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George Neil and Gwyn Williams JSA Science Council

UV FEL Status and Plans. George Neil and Gwyn Williams JSA Science Council . January 7, 2011.

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George Neil and Gwyn Williams JSA Science Council

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  1. UV FEL Status and Plans George Neil and Gwyn Williams JSA Science Council January 7, 2011 * This work was supported by U.S. DOE Contract No. DE-AC05-84-ER40150, the Air Force Office of Scientific Research, DOE Basic Energy Sciences, the Office of Naval Research, and the Joint Technology Office.

  2. Existing JLab IR/UV Light Source E = 135 MeV present limit Up to135 pC pulses @ 75 MHz 20 μJ/pulse in (250)–700 nm UV-VIS 120 μJ/pulse in 1-10 μm IR 1 μJ/pulse in THz The first high current ERL 14 kW average power Ultra-fast (150 fs) Ultra-bright

  3. Initial UV FEL Specifications • Specification from UV Demo proposal (May, 1995) • Average Power> 1000 W • Wavelength range 1–0.25 mm • Micropulse energy ~25 mJ • Pulse length ~0.1-1 ps FWHM nominal • PRF 74.85, 37.425, 18.7, 9.36, 4.68 MHz • Bandwidth ~ 0.2–1.5 % • Timing jitter < 1 ps • Amplitude jitter < 2 % p-p • Wavelength jitter 0.02% RMS • Polarization linear, > 100:1 • Transverse mode quality < 2x diffraction limit • Beam diameter at lab 2 - 3 cm

  4. Initial UV FEL Performance Projected

  5. IR Demo Harmonic Power Measurements Third harmonic power is down by about a factor of 1000. We get about 50 W at 372 nm so we expect about 50 mW of VUV light.

  6. Projected harmonic performance - water cooled mirrors

  7. Working in the UV is challenging • Short wavelengths require higher electron beam energies; the higher the better. IR Upgrade was fine with 110 MeV; we are limited to 135 MeV at present • The transverse emittance and energy spread needs to be lower by ~ 2X compared to the IR Upgrade. • Achieve this by operating at ½ the IR Upgrade FEL charge/bunch. • The vacuum requirement is high and must be achieved to maintain a stable output and avoid mirror degradation. • Manufacturing mirrors with l/10 figure in the UV is a challenge. • Must also have metrology capable of verifying specs. • Must mount without inducing aberrations. • UV coatings are more lossy than those in the visible, although exact numbers are hard to pin down. They may be only a few 100 ppm • We use mirrors with hole outcoupling to let the VUV out. FELs with high gain don’t like this; the mode tries to avoid the hole. A careful match is required for optimal performance

  8. Estimates of FEL performance • Both pulse propagation and one-dimensional spreadsheet models are first used to estimate the gain and power. • Gain is (photon power out of wiggler)/(power going in) measured at low power before saturation effects enter the picture • Efficiency is [1- (ebeam power exiting wiggler)/(ebeam power entering wiggler)] measured at saturation or equivalently (photon power out)/(ebeam power in) if mirror losses are small

  9. 400nm 3D simulation results from Genesis/OPC • Assumes 0.3% energy spread. Small-signal net gain = 139% Electronic gain = 165% Efficiency = 0.704%

  10. Cornell Undulator A Prototype

  11. UV Demo Commissioning Timeline • January 2006 - Install and commission Cornell wiggler with new gap mechanism. • Spring and Summer 2009 – Install beamline components except for optical cavity and wiggler chamber. • Fall 2009 – CW beam through UV beamline. • Spring 2010 – Install new zone 3 module and commission. • June 2010 – Lase at 630 nm, 67 pC in IR laser with 135 MeV beam. • July 2010 – Recirculate laser quality 1 mA CW beam through wiggler sized aperture. • August 17, 2010 – First electron beam through wiggler. • August 19, 2010 – First lasing, 150 W CW at 700 nm. • August 31, 2010 – First lasing in UV, 140 W @400 nm, 68 W @372 nm • December 9, 2010 – First measurement of 124 nm light

  12. FEL performance at 700nm Gain at low power is ~100%, detuning curve is 12.5 µm in length

  13. Images while lasing at 100W Light scattered from HR mirror Power meter Time dependent diagnostics Light scattered from power probe

  14. FEL performance at 400nm • We had to run with the OC mirror de-centered, as the metallization technique created a damage spot at the mirror center.

  15. Characterization of 10eV photons Bob Legg had built a chamber for the SRC at Univ. Wisconsin that we adapted for our purposes: Just measure diode photoelectric current. No filter required; only responsive to photons > 10 eV. Calibration is traceable to NIST. VUV photodiode VUV Chamber 10eV viewer Ce:YAG viewer Viewport

  16. Code Comparison with Experiment • Besides the aforementioned spreadsheet and 1-D pulse propagation codes, we have 3D & 4D codes that better model the FEL interaction. • These codes are: a code developed at NPS, as well as Genesis and Medusa. • In conjunction with a resonator simulation code we can also model the effects of aberrations (from thermal absorption, off-axis tilts, etc) and the mode shape within or outside the optical cavity. • This is the Optical Propagation Code (OPC). • Performance of the UVFEL has greatly exceeded the predictions of simulations. Parameter Simulations Experiment Turn-on time 8.6 µsec. 5 µsec. Gain ~100% ~180% Detuning curve 4.5 µm >7 µm Efficiency 0.4-0.7% 0.8%

  17. Very High Gain Seen at 400 nm

  18. from the announcement: “ 5 nanoJoules of fully coherent light was measured in each 10eV micropulse, which represents approximately 0.1% of the energy in the fundamental, as expected. These numbers allow us to anticipate being able to deliver 25 - 100 mW by operating CW at up to 4.687 MHz with more optimized water-cooled optics, and several 100's of mW with cryogenically-cooled optics. Optics upgrades, and installation of an optical transport beamline to a user laboratory for full characterization, including bandwidth, are in progress. We note that for many applications the anticipated narrow bandwidth eliminates the need for a spectrometer. This allows substantially higher flux to be delivered to user experiments than is possible at storage rings. “

  19. What happens next on the UV FEL? • Present mirror is lossy and hole size is somewhat mismatched for proper outcoupling at these high gains. As a result we cannot lase stably at as high a power as may be possible even with water cooled mirrors. We are obtaining a better water cooled mirror set and will have ROC control. • We are presently installing Optical Transport to Lab 1 and will test it in February • We are returning UV wiggler to Cornell and adapting an APS Undulator A at the manufacturer (STI Optronics). • A high power test in the IR for the ONR will follow in April and early May followed by a shutdown till mid July during which time the cooled mirrors and new undulator will be installed. We will recommission and perform User runs. (Gwyn’s talk) • We also intend to install a new R100 cryomodule and get higher beam energy for shorter wavelength lasing. Perhaps in June if assembly /installation schedule permits. Lasing in fundamental down to 250 nm may be achievable depending on energy. If not June then October.

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