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

Intense FEL light pulses with a length of three optical cycles produced at zerodetuning of an optical cavity length
Nobuyuki Nishimori, Ryoji Nagai, JAERI, lbaraki, Japan

Abstract
We have observed sustained saturation in a Free-Electron
Laser (FEL) oscillator at zero detuning of an optical cavity
length for the first time, and confirmed that the optical pulse
length at zero detuning is 250 fs FWHM at wavelength of
22 JLm. The optical length corresponds to three optical cycles, which is the fewest cycles ever achieved at FELs in theworld. The peak FEL power reaches 0.8 GW with the single optical pulse energy of 200 J,LJ. The sustained lasing atzero detuning is not allowed in conventional FEL oscillator
theories and the physical mechanism is under investigation.

1 INTRODUCTION
In a Free-Electron Laser (FEL) oscillator, spontaneous undulator radiation produced by an electron bunch is stored
in an optical cavity and amplified through interactions withsuccessive fresh electron bunches in the undulator. The
FEL laser oscillations are, therefore, realized when the cavity length is tuned in a finite range near the length matchedwith the electron bunch interval. The lasing range is restricted to cavity lengths shorter than the zero detuning,where the cavity length is completely matched with theelectron bunch interval, in the following reason. The trailing edge of the FEL light is mainly amplified during aninteraction with a fresh electron bunch due to the slippage(Nw>.), which is a delay distance of the electron bunch to
the FEL light in the undulator. The group velocity of the
FEL light, therefore, becomes somewhat smaller than thevacuum speed of light This is known as "laser lethargy"
and optical cavity shortening is required to compensate thelethargy delay [1]. Here Nw is the number of undulator
periods and ).. is radiation wavelength.
The laser performances of FEL oscillators strongly depend on the length detuning (t5 L) of the optical cavity fromthe zero detuning (oL = 0). The FEL gain becomes maximum at the detuning length where the lethargy delay ismost effectively compensated. The efficiency from electron beam power to FEL radiation becomes maximum nearoL = 0, where the gain is small though, because the FEL
light can interact with fresh electron bunches many roundtrips before the FEL light is pushed out by oL with theround-trips. The FEL optical pulse length becomes shorternear L = 0, because the trailing edge of the pulse grows
mainly [2, 3].
At the zero detuning of FEL oscillators, the optical pulse
centroid is retarded on successive passes and the opticalpulses finally dissipate. Only a transient state therefore ex-oists at oL = 0. This transient evolution of the optical pulses
at oL = 0 is supported by numerical and analytical studies[1, 2].
We describe improvements of the JAERI-FEL system
which contributed to "over 2 kW" lasing, in the next section. A numerical analysis on JAERI-FEL, which reproduces the high-power lasing observed in the JAERI-FEL
oscillator, has clearly shown that the maximum efficiency
is obtained at oL = 0 despite the lethargy in conventional
FEL oscillator theories. This is discussed in section 3. The
experimental confirmation of the sustained lasing at oL =0 is presented in the section 4. The measured FEL pulse
length at oL = 0 is only 3.4 cycles FWHM, which is the
fewest cycles ever achieved at existing FELs. This is alsopresented in the section 4. A conclusion is given in the lastsection.

2 HIGH-POWER FEL OSCILLATION [4]The project of JAERI-FEL, high average power FEL driven
by a superconducting linac with frequency of 499.8 MHz,
started in 1987 to provide a quasi-CW far infrared (FIR)
laser of a 1 ms long macropulse at 10 Hz repetition rate.
The early years of the project were devoted to the design
study and construction of the linac system. The first lasing
was obtained in Feb. 1998 [5] after improvements of theelectron gun and RF feed-back circuits. The FEL power
was gradually increased by achieving higher longitudinalbrightness of driver electron bunches and optimizing output coupling scheme of FEL from an optical cavity, andeventually exceeded 2 kW in Feb. 2000 [4].

