To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, December 7 - 11, 1998
Date: December 11, 1998
Management
Highlights for the week include achievement
of 124 watts cw lasing (as measured in the optical control room)
with a recirculated electron beam current of 1.3 mA.
A second meeting was held with Northrop Grumman
and Advanced Energy Systems personnel on Jan. 9th to discuss possible
collaboration with the 20 kW upgrade project. The collaboration
meeting was held during a meeting of SURA's Maritime Technical
Advisory Committee (MTAC) so that we could benefit from MTAC's
critique of the proposed joint venture.
We received an invitation from Steve Leone,
the chair of a BESAC panel on "Novel and Coherent Light Sources"
which is scheduled for Jan. 18-21 in Gaithersburg, MD. JLab has
been given an hour before the panel (the same time allotted to
the DOE light source labs) to present a summary of the Demo project
and the proposed science with the IR and UV FELs.
FEL Installation/Maintenance Activities
Recesiation of the photocathode is taking
place today (Friday, 11 Dec. 98). In conjunction, given the FEL
vault is open, we are doing some minor tasks, such as replacing
the air filters in the clean hoods over the optical cavity, checking
the connections on one of the beam-position monitors, etc. We
also had a short maintenance day Monday, again to take care of
some minor details.
FEL Commissioning Activities
Commissioning of the recirculation loop in
the presence of cw lasing continued for most of the week. At
week's beginning the cathode was producing 45 pC maximum bunch
charge, less than the nominally desired 60 pC, and by week's end
it was producing 30 pC maximum. The cathode has, in actuality,
performed remarkably well over its history given that it has supported
many tens of hours of cw running at the milliamp current level.
Today we are recesiating the cathode in hopes of bringing the
gun back to 60 pC performance this evening. Should we run into
difficulty, we will heat clean over the weekend and do high-voltage
processing Monday, followed by a second brief heat clean and another
cesiation. Plans are to continue commissioning through 22 Dec.
98, and then shut down until 18 Jan. 99. This coincides with
a recently planned shutdown/maintenance period of the nuclear-physics
machine, so we will avoid possible interference or staffing difficulties.
We will use the FEL shutdown first to replace the cathode wafer
and bring the gun back up (nominally 10 working days), plus add
some key machine diagnostics and software that should aid our
efficiency in achieving full kW-level operation. The Lab will
be closed during the last week of December.
This week, the peak machine performance was
124 W cw as measured upstairs in the Optical Control Room, during
which 1.3 mA of recirculated, energy-recovered beam was being
run at a bunch charge of about 30 pC. We could not go higher
in average current because we reached a hardware limitation in
the rf waveguides of the cryomodule cavities. Specifically the
infrared detectors in the waveguides began tripping the machine.
Detailed studies conducted yesterday strongly suggest the trips
are due to heating some small piece of hardware in the waveguides,
the source of heating being higher-order cavity modes excited
by the electron beam. Today we are doing some off-line experiments
to try to ascertain what specifically is heated, the results of
which will tell us how to work around the problem. Had this infrared-detector
problem not shown up, we could not have gone much higher in current
anyway due to the reduced quantum efficiency of the cathode.
The week also saw significant progress in developing the methodology for measuring transverse emittance before and after the first recirculation arc, i.e., the "CSR experiment". An unambiguous emittance growth was seen from the arc with 30 pC bunch charge, and it is roughly at the same level as might be expected if the source of the emittance growth were indeed coherent synchrotron radiation. This is a very tentative statement at this early stage of investigation -- the uncertainties in both the measurements and the calculations need to be refined, and the source of the emittance growth will need to be identified conclusively by way of parametric studies. Plans are next week carefully to take data with 60 pC bunch charge. A self-consistent simulation of the beam dynamics with CSR around the first arc is coming very soon.