surface becomes
fairly flat in nano-meter scale after usage at hightemperature, is suitable to produce such a high "internal"
quality beam.
In the HV pulse gun, 3 Amp. with 300 nsec flat-top
pulse beam is pulled out from a single-crystal CeB6
cathode of 3 mm diameter at 1450 deg.-C. The beam

We use four units of the C-band accelerator system.
Each unit is capable of accelerating beam 250 MeV, total
four units provide 1 GeV beam. The C-band (5712 MHz)
accelerator technology has been developed at KEK Japanas the main linac of thefuture 500 GeV e+eLinear Collider project.
summarises
Table-2
results of the phase-I
C-bandon
R&D
1996-2000.
duringThe SCSS will perform
a best string test of themain linac system for
the Linear Collider
project.

2.5 Bunch
Compressor BC2Fig. 2 Single crystal CeB6 cathode.

In BC2, the bunch is Fig. 4 Internal structure of thecompressed to 0.5 in-vacuum undulator.
psec.fWHM. Since the-115-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

coherent synchrotron radiation (CSR) effect dominates,
we need careful design to minimize transverse emittancebreak due to CSR. We are currently working to findoptimum design, including double chicane configuration.

This is longer than the estimated saturation length at theshortest wavelength. The expected saturation peak-poweris about 2 GW.

REFERENCES

2.6 Undulator [3]After four units of the C-band accelerator, the beam isaccelerated to 1 GeV. The bunch is accelerated not on thecrest, but delayed phase to correct the residual energy
slope used in BC2.
The undulator consists of five unit of 4.5 m long invacuum undulator, and the total active length is 22.5 m.

[1] T. Shintake, "SPring-8 Compact SASE Source",
SPIE2001, San Diego, USA, June 2001.
[2] http:/c-band.kekJp[3] H. Kitamura, T. Bizen, T. Hara, X.Marechal, T. Seikeand T. Tanaka, "Recent Developments of Insertion
Devices at SPring-8" to be published in N.I.M.
to scss.
SCSS Application(Phase- II)

Items

Output 50 MW, Efficiency >45%
Refine design details for the massproduction and reducing cost. PPMPulse width >2.5 J.lseC
klystron is an option.
Pulse repetition 100 pps
Power<5 kW
All of No.I, 2, 3 tubes achieved 50
MW output, pulse width 2.5 J.lsec
and 50 pps. No.3 tube showed 47%
power efficiency.
Focusing power 4.6kW.
Life test No.2, 3 > 5000 hours.
Example of output power.
350 kV, 2.5 J.1Sec pulse generation,>50%
Smart Modulator, No. 1
Inverter HV power supply wasfirstly used in klystron modulator.
Operation for klystron life-test wasvery successful.
Power efficiency >52.4%

Oil-filled closed design will

Power gain >3.5,

Temperature stabilized design will beInvar body with copper
Cold Model Test (1997)
plating will provide better than one tenthPower Gain 3.25, Efficiency 65%
phase-sensitivity on the temperature
Not yet performed the high-power variation.
test is scheduled in 2001.
test.
Multi-bunch 1.6 nC, 80 bunch
Refine design details.
Acceleration gradient> 35 MV/m
Optimization for mass-production.
Lowering
cost.
ASSET test at SLAC demonstrated
Themulti-bunch
option in SCSS will
damping performance of the chokeprovide high average brightness.
mode cavity.

~~~?!!!~~c.:>:::7~0~o/!!_o_ _ _ _~ employed.

Resolution - 25 nm (FFTB test)
Position< 10-116-