Management

Highlights of the week include the first demonstration of lasing of the FEL at its 5th harmonic. Previous FELs have demonstrated 3rd harmonic operation but 5th harmonic operation has never been demonstrated until
now.  This provides a means of providing shorter wavelength operation prior to installing additional acceleration hardware in the FEL driver linac (which is planned in the Upgrade).  Additional results during this week's run were obtained in accelerator physics studies needed for the FEL Upgrade design and with a number of user tests.
 

FEL Commissioning Activities

This week L. Merminga and I. Campisi continued their beam-breakup (BBU) experiments and have been collecting data all week during the first shift.  They established experimental techniques that allow transverse
beam modulation at the frequencies of the higher order modes (HOMs) that are not damped by the loads, ~< 2 GHz.  Unambiguous signals from several of the strongest modes were observed, both from the cavity's probe and
from the Beam Position Monitor (BPM) in the energy recovery dump.  These were used to obtain beam transfer function measurements as a function of cw beam current, up to 4 mA.  Preliminary analysis of the data shows a
clear nonlinearity from which estimates of the instability threshold can be extracted.  Data were obtained from 3 cavities, 2 HOMs per cavity, at 2 different energies, to verify scaling of the threshold current with beam energy, and frequency, Q and shunt impedance of the modes.  Difference orbits data were obtained to experimentally determine the optics of the recirculator. The recirculation path length was also measured and found to agree with the design value.

Next week data will be obtained for different optics of the recirculator to determine scaling with M_34.  In addition, beam transfer function measurements between the kicker and the BPM in the energy recovery dump
will be done.  Pulsed modulation of the beam excitation will be used to try to correlate the decay rate of the cavity fields and the instability threshold.

Our campaign to measure electron beam emitance growth due to CSR continued on the owl shift with the efforts of P. Piot.  We tried to precisely measure the energy spread of the beam around the driver-accelerator to study any potential growth (due to wakefield or coherent synchrotron radiation).  A preliminary analysis indicates that
within the error bar of the measurement (10% relative) there is no growth.

The second part of week was dedicated to measure transverse emittance at several locations.  The measurement is still under optimization (it has never been performed with the 48 MeV lattice setup), and we should be ready to start measuring emittance during owl shift tonight.
 

FEL User Tests

On Monday night we successfully initiated 5th harmonic lasing.  Approximately 6-7 watts was generated at the output of the FEL at 1 micron when the linac was operated at an energy equivalent to fundamental lasing at 5 microns.  The 3 micron mirror set was used for this test, which happed to have reasonable reflectivity at the 5th
harmonic and negligible reflectivity at the fundamental.  A number of FELs have demonstrated the ability to lase at the third harmonic but no FEL until now has demonstrated 5th harmonic lasing.  Harmonic lasing allows the FEL facility to deliver shorter wavelength light without the necessity of upgrading the linac energy.  If lasing is not limited by electron beam emittance or energy spread considerations then it is possible to generate harmonic power about 1/10th of strong as the fundamental power.  The 5th harmonic operation we demonstrated this week
shows that the electron beam quality and stability is excellent.

On Tuesday and Wednesday night a team from RPI installed their apparatus for generation of terahertz radiation using the FEL output at 3 microns.  Much of the time was spent on alignment and no unambiguous signal was detected.  The group is anxious to re-schedule beam time with a new detector and try again during the next run. The RPI set-up has the possibility of generating a thousand times more terahertz radiation than can be generated with a table top laser. This radiation is useful both for spectroscopy and imaging of complex objects.

On Thursday and Friday night, our partners from Armco began their experiments on laser induced oxide removal on steels.  The LSS was certified in the metals lab (Lab 3) and the first FEL light was delivered as a line source onto an array of steel samples.  This is the first type of optical set-up to be commissioned in the metal workstation
in Lab 3.  This workstation also includes a fast scanning polygon mirror for laser spot scanning.

Next week user tests will continue with additional work on harmonic lasing and polymer surface studies at 5-6 microns.
 

FEL Maintenance/Installation Activities

Alignment mode was successfully commissioned. This allows users to perform alignment and set-up operations within the users labs using a low power low duty FEL beam.  Additional activities that were fit-in
during operations this week included:

Replacement of the foil in ITV2G00 . The old foil was too wrinkled for emittance measurements.

The 480 VAC power line to the Cathode Power Supply for cryounit cavity 4 was replaced.  A connector in its circuit breaker loosened causing a higher resistance spot which melted the line's insulation.

The Gun's HVPS was repaired.  Its high voltage driver cards and high voltage cable connectors were replaced.  A new flange is being made for higher voltage connectors.

Connections were tightened for magnet correctors 1F02H and 4F04H.  They had loosened and were erratic.

The trim cards for both solenoids were replaced.