2.1

Improvements of injection system [4, 6]

Two modifications mainly contributed to generation of highbrightness electron multi-bunches with a long macropulse
duration. One is an improvement of an electron gun, theother is an optimization of a drift length between a 6th subharmonic buncher (SHB) and a first cell of the superconducting accelerator.
The electron gun at JAERI-FEL is a DC gun with athermionic cathode driven by a grid pulser, and its DC voltage is 230 kV. The high voltage is very useful not only to
reduce space charge effects, but also to keep a high speed of(3 = 0. 71 in the low energy region. The accelerator tube andthe grid pulser are contained in a tank filled with SF 6 insulation gas to avoid a discharge. An input signal into the grid
pulser on the high voltage terminal is fed through an optical fiber, and has to go across an extra connector attached- 95-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001?,-~====~======~------~-------.

~:(~1

Table 1: JAERI-FEL parameters.
Parameter
Kinetic energy
Average current
Bunch charge
Bunch length (FWH~)
Peak current
Energy spread (rrns)
Normalized emittance (rrns)
Bunch repetition
Undulator period
Number of undulator periods
Undulator parameter (rrns)
Optical cavity length
Rayleigh range~irror radii
Output wavelength~easured

16.5 ~eV
5.3 rnA
0.51 nC< 5 ps
> I 00 A
1.2 %40 rnrn rnr (x)22 rnrn rnr (y)

1t

400

__.............,~"""":'!>

~

'
2

0

0

0

0
00

00

o..,.,

00

10.4I25~Hz~1~5-------_-;-:1o=---------s=---------:'-o------:....e.....J5

3.3 ern52
0.7
14.4 rn
1.00 rn
6crn
22- 17 J.Lnl

detuning length ( ~m)to the tank. As the input signal is attenuated too much at
the connector, the grid pulser is affected by a slight error
source such as a stray capacitance. This causes the jitter ofbunch intervals from the gun.
The time jitter at the gun causes arrival time fluctuationsof electron bunches at an undulator. The fluctuation corresponds to the shift of the effective optical cavity length.
The fluctuation should be reduced for high efficiency operation, because the efficiency is very sensitive to a slightshift of cavity length near 8£ = 0 J.Lrn [3]. Our numerical simulation indicates that the jitter inside the undulator
should be smaller than I 00 fs rrns for the simulation to reproduce the shape of the detuning curve presented in Fig. I[7]. Realization of such very low time jitter at the undulator
is mainly due to jitter reduction at the gun (see Ref. [6] formore details).
The drift length after the SHB must be adjusted so thatthe bunch is compressed effectively, and it was shortened
from 7.4 rn to 4.5 rn in 1999 [6]. The FEL power was gradually increased by adjusting accelerator phases, optimizing
output coupling methods [8], reducing the time jitter of thegun, and removing frequency lluctuation of a master oscillator. The FEL power finally became 10 times higher
than it obtained in I999, and the peak current of electron
bunches was obtained to be 100 A from the bunch length,
5 ps FWH~, measured with a synchroscan streak camera (~I954-IO, Harnarnatsu) at the undulator center. The
above bunch length is almost comparable to trigger jitterof the streak camera and the bunch may have the shorter
length and the higher peak current. The parameters for theelectron bunch and the FEL are listed in Table I.

2.2

time (~ s)

0

-~ 3

Optimum output coupling of FEL light [8]The wavelength in JAERI-FEL ranges from 22 to 17 J.Lrn
where dielectric mirrors are not available as output couplers. A center hole on an optical cavity mirror or a scraper

Figure 1: The open-circles are typical measured efficiencies as a function of 8£ in JAERI-FEL. The solid-triangles
are our numerical results, which agree with the measured
efficiency curve well. The solid-line is a guide to the eye.
The above left shows typical optical rnacropulses at 8£= 0 from our simulation (solid-line) and an experiment(dashed-line). It should be noted that the transversal axis
for experimental data is relative detuning length, while itfor the numerical data is absolute one.
mirror are used as output couplers in JAERI-FEL. The
output coupler using the hole or the scraper mirror offers
broadband operation, but diffraction loss due to scatteringfrom the coupler edge becomes comparable to or larger
than the coupled FEL power. The ratio between the diffraction loss and output FEL power decreases with increasing
radius of the hole or the scraper mirror, while the total loss
also increases and the FEL efficiency decreases. There isan optimum coupling scheme for maximum FEL output.
The scraper can couple out the outer part of the transversal profile of the FEL light, while the center hole couples
out the most intense part of the FEL for the lowest transversal eigen-rnode of the optical cavity. This means that radiusof the scraper mirror is larger than the hole for the same
FEL output power, and the scraper mirror is more efficient.
An iterative calculation based on Fox-Li procedure [9] hasbeen made to obtain an optimum coupling scheme (see Ref.
[8] for more detail).

