LSCCSR Instability

1
LSC/CSR Instability Z. Huang, M. Borland (ANL),
P. Emma, J. Wu C. Limborg, G. Stupakov, J. Welch

• Introduction (origin of the instability)
• CSR/LSC
• Cure (laser heater)

2
Introduction

• FEL instability in the undulator requires very
  cold electron beams (small emittance and energy
  spread)
• Such a cold beam can be subject to other
  undesirable instabilities in the accelerator
• Bunch compression gives rise to a microbunching
  instability that may be harmful to LCLS
• A laser heater at the end of LCLS injector can
  be used to add incoherent energy spread to
  control LSC/CSR instability while preserving the
  FEL lasing

3
How cold is the photoinjector beam?
Parmela Simulation
TTF measurement
3 keV
DE/E
measured
simulation
mean
(sec)
4
Microbunching instability

• Initial density modulation induces energy
  modulation through long. impedance Z(k),
  converted to more density modulation by a chicane
  ? growth of local energy spread/emittance!

Energy
l
t
Current modulation
Gain10
10
1
t
5
LCLS accelerator systems
SC wiggler at 4.5 GeV
End of injector
DL1
DL2
Laser heater at 135 MeV
Linac 1
Linac 2
Linac 3
BC1
BC2

• At the end of injector, e-beam carries some
  residual density modulations which can be
  amplified in the downstream accel.
• Sources of impedance CSR in dipoles,
  longitudinal space charge (LSC) and linac
  wakefields in linacs
• Landau damping options a SC wiggler before BC2
  at 4.5 GeV or a laser heater before DL1 at 135 MeV

6
Heating within FEL tolerance

• FEL parameter r 510-4, not sensitive to
  energy spread until sd 110-4

M. Xies fitting formula

• 3 keV initial energy spread after compression
  120 keV,
• corresponding to sd 110-5 at 14 GeV
• ? can increase sd by a factor of 10 without FEL
  degradation

7
CSR instability
SC-wiggler damps bunching
8
SC wiggler

• SC wiggler increases sd 10 times at 4.5 GeV
  (BC2), suppresses the CSR gain

Initial modulation wavelength (mm)

• Ineffective for LSC instability occurred earlier
  in the beamline

9
Longitudinal space charge
Current modulation
Energy modulation

• Space charge oscillation at low energies (in the
  photoinjector), little accumulation in energy
  modulation

10
LSC instability

• Acceleration in linacs freezes density
  modulation and accumulates energy modulation,
  amplified by the chicane

Saldin Schneidmiller Yurkov
11
LSC instability in LCLS

• 3 keV energy spread is too small to suppress the
  LSC instability in BC1, which could induce too
  much energy modulation in L2 before the wiggler

110-4
Elegant tracking of final energy spread with 1
initial density modulation
l015 mm from 3 keV
12
Laser Heater
50 cm
10 cm
2 cm
10 period undulator
q ? 5.7º
10 cm
120 cm

• Laser-electron interaction in an undulator
  induces rapid energy modulation (at 800 nm), to
  be used as effective energy spread before BC1 (3
  keV? 40 keV rms)
• Inside a weak chicane for easy laser access,
  time-coordinate smearing (Emittance growth is
  completely negligible)

13
P0 1.2 MW w0 350 mm matched spot sx,y ? 200 mm
P0 37 MW w0 ? 3 mm large laser spot sx,y ? 200
mm
60 keV
spread by laser transverse gradient
In Chicane
-60 keV
After Chicane
less uniform heating
more uniform heating
14
Microbunching Gain after Laser Heater
40 keV
large laser spot
matched laser spot
15
THE GOOD (w0 350 mm, P0 1.2 MW)
THE BAD ( w0 3 mm, P0 37 MW)
Final phase space for initial 15 mm seed
AND THE UGLY (no heater)
16
Sliced final energy spread

• No heater
• w0 3 mm,
• P0 37 MW
• (c) w0 350 mm,
• P0 1.2 MW

17
Choices of transverse laser profile

• For an initial white noise spectrum, heating
  with a matched laser spot is generally more
  effective
• Laser spot size may be used to shape the
  sliced energy distribution to suppress a
  particular range of modulation spectrum

A 60-fs section of the final phase space with
initial150-mm seed
(w0 350 mm, P0 1.2 MW)
(w0 3 mm, P0 37 MW)
18
Summary

• Microbunching instability driven by LSC, CSR and
  machine impedance can be a nightmare for LCLS

• The photoinjector beam is too cold in energy
  spread, heating within the FEL tolerance (10X)
  can damp the instability

• SC wiggler is too late for the LSC instability
  occurs in the lower energy end of the linac (L1,
  BC1 and L2)

• A laser heater can be effective to suppress the
  microbunching and is under technical design (R.
  Carr et al.)
• It also adds flexible control of sliced energy
  spread to study FEL physics