Jang- Hui Han , Juho Hong, Munsik Chae , Sung- Ju park Pohang Accelerator Laboratory

Accelerator Physics Seminar, DESY, 14 November 2012

Jang-Hui Han, Juho Hong, Munsik Chae, Sung-Ju park

Pohang Accelerator Laboratory

Low Emittance RF Gun Develop-ments for PAL-XFEL

Accelerator Physics Seminar, DESY, 14 November 2012 2

Contents

• PAL-XFEL project overview

• Requirements of PAL-XFEL injector

• Baseline Injector

• Methods to further improve injector performance

• Alternative (Low Emittance) Injector

• PAL-XFEL injector test facility

Jang-Hui HAN


PAL-XFEL Project

• S-band 10 GeV linac + 1 hard & 1 soft X-ray undulator beamlines (s-pace for two more hard and one more soft lines reserved)

• 60 Hz rep. rate with single or two bunches• The project started in 2011 aiming at construction by 2014 • Building construction ongoing• Commissioning to start in 2015

PAL-XFEL PLS II

Jang-Hui HAN


Baseline Linac Layout

– Injector to 139 MeV + Laser heater– S-Band cavities for beam acceleration– X-band cavity for linearization in time-energy phase space– BC1 (1st magnetic bunch compressor) at 0.33 GeV– BC2 at 2.5 GeV– Beam branch to soft X-ray beamlines at 3 GeV– BC3 at 3.5 GeV– Linac end at 10 GeV

BC1

330 MeV

XRF Gun BC2

Hard X-ray FEL10 GeV2.52 GeV

Laser Heater

3.45 GeV

Soft X-rayFEL

L1 L2 L3 L4BC3_H

BC3_S De-Chirper

kicker

710 m 780 m 1100 m1000 m

Linac BTL Undulator Hall Beamline

L3:S

3.15 GeV

Jang-Hui HAN


Two Undulator Lines for Phase 1

Undulator Line HX1 SX1Wavelength, nm 0.1 3.0Beam energy, GeV 10 3.15Undulator period, mm 24.4 34.0Undulator gap, mm 7.2 8.3Undulator module length, m 5 5

Soft X- ray Undulator Hall(~110 m including Dump Section)

Hard X- ray Undulator Hall(~225 m including Dump Section)

e-gun

e-

Exp.Hall

Exp.Hall

Planar Undulator x 24λ = 0.6 nm ~ 0.06 nm

PU + EPUλ = 4.5 nm ~ 1.0 nm

HX1HX2HX3SX1

SX2

PU EPU

3 GeV 10 GeV

Kicker

Two bunches at 60 Hz

Jang-Hui HAN


How good Injector needed?

• Electron beam quality for – 0.1 nm FEL using 10 GeV electron beam– 1012 photons per pulse at 0.1 nm

30 GW with 0.2 nC bunch– Saturation within 120 m (24 x 5 m long undulators)

• Thermally stable at 60 Hz +

• Robust and reliable for user service

Jang-Hui HAN


PAL ITF Gun1

• Developed during last few years at PAL• 1.6 cell, 2.856 GHz• 2 RF coupler ports & 2 vacuum ports at the

full cell for RF symmetry• Gun solenoid at 205 mm from cathode

(no magnetic field compensation solenoid)• Basic test done at GTF in 2011• Commissioning started at ITF

e beam

Jang-Hui HAN


Baseline Injector Layout

• 2 accelerating sections with focusing solenoids• 120 MV/m max field at cathode, 5.7 MeV beam energy at gun exit• 20 MV/m & 25 MV/m gradients at 1st & 2nd accelerating tubes, 139 MeV

beam energy at injector end• Drive laser beam size is minimized to reduce thermal emittance (0.12 mm

rms for 200 pC beam 0.11 mm mrad thermal emittance)• 1st accelerating section starts at 2.2 m from the cathode (longer e-

beam focal length due to the smaller laser beam at cathode)• Two linac solenoids for beta-function control at the entrance of the laser

heater systemJang-Hui HAN


Beam Dynamics, Flat-top Laser – 1

Cathode laserLongitudinal shape:- 8 short Gaussian pulses stacked- 8 ps fwhmTransversely uniform

200 pC beam simulation at injector end 0.26 mm mrad transverse projected emittance

Jang-Hui HAN


Beam Dynamics, Flat-top Laser – 2

Emittance at central slices below 0.3 mm mrad

Mismatch parameter close to 1

Peak current 26 A

200 pC

0.3 mm mrad

Jang-Hui HAN


Beam Dynamics, Flat-top Laser 20 pC

4 ps flat-top laser pulse profile (4 Gaussian pulses stacked)

Peak current 5 A

Emittance at central slices below 0.1 mm mrad

0.1 mm mrad

Jang-Hui HAN


Beam Dynamics, Gaussian Laser

Peak current 26 A

Gaussian laser pulse profile instead of flat-top

200 pC

0.3 mm mrad

Jang-Hui HAN


Can We further improve Injector?

• Thermal emittance

• Gun cavity geometry

• Position of gun solenoid

• Position of first accelerating section– already done for the PAL-XFEL baseline injector

Jang-Hui HAN


Low Thermal Emittance

• Small drive-laser beam size– Electron beam dynamics optimization

• Small kinetic energy of emitted electrons from the cathode– Longer wavelength of drive-laser – Higher work function (and high quantum efficiency)

Need for R&D of drive-laser and photocathode Photocathode plug enables using various photocathode material

with different surface treatment

Jang-Hui HAN


Keeping Beam Quality through Gun

• Immediately after the beam emission, the beam is affected by space charge and mirror charge Strong acceleration field required

• On the other hand, strong surface field produces Schottky effect. The kinetic energy of emitted electrons increases.

