OVERVIEW OF LINAC DESIGN, BEAM DYNAMICS

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16 Νοε 2013 (πριν από 3 χρόνια και 4 μήνες)

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1



M.
Venturini
, Sept. 26, 2013,
SLAC

Marco
Venturini

LBNL

Sept. 26, 2013

THE LATE NGLS:


OVERVIEW OF
LINAC

DESIGN,
BEAM DYNAMICS

2



M.
Venturini
, Sept. 26, 2013,
SLAC

Outline

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







3



M.
Venturini
, Sept. 26, 2013,
SLAC

Requirements informing choice
of
l
inac

design


All bunches exiting the
l
inac

have same design characteristics,
are adequate to feed any of the FEL
beamlines

(1keV photon /energy)


D
ifferent kinds of beam tailored to specific FEL
beamlines

are a speculative
possibility. Not investigated yet.



A
s 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



4



M.
Venturini
, Sept. 26, 2013,
SLAC

RF
vs.

m
agnetic compression

A
t cathode of proposed gun bunch
current

is very low
I ~ 5
-
6A


Substantial compression is needed



M
agnetic compression:


E
nergy 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
c
ompression


Right balance depends on various factors (
e.g.
how much chirp can be
removed after compression
)


RF compression to 40
-
50A range has shown overall best results



5



M.
Venturini
, Sept. 26, 2013,
SLAC

Single vs. multiple stage magnetic
compression

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).

6



M.
Venturini
, Sept. 26, 2013,
SLAC

Machine layout,

highlights of
linac

settings

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
l
arge angle (36 deg)
between
l
inac

axis and FELs (CSR)




Linearizer off

Large
dephasing

to remove

energy chirp

7



M.
Venturini
, Sept. 26, 2013,
SLAC

Baseline beam out of injector

(
used in Elegant simulations of
l
inac
)

Out of injector beam

(ASTRA simulations)

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

r
elatively long tail

i
s a signature of velocity

compression



head

flat core


head

c
urvature


s
lice
e


≤0.6
m
m

p
roj
.
e

=0.72
m
m

I
pk
~45 A

8



M.
Venturini
, Sept. 26, 2013,
SLAC

Elegant tracking:

Longitudinal dynamics through BCs

BC2 exit (~5 compression)

BC1 exit (factor ~2 compression
)

I
pk
~90 A

I
pk
~500 A

s
ubstantial portion

of bunch is in the tail


Curvature of energy profile,

t
o cause current spikes,

h
arm radiation coherence

if we compressed much more

f
lat current profile as desired




(current not very
high
but adequate)

head

head

9



M.
Venturini
, Sept. 26, 2013,
SLAC

Elegant tracking: Longitudinal dynamics

through
l
inac

and Spreader

Entrance to FEL
beamlines


Exit of
l
inac

Energy profile relatively
f
lat

within beam core


Note: tracking done through

fast
-
kicker

based spreader

Flat core is

>

300fs long

CSR long. wake in

spreader
helps
somewhat with

energy
chirp
removal

head

head

10



M.
Venturini
, Sept. 26, 2013,
SLAC

Careful lattice design keeps
p
rojected
emittance

almost
unchanged by the exit of
spreader (<0.8
m


(two
-
stage compression)

10

x/z and x’/z sections

vertical

horizontal

P
rojected
emittances

through spreader

horizontal

vertical

Slice* x
-
emittance

(exit of spreader)

10

*slice is 5
m
m

head

head


𝜺

=
𝟎
.
𝟔𝝁𝒎

11



M.
Venturini
, Sept. 26, 2013,
SLAC

Aside on
setting of linearizer

wakefields

(RF, CSR)

g
enerate energy chirp

w/ positive quadratic term within bunch

Turning on linearizer


would add to positive

quadratic chirp, pushing beam

t
ail forward upon compression,

a
nd causing current spike*

*Details depend on machine settings

Elegant simulations for baseline working point
; linearizer off

head


Exit of BC1


Exit of BC2

Exit of
l
inac

head

12



M.
Venturini
, Sept. 26, 2013,
SLAC

One
-
stage compression

causes 25%
growth of projected
emittance

BC1 at beam energy ~ 250MeV;


