**MEMORANDUM**

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- June 9-13, 2003

**Date:** June 13, 2003

**Highlights:**

We are pleased to report that yesterday (June 12^th^) we produced
spontaneous emission of 6 micron light from the FEL Upgrade --an
important accomplishment as we progress towards our "first lasing"
milestone. In last week's report we noted that we had pushed the
electron beam through the wiggler into the first light dump. Since last
Friday night we have had four days of near continuous beam operations as
we optimized the electron beam parameters and performed several length
scans on the optical cavity looking for lasing. We were limited in our
ability to set the cavity length to its micron level tolerance because
our laser interferometer suffered an infant mortality in its diode pump
and (spare promises by the manufacturer to the contrary) can not be
repaired until next week. We decided to go ahead and try to manually
scan the cavity length anyway expecting to gain experience in other
operational characteristics in the process. A summary of these
commissioning activities is given below for those interested in the
details. Today and through the weekend we are performing a check on our
optical cavity length survey measurements, fixing some minor problems
accumulated through the week, and preparing to restart commissioning on
Monday.

All in the team are pleased with how the Upgrade hardware is performing
during this shakedown cruise.

**Commissioning:**

Last weeks activities centered on roughly setting up the injector and
linac, threading the beam to the tune-up dump past the optical klystron,
and commissioning the optical klystron.  This week, including last
Saturday, was involved with careful setup of the injector, linac and
beam transport, and optical systems, including many hours of length
scans looking for lasing.  The modeling tools available have greatly
aided in setting up the machine to match the design.\
\
On Saturday we carefully set up the linac phasing and optimize the bunch
length using the 1st arc multi-poles.  We then centered in all the
machine quadrupoles and saved the orbit.  Finally we obtained a decent
match to the wiggler and looked for lasing using an IR camera and the
electron spectrometer.  No lasing was seen.

Monday was spent in maintenance and PSS re-certification required when a
logic unit failed.  We ran briefly and took data on current linearity in
the diagnostics (they all track except for the laser power).   All the
magnets whose polarity was found to be wrong last week were fixed and
one missing trim channel was added.

On Tuesday we carefully set up the injector to match the Parmela model
as closely as possible.  The match to the linac was good and the spots
throughout the machine were greatly improved.   The injector energy was
also shifted closer to design.  We then set up the rest of the machine
and re-optimized the bunch length using multi-pole values closer to the
model.  The design values are much stronger than those used last week. 
We also measured the sensitivity to the gun voltage and determined that
the voltage droop caused by the conditioning resistor should not hurt
us.  After shutdown we installed the spectrometer and a more sensitive
detector so that spontaneous radiation could be seen.

Wednesday we studied reproducibility and found that most magnets
reproduced well.  The DY used in the IR Demo did not reproduce quite as
well (one part in a thousand) and a pair of iron core correctors did not
reproduce well at all.  We developed a procedure to bring the machine
back within a few minutes of bringing the machine back up.  The stronger
sextupoles in the

1st arc created some distortion in the beam and some time was spent
trying to center in them.  Once this was done we tried again to look for
lasing.  We could not see the spontaneous and found no lasing.

Thursday we fixed a problem in the RF system that was leading to
frequent trips and, using a more compact beam transport to the detector,
succeeded in seeing a strong spontaneous radiation signal.  We scanned
the cavity length and found a few candidates for a length enhancement
but could not obtain lasing on any of them.  We also measured the spots
sizes in the machine and used these and the model to derive a better
match to the wiggler.  We also discovered that we could better optimize
the bunch length by steering the electron beam to center in the detector
aperture.  Despite having a very nice electron beam in the wiggler we
were unable to find a length enhancement.

Analysis of surveying data indicated that the proper length might be
outside of the scanned range.  We are now refining the surveying data to
allow us to set the cavity length to better than 1 mm.  We are also
going to make a new cathode (heat clean followed by recesiation) to
allow higher charge.  Finally, we found that the high reflector in the
optical cavity is slightly astigmatic and we are installing our spare
high reflector so that the optical mode is closer to ideal.

