Advanced FEL Beamline for Infrared SASE Experiments

1
Advanced FEL Beamline for Infrared SASE
Experiments

• L-band linac
• 17-MeV e-beam
• Solenoid focusing
• 2-m wiggler
• Sextupole focusing
• OTR diagnostics
• Streak camera
• Infrared detectors

2
Abstract

• A joint effort between LANL, SLAC and UCLA to
  achieve SASE saturation in the infrared is
  underway at the Los Alamos Advanced FEL facility.
  This experiment is an extension of two previous
  SASE experiments that have achieved very large
  single-pass gains, e.g. 300 at 16 µm and 3 x 105
  at 12 µm. The new experiment will use a
  two-meter-long uniform permanent-magnet wiggler
  with natural (sextupole) two-plane focusing. By
  reducing the wiggler gap, we achieve an on-axis
  magnetic field of 0.75 T and a corresponding rms
  wiggler parameter of 1.0. Analytical
  calculations indicate that with a 270-A, 16.5-MeV
  electron beam, the SASE radiation at 18 µm will
  reach saturation in 1.9 meters of wiggler length.
  A low-power re-injection of the SASE output can
  also be used to verify saturation of the
  single-pass SASE.

3
Analytic Model for Calculating SASE Power vs.
Wiggler Length
Dattoli Model for Saturation Yu Model for
Startup Calculations agree with Measurements
4
Saturated SASE Experimental Setup and Beam
Parameters

• Beam Energy 16.5 MeV
• Peak Current 270 A
• Bunch Length 17 ps FWHM
• Emittance 8 mm-mrad
• Energy Spread 0.3
• Betatron Period 0.94 m
• Wiggler Period 2 cm
• Wiggler rms aw 1.0
• 1-D r 0.021
• 3-D Gain Length 7.7 cm
• Saturation Length 1.7 m
• Saturated Power 50 MW

5
First SASE Experiment at LANL Achieves 300X
Single-Pass Gain

• 1-m Uniform wiggler,
• rms aw 0.92
• SASE pulse energy was measured as a function of
  charge.
• From current dependence on Q, plot SASE pulse
  energy versus I.
• Fit measurements to predicted dependence and
  obtain SASE gain versus I.
• Analytical calculations agree with measurements.

Log-log plot of measured and predicted SASE pulse
energy versus current
6
UCLA-LANL-RRC-SLAC Joint SASE Achieves 3x105 Gain

• 2-m Uniform wiggler rms aw 0.7
• Calculated saturation length 2.9 meters
• Highest SASE gain in the infrared to date
• Fluctuations agree with prediction of 8.8
  cooperation lengths
• Measured pulse energy agrees with predictions.

Log-log plot of measured and predicted pulse
energy vs current
7
High-brightness Electron Beams for Saturated SASE
Experiments
Measured rms radius and inferred normalized
emittance vs charge
Measured FWHM pulse length and inferred peak
current vs charge
8
SASE Wiggler Has 2-m Uniform Length with
Two-plane Focusing

• lw 2 cm
• SmCo PM
• Bpeak 0.75 T
• aw 1.0
• a 5.4 mm
• b 3.9 mm
• c 1.5 mm
• d 5.5 mm
• bx 0.19 m/rad
• by 0.15 m/rad

9
Taut-wire Measurements of Corrected 50-cm Wiggler
Section

• First integral indicates uniformity of aw
• Second integral is a measure of straightness of
  electron trajectory
• Quadrupole field is determined by off-axis 2nd
  integral measurements
• Sextupole field is determined by off-axis
  magnetic field measurement.

Plots of first and second integrals of fourth
50-cm section of wiggler
10
Conclusion
An experiment underway at the Los Alamos
National Laboratory is aimed at achieving SASE
saturation at 18 µm. This experiment, a joint
effort between LANL, SLAC and UCLA, is an
extension of two previous SASE experiments that
have achieved very large single-pass gains. The
new experiment will use a two-meter-long uniform
permanent-magnet wiggler with natural (sextupole)
two-plane focusing. A successful demonstration of
SASE saturation in the infrared will provide
validation of SASE theory as well as the
capabilities to measure the SASE spectral,
temporal and spatial characteristics. Also,
using the regenerative amplifier FEL feedback
optics to reinject a low-power signal to restart
the amplification process, in principle one can
reduce the pulse-to-pulse amplitude and spectral
fluctuations of the single-pass SASE signal.