Experience with Laser Heater at FERMI

S. Di Mitri, Elettra Sincrotrone Triesteon behalf of the FERMI team

PROBLEM:

Since the uncorr. energy spread (sE) from the gun is very

small, m-bunching instabilities (MBI) driven by Longitudinal

Space Charge and Coherent Synchrotron Radiation in the

magnetic bunch compressor may grow and enhance sE,

degrading the quality of FEL.

Measured at LCLS

D. Ratner et al., Phys.

Rev. ST-AB 18, 030704

(2015)

SOLUTION:

“Resonant laser-electron interaction in a short undulator induces rapid

energy modulation at the optical frequency, which can be used as an

effective energy spread for beam ‘heating’….”

Z. Huang et al. PRST-AB 7 074401 (2004).

LH

,3,4

Laser heater setup

• The chicane “washes out” the optical frequency modulation

• Induce a controllable increase of sE to Landau damp the MBI:

A modulation to cure a modulation

1.4 m 0.8 m 1.8 m

(Z.Huang, FEL09)

30

mm

Ti:Sa783 nm

<20 MW

FERMIfew keV rms

E [M

eV]

15

keV

E [M

eV]

50

keV

E [M

eV]

50

keV

modulation smeared by half chicane R52 and angular spread

laserlaser

e-

e-

Chromox screen

e-

Chromox screen

•Heating measurements: slice energy

spread

6

4.5 keV

10.5 keV

29.5 keV

a

b

c

• end of linac (DBD)• 500 pC• CF=10 in BC1

•Calibration: added se,LH vs. laser energy

7

s e =s r

2(x)

s x

2 +s r

2(x)

PL

P0

×K

g0

1

s r (x)m0c

2 JJ[ ]ds0

LU

ò ,

laser spot size (in LH)

e- spot size (in LH) e- energy

laser peak powerPL undulator Bessel functionJJ[ ]

•LINAC energy spread vs. LH induced

energy spread

8

OFF

5 keV

15 keV

e-in

ten

sity

[a.

u.]

e- energy [MeV]

at low heating, the instability dominates and final energy spread remains large

at large heating, longitudinal Landau damping dominates and final energy spread LH-induced energy spread

• end of linac (DBD)• 500 pC• CF=6.5 in BC1+BC2• X-band off

FERMI works here

(119 keV)

•COTR suppression

9

OTR screen (downstream BC1)

sx=160mm

LH off

sx=100mm

LH on

OTR screen (TLS)

a small amount of heating is sufficient to dump the COTR at screens downstream thebunch compressor

• Laser heater reduces COTR emission from the OTR screen located downstream the

bunch compressor

• At FERMI emittance measuraments in the linac are done with OTR screen

• Good reability of this measuraments thanks to laser heater

LH ONLH OFF

LH OFF LH ON

LINAC

FEL-1

FEL-2

Spectrometer

e-Dump

BC1 BC2Mirror

MagneDYN

Fel enhancement

High Gain Harmonic Generation (HGHG). Echo Enabled Harmonic Generation (EEHG).

FERMI Layout

•Laser heater – FEL spectrum

12

LH OFF

• 100 spectra• 500 pC• CF=10 in BC1• X-band on

LH ON

• FEL1 and first stage of FEL2: The spectrum is

clean even for very low laser heater when the

beam and the optics are properly optimized. Laser

heater increases the pulse energy of 20-60%

• Second stage FEL2: Pulse energy can be

increased by a factor 3-4 for the same laser

energy that maximizes the first stage. A

greater laser energy is required to optimize

the spectrum

• EEGH require laser heater for a

reasonable power and spectrum. It

tolerates higher energy spread

H11

H18

H44

FERMI status

• The laser is a portion of the photocathode drive laser (infrared) naturally synchronized with the electron beam

• The laser is kept after the compressor of the PIL before the harmonic conversion

To UV

To UV

2-4% splitter

stretcherTo Laser heater

Pulse duration 12-17psPulse energy<100µJ

Reference M. Danailov,A. Demidovich