An ERL-Based High-Power FreeElectron Laser for EUV Lithography	
Norio Nakamura
High Energy Accelerator Research Organization(KEK)	

2015 EUVL Workshop, Maui, Hawaii, USA, June 15-19, 2015.

ERL-EUV Design Group	(KEK) H. Kawata, Y. Kobayashi, T. Furuya, K. Haga,
I. Hanyu, K. Harada, T. Honda, Y. Honda, E. Kako,
Y. Kamiya, R. Kato, S. Michizono, T. Miyajima, H. Nakai,
N. Nakamura, T. Obina, K. Oide, H. Sakai, S. Sakanaka,
M. Shimada, K. Tsuchiya, K. Umemori, M. Yamamoto,
S. Chen, T. Konomi, T. Kubo(JAEA) R. Hajima, N. Nishimori 	The design study has been done under collaboration with a Japanese company.	

Outline	• Introduction• Injector Design• Main Linac Design• Bunch Compression & Decompression Scheme• Design of Arc & Chicane• Bunch Compression Simulation• FEL Performance• Summary and Outlook	Motivation	
• 10-kW class EUV sources are required inthe future for lithography• The order of EUV-FEL size and cost can beacceptable• ERL-FELs have merits of energy recovery,low dumped beam power and activation

Design Concept	
• Target : 10kW power @ 13.5 nm, 800 MeV• Use available technology without too muchdevelopment• Make the most of the designs, technologies
and experiences of the Compact ERL(cERL)at KEK	Compact ERL at KEK	
Beam Energy: 20 MeV
Max. Beam Current: 80 µA ( à 10 mA )RF Frequency: 1.3 GHz	in operation since 2013	

Injector Design	• DC Photocathode gun with the same structure of 2nd gun at cERL
• Two cERL cryomodules with six 2-cell SC cavities for Einj=10.5 MeV• Two solenoid magnets and one buncher cavity• New merger (under design)
500 kV Gun

Two solenoid magnets
Buncher

Two injector cryomodule(Max. Eacc: 8 MV/m)	

Beam
Einj=10.5 MeV	Injector system of EUV source (merger not included) 	

Injector Parameters	
Optimization of injector parameters before merger	
Qb=60 pC
E=10.511 MeV	

0.5	
0.4	
0.3	
0.2	
0.1	
0.0	

1ps	

2ps	

0	  	  	  	  	  	  	  	  	  	  	  	  	  0.2	  	  	  	  	  	  	  	  	  	  	  	  0.4	  	  	  	  	  	  	  	  	  	  	  	  	  0.6	  	  	  	  	  	  	  	  	  	  	  	  	  0.8	  	  	  	  	  	  	  	  	  	  	  	  	  1	  	  	  	  	  	  	  	  	  	Bunch length[mm]	

Momentum spread[%]	

Normalized emittance[mm·mrad]	

0.6	

0.5	

Qb=60 pC
E=10.511 MeV	

0.4	
0.3	
0.2	
0.1	
0.0	1ps	2ps	0	  	  	  	  	  	  	  	  	  	  	  	  	  0.2	  	  	  	  	  	  	  	  	  	  	  	  0.4	  	  	  	  	  	  	  	  	  	  	  	  	  0.6	  	  	  	  	  	  	  	  	  	  	  0.8	  	  	  	  	  	  	  	  	  	  	  	  	  1	  	  	  	  	  	  	  	  	  	Bunch length[mm]	
Tracking by GPT	60pC/bunch
1 ps : 0.30 mm mrad, 0.25 % à εn = 0.60 mm·mrad, σp/p = 0.25 % @ merger exit
2 ps : 0.25 mm mrad, 0.25 % à εn = 0.55 mm·mrad, σp/p = 0.25 % @ merger exit
100pC/bunch
1 ps : 0.57 mm mrad, 0.35 % à εn = 0.80 mm·mrad, σp/p = 0.35 % @ merger exit
2 ps : 0.35 mm mrad, 0.16 % à εn = 0.60 mm·mrad, σp/p = 0.16 % @ merger exit

The	  results	  are	  used	  as	  ini8al	  values	  for	  simula8ons	  including	  bunch	  compression.	