3 NUMERICAL STUDY ON JAERIIDGH-POWER FEL [7]
An interesting feature of the efficiency detuning curve in
JAERI-FEL is the sharp peak observed at the maximum
efficiency (see Fig. I). Here the detuning length of themeasured efficiency curve (open circles) is relative value,
and the detuning length at the maximum efficiency is tentatively defined as 8£ = 0 J.Lrn. The efficiency gradually
increases from 8£ = -30 J.Lnl to -1 J.Lrn, but is steeply enhanced from -1 J.Lrn to 0 J.Lrn. The enhancement of theefficiency is about 80 %. The sharp peak has not been observed in other FELs [10]. A numerical analysis is madeto characterize the lasing behavior by using a 1-D time-- 96-

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

scrapermirroroptical cavity 14.4m

7

i

6

6

5

4

#
energy power
analyzer meterphoto diode

2

·o 3

~10

.,

0.

dependent code based on a macro-particle model [II]. A
shot-noise of the bunch is also introduced into the simulation according to Penman's method [12], which gives theeffective shot-noise derived from statistical consideration.
An efficiency curve simulated with the dimensionless beam
current [13], j 0 =50, is shown as solid-triangles in Fig. I
and agrees with the experimental data well.
The important indication of the numerical simulation isthat the efficiency is maximum at 6£ =0 J.tm and the lasing
is sustained despite the laser lethargy. However, no measurement has been made at FEL oscillators with enoughaccuracy to claim the lasing at 6£ 0 [10, 3]. For an experimental confirmation of the las~ng at 6£ = 0, we madea simultaneous measurement of FEL power and absolutedetuning length, as described in the next section.
It was also found that our numerical simulation without the shot-noise gives a result consistent with the studies by Colson [1] and Piovella [2], in which no stationary
lasing exists at 6£ = 0 with non-zero optical cavity loss.
Thus the small shot-noise largely affects the lasing dynamics at the zero detuning, although the coherent componentof the shot-noise has small power, w- 10 of the saturated
FEL power in our simulations, and does not affect the efficiency curve directly. Typical optical macropulses at 6£
0 J.tm from the simulation (solid-line) and an experiment(dashed-line) are shown in the above left in Fig. 1 withan arbitrary unit in the vertical axis. They are similar witheach other.
The temporal profile at 6L 0 J.tm obtained by the simulation is a short, single optical spike with length of 300 fs
FWHM without subpulses even after saturation. We measured the temporal profile to confirm the numerical indication, as described in the next section. Details of our simulation will appear in elsewhere [7].

=4 SHORT, INTENSE FEL AT dL

4.1

=0

Sustained lasing at bL = 0 [14]The experimental setup for verification of lasing at 6L =0 is shown in Fig. 2. The absolute detuning length wasmeasured with a mode-locked Ti:sapphire laser (Tsunami
3960, Spectra-Physics) synchronized with the frequency of
83.30000 MHz, which is the eighth harmonic of the electron bunch repetition rate and supplied from the same RF
source used for the accelerator system [15]. The pulse
length of the laser is about 80 fs FWHM. The laser pulses

I~

'iii

11i

·i'

0

·5

·50

10

5

0.

0

r~

I!!

~
Q.

11ti\

OY'

ooooooo.... ....J

!ll

i=

i

-30
-10
-20
detuning length (11m)

-40

c:

~Q

-~

0

Figure 3: FEL efficiencies (open-circles) and Ti:sapphiresignals (solid-circles) as a function of detuning length. The
enlargement around 6L 0 J.tm is also shown. The symbols
without error-bars have error less than their size. The absolute vertical scale was calibrated by average energy loss ofthe electron beam over an entire macropulse (solid-squares)
at several detuning lengths.

