Low Emittance RF Gun
Developments for PAL-XFEL
Jang-Hui Han, Juho Hong,
Munsik Chae, Sung-Ju park
Pohang Accelerator Laboratory
Accelerator Physics Seminar, DESY, 14 November 2012
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
Accelerator Physics Seminar, DESY, 14 November 2012
2
PAL-XFEL Project
• S-band 10 GeV linac + 1 hard & 1 soft X-ray undulator beamlines
(space 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
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
PLS II
3
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
L3:S
BC3_S
De-Chirper
Soft X-ray
FEL
3.15 GeV
RF Gun
L1
X
Laser
Heater
BC1
330
MeV
L2
BC2
2.52
GeV
BC3_H
L3
kicker
3.45
GeV
L4
10 GeV
Linac
Undulator Hall
BTL
710 m
Jang-Hui HAN
Hard X-ray FEL
780 m
Accelerator Physics Seminar, DESY, 14 November 2012
Beamline
1000 m
1100 m
4
Two Undulator Lines for Phase 1
e- gun
e
3 GeV
10 GeV
-
PU
Two bunches
at 60 Hz
Planar Undulator x 24
λ = 0.6 nm ~ 0.06 nm
PU + EPU
λ = 4.5 nm ~ 1.0 nm
Kicker
EPU
SX1
SX2
HX1
HX2
HX3
Exp.
Hall
Soft X - ray Undulator Hall
(~110 m including Dump Section)
Hard X - ray Undulator Hall
(~225 m including Dump Section)
Undulator Line
HX1
SX1
Wavelength, nm
0.1
3.0
Beam energy, GeV
10
3.15
Undulator period, mm
24.4
34.0
Undulator gap, mm
7.2
8.3
5
5
Undulator module length, m
Jang-Hui HAN
Exp.
Hall
Accelerator Physics Seminar, DESY, 14 November 2012
5
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
Accelerator Physics Seminar, DESY, 14 November 2012
6
PAL ITF Gun1
e beam
• 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
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
7
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 system
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
8
Beam Dynamics, Flat-top Laser – 1
Cathode laser
Longitudinal shape:
- 8 short Gaussian pulses stacked
- 8 ps fwhm
Transversely uniform
200 pC beam simulation at injector end
 0.26 mm mrad transverse projected
emittance
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
9
Beam Dynamics, Flat-top Laser – 2
Emittance at central slices
below 0.3 mm mrad
200 pC
0.3 mm mrad
Peak current 26 A
Mismatch parameter
close to 1
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
10
Beam Dynamics, Flat-top Laser 20 pC
Emittance at central slices
below 0.1 mm mrad
4 ps flat-top laser pulse profile
(4 Gaussian pulses stacked)
0.1 mm mrad
Peak current 5 A
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
11
Beam Dynamics, Gaussian Laser
200 pC
0.3 mm mrad
Gaussian laser pulse profile
instead of flat-top
Peak current 26 A
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
12
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
Accelerator Physics Seminar, DESY, 14 November 2012
13
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
Accelerator Physics Seminar, DESY, 14 November 2012
14
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
Accelerator Physics Seminar, DESY, 14 November 2012
15
Optimization of a New Gun
1st cell
2nd cell
LCLS
2.856 GHz gun
Scaled DESY
1.3 GHz gun
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
16
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 guns
Coaxial RF coupler
 perfect RF symmetry
Optimum solenoid position
 minimized transverse emittance
Cathode plug exchangeable under UHV
 more reliable operation
 possibility using other cathode materials
Improved water-cooling
 more thermal stablility
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
17
Gun + Coupler
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
18
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
Accelerator Physics Seminar, DESY, 14 November 2012
19
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 sections
Mild velocity bunching at 1st section (67), energy chirp corrected by 2nd & 3rd sections (16)
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
20
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
Accelerator Physics Seminar, DESY, 14 November 2012
21
Low Bunch Charge, 20 pC
• 4 ps flat-top: four Gaussian pulses stacked
• Short Gaussian (no complicated pulse shaping)
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
22
PAL Injector Test Facility
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Accelerator Physics Seminar, DESY, 14 November 2012
S. J. Park
23
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
Accelerator Physics Seminar, DESY, 14 November 2012
24
ITF Status I
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
25
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
Accelerator Physics Seminar, DESY, 14 November 2012
26
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
Accelerator Physics Seminar, DESY, 14 November 2012
27
Thanks
Jang-Hui HAN
Accelerator Physics Seminar, DESY, 14 November 2012
28