Design of Main Linac Cavity	
ERL-EUV cavity (Model 1) – TESLA-type 9-cell cavity + 108ϕ beam pipe

Under design. A large-aperture beam pipe will be also applied to the left side.	

cERL cavity (Model 2) – stably operated at ~8.5 MV/m

Parameters for acceleration mode	Model 2Model 11300 MHz

1300 MHz

Rsh/Q

897 Ω

Ep/Eacc

3.0

Frequency

Model 2Model 1

Iris diameter

80 mm

70 mm

1007 Ω

Qo×Rs

289 Ω272 Ω

2.0

Hp/Eacc

42.5 Oe/(MV/m)

42.0 Oe/(MV/m)

Stable operation at 12.5 MV/m seems achievable due to reduced Ep/Eacc.	

Main Linac Optics	
Main linac optics

2 cryomodules	

Quadrupole triplet	n Main Linac
 64 cavities in 16 cryomodules (4 cavities/cryomodule) Eacc ≈ 12.5 MV/mn Optics
 Focusing of quadrupole triplet at every two cryomodules
 Body/edge focusing of cavities
 Betatron function optimization against BBU à Ith,BBU > 190 mA
 Symmetric for acceleration and deceleration

HOM Heating	Non-resonant heating	Heating resonant to monopole HOMs 	

Parasitic loss absorbed at HOM damper

Difference between monopole HOM frequency
and harmonics of bunch frequency	

Ploss = klossQb2 fbkloss: Loss factor, Qb : bunch charge
fb : bunch frequency	

Estimation of loss factor	

kloss ~ 20 V/pC @ 1 ps
~ 15 V/pC @ 2 ps	
Examples of parasitic loss power	

momopole
fHOM [MHz]	 325

Bunch frequency fb[MHz]260 162.5 135.4 130

100 81.25

2393

207

207

118

44

53

7452427

173

173

152

10

87

27

112442

158

158

158

5

102

42

52447

153

153

153

10

107

47

102452

148

148

148

15

112

52

152453

147

147

147

16

113

53

162459

141

141

141

22

119

59

22

Bunch
length@cavity	

9.75mA x 260pC
162.5MHz	8mA x 2100pC
81.25MHz	

1 ps	

23.4 W	

32 W	

3848

52

208

52

57

52

48

52

2 ps	

17.6 W	

24 W

3851

49

211

49

60

49

51

493852

48

212

48

61

48

52

483853

47

213

47

62

47

53

47

Max. absorption power of HOM damper :30 W (first target), 100 W (final goal) 	

:	  frequency	  diﬀerence	  within	  10	  MHz	Max. absorption power of the HOM damper restricts the bunch charge, length and frequency.
The bunch frequency should be selected so as to avoid the resonant heating.

FEL Parameters	
FEL power at saturation
Psat ≈ ρ FEL Pelectron , Pelectron = EI av

Pierce parameter
1/3! 1 I K 2 [JJ ]2 λ 2 $puρ FEL = #&32#"16 I A γ σ xσ y (2π ) &%Qb
Ip =, I A = 17kA σ x = γε nx β x , σ y = γε ny β y2πσ t[JJ] = J 0 (ξ ) − J1 (ξ ) , ξ = K 2 / (4 + 2K 2 )[JJ ] = 1Planar undulator

Helical undulator

Photon wavelength and undulator parameters
λu ! K 2 $λ = 2 #1+&2γ "2 %K = KyPlanar undulator

K = 2K x = 2K yHelical undulator

High peak current and low emittance are important for FEL power.