In conclusion, we have used the modeling techniques developed in the IR
Demo to set up the machine to match the model in an efficient manner. 
The system is now close to the design point but running at low charge. 
The optical cavity has some problems that are being addressed.  With
these problems resolved and the charge increased we should see lasing
soon.

**Management:**

We prepared draft presentations on the FEL program for our annual DOE
Science and Technology Review which is scheduled for June 25-26^th^.

The kick-off meeting for our joint project with AES on terahertz imaging
funded by the US Army is scheduled for July 22 at Jefferson Lab.

**WBS 4 (Injector):**

The photogun has performed extremely well since it was commissioned on
May 7^th^.

Generally, quantum efficiencies have been obtained in the 3% range after
cesiation. After the last two weeks of operation the QE has fallen to
1.1%, so we are processing a new cathode during today's shut-down with a
standard (670C) heat clean, followed by re-cesiation and HV conditioning
up to 360 kV.

**WBS 8 (Instrumentation):**

The commissioning the BLM Beam-Veto function continues. The beam loss
signals from the PMT heads arrives a few hundred nanoseconds ahead of
the 50 ohm Beam Sync signal. As a result, the BLM cards can not generate
a veto pulse for the same macro pulse that triggered it. The initial
solution was to make use of the periodic nature of our tune-up mode at
2Hz and predict the arrival of the next pulse. In attempting to
implement this method, it was observed that the pulse to pulse accuracy
of the existing pulse control electronics is only 0.1%. This means that
in a pulse mode with a period of 0.5sec (2Hz), the pulse-to-pulse jitter
of the macro pulse will be as much as 500 us. The BLM electronics was
then trying to place a 400 us veto window over a 250 us signal whose
arrival time was fluctuating within 500 us. The correct solution will be
to implement the new pulse control electronics in the drive laser clean
room which will not only have better pulse-to-pulse accuracy but will
also be able to delay the gating of the EO cells which will delay the
light pulses to the cathode, giving all the other electronics in the
machine including the BLM cards a *pre-trigger* beam sync signal to use.
The upgrade of the pulse control electronics is scheduled to occur
during the July shut-down.

The firmware in the DLPC chassis has been upgraded to include Beam Mode
3 into our tune-up mode commissioning use. The redefined mode will be
the same as Beam Mode 2 which has a 0.05% duty factor limit (i.e. 250us
at 2Hz) except that Beam Mode 3 will allow pulsed beam with duty factors
as much as 0.15% or 750 us at 2Hz. The MPS will provide the proper
permissions to enter this mode.

There were errors identified with the QT/GC Panofski quadrupole &
vertical trim combo magnet. The magnet measurement group made an access
to check it out but was unable to identify what was wrong. The work
around was to use other elements to give the required vertical trim.
Upon further investigation the jumper cables that ran upstairs were
wired incorrectly. This has been fixed. Work continues on the GC trim
power supplies. These need to be 4 quadrant floating supplies to be able
to ride on the 75 volt bipolar quad supply. The high power dump Vertical
and Horizontal Fast Raster Control chassis and power supplies were
installed and are awaiting the magnet for testing.  The schematic for
the HVPS Control Chassis Analog I/O board was completed and signed off. 
New boards need to be ordered so one can be stuffed and tested for
installation.

The crystalline quartz windows for GN/HR experiment are in. The amount
of pressure that they can withstand must be determined. An apparatus was
designed to apply a known pressure to the piece of quartz in order to
determine its maximum load capacity. The device has been partly
assembled, and the procedure documented. The experimental procedure has
been written and will be performed shortly.

The W&M implantation station is down. The I&C group is attempting to
help in troubleshooting in between the 16+ hour days commissioning the
FEL. People are still smiling - could be rigor mortise.

The documentation on the Optical Cavities Analog Breakout Chassis is
proceeding well. The check prints on the HR & OC Analog Buffer Board are
in the progress of being marked up so the schematic can be completed. 
The fabrication of the chassis is completed and the wiring is 95%
completed, the chassis will be turned into EECAD for schematics and
fabrication drawings as soon as it is ready.

Performance Integration of the I&C systems - The new database at:
<http://laser.jlab.org