To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, November 16-20, 1998
Date: November 20, 1998
Management
This week's report is being composed a day early because F. Dylla,
C. Bohn, and G. Neil will be attending the SURA Workshop on UV,
X-Ray, and Gamma Ray Applications of FELs to be held tomorrow
in Washington, DC. No report will be composed next week due to
the Thanksgiving holidays. Plans are to continue commissioning
activities from 0800 Monday through 1600 Wednesday, then shut
down until 0800 Monday, 30 Nov. 98.
Highlights for the week (thus far) include delivering up to 0.75
mA cw electron beam through the recirculation loop and into the
energy-recovery dump, as well as doing pulsed lasing with recirculated
electron beam.
A talk summarizing plans for upgrading the FEL for 20 kW IR, 1
kW UV was drafted this week; it will be refined prior to any formal
presentation to potential sponsors.
FEL Installation/Maintenance Activities
A charge monitor was installed to integrate total charge delivered
from the cathode. It will help in tracking cathode performance.
The optics in Happek #2, the bunch-length interferometer near
the wiggler, were realigned.
The cathode scanner is now in use, though debugging continues.
The first DQ dipole suffered a ground fault between the coil leads
and the downstream lower field clamp. Some slight bending and
two layers of kapton taped in place cleared the fault.
A faulty filter in one of the klystrons was found and removed.
It had been causing sporadic RF trips.
FEL Commissioning Activities
Commissioning of the recirculation loop continued through the
week. As of this writing (0930, 19 Nov. 98) up to 750 µA
cw electron beam had been delivered to the energy-recovery dump
(without lasing), larger than the previous "record"
of 600 µA achieved in August. Work continues toward cleaning
up the beam transport, both without and with lasing, and raising
the average current. Thus far, the magnet settings are in reasonable
agreement with optics (DIMAD) calculations, a situation that was
not true during the August runs. Diagnostics are also being continually
checked out and procedures for using them are being refined.
Some of the latest measurements indicate a transverse emittance
after the injector cryounit that is roughly a factor of two larger
than predicted with PARMELA and a bunch length at the wiggler
that is about 0.5 ps.
The photocathode gun has continued to operate well. The attainable bunch charge has just begun to fall below 60 pC. At this writing the machine is off for a few hours due to a trip of the Central Helium Liquifier (not caused by the FEL), and we are taking advantage of the down time to do another scan of the cathode photoresponse and recesiation of the cathode. This week we also did an experiment to ascertain the feasibility of moving the drive-laser spot around on the cathode. The result is favorable; the corrector magnets in the gun line are sufficient to resteer the beam after moving the laser spot. Accordingly, if the cathode is found to degrade locally coincident with drive-laser irradiation, we will be able to recover performance by moving to a fresh region of the cathode. In the case of this particular experiment, the move resulted in only a few percent increase in charge per bunch, but we will keep the option in mind during future operations. The technique is used routinely with the CEBAF gun.