Accelerator Physics Challenges of X-Ray FEL SASE Sources

1
Accelerator Physics Challenges of X-Ray FEL SASE
Sources
Paul Emma Stanford Linear Accelerator Center
2
Why a Linac-Based Free-Electron Laser (FEL) ?

• Longitudinal emittance from linac is much smaller
  than ring

• Bunch length can approach 100 fsec with small
  energy spread

• Potential for 1010 brightness increase and 102
  pulse length reduction

• Much experience gained from linear collider
  operation and study (SLC, JLC, NLC, TESLA, CLIC)

• At SLAC, the linac is available

• at DESY, XFEL fits well into TESLA collider plans

Use SASE (Self-Amplified Spontaneous Emission)
? no mirrors at 1 Å
Motz 1950 Phillips 1960 Madey 1970
Kondratenko, Saldin 1980 Bonifacio,
Pellegrini 1984
3
SASE Saturation Results
XFEL ? 0.1 nm
(or 1 Å)
Just 20 months ago SASE saturation not yet
demonstrated
Since September 2000 3 SASE FELs demonstrate
saturation
4
Proposed/Planned SASE X-Ray FELs

• TESLA-XFEL at DESY (0.85-60 Å)

• LCLS at SLAC (1.5-15 Å)

• INFN/ENA FEL in Roma (15 Å)

• Fermi FEL at Trieste (12 Å)

• SCSS at Spring-8 (36 Å)

• 4GLS FEL at Daresbury (SXR)

• SASE-FEL at BESSY (12-600 Å)

5
Peak Brilliance of FELs
photons per phase-space volume per band-width
courtesy T. Shintake
6
TESLA XFEL at DESY
0.85-60 Å
multiple undulators
X-FEL Integrated into linear collider
7
LCLS at SLAC
1.5-15 Å
X-FEL based on last 1-km of existing SLAC linac
8
SASE FEL Electron Beam Requirements
(1.5 mm realistic goal)
eN lt 1 mm at 1 A, 15 GeV
lt0.08 at Ipk 4 kA, K ? 4, lu ? 3 cm,
20Lg gt 100 m for eN ? 1.5 mm
Need to increase peak current, preserve
emittance, and maintain small energy spread, all
simultaneously
AND provide stable operation
9
Slice versus Projected Emittance
For a collider
collision integrates over bunch length
emittance projected over the bunch length is
important
For an FEL
e- slips back in phase w.r.t. photons by lr per
period
FEL integrates over slippage length slice
emittance (and E-spread) is important
10
Slice Emittance is Less Sensitive
transverse wakefield effect
TAIL
as can b variations along bunch
emittance of short slice not affected by
transverse wakes
also true for quad-misalignments, CSR, and RF
kicks
11
RF Photo-Cathode Gun
rapid RF-acceleration to avoid space-charge
dilution
Ipk ? 50-100 A Q ? 1 nC eN ? 1 mm
courtesy J. Rossbach
12
Thermionic pulsed high-voltage gun
at Spring-8 SCSS (0.1-0.5 nC), CeB6 cathode and
sub-harm. bunchers
T. Shintake, TUPRI116
13
Emittance Results from Gun Test Facility at SLAC
projected emittance at reduced charge levels
Parmela
gaussian bunch at GTF limits gex
Measurements
1 mm at 1 nC possible, but not demonstrated yet
C. Limborg, TUPRI041, TUPRI042, TUPRI043
courtesy S. Gierman, J. Schmerge
14
Slice ey Measurements at BNL
projected gey ? 3.0 mm
200 pC, 50 A, 75 MeV
care needed with image processing
Data from SDL at BNL W. Graves, et al.
15
Magnetic Bunch Compression
16
LCLS Linac Parameters for 1.5-Å FEL
single bunch, 1-nC, 120-Hz
(RF phase frf 0 at accelerating crest)
17
Coherent Synchrotron Radiation
coherent power
incoherent power
18
Coherent Synchrotron Radiation (CSR)