• 120 MV/m peak field at the PAL-XFEL S-band gun

• Fast acceleration as well as keeping the electron beam size through the gun cavity is crucial to generate a small emittance beam.

Jang-Hui HAN


Optimization of a New Gun

1st cell 2nd cell

Scaled DESY 1.3 GHz gun

LCLS 2.856 GHz gun

Jang-Hui HAN


Low Emittance Gun (Alternative Gun)

In parallel with the baseline injector commissioning at ITF, a new injector with an alternative gun is being developed.

Alternative gun (2.856 GHz):A scaled down version of DESY L-band gunsCoaxial RF coupler perfect RF symmetryOptimum solenoid position minimized transverse emittanceCathode plug exchangeable under UHV more reliable operation possibility using other cathode materialsImproved water-cooling more thermal stablility

Jang-Hui HAN


Gun + Coupler

Jang-Hui HAN


Gun Development Status

• Gun cavity & coupler– First machining ongoing at a local machining company– Vacuum brazing to be done at PAL – Production to be ready January 2013

• Gun solenoids – Production ready at a local company– Magnetic field measured

• Gun test beamline– Under design– First operation planned summer 2013

Jang-Hui HAN


Low Emittance Injector

Alternative gun + three accelerating sections (focusing solenoids at the first two sections)120 MV/m max field at gun, 14 MV/m gradient at 1st section, 20 MV/m at 2nd & 3rd sectionsMild velocity bunching at 1st section (67), energy chirp corrected by 2nd & 3rd sections (16)

Jang-Hui HAN


Nominal Bunch Charge, 200 pC

• 8 ps flat-top: eight Gaussian pulses stacked

• 4 ps flat-top: four Gaussian pulses stacked

• Short Gaussian (no complicated pulse shaping)

Jang-Hui HAN


Low Bunch Charge, 20 pC

• 4 ps flat-top: four Gaussian pulses stacked

• Short Gaussian (no complicated pulse shaping)

Jang-Hui HAN


PAL Injector Test Facility

S. J. Park

Ca th o de t o c ou p le r ce n te r : 5 9 .3Fu ll ce ll le n g th : 3 2. 52

Ha lf ce ll le n g th : 2 3Iri s th ic kn e ss : 2 0P u mps T y pe ( B ea m li ne & WG ):P a : 2. 75 " 3 0 l/s - > 9 E A

P b : 4. 5" 1 5 0 l/s - > 7 E AP c : 6" 1 50 l /s -> 7 EA

P d : 6" 8 0 l/s - > 2 E A

G un So len oi d

G un

S PO OL

LE C or rec tor _3w / B ello w s_2

H B -S -D C - H B

W/ VPW/ VP

H B -S -D C - H B

W_ C P R

IS O L S

LE C or rec tor _1w / B ello w s_1

IC T_ 1G V_ 1

PcPbPc

PbPb

A /C S o le n oi d #1

Pc

Pa Pa

PaPa

PaPa

A /C # 1

Pb

E B

H B -T WI ST - AT T P D

C olli ma tor

S cre en _2La se r M irr or S cre en _1LE C or rec tor _2

G V_ 2

LE B en d B PM _1

P M_ 1

V P

S

W / VP _ C P R

TW

IST

P S -H B

Pa

Pc

C orr ec tor_ 4 (H E)w / B ello w s_3

B PM _2

P M_ 2

S PO OL

Pa

A /C S o le n oi d #2

A /C # 2

Pb

T C A V

IC T_ 2

F ara da y C up 2&

B eam D um p 2

S cre en _4

S cre en _3B PM _5

Wire S can ne r

Pb

PbG V_ 5

Pc

G V_ 3 G V_ 4

W/ VP

H B -S -D C - H B

S

E B

S

PcH B -T WI ST - AT TP D

TW

IST

P S -H B

V P

S

LCW

19200

V P

D C

S iC

V P

D C

S iC

V P

F ara da y C up 1&

B eam D um p 1

17770150

L H s po o l

L as e r H e a te r (to b e in s ta lle d in 2 0 1 3)

LASER CLEAN ROOM

CONTROL ROOM

E lb o wE lb o w

D C

S iC

E B

S

H B -S -D C - H B

W/ VP

Pc

Pa

V P

PcE B (1 4 4 x1 4 4)

D C ( 16 0 )

V P (2 1 0 )PcE B (1 4 4 x1 4 4)

D C ( 16 0 )

V P (2 1 0 )

19.2 m

Jang-Hui HAN


Purpose of ITF

• Commissioning of PAL-XFEL injector (one gun + two accelerating sections) including laser heater and RF deflector

• Test of new guns, laser, diagnostics, control, cooling, … • Practice of beam operation, education of new operators

Jang-Hui HAN


ITF Status I

ITF tunnel Gallery

Jang-Hui HAN


ITF Status II

• RF station installed• GTF gun with solenoid installed• 2 linac tubes with solenoids installed• Laser system installed in the laser room• Laser optics being installed in the tunnel• RF network being installed• RF conditioning being started First beam foreseen in the next weeks

Jang-Hui HAN


Summary

• Baseline injector– PAL ITF gun1 + two accelerating sections– Fully satisfy PAL-XFEL basic requirement– Installed at PAL-XFEL ITF, RF conditioning ongoing, first

beam foreseen this month

• Alternative injector– Alternative gun + three accelerating sections– Will provide further low emittance– Thermally more stable, photocathode easily exchangeable– Under production, first beam foreseen summer 2013

Jang-Hui HAN


Thanks

Jang-Hui HAN

Documents

Jang- Hui Han , Juho Hong, Munsik Chae , Sung- Ju park Pohang Accelerator Laboratory