=

=

=

o~...

2

0

.e.!!.

:

0

__./i

..

c:

Figure 2: An experimental setup for a simultaneous measurement of FEL power and absolute cavity length.

*

~

:o

.f!

...,..-:o

~4

i

~

.r
<io

5,---------------~-------,-------,

0.3 ,------.-.--.,----,• • "!1:,,.1:•"~3 0.1
~
0

0
-4

Q.

•

t

q

•i

4 0.2
8w=0.31

'!i•

·2

0

1! 0

iy

0 1

2

u:J2

00

:

t.

0

u.

Q

~0 i•
QQ

,: 0

0

aw=O.~o i c

0

c

c

c

cc

aw=0.31

l

c

! ""

00
00

0

"<>'

o~~~~~-------·~·-·-·----~'~~~~~·~
·15
·10
·5
5
detuning length (11m)

Figure 4: FEL power measured as a function of detuning
length with different undulator parameters: 0.70 (opencircles), 0.49 (open-squares), and 0.31 (crosses). The
macropulse duration is 0.4 ms at 10 Hz repetition rate. The
enlargement around 6L =0 J.tm for aw =0.31 is also shown.

were injected into the optical cavity through a glass window, which does not transmit FIR light, and a center holewith 2 mm in diameter on an upstream cavity mirror. The
stored pulses were coupled out through the same hole anddetected by an avalanche photo detector (C 1536-01, Hamamatsu). The detector signal is enhanced by the pulse stacking, if an optical cavity is tuned at an appropriate length[16]. Mean values of the photo detector signal over 200
ms duration were measured by a digital oscilloscope (IDS
684B, Tektronix) at a peak detection mode. The measurement was repeated five times at each detuning length.
The experimental results exhibit a clear resonance peak asshown in Fig. 3, where each error-bar is the standard deviation of the five repeated measurements. The centroid of theresonance peak was determined with the accuracy of 0.01

-97-

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

f.lill, by fitting the data with Gaussian distribution.
The FEL light was coupled out by a gold-coated scraper
mirror of 18 mm in diameter, which was installed at 0.5
m away from the downstream mirror and 23 mm from thecenter axis of the optical cavity, and was extracted througha KRS5 window. The FEL power was measured with apower-meter placed near the window and acquired by theoscilloscope in the same manner as the Ti:sapphire signal.
The typical maximum power was 4.4 W at macropulse duration of 0.4 ms with 10 Hz repetition rate. This measured
power is equal to 1.1 kW at a macropulse average. The experimental data are plotted as open-circles in Fig. 3. For
the calibration from the FEL power to the efficiency, wemeasured average energy loss of the electron beam overan entire macropulse at several detuning lengths. The loss
measurement was made with the energy analyzer placed after the undulator, which has the energy acceptance of 20 %,and the measurement was restricted to the larger detuning
length beyond 6L = -10 f.lill where the total energy spread
was 17 %. The FEL power and Ti:sapphire signal were
simultaneously measured at equal steps of 0.2 f.lill aroundthe maximum FEL power within two minutes. This quick
measurement is necessary to reduce the uncertainty causedby slowly evolving drifts of the cavity length due to thermal
strain and other sources. A separate measurement confirmsthat the maximum FEL power is held with variation of 2.3% rms and the amplitude change of the Ti:sapphire signal
is 2.0 % rms over two minutes.
As shown in Fig. 3, the peak of the FEL efficiency curve
coincides with the resonance peak of the Ti:sapphire signal
within the accuracy of 0.1 f.lill. This is the first demonstration of sustained saturation at the zero detuning. As the
maximum efficiency is 6 % with the electron beam powerof 88 kW, the extracted FEL power should be 5 kW at a
macropulse average. The total loss of the optical cavity
with and without the scraper mirror was measured to be 7.6± 0.3 % and 3.3 ± 0.2 %, respectively, from a cavity ring
down of FEL with a Ge-Cu detector. The FEL coupling
rate from the cavity is therefore estimated to be about 30 %on the assumption of coupling rate of 50 % at the scraper
mirror itself [17]. The transmittance of a KRS5 window for>. = 22 f.lill light is about 70 % in normal incidence. Consequently, the estimated output FEL power from the measured efficiency is consistent with the experimental valueof 1.1 k W with the power meter.
It should be noticed that the numerical study by Colson, which shows only a transient FEL evolution at 6L =0, also includes a shot-noise effect and uses the coupling
mode parameter f.lc = 1 similar to the present study [1].
The parameter f.lc is defined as f.lc = N>J2ue, where N is
the number of undulator periods and u e is the rms electron
bunch length [18]. The main difference is the dimensionless beam current, j 0 =50, in JAERI-FEL, while j 0 5 isused in the simulation by Colson. In order to study the lasing dynamics at smaller j 0 , we measured efficiency curves
at three different rms undulator parameters of aw = 0.70,
0.49, and 0.31 by widening the undulator gap. The cur-=

