Review of Experimental Results from High Brightness DC...

JAEA Nobuyuki Nishimori

Review of experimental results from high brightness dc guns: Highlights in FEL applications

Outline • Introduction • The DC gun-based Cornell photoinjector • High voltage DC gun at JAEA • High gradient and high voltage DC guns at KEK and Cornell • Summary

N. Nishimori FEL15 at Daejeon 2 Aug. 24, 2015

High repetition rate FEL Aug. 24, 2015 N. Nishimori FEL15 at Daejeon 3

10 kW FEL for EUV lithography

https://portal.slac.stanford.edu/sites/lcls_public/lcls_ii/Pages/design.aspx

MHz XFEL

N. Nakamura, “An ERL-based high-power free-electron laser for EUV lithography”, 2015 EUVL Workshop

LCLS-II injector specifications bunch charge 95 % en (mm) peak current (A) average current (mA) 20 pC 0.25 5 0.02 100 pC 0.40 10 0.1 300 pC 0.60 30 0.3

Aug. 24, 2015 N. Nishimori FEL15 at Daejeon 4

• The Cornell photoinjector demonstrated cathode thermal emittance dominated beam satisfying LCLS-II specifications in 2015.

• The Cornell photoinjector demonstrated average beam current of 75 mA much higher than LCLS-II specifications in 2013.

C. Gulliford et al., Appl. Phys. Lett. 106, 094101 (2015).

L. Cultrera et al., Appl. Phys. Lett. 103, 103504 (2013); B. Dunham et al., Appl. Phys. Lett. 102, 034105 (2013).

• Reliability (long-term operational experience) is important especially for industrial application such as 10 kW EUV FEL for lithography.

Cathode thermal emittance

bunch charge en,th (mm) 95 % en (mm) LCLS-II specifications

20 pC 0.11 0.25 100 pC 0.24 0.40 300 pC 0.41 0.60


I.V. Bazarov et al., Phys. Rev. Lett. 102, 104801 (2009).

𝜀𝑛,𝑡ℎ = 𝜎𝑥

𝑀𝑇𝐸

𝑚𝑐2

MTE: cathode mean transverse energy sx: initial rms beam size q: bunch charge E: applied cathode field

MTE = 140meV (corresponds to 0.5mrad) : NaKSb cathode E = 4.3 MV/m: 400 kV DC gun @Cornell

The applied cathode field E should be greater than image charge field . 𝑞 𝜀0 𝜋 2𝜎𝑥 2

q 𝐸 𝐸 −

𝑞

𝜀0𝜋 2𝜎𝑥 2

D. Nguyen et al., “RF Linac for High-Gain FEL Photoinjectors”, LA-UR 14-23995

≥ 1

2

𝑞

𝜋𝜀0𝐸

𝑀𝑇𝐸

𝑚𝑐2


Cornell photoinjector

Courtesy of Adam Bartnik


400 kV DC Gun + Bunching/Focusing

400 kV DC Gun

50 MHz 520 nm



NaKSb: MTE = 140 meV, QE roughly 5%



Short SRF Linac 5 – 15 MeV

400 kV DC Gun

50 MHz 520 nm



Superconducting RF Cavities (niobium)




400 kV DC Gun

50 MHz 520 nm






6 Dimensional Phase Space Diagnostics + High Power Beam Dump

Viewscreen

Emittance Measurement System

1st Slit 2nd Slit Deflecting Cavity

Faraday Cup + Viewscreen

Faraday Cup

400 kV DC Gun

50 MHz 520 nm



Beam based alignment with near-zero bunch charge

• Measured beam response of each injector component.


C. Gulliford et al., Phys. Rev. STAB 16, 073401 (2013).

Bunch charge εn,x(100%) εn,y(100%) 19 pC 0.33 mm 0.20 mm 77 pC 0.69 mm 0.40 mm 300 pC 1.36 mm 0.79 mm


• Performed beam based alignment of gun, buncher, SRF cavities, and solenoids within 50 mm accuracy.

• Confirmed the emittance and beam sizes are reproduced by GPT simulations.

• Loaded optimum parameter set for each bunch charge and measured emittance and longitudinal profile.

Increasing solenoid current


Origin of asymmetry • Found a beam asymmetry after the first solenoid. • Likely culprit: stray quad field in the solenoid.


Increasing solenoid current

Quad wiring

• Mitigated with a correcting quadrupole coil.