Bunch compression and decompression scheme (1)	  p 2nd turn	pt

1st turn	

1st turn	

2nd turn	

pbunch	tt

RF field	tt

Injector

Beam Dump

Main Superconducting Linac

1st Arc
R56 > 0, T566 > 0(R56 < 0, T566 < 0 )EUV Source (ERL)
Undulator（FEL)pt

Bunch compressor : 1st Arc
Bunch decompressor : 2nd Arc	

2nd Arc
R56 < 0, T566 < 0(R56 > 0, T566 > 0 )FEL light	pt

Bunch compression and decompression scheme (2)	  p 2nd turn	p

1st turn	

bunch	tt

p

1st turn	

2nd turn	t

RF field	tt
Injector

Beam Dump

Main Superconducting Linac
R56 < 0, T566 < 0

1st Arc + Chicane

EUV Source (ERL)

2nd Arc
R56 > 0, T566 > 0

Undulator（FEL)pt

Bunch compressor : Chicane: 1st Arc + Chicane
Bunch decompressor : 2nd Arc	FEL light	pt

Design of Arc Sections (1)	Dispersion function[m]	
Betatron function[m]	

2-cell TBA lattice and optics（R56=0.0 m）
ρ =3 m, θ =π/8 rad, LB=1.178 m, LQ=0.4 m, LSX=0.2 m	50	40	30	20	10	0	TBA cell	

0.5	0	ηcTBA cell	

R56 = 4 ρ (θ − sin θ ) + 2ηc sin θ-0.5	-1.0	-1.5	

SX3	B	 Q	 Q	Q	 B	

0	 SX1	
SX2	5	

SX4	SX1	

SX2	B	 Q	 Q	
Q	
B	 Q	
Q	
Q	
Q	
B	 Q	 Q	Q	 B	

10	15	s[m]	

20	B	 Q	 Q	
Q	 B	

25	
SX3	
SX4	30	

B: Bending magnet, Q: Quadrupole magnet, SX: Sextupole magnet	

Eight sextupole magnets can be inserted in the arc to optimize T566.

50	
40	
30	
20	10	0	R56=0.3m	0.5	0	-­‐0.5	

-­‐1.0	Dispersion	  func8on[m]	
Betatron	  func8on[m]	

-­‐1.5	50	40	
30	
20	10	0	50	40	
30	
20	
10	0	

R56=-0.3m	

0.5	0	-­‐0.5	

-­‐1.0	0	5	10	15	s[m]	

20	25	

30	

R56=0.6m	

0.5	0	-­‐0.5	

-­‐1.5	50	40	
30	
20	
10	0	0	5	10	15	s[m]	

20	25	30	10	15	s[m]	

20	25	

30	

R56=-0.6m	

0.5	0	-­‐0.5	

-­‐1.0	-­‐1.0	-­‐1.5	Dispersion	  func8on[m]	
Betatron	  func8on[m]	

Examples of 2-cell TBA optics with different R56

Dispersion	  func8on[m]	
Betatron	  func8on[m]	

Dispersion	  func8on[m]	
Betatron	  func8on[m]	

Design of Arc Sections (2)	0	5	10	15	s[m]	

20	25	30	-­‐1.5	0	5	The 2-cell TBA lattice has a wide dynamic range of R56.
Momentum acceptance is more than 4% for horizontal half-aperture of ~5cm.	