BC2 off


Linearizer on
(
20MV), decelerating mode


Reduced
d
ephasing

of L3S (20
deg
)

angle
rad
R
56
m
BC1
0.0955
0.0856392
BC2
0.
Indeterminate
Voltage
MV
phase
deg
E
MV
Acc
.
Grad
MV
m
no
.
modls
L1
201.6
30.
174.591
13.8763
2
L2
549.998
0.
549.998
12.619
6
HL
20.
180
20.
8.25971
1
L3
1706.67
20.
1603.75
13.0524
18
P
rojected
emittances

through spreader

Longitudinal
phase space is
comparable


to that of 2
-
stage compression

Exit of spreader

vertical

horizontal

head

head

13



M.
Venturini
, Sept. 26, 2013,
SLAC

Transverse
space
c
harge effects in
low
-
energy
section of
l
inac



some effects in section between
Laser
H
eater (~95MeV) and BC1
(~210MeV)


IMPACT simulations (
Ji

Qiang
)



E
mittance

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
?

E=94 MeV

Space charge affects:




,


matching


emittances

with space charge

(dashed)

w/o space charge

(solid lines)

y

x

y

x

with space charge

(dashed)

w/o space charge

Entrance of L1

Exit of injector

14



M.
Venturini
, Sept. 26, 2013,
SLAC

T
he
microbunching

instability can damage
the longitudinal phase space

Seeded by shot noise and perturbations at the source
(e.g. non
-
uniformity in photo
-
gun laser pulse)


Consequences


Slice energy spread
(penalty on lasing efficiency)


Slice average energy
(penalty on radiation spectral purity,
in particular in externally seeded FELs
beamlines
)


Modeling primarily by
IMPACT;

simulations w/ multi
-
billion
macroparticles

to minimize numerical noise.

Linear gain for 2
-
stage compression

2
-
stage

compression

5
keV

10keV

head

Current profile

Longitudinal phase space

head

5
keV

10keV

Compare various
degree of
heating (
rms
)

Note: beam not fully compressed

15



M.
Venturini
, Sept. 26, 2013,
SLAC

Microbunching

seeded

by
shot noise

Two
-
stage compression:


Slice energy spread
is minimum for
s
E
=15keV
heating


Variations of
slice energy

are on the order of
the energy spread (~200keV). Too big?


5
10
15
20
100
150
200
250
300
350
400
450
E
Laser
Heater
keV
final
E
keV
Slice* energy spread
vs. Heater setting

Two
-
stage

compression

*Slice is 1
m
m ~ coop length

Slice energy
along bunch


One
-
stage compression:


I
nstability is effectively suppressed for
s
E
=10keV heating


IMPACT simulations

s
E

=15keV

heating

D
E~200keV

16



M.
Venturini
, Sept. 26, 2013,
SLAC

Microbunching

seeded by
sinusoidal

current perturbation
at cathode (I)

Amplification of modulation
depends strongly on period
of perturbation

I
nitial current profile w/ perturbation

5% perturbation,
3.4ps
period

Current profiles at exit of
l
inac

(Two
-
stage compression)

5% perturbation,
0.8ps
period

z

(mm)

z

(mm)

head

head

IMPACT simulations

17



M.
Venturini
, Sept. 26, 2013,
SLAC

Microbunching

seeded by sinusoidal current
perturbation at cathode (II)

5% amplitude
perturbation on current at
cathode


0.8
ps

period

s
E
=15keV
heating



Energy profile for one
-
stage

compression remains

Relatively smooth

Energy profile for two
-
stage

compression shows

~200keV ripple

(comparable to instability

Seeded by shot noise)

Slice energy along core of bunch


(
exit of Spreader)

IMPACT simulations

17

18



M.
Venturini
, Sept. 26, 2013,
SLAC

Specs for Heater with
s
E
~15keV heating
power are not too demanding

~0.16 MW laser peak power


f
or ~15keV
rms

energy spread


~
2
.
2𝜇𝐽

/laser pulse

~2.2 W laser average power @1MHz
(at LH
undulator
)


Dedicated laser system

C
ommercially existing, high
-
repetition rate, short
-
pulse, high
-
power laser

l
u

5.4 cm

l
L

1.064
m
m

E
b


94 MeV

s


160
m
m

Laser peak power* requirement for
s
E
=12keV

PM
Undulator

gap vs. e
-
beam energy @LH

*Neglecting diffraction effects

Accurate simulation

o
f 3D laser
-
beam

interaction w/ collective

f
orces (“trickle” effect)

s
till missing.