Q (pC) Ipeak Target (A) Ipeak (A) εn Target (95%, mm) εn (95%, mm) εn,th /εn 20 5 5 0.25 H: 0.18, V: 0.19 60%

100 10 11.5 0.40 H: 0.32, V: 0.30 80% 300 30 32 0.60 H: 0.62, V: 0.60 70%



Emittance and longitudinal profile measurements

Aug. 24, 2015 15

Effects of the Laser Shape N. Nishimori FEL15 at Daejeon



Effects of the laser shape

Aug. 24, 2015 16


Ideal Shape




Aug. 24, 2015 17


Ideal Shape




Aug. 24, 2015 18


Ideal Shape Measured Shape




Development of a 500 kV gun at JAEA

Field emission seg

men

ted

cera

mic

s

guar

d rin

gs

R. Nagai et al., RSI 81, 033304 (2010). N. Nishimori et al., PRSTAB 17, 053401 (2014).

• a segmented insulator • 160 mm acceleration gap

stal

k

N. Nishimori FEL15 at Daejeon 19

acceleration gap

Time (hours) 0 50 200 100 150

0

100

300

500

600

Hig

h vo

ltage

(kV

) 200

400

Aug. 24, 2015

1.8mA@500keV

0

100

300

500

Hig

h vo

ltage

(kV

)

Bea

m c

urre

nt (m

A)

0.0

0.5

1.0

1.5

2.0

2.5

200

400

0 2 6 1 5

600

Time (min.)

500keV beam generation N. Nishimori et al., APL 102, 234103 (2013)


500-kV gun

beam dump radiation shield

bending magnet

solenoid

1.2

0 20 10 30 40 50

1mA@440keV

Time (min.)

Bea

m c

urre

nt (m

A)

0.0

0.4

0.8

30 min.

60 0

100

300

500

Hig

h vo

ltage

(kV

)

200

400

600

Aug. 24, 2015

Injector acc.

compact ERL (cERL) at KEK N. Nishimori FEL15 at Daejeon 21

gun

Gun assembly at cERL

Oct. 2012 Transport of gun from JAEA to cERL at KEK Jun. 2013 Injector commissioning 5MeV-0.3μA Mar. 2014 ERL loop commissioning 20MeV-4.5μA Mar. 2015 Laser Compton Scattering (LCS) 20MeV-80mA

Gun voltage is limited to 390kV at cERL.

Aug. 24, 2015

bunch charge en (mm) peak current (A) average current (mA) 60 pC 0.60 30 10


Reliability and high average current are important as well as brightness.

N. Nakamura et al., “Design work of the ERL-FEL as the high intense EUV light source”, Proc. of ERL2015 (2015).

10 kW EUV FEL injector specifications after merger

cERL prototype for future ERL light sources: LCS, THz, and EUV FEL


Gun operational status for two weeks Stable operation

Interruption to enter the acceleration hall

Pressure rise caused by gate valve open and close

500 kV

Gun HV

Vacuum pressure

10-10 Pa

0 kV

390 kV

10-8 Pa

0.7x10-9 Pa

10-7 Pa

N. Nishimori FEL15 at Daejeon

Gun operational status during LCS experiment 24

100

QE

(%)

GaA

s @

530n

m

0 2

time (hours)

6 B

eam

cur

rent

(μA

)

80

60

40

20

0

Aug. 24, 2015

1

5

4

3

2

1

0

10

8

6

4

2

0

12

14

5

4

3

2

1

0

6

7

Qua

ntum

effi

cien

cy (%

)

Cha

rge

extra

ced

(C)

Jan. 2015

Feb. Mar. Apr. May

1/e life=6,000 hours

July


laser position

June

QE during the cERL operation GaAs @ 530nm

a short

a short

A Plan to 500kV operation at the cERL 26 N. Nishimori FEL15 at Daejeon Aug. 24, 2015

a short

a short

A Plan to 500kV operation at the cERL 27 N. Nishimori FEL15 at Daejeon Aug. 24, 2015

addition

addition

stal

k

a short

a short

A Plan to 500kV operation at the cERL 28 N. Nishimori FEL15 at Daejeon

• Installed an additional ceramics in July 2015 • Performed HV test up to 550 kV without stalk

Aug. 24, 2015

• Install a stalk in Sep. 2015 • Perform HV test with cathode electrode • Perform beam generation in Nov. 2015

addition

addition

stal

k

Aug. 24, 2015

Time (hours)