Design of Chicane	
Four-magnet chicane	
d	LD	
LB	

4LB
4LB2 LD
R56 = −− 2+ 4 ρθ
3cosθ ρ cos θBeam orbit	
Bending magnet	

ρ	 θ	Chicane optics for LB=1m and LD=d=0.51m
R56=-­‐0.30	  m,	  ρ=3	  m,	  θ=0.34	  rad	
R56=-­‐0.15	  m,	  ρ=4.1	  m,	  θ=0.246	  rad	

Bunch Compression by Arc	
simulation by Elegant

Main Linac + 1st Arc （R56=0.30 m）
Qb=60 pC	E=10.5 MeV	
E=800 MeV	Main linac	

entrance of main linac

entrance of 1st arc

σt=1.00	  psσp/p=0.250	  %	  σt=1.01	  ps	  σp/p=0.099	  %	  εnx=0.60	  mm	  mrad	  εny=0.60	  mm	  mrad	  εnx=0.60	  mm	  mrad	  εny=0.60	  mm	  mrad	  M1exit of 1st arc
σt=43.9	  fs	  σp/p=0.107	  %	  εnx=2.27	  mm	  mrad	  εny=0.60	  mm	  mrad	  RF phase and sextupole strengths maximizing the Pierce parameter
K2 (SX1)=-54.6 [m-3], K2 (SX4) =26.4 [m-3], φRF=96.7 [deg]	

1st arcBunch Compression by Chicane	
Main Linac + 1st Arc （R56=0.0 m） + Chicane (R56=-0.3 m)
Qb=60 pC	E=10.5 MeV	
E=800 MeV	Main linac	

entrance of main linac

entrance of 1st arc

σt=1.00	  psσp/p=0.250	  %	  σt=0.997	  ps	  σp/p=0.104	  %	  εnx=0.60	  mm	  mrad	  εny=0.60	  mm	  mrad	  εnx=0.60	  mm	  mrad	  εny=0.60	  mm	  mrad	  M11st arc

exit of chicane
σt=43.8	  fs	  σp/p=0.110	  %	  εnx=1.72	  mm	  mrad	  εny=0.60	  mm	  mrad	  RF phase and sextupole strengths maximizing the Pierce parameter
K2 (SX1)=-91.2 [m-3], K2 (SX4) =23.6 [m-3], φRF=82.4 [deg]	

M2 Chicane

Bunch Compression by Arc & Chicane	
Main Linac + 1st Arc （R56=-0.15 m） + Chicane (R56=-0.15 m)
Qb=60 pC	E=10.5 MeV	
E=800 MeV	Main linac	

entrance of main linac

entrance of 1st arc

σt=1.00	  psσp/p=0.250	  %	  σt=0.997	  ps	  σp/p=0.104	  %	  εnx=0.60	  mm	  mrad	  εny=0.60	  mm	  mrad	  εnx=0.60	  mm	  mrad	  εny=0.60	  mm	  mrad	  M11st arc

exit of Chicane
σt=43.2	  fs	  σp/p=0.108	  %	  εnx=1.67	  mm	  mrad	  εny=0.60	  mm	  mrad	  RF phase and sextupole strengths maximizing the Pierce parameter
K2 (SX1)=-110.5 [m-3], K2 (SX4) =41.4 [m-3], φRF=82.4 [deg]	

M2 Chicane

Peak Current & Slice Emittance	Bunch Compression by Arc	

Bunch Compression by Chicane	

Bunch Compression by Arc + Chicane	

Qb=60 pC	
projected normalized
horizontal emittance	

Slice emittance at high peak currents is lower than the projected emittance.	

FEL Performance 	
Electron beam parameters: E=800 MeV, Qb=60 pC, fb=162.5/325 MHz
Helical undulator parameters: K=1.652, λu=28 mm, Lu=2.8m
Bunch compression scheme: 1st Arc + Chicane	

67.6 µJ	
55.5 µJ	FEL power without tapering:
9.0/18.0 kW @ 9.75/19.5 mA
FEL power with 10% tapering: 11.0/22.0 kW @ 9.75/19.5 mA

Image of ERL-EUV Design	
1st Arc	

Beam Dump	Merger	

2nd Arc	Injector Linac	Gun	Summary & Outlook	• Design of ERL-EUV– Injector (gun, SRF cryomodule, tracking)– Main linac (cavity, optics, HOM BBU and heating)– Arcs and chicane (lattice, optics)– Bunch compression simulation• Performance of the designed ERL-EUV– 9 kW power at 9.75 mA without tapering– 11 kW at 9.75 mA with tapering• Further design work and optimization– Improvement of FEL power(tapering, optics, beam&undulator parameters etc.)– Bunch decompression simulation à S2E simulation

Thank you for your attention!	