• Powerful radiation generates energy spread in
  bends

• Energy spread breaks achromatic system

• Causes bend-plane emittance growth (short bunch
  worse)

bend-plane emittance growth
coherent radiation for l gt sz
Dx R16(s)DE/E
overtaking length L0 ? (24szR2)1/3
19
CSR Microbunching Animation
First observed by M. Borland (ANL) in LCLS
Elegant tracking
20
Energy Profile also modulated
energy profile
DE/E vs. z
Next set of bends will magnify this again ?
slice effects
current profile
21
CSR Microbunching Gain vs. l
see also E. Saldin, Jan. 02, and Z. Huang, April
02
Initial modulation wavelength prior to compressor
theory S. Heifets et al., SLAC-PUB-9165,
March 2002
22
Evolution of LCLS Longitudinal Phase Space
after L2
energy profile
phase space
time profile
after DL1
sz 830 mm
sz 190 mm
after L1
after BC2
sz 830 mm
sz 23 mm
after X-RF
after L3
sz 830 mm
sz 23 mm
after BC1
at und.
sz 190 mm
sz 23 mm
23
CSR Micro-bunching in LCLS
SC-wiggler damps bunching
tracking with Elegant code, written by M.
Borland, ANL
24
CSR in Chicane (animation through LCLS BC2)
25
CSR Projected Emittance Growth (simulated)
projected emittance growth is simply steering
of bunch head and tail
Tracking using Elegant
26
Energy Spectrum at TTF-FEL (DESY)
TraFiC4 simulation
27
SPARC Project at INFN-LNF
1st stage bunch compression without bend magnets
122 MeV
(simulation)
SPARC Project _at_ INFN-LNF Collab.
Among ENEA-INFN-CNR- Univ. Roma2-ST- INFM 9.4 M
funding
ge ? 0.6 mm
peak current gt500 A
100 mm
C. Ronsivalle
M. Ferrario, TUPRI056
L Serafini, WEYLA001
28
Harmonic RF used to Linearize Compression
RF curvature and 2nd-order compression cause
current spikes
Harmonic RF at decelerating phase corrects
2nd-order and allows unchanged z-distribution
3rd harmonic used at TTF/TESLA 4th harmonic used
at LCLS
0.5-m X-band section for LCLS (22 MV, 11.4 GHz)
29
Diagnostics Transverse RF Deflector
off-axis screen
sz ? 20 mm, E ? 5 GeV, V0 ? 15 MV
single-shot, absolute bunch length measurement
RF streak
R. Akre, THPRI097
30
Measurements with Deflector at SLAC
46 GeV
built in 1960s (G. Loew)
20 MV
Planned also for TTF at DESY
bunch length measurements in the SLAC linac
szlt500 mm
31
Machine Stability Simulations (M. Borland, ANL)

• Track 105 particles with Parmela? Elegant? Genesis

• Repeat 230 times with jitter in gun, RF,
  magnets, etc.

• Include wakefields and CSR

Provides realistic estimate of operational
stability and verifies machine jitter budget
lt10
lt10
25
lt4
32
Undulator Beam-Based Alignment (LCLS)
uncorrected undulator trajectory
sx gt 500 mm
Undulator trajectory must be straight to 5 mm
level ?
simulate beam-based alignment procedure with
realistic errors BPMs, quads, poles
trajectory after 3rd pass of BBA
sx ? 1.5 mm ?Df? ? 82º
BBA also used for TESLA-FEL DESY (B. Faatz)
33
Resistive-Wall Wakefields in the Undulator

• Strong magnetic field in undulator requires small
  gap

• Need small radius pipe (r ? 2.5 mm, Lu ? 120 m
  LCLS)

Wake changes slice energy during exponential
gain regime more damaging than incoming chirp
Courtesy S. Reiche
Need smooth copper pipe
34
FEL Output Power with Undulator Wakefields
Genesis 1.3, S. Reiche, NIM A 429 (1999) 242.
35
Emittance Exchange Transverse to Longitudinal
Electric and magnetic fields
hk 1
gex0 5 mm
gez0 1 mm
M. Cornacchia, P. Emma, SLAC-PUB-9225, May 2002
gex0 1 mm
gez0 5 mm
36
Final Comments
For LCLS, slice emittance gt1.8 mm will not
saturate (TESLA ?)
Merci Beaucoup
SASE FEL is not forgiving instead of mild
luminosity loss, power nearly switches OFF
electron beam must meet brightness requirements