MEMORANDUM
To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, March 9-13, 1998
Date: March 13, 1998
FEL Management
Highlights for the week include (1) completion of the bake cycle
for the gun and start of its high- voltage processing following
its reassembly; and (2) completion of the initial longitudinal
alignment of the FEL optical cavity.
Draft Field Work Proposals (FWP's) for FY 99-00 FEL activities
were sent to the DOE BES Program Office and our collaborators
for review. One FWP dealt with the design and implementation
of beamline infrastructure that would support IR spectroscopy
in the user lab. The second FWP dealt with extending the capabilities
of the IR Demo to the far-infrared.
On Thursday, March 12, Howard Schlossberg, a program officer from
AFOSR and Henry Helvajian from Aerospace Corp. visited the lab
to discuss laser processing applications with the IR Demo.
FEL Installation Activities
Highlights for this week include:
Alignment of the optical cavity was finished.
The first 180 bend vacuum chamber was removed from its magnet
to install a view port and cross hairs on it as diagnostic aids
for energy recovery. Plans are to reinstall the chamber this
afternoon and to pump down the region on Monday.
Recertification of the Personnel Safety System was completed.
A SEG block was placed over the insertable dump at the end of
the recirculation loop. Low- conductivity water is scheduled
to be connected today.
FEL Commissioning Activities
This week was devoted to continued preparation for resumption
of commissioning. Bake of the gun is complete, and high-voltage
processing commenced last night, progressing to 385 kV. Plans
are to go to 420 kV tonight and let the gun set at that voltage
for several hours. Plans are to turn on electron beam at swing
shift Monday, 16 Mar 98. The basic task is to get 2 µA,
38 MeV beam down to the straight-ahead dump as expeditiously as
possible, hopefully by owl shift Thursday, then install the GEN
target chamber Thursday and Friday, then irradiate GEN targets
through Wednesday of next week.
As wiring for the Machine Protection System (MPS) progressed yesterday,
problems with integrating various subsystems arose. They are
being worked out with the goal of completing the MPS before Monday
turn on.
All first-light magnets are now in place except the oval air cores,
one of which goes on each optical chicane. At present there are
spare CEBAF air cores at these locations, and the spares are sufficient
for supporting irradiation of the GEN targets.
We still need to finish preparing the "operator page"
(www posting of viewer images) and initial save/restore file (from
PARMELA simulation). B. Yunn discovered an inconsistency with
how PARMELA establishes cavity phases and is in process of verifying
all of the calculated machine settings up to the wiggler location.
D. Douglas awaits handoff from Yunn so he can update his spreadsheet
and calculate all of the post-cryomodule 60 pC settings and beam
dynamics.
An extensive set of higher-order-mode measurements in the FEL
cryomodule cavities indicates many more resonances are present
than in the geometry of the conventional CEBAF cryomodule, and
one question was whether cross-coupling between cavities was the
cause. The data suggest that cross-coupling is weak, at the 80
dB level. Rather, the lesson seems to be that a small change
in geometry can lead to considerable change in Q of strong modes
and large change in Q of weak modes. For example, the strong
1890 MHz mode has Q~10^5, about ten times larger than in the original
CEBAF test cavities, and the weak 1730 MHz mode has Q~10^7, about
1000 times larger than in the original test cavities. For the
cryomodule itself, the beam-breakup-threshold current is estimated
to be about 60 mA (i.e., >>5 mA) assuming a somewhat pessimistic
frequency distribution in the eight constituent cavities.