19



M.
Venturini
, Sept. 26, 2013,
SLAC

How could availability of passive “
dechirping

insertions affect the
linac

design?

1.
Save on no. of
cryomodules

in last
l
inac

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

add 5
-
m long
de
-
chirper

(
r

= 3 mm)

L3 on crest

…or 35
-
deg off
crest

Longitudinal Phase Space

P.Emma

20



M.
Venturini
, Sept. 26, 2013,
SLAC

Tracking the origin of the long bunch tail:
longitudinal dynamics in the injector

20



0
1
2
3
4
0.52
0.54
0.56
0.58
z
mm
E
MeV
s
1.2
mm
Fig. from C. Papadopoulos

Energy profile

Current profile

head

head

(kinetic E)

21



M.
Venturini
, Sept. 26, 2013,
SLAC

A walk down the injector (1):
half
-
way through the gun

21



Fig. from C. Papadopoulos



14
16
18
20
22
24
26
0.75
0.80
0.85
0.90
0.95
1.00
z
mm
E
MeV
s
2
cm
Energy profile

Current profile

head

head

22



M.
Venturini
, Sept. 26, 2013,
SLAC

A walk down the injector (2):
past the exit of the gun

22







65
70
75
1.235
1.240
1.245
1.250
1.255
z
mm
E
MeV
s
7
cm
Energy profile

Current profile

head

head

Space
-
charge

induced

energy
chirp

Fig. from C. Papadopoulos

23



M.
Venturini
, Sept. 26, 2013,
SLAC

A walk down the injector (3):
right before the
buncher

23







695
700
705
1.23
1.24
1.25
1.26
z
mm
E
MeV
s
70
cm
Energy profile

Current profile

head

head

Fig. from C. Papadopoulos

24



M.
Venturini
, Sept. 26, 2013,
SLAC

A walking down the injector (4):
right after the
buncher

24







995
1000
1005
1.30
1.32
1.34
1.36
z
mm
E
MeV
s
1
m
energy chirp
imparted by
buncher

(@
about zero
-
crossing
)

Energy profile

Current profile

head

head

Fig. from C. Papadopoulos

25



M.
Venturini
, Sept. 26, 2013,
SLAC

A walking down the injector (5):
ballistic compression begins

25







1392
1394
1396
1398
1400
1402
1404
1406
1.30
1.31
1.32
1.33
1.34
1.35
1.36
1.37
z
mm
E
MeV
s
1.4
m
Energy profile

Current profile

head

head

Fig. from C. Papadopoulos

26



M.
Venturini
, Sept. 26, 2013,
SLAC

A walking down the injector (6):
a tail in current profile develops

26







2186
2188
2190
2192
1.315
1.320
1.325
1.330
1.335
1.340
1.345
1.350
z
mm
E
MeV
s
2.19
m
Current profile

head

head

Energy profile

Long tail is associated
with 2
nd

order chirp

Fig. from C. Papadopoulos

27



M.
Venturini
, Sept. 26, 2013,
SLAC

A 650MHz booster
for the APEX injector?

O
ption 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
)


B
unches are too long for a 3.9GHz linearizer


c
hoose 1.3GHz
rf

frequency for the linearizer (same as in
Linac

structures)


i
njector booster at 650MHz


(Very) preliminary
study
using
LiTrack

and parabolic model
of beam


l
ayout with three magnetic BCs (BC1 functionally replacing
most of the RF compression in the injector)


s
imulations show improvement in longitudinal phase space


t
ransverse
emittance

could suffer from low
-
energy
compression


27

With moderate (
𝐶

2
)

RF compression,
beam is close to parabolic.

Snap
-
shot of NGLS baseline beam @0.4m
downstream the
buncher

(
IMPACT
simulations
)

Possible layout for injector, first
linac

Section
.

Long. phase space at exit of
linac


Passive insertion used for
dechirping


28



M.
Venturini
, Sept. 26, 2013,
SLAC

Conclusions

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


N
onlinear energy chirp in the beam tail


e
x
=0.6
m
m (slice) preserved;
e
x
=0.8
m
m projected (two
-
stage
compression)



e
x
=1
m
m
(projected) for 1
-
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.