Appendix	

HOM BBU	HOM-BBU threshold current is calculated by Simulation code bi.	
HOM parameters of Model 1 cavity	

Calculation of BBU threshold current	BBU Current / A

0.45

0.5

0.45

0.4

0.4

0.35

0.35

0.3

0.25
0.3

0.2

0.152
1.5

11

HOM randomization effects	

Average threshold <Ith> / A

1.8

0.25

0.8
0.6

Δψ (πrad) 0.5

0.4
0 0

0.2

ΔT/T00.2

Scan over the betatron phase advance (0-2π) andreturn loop length (in one period of the base mode).
Minimum BBU current is found to be about 195 mA.
(478mA maximum).	

1.6
1.4
1.2
10.8
0.60.4
0.2
00.5
1
1.5
2
2.5
3
3.5
Frequency randomization std. mf / MHz

Considering a Gaussian frequency distribution between linaccavities, the average BBU threshold current grows with thefrequency spread σf increases, reaching about 1.1A when σf
4 =2MHz.	

BBU threshold current is well above the expected average current.	

Bunch Compression and Decompression at cERL	
after deceleration	
②	⑥

before acceleration	
①	⑤	

σt=1.34[ps],	  σp/p=0.00385	 σt=1.20[ps],	  σp/p=0.00115	 σt=1.02[ps],	  σp/p=0.001	 σt=0.86[ps],	  σp/p=0.00388	

K2(SXIF2)=-52.29 [m-3]K2(SXIF4)=-34.97 [m-3]	

φRF(ACC)=24.62 [deg]	
Initial Condition at ① :Bunch charge : 7.7 pC
Initial bunch length : 1 ps
Initial momentum spread : 0.1 %σt=0.51[ps],	  σp/p=0.00391	 Initial norm. emittance : 1 mm·mrad③	

K2(SXIR2)=-64.41 [m-3]K2(SXIR4)=-40.76 [m-3]	

φRF(DEC)=205.95 [deg]	
④	

σt=45.2[fs],	  σp/p=0.00384	

Bunch compression and decompression are successfully simulated at cERL.	

Compensation of CSR effects (1)	Minimization of CSR-induced horizontal emittance growth	
Phase ellipse angle vs beam/FEL parameters at chicane exit
(bunch compression by chicane) 	
Q =60 pC	bx’	 φCSR=63.4°	

x’	 φCSR=63.4°	

x	x	φphase=148.6°	

φphase=32.9°	

x’	

2 tan(θ / 2)ρ (1− cosθ )(rectangular magnet)
tan φCSR =φCSR=63.4°	x	

φphase=56.1°	

∝1/ρFEL	tan 2φ phase =2α x
γ x − βx

The phase ellipse and CSR kick directions can be matched at chicane exit.	
Such optics adjustment is difficult for bunch compression by arc. 	

Compensation of CSR effects (2)	Minimization of CSR-induced horizontal emittance growth	
Phase ellipse angle vs beam/FEL parameters at chicane exit
(bunch compression by arc & chicane) 	
Q =60 pC	bx’	 φCSRφ=63.4°	
=36.9°	x’	 φCSR=63.4°	

phase

x	x	

φphase=151.0°	

x’	
φCSR=63.4°	x	

φphase=63.6°	

2 tan(θ / 2)ρ (1− cosθ )(rectangular magnet)
tan φCSR =∝ 1/ρFEL	

tan 2φ phase =2α x
γ x − βx

The phase ellipse and CSR kick directions can be matched at chicane exit.	
Such optics adjustment is difficult for bunch compression by arc.