Spontaneous Radiation at LCLS

1
Spontaneous Radiation at LCLS

• Sven Reiche
• UCLA - 09/22/04

2
General Properties

• Resonant wavelength
• Maximum signal when directions of observation and
  trajectory are parallel? with a characteristic
  opening angle of ??????
• Maximum angle in electron trajectory is K/?
• Effective solid angle of radiation is ? 1/? x
  K/?

3
The Signal in Time Domain
trajectory
For larger angle in x the uni-polar signals move
closer together, merging into a bi- polar signal
for ?gtK/?
Above plane of oscillation
4
Angular distribution (Far Field)

• Only odd harmonics are visible on-axis
• All harmonics are present for off-axis angles.
• The nth harmonic has n-1 knots in the yz-plane.

fundamental
2nd harmonic
3rd harmonic
5
Intensity Spectrum

• LCLS-lattice with super period. Detector 113
  behind exit of undulator.
• Rich harmonic content on-axis.
• Wider spikes for off-axis radiation due to red
  shift
• Reduced harmonic content for off-axis emission.

6
Full Spectrum

• Summing over all emission angles, the full
  spectrum resembles that of a bend dipole.

Simplified LCLS lattice (far field)
7
Power Consideration

• The total power is
• For LCLS the total power is 75 GW, 10x larger
  than the FEL signal at 1.5 Å.
• The effective solid angle is 1/?2 1.510-9 rad2,
  3 orders of magnitude larger than for the FEL
  signal (10-12 rad2)

At saturation the FEL intensity is about 100
larger than the spontaneous background signal
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Intensity Distribution

• Angular distribution, 113 m behind undulator
  exit, using real LCLS lattice

The peak intensity is 73 kW/mm2 The distribution
is almost like in the far field zone. Total
energy 75 GW
9
Spectral Power Cut

• The opening angle for a single frequency is
• For LCLS the angle is ?? 1.5 ?rad.
• The emitted power at the fundamental is about 1
  MW per 0.1 bandwidth (the full FEL signal of
  about 10 GW falls within this bandwidth).
• Higher harmonics contribute less than 5 to the
  total background signal and are most likely
  filtered out by spatial apertures.

10
Spatial Power Cut

• Array detectors (e.g. X-ray CCD cameras) or
  spatial collimator improve the signal to noise
  ratio.
• For LCLS, any cut below 1 mm2 at the first
  detector position (113 m behind undulator) would
  reduce also the FEL signal.

11
Signal-Noise (Full Undulator)
The noise signal for spatial cuts can be
lower, depending on the spectral response of the
detector.
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Detecting the FEL Signal
Solid - electron beam mis-steered Dashed -
undulator modules removed
Spectral Cut 0.1 Spectral Cut 1.0 Spatial
Cut 1 mm2 Spatial Cut 4 mm2 FEL Signal
13
Detecting the FEL Signal

• For LCLS no information can be obtained from the
  FEL signal for the first 20 m with respect to
  undulator alignment and field quality.
• Operating at longer wavelength reduces the
  distance but makes the FEL signal less sensitive
  to the field quality.
• Short pulse operation of the FEL (e.g. two-stage
  pulse slicing or slit in dispersive section)
  reduces the signal-noise ration by 1-2 orders of
  magnitude.
• Information on undulator modules can be obtained
  by the spectrum of the spontaneous radiation.

14
Module Detuning Tolerance

• Detuned modules yield a local phase slippage of
  the radiation field with respect to electron
  beam, yield a degradation in the synchronization
  of the resonance condition.
• Simulations yield tolerance of ?K9.10-4

15
Undulator Module Tuning

• Possible method to tune undulator modules with
  the spontaneous radiation.
• Following method, proposed by TESLA (thanks to
  Markus Tischer, Kai Tiedke - HASYLAB, DESY)
• Prerequisite set-up Non-destructive measurement
  of X-ray path (e.g. X-ray BPM, resolution lt 1 mm)
• To measure changes in K of 9.10-4 the orbit of
  the photon beam has to be stable by about 2.1
  ?rad.

Gas
X-ray beam









Pick-up line
16
Single Module Spectrum

• Bandwidth of 1/Nu1
• Angular distribution
• after monochromator

At 5th harmonic
Ideal case of zero energy spread and emittance
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Detuned Module

• Monochromator selects frequency slightly above
  5th harmonic (shift of about 6.10-4)
• Variation in detected power and width of
  distribution
• Works best for monochromator tuned to the half
  value point of the high-frequency side of the
  spectrum

Same at 1st harmonic
?K/K 10-4
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Emittance and Energy Spread

• Line width and distribution size are dominated by
  emittance (energy spread is negligible) for the
  10th or higher harmonics.
• At 5th harmonic no degradation by emittance and
  energy spread.
• No benefits by going to higher harmonics

?K/K 10-4
1st 5th 10th 20th
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Machine Jitter

• Energy jitter of 0.1 has same wavelength shift
  as detuning of ?K/K10-4, but can be eliminated
  by statistic
• Same argument applies to charge jitter
• Alternatively the radiation measurement can be
  binned by measuring charge and energy of the
  spent beam.
• Jitter in beam angle (0.12-0.24 ?rad) is
  sufficiently small for the measurement.
  Transferred jitter on the radiation beam might be
  detectable if a X-ray BPM is installed.
• Other machine jitter not of relevance for tuning
  the modules.

20
Tuning the Undulator

• After BBA the orbit must be straight enough to
  have a beam divergence less than 1 ?rad.
• X-ray BPM are complimentary measurement of the
  orbit straightness. Improvement in resolution
  when installed in far hall, but not necessary
  when BBA is successful.
• Tuning works only for one module per time. If
  tuned modules remain in beam line than line width
  and distribution are determined by emittance and
  change in signal is too weak.
• Emittance effects can be slightly suppressed by
  increasing the beta-function for tuning.

21
Micro-Taper

• The energy loss due to spontaneous energy
  radiation requires to taper the undulator.
• The required taper is ?K1.710-4 per module.
• Defines the required precision for undulator
  alignment.
• Denies module detuning at lower energy.

Ideal Case
Tapered
Not tapered
22
Coherent Radiation

• Coherent radiation arises from
• Undulator radiation, emitted under large angles
• Transition undulator radiation in the forward
  direction
• The coherently emitted energy of 40.5 ?J for CUR
  and 1.3 ?J for CTUR are negligible with respect
  to the incoherently emitted radiation.
• Although CTUR emits although at the resonant
  wavelength and is proportional to the bunching
  factor, the emission is strongly suppressed due
  to the finite extend of the electron bunch.

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Conclusion

• Strong background signal from spontaneous
  undulator radiation. Requires some spatial and/or
  spectral cut to select FEL signal.
• No information on the undulator quality can be
  obtained from the FEL signal for the first
  section of the undulator.
• Individual undulator modules can be tuned by
  spectral analysis of the 5th (or 3rd) harmonics.
• Tuning for multiple modules in the beam line
  somehow limited by emittance