0

100

200

300

400

500

600

Hig

h vo

ltage

(kV

)

10-9

10-8

10-7

vacu

um (P

a)

0 1 2 3 4 5 6 7

550kV for > 3hours

HV test with the additional insulator without stalk N. Nishimori FEL15 at Daejeon 29


MOGA optimization for EUV FEL photoinjector Aug. 24, 2015

Courtesy of Tsukasa Miyajima

N. Nakamura et al., Proc. of ERL2015, MOPCTH010

bunch charge en (mm) peak current (A) 60 pC 0.60 30

10 kW EUV FEL injector specifications after merger

EUV FEL injector (merger not included)

500 kV Gun Two solenoid magnets Two injector cryomodule (Max. Eacc: 8 MV/m) Buncher

Beam Einj=10.5 MeV

1ps 2ps

en=0.30mm, 30A before merger

Q=60 pC、E=10.5 MeV 0.6

0.5

0.4

0.3

0.2

0.1

0.0

100%

em

ittan

ce(m

rad)

0 0.2 0.4 0.6 0.8 1 rms bunch length [mm]

Cathode Anode

e- beam

6.9 MV/m @ photocathode center (=E)

11.0 MV/m @ cathode ball surface

31

70 mm gap 500 kV gun at KEK

Courtesy of Masahiro Yamamoto

𝜀𝑛,𝑡ℎ ≥ 1

2

𝑞

𝜋𝜀0𝐸

𝑀𝑇𝐸

𝑚𝑐2

Vacuum pressure: ~4x10-10 Pa

32

HV conditioning test 1st day (~2.5 h)

2nd day (~4 h)

Reached 550kV in 48 trips / a short time (~7hs.)


10H 2H 4H 8H 8.7H 8H 8H

No breakdown happened during totally 50 hours of 500 kV-holding test.

33

HV holding test


Variable gap segmented insulator gun at Cornell

34

Stability test with various gaps:

Dis

char

ge R

ate

(per

min

)

50mm 30 mm 35 mm 40mm

J. Maxson et al., RSI 85, 093306 (2014)

50 mm gap

P. Slade, The Vacuum Interrupter, CRC Press, 2008

Aug. 24, 2015 N. Nishimori FEL15 at Daejeon

Courtesy of Jared Maxson

Variable gap segmented insulator gun at Cornell

35

Stability test with various gaps:

Dis

char

ge R

ate

(per

min

)

50mm 30 mm 35 mm 40mm • Surprisingly good agreement between different HV systems.

• But what configuration is best for the beam emittance? –Turn to simulations.

J. Maxson et al., RSI 85, 093306 (2014)

50 mm gap

Recent Result from KEK




MOGA optimizations with various gaps • Choose 3 Cornell style guns as the injector source use MOGA

• 500 kV: 70mm • 450 kV: 50 mm • 400 kV: 30 mm

36


larger field…

…smaller V

Z position within gun (mm)

𝐸𝑧 (

MV

/m

)



Emittance vs. bunch charge with various gaps

37

Approaching thermal emittance!



100

% e

mit

tan

ce (𝜇𝑚

)

Co

re e

mit

tan

ce (𝜇𝑚

)

𝜖𝑐𝑛𝑥 =1

4 𝜋𝜌𝑚𝑎𝑥

Summary


• The DC gun-based Cornell photoinjector demonstrated cathode emittance dominated beam satisfying LCLS-II specifications with a 400 kV gun.

• Higher voltage and higher gradient gun developments with segmented insulators are in progress at JAEA/KEK and Cornell University to go beyond the Cornell photoinjector. • 500 keV beam generation from JAEA gun with 160 mm gap • 50 hours holding at 500kV with 70 mm gap at KEK gun • Experimental and numerical studies with 20-50 mm gaps at Cornell gun

• Gun operational experience for users is accumulated at the cERL.

Acknowledgements


We acknowledge Adam Bartnik, Colwyn Gulliford, Jared Maxson, Bruce Dunham, and Ivan Bazarov at Cornell University for useful discussion on the Cornell photoinjector developments. We thank Masahiro Yamamoto, Tsukasa Miyajima, Yosuke Honda at KEK, Masao Kuriki at Hiroshima University, Ryoji Nagai and Ryoichi Hajima at JAEA for helpful discussion on KEK/JAEA gun developments. This work is partially supported by JSPS Grants-in-Aid for Scientific Research in Japan (15H03594).

Review of Experimental Results from High Brightness DC...

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