Management

Highlights for this week include raising the electron-beam energy to 48.2 MeV for lasing at 3.0 µm, running 4.7 mA cw recirculated beam at this energy while lasing, and running up to 90 pC bunch charge in pulsed mode while lasing strongly.  Presently we have achieved about 300 W cw power at 3.02 µm.
 

FEL Installation/Maintenance Activities

The drive laser was repaired Monday with on-site help from a vendor representative.  Repair consisted of installing a new rod, with the result that the drive laser can now put out 6 W in the green.  We have
been running it at about 5.6 W this week, as compared to typically 4.5 W in the past.  Combined with better cathode quantum efficiency, we can now generate considerably higher bunch charge than in the past, though we are now running the gun at lower voltage (320 kV).
 

FEL Commissioning Activities

Commissioning resumed (finally!!) on Tuesday afternoon after repair of the drive laser.  First, we set up the injector to accommodate the lower (320 kV) gun voltage.  Then we set up the accelerator to generate and recirculate 48 MeV beam for lasing at 3 µm.  Yesterday (Thursday) we recirculated up to 4.7 mA cw current (a new record) while lasing weakly.  At that point the gun's high-voltage power supply (HVPS) tripped on its current limit.  Thus, without modifying the resistor in the current circuit of the HVPS, this is the highest current we can run.  Presently it is "adequate" in that it is not limiting machine performance.  We apparently have been having some difficulty with oscillations in the buncher field and are exploring the matter further.

Last night we ran up to 90 pC bunch charge in pulsed mode while lasing strongly, achieving better than a kilowatt over the msec macropulses.  Presently it looks like 70 pC is the limiting charge for cw runs (~2.5mA at 37.4 MHz) due to scraping in the cryounit at higher charge.

As of this writing (1030 Friday, 21 May 99), it appears that the fundamental limit in machine performance is thermal distortion of the sapphire mirrors, probably attributable to the thermal conductivity of their coatings.  We have thus far found we can lase cw at 3 µm at up to
~300 W, at which point raising the electron-beam current seems not to increase the output power.  Consequently it is looking like mirror distortion is the limit.  This "conclusion" is a very tentative one
based on very limited run time.  Today we will be working toward confirmation/clarification.  If thermal distortion is indeed the problem, we have a few ideas how to proceed for higher power and will
institute one or more of them early next week.

Electron-beam-diagnostic efforts have focused on comparing the measured injector setup to simulation (PARMELA) results and on preliminary measurements in preparation for CSR studies.  Time-of-flight
measurements in the injector now agree well with PARMELA for the putative gradients in the buncher cavity and the cryounit cavities.  Dispersion in the back leg is very low (about 6 cm), which aids
interpretation of emittance measurements there.  The bunch length at the wiggler is also very low, about 80 µm rms.

Quadrupole-viewer emittance measurements have been hampered by ghost pulses from the drive laser, and we have taken steps to reduce the ghost pulses, including realigning the electrooptic cells and installing a mechanical shutter in the drive-laser beamline.  As of last night, the ghost- pulse intensity was still prohibitive, so this morning a filter was placed over the relevant camera.  Hopefully it will help.