1.2 r·-•-r•T~~-,-m '"T'·~~Tn-c ~r~···~-,-r~-c·•r( ···)DetectorfcT~ZnSe

f"!"Te !Delay

1.o i';' 0.8 t

T'locl'l~Q

FELiight
ff L_____________________-~ 0.6

i
l

.2

~

l

:

0.4

r

0.0 L~_L<~-U~~~--'-'--~~.L •.~

-100

0

100

200

..LL~~~--L~--~-'-'-•-L-"--'~

300

400

500

600

De1ay(f.1m)

Figure 5: Second-order autocorrelation measurement for awavelength of 22 f.lill at zero detuning of an optical cavity. The pulse length is 250 fs FWHM with assumption ofsech2 (t) pulse. This corresponds to 3.4 field cycles. The
above left shows second-order autocorrelation setup, basedon second harmonic generation in a Te crystal.

rents j 0 are estimated to be 50, 22, and 10, respectively,on the definition of jo ex (aw[JJ]? /(1 +a~) [13]. Here,[J J]. is the Bessel function for a planar undulator. The detuning length of each efficiency curve was calibrated by theTi:sapphire laser, independently. The experimental results
shown in Fig. 4 exhibits that the lasing dynamics at 6L =0 depends on the dimensionless beam current, and the sustained saturation at 6L = 0 disappears at j 0 = 10. This result
is consistent with the Colson's simulation at j o = 5.

4.2

Intense FEL with three optical cycles [19]Short, intense optical pulses in the FIR spectral range areuseful for studies on dynamic properties of a wide variety of semiconductor systems. An FEL oscillator is appropriate for producing such short, intense FIR lights. The
FEL pulse length strongly depends on 6L, because the FEL
pulse is pushed forward due to 6L and evolves into a trainof subpulses. A short-pulse FEL with a length of six optical
cycles has been already demonstrated in FELIX at wavelengths of 10.4 and 24.5 f.lill at 6L = -1 and -3 f.lill, respectively [20]. The reason why the temporal profiles in
FELIX are subpulse-free smooth single pulses is that themeasurements in FELIX were made in start-up phase of the
FEL evolution due to the restricted macropulse duration.
The macropulse duration in JAERI-FEL is enough longto reach saturation near 6L =0, and the experimental confirmation ofthe lasing at JL = 0 stimulated us toward studyof the temporal FEL profile at 6L = 0: whether a short,
subpulse-free single pulse is obtained at 6L even after saturation, as shown in our simulation [7]. We measured the- 98-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 200I

pulse length of the JAERI-FEL FIR light using secondorder autocorrelation, in a time range after the FEL saturation. The experimental setup is shown in the above leftof Fig. 5. The second-harmonic generation in a 2-mmlong Te crystal was used to provide the nonlinear autocorrelation signal. The Te crystal is birefringent and phasematching can be realized easily by setting orientation of
the crystal to an appropriate angle. Residual fundamental
FEL through the Te crystal were blocked by a ZnSe filtter.
Higher-harmonic signals of the FEL light are negligible,
because the second-harmonic conversion efficiency of theTe crystal is sufficiently high [21].
The observed autocorrelation signal is shown in Fig. 5.
The pulse length is 250 fs FWHM, which corresponds toonly 3.4 optical cycles, with assumption of sech 2 ( t)-pulse
shape. This agrees with the numerical result well [7], andshows that a smooth single pulse is quasi-continuously generated during a macropulse duration. The maximum pulse
energy is 200 ttl [4], and the peak FEL power reaches 0.8
GW.

5

CONCLUSION

We have observed sustained saturation in FEL oscillators
at zero detuning of an optical cavity for the first time, although it has been considered that a transient solution only
exists at the zero detuning due to the laser lethargy, so far.
The FEL efficiency becomes maximum at §L = 0, if the dimensionless beam current is large enough. A train of short,intense FEL pulses with only three optical cycles has been
quasi-continuously generated at §L = 0.
The efficiency detuning curve and the short-pulse at §L= 0 are well reproduced by our numerical simulation including a shot-noise effect. The physical mechanism of thelasing at §L = 0 is under progress (see Ref. [22] for more
informations and "http://hobbittokai.jaeri.go.jp/papers"
about JAERI-FEL activity).

6 ACKNOWLEDGEMENTS
We would like to acknowledge Dr. Ryoichi Hajima, oneof our colleagues, for his numerical analysis on JAERI
high-power FEL, which is indispensable for the present
research, and helpful suggestions and discussions about
the present experiments and analyses. We are indebted toDrs. M. Sawamura, N. Kikuzawa, and T. Shizuma at the
JAERI-FEL facility for helpful discussions, especially to
Dr. Eisuke Minehara, who is our group leader, offered us
an FEL system equipped with potential of high-power FEL
operation and encouraged us during the present research.

7 REFERENCES[3] N. Piovella, P. Chaix, G. Shvets, and D. A. Jaroszynski,
Phys. Rev. E 52, 5470 (1995); P. Chaix, N. Piovella, and G.
Gregoire, Phys. Rev. E 59, 1136 (1999).
[4] N. Nishimori, R. Hajima, R. Nagai, and E. J. Minehara, inProceedings of the 22nd International Free-Electron Laser
Conference, (North Carolina, 2000) (in press).
[5] E. J. Minehara et a!., Nucl. Instrum. Methods Phys. Res.,
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[6] N. Nishimori et al., Nucl. Instrum. Methods Phys. Res., Sect.
A 445, 432 (2000); in Proceedings of 7th European Particle
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[7] R. Hajima, N. Nishimori, R. Nagai, and E.J. Minehara, inProceedings of the 22nd International Free-Electron Laser
Conference, (Ref. [4]) (in press).
[8] R. Nagai eta!., in Proceedings ofthe 22nd International FreeElectron Laser Conference, (Ref. [4]) (in press); J. Nucl. Sci.
and Tech. 38, 15 (2001).
[9] G. Fox and T. Li, Bell Syst. Tech. J. 40,453 (1961).

[10] G. R. Neil et al., Nucl.lnstrum. Methods Phys. Res., Sect. A
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[13] (Ref. [11]), p. 93.
[14] N. Nishimori, R. Hajima, R. Nagai, and E. J. Minehara,
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[17] M. Xie and K.-J. Kim, Nucl. Instrum. Methods Phys. Res.,
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L. Stitch and M. Bass (North Holland, Amsterdam, 1985),
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[19] R. Nagai et al., in Proceedings of the 23rd International
Free-Electron Laser Conference, (Darmstadt, 2001) (to be
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[20] G. M. H. Knippels et al., Phys. Rev. Lett 75, 1755 (1995);
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[21] C. K. N. Patel, Phys. Rev. Lett. 15, 1027 (1965).
[22] R. Hajima et al., in Proceedings of 2001 Particle Accelerator Conference (Chicago, 2001); R. Hajima, N. Nishimori,R. Nagai, and E. J. Minehara, in Proceedings of the 23rd International Free-Electron Laser Conference, (Ref. [19]) (tobe published); N. Nishimori, R. Hajima, R. Nagai, and E. J.
Minehara, ibid, (Ref. [19]) (to be published).

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