THE LATE NGLS:
Marco Venturini
LBNL
Sept. 26, 2013
1 ─ M. Venturini, Sept. 26, 2013, SLAC



Guiding principles for choice of main parameters, lattice design, bunch compression
RF vs. magnetic compression
Single vs. multiple stage magnetic compression
Description of layout, lattice, working point baseline


Preservation of beam quality and beam dynamics issues (single bunch)
 Longitudinal dynamics
 CSR-induced emittance growth
The microbunching instability
Transverse space-charge effects in the low-energy section of the linac
Impact of availability of passive de-chirping insertion on machine design
 Lowering degree of RF (velocity bunching) compression
2 ─ M. Venturini, Sept. 26, 2013, SLAC

All bunches exiting the linac have same design characteristics, are adequate to feed any of the FEL beamlines (1keV photon /energy)
 Different kinds of beam tailored to specific FEL beamlines are a speculative possibility. Not investigated yet.




As high as possible peak current consistent with:
 Flat current profile
Flat energy profile
Minimal degradation of transverse emittance (both slice and projected)
Sufficiently small energy spread
Sufficiently long bunches to support two- (three-?) stage HGHG external-laser seeding
2.4 GeV beam energy
Q=300 pC/ bunch
3 ─ M. Venturini, Sept. 26, 2013, SLAC

At cathode of proposed gun bunch current is very low I ~ 5-6A
 Substantial compression is needed


Magnetic compression:
 Energy chirp at exit of last compressor
CSR effects
Low-frequency SC RF structures would be needed for acceptance of very long initial bunches
RF compression in injector (velocity bunching):
 Less than ideal current profile
 Space-charge effects, emittance compensation
Adopted approach : do both RF and magnetic compression
 Right balance depends on various factors ( removed after compression ) e.g. how much chirp can be
 RF compression to 40-50A range has shown overall best results
4 ─ M. Venturini, Sept. 26, 2013, SLAC

Overall magnetic compression ~ 10 or higher.
One-stage compression:
 Minimizes microbunching instability
Two-stage compression:


More favorable to preservation of transverse emittance
Better beam stability
Three-stage compression
 Adds complexity; may aggravate microbunching instability
Adopted approach :
 Two-stage compression with flexibility for single-stage compression
(disabling second chicane).
5 ─ M. Venturini, Sept. 26, 2013, SLAC

Linearizer off
Magnetic compressors are conventional
C-shaped chicanes
 BC1 @ 215MeV (Sufficiently high to reduce
CSR effects on transverse emittance)
 BC2 @ 720MeV (There may be room for optimizing beam energy)
Potential harm from large angle (36 deg) between linac axis and FELs (CSR)
6 ─ M. Venturini, Sept. 26, 2013, SLAC
Large dephasing to remove energy chirp

Out of injector beam
(ASTRA simulations) curvature  head head
I pk
~45 A flat core 
Physics model in Elegant simulations (next 4 slides) includes:
• 2 nd order transverse dynamics
• Ideal (error free) lattice
• Longitudinal RF wakefields
(using available models for TESLA cavities)
• CSR
Not included:
• LSC, RW wakes, transverse RF wakes slice proj. e e
┴
≤0.6 m m
┴
=0.72 m m relatively long tail is a signature of velocity compression 
7 ─ M. Venturini, Sept. 26, 2013, SLAC

BC1 exit (factor ~2 compression) BC2 exit (~5 compression) head
I pk
~90 A head flat current profile as desired 
(current not very high but adequate)
8 ─ M. Venturini, Sept. 26, 2013, SLAC
Curvature of energy profile, to cause current spikes, harm radiation coherence if we compressed much more
I pk
~500 A substantial portion of bunch is in the tail 

Exit of linac Entrance to FEL beamlines head CSR long. wake in spreader helps somewhat with energy chirp removal head
Energy profile relatively flat within beam core 
Note: tracking done through fast-kicker based spreader
9 ─ M. Venturini, Sept. 26, 2013, SLAC
Flat core is
> 300fs long

Careful lattice design keeps projected emittance almost unchanged by the exit of spreader (<0.8
m m)
(two-stage compression)
Projected emittances through spreader vertical vertical x/z and x’/z sections head
Slice* x-emittance (exit of spreader) 𝜸𝜺 𝒙
= 𝟎. 𝟔𝝁𝒎 head
10 ─ M. Venturini, Sept. 26, 2013, SLAC
*slice is 5 m m 10
10

wakefields (RF, CSR) generate energy chirp w/ positive quadratic term within bunch
Turning on linearizer would add to positive quadratic chirp, pushing beam tail forward upon compression, and causing current spike*
Exit of BC1 Exit of BC2 Exit of linac head head
Elegant simulations for baseline working point ; linearizer off
11 ─ M. Venturini, Sept. 26, 2013, SLAC
*Details depend on machine settings

Longitudinal phase space is comparable to that of 2-stage compression
BC1 at beam energy ~ 250MeV;
 BC2 off
Linearizer on (20MV), decelerating mode
Reduced dephasing of L3S (20 deg)
Exit of spreader head
Projected emittances through spreader head
BC1 angle rad
0.0955
R
56 m
0.0856392
BC2 0.
MV Indeterminate MV Acc. Grad MV m no. modls
L1
L2
HL
L3
201.6
549.998
20.
1706.67
30.
0.
180
20.
174.591
549.998
20.
1603.75
13.8763
12.619
8.25971
13.0524
2
6
1
18
12 ─ M. Venturini, Sept. 26, 2013, SLAC vertical horizontal

Transverse space charge effects in low-energy section of linac
Space charge affects: 𝜷 𝒙,𝒚
− matching y with space charge
(dashed) x
 some effects in section between
Laser Heater (~95MeV) and BC1
(~210MeV)
 IMPACT simulations (Ji Qiang)
Exit of injector
 Emittance growth not large (~10%)
 but a portion of it is slice rather than projected emittance growth.
 Possible remedy: Increase beam energy at exit of injector
 2 vs 1 cryomodules?
w/o space charge
(solid lines)
E=94 MeV
Entrance of L1 emittances with space charge
(dashed) y x w/o space charge
13 ─ M. Venturini, Sept. 26, 2013, SLAC

Linear gain for 2-stage compression
Seeded by shot noise and perturbations at the source
(e.g. non-uniformity in photo-gun laser pulse)
Consequences


Slice energy spread
Slice average energy
(penalty on lasing efficiency)
(penalty on radiation spectral purity, in particular in externally seeded FELs beamlines)
Modeling primarily by IMPACT; simulations w/ multibillion macroparticles to minimize numerical noise.
Current profile
2-stage compression
Longitudinal phase space head head
10keV
5keV
14 ─ M. Venturini, Sept. 26, 2013, SLAC
10keV
Compare various degree of heating (rms)
5keV

Slice energy along bunch
Two-stage compression:
 Slice energy spread is minimum for s
E heating
=15keV
 Variations of slice energy are on the order of the energy spread (~200keV). Too big? s
E
=15keV heating
One-stage compression:
 Instability is effectively suppressed for s
E
=10keV heating
IMPACT simulations
Slice* energy spread vs. Heater setting
450
400
350
300
250
200
Two-stage compression
150
100
5 10 15
E
Laser Heater keV
15 ─ M. Venturini, Sept. 26, 2013, SLAC
20
*Slice is 1 m m ~ coop length
D
E~200keV

Initial current profile w/ perturbation
Amplification of modulation depends strongly on period of perturbation
Current profiles at exit of linac (Two-stage compression)
5% perturbation, 3.4ps period 5% perturbation, 0.8ps period head head
16 ─ M. Venturini, Sept. 26, 2013, SLAC z (mm)

Microbunching seeded by sinusoidal current perturbation at cathode (II)
Slice energy along core of bunch
(exit of Spreader)
5% amplitude perturbation on current at cathode
 s
0.8 ps period
E
=15keV heating
IMPACT simulations
17 ─ M. Venturini, Sept. 26, 2013, SLAC
Energy profile for two-stage compression shows
~200keV ripple
(comparable to instability
Seeded by shot noise)
Energy profile for one-stage compression remains
Relatively smooth
17

s
E
PM Undulator gap vs. e-beam energy @LH
~0.16 MW laser peak power
 for ~15keV rms energy spread
~2.2𝜇𝐽 /laser pulse
~2.2 W laser average power @1MHz
(at LH undulator)
Dedicated laser system
Commercially existing, high-repetition rate, shortpulse, high-power laser
Laser peak power* requirement for s
E
=12keV
18 ─ M. Venturini, Sept. 26, 2013, SLAC
Accurate simulation of 3D laser-beam interaction w/ collective f orces (“trickle” effect) still missing.
*Neglecting diffraction effects l u l
L
E b s
┴
5.4 cm
1.064 m m
94 MeV
160 m m

How could availability of passive “dechirping” insertions affect the linac design?
Longitudinal Phase Space
L3 on crest add 5-m long de-chirper
(r = 3 mm)
…or 35-deg off crest
P.Emma
1.

Save on no. of cryomodules in last linac section (or allow for higher beam energy)
5m long, r=3mm corrugated pipe would do the dechirping job (L3S on crest )
2.

Allow for compression through the spreader lines (a bit far fetched…)
Different FEL lines with differently compressed bunches
3.

Increase amount of magnetic compression relative to RF compression as a way to increase beam quality
Deliver beams with more compact current profile and possibly higher peak current
19
19 ─ M. Venturini, Sept. 26, 2013, SLAC

longitudinal dynamics in the injector
(kinetic E)
0.58
0.56
0.54
0.52
head
0 1 2 3
20 z mm
SLAC
4
Fig. from C. Papadopoulos
Current profile
20 head

half-way through the gun
1.00
0.95
0.90
0.85
0.80
head
0.75
14 16 18 20 22 24 26 z mm
21 ─ M. Venturini, Sept. 26, 2013, SLAC
Fig. from C. Papadopoulos
Current profile
21 head

past the exit of the gun
1.255
1.250
1.245
1.240
1.235
65 70 head
75
Space-charge induced
22 z mm
SLAC energy chirp
Fig. from C. Papadopoulos
Current profile
22 head

right before the buncher
1.26
1.25
1.24
head
1.23
695 700 705 z mm
23 ─ M. Venturini, Sept. 26, 2013, SLAC
Fig. from C. Papadopoulos
Current profile
23 head

right after the buncher
1.36
1.34
1.32
energy chirp head buncher
(@
995 1000 1005
24 z mm SLAC
Fig. from C. Papadopoulos
Current profile
24 head

ballistic compression begins
Energy profile s 1.4m
1.37
1.36
1.35
1.34
1.33
1.32
1.31
1.30
head
1392 1394 1396 1398 1400 1402 1404 1406
25 ─ M. Venturini, Sept. 26, 2013, SLAC
Fig. from C. Papadopoulos
Current profile
25 head

a tail in current profile develops
1.350
1.345
1.340
1.335
1.330
1.325
1.320
1.315
head
2186 2188 2190 2192
26 z mm SLAC
Long tail is associated with 2 nd order chirp
Fig. from C. Papadopoulos
Current profile
26 head

A 650MHz booster for the APEX injector?
Option of very low RF compression
 enabled by availability of passive dechirpers (we could afford making more magnetic compression)
~10A peak current, ~1.2cm FW bunch length (300pC)
Bunches are too long for a 3.9GHz linearizer
 choose 1.3GHz rf frequency for the linearizer (same as in
Linac structures)
 injector booster at 650MHz
(Very) preliminary study using LiTrack and parabolic model of beam
 layout with three magnetic BCs (BC1 functionally replacing most of the RF compression in the injector)
 simulations show improvement in longitudinal phase space
 transverse emittance could suffer from low-energy compression
Possible layout for injector, first linac Section .
With moderate ( 𝐶 ≃ 2)
RF compression, beam is close to parabolic.
Snap-shot of NGLS baseline beam @0.4m downstream the buncher (IMPACT
simulations)
Long. phase space at exit of linac
27
27 ─ M. Venturini, Sept. 26, 2013, SLAC


Delivered beam meets FEL design requirements


I=500A flat current profile over about 300fs core
Relatively long tail is harmless but wastes a good fraction of charge
 Relatively flat energy profile in core
 Nonlinear energy chirp in the beam tail
 e x
=0.6 e x
=1 m m (slice) preserved; e x
=0.8 m m (projected) for 1-stage compression m m projected (two-stage compression)
CSR in spreader not harmful at this current
 CSR longitudinal wake helps with energy chirp removal from beam core (but adds some nonlinearity on energy chirp)
The microbunching instability seeded by shot noise is effectively suppressed by heating to
 s
E
= 10keV in one-stage compression mode
In two-stage compression, heating to s
E
= 15keV yields ~150 keV final slice rms energy spread (acceptable) but also slice average energy variations of the same magnitude.
 B eam current at cathode should be smooth within a few %’s, or much less depending on spectral content of noise
Availability of reliable dechirper-insertion would open up interesting possibilities
 Reduce RF compression for better beam quality.
28 ─ M. Venturini, Sept. 26, 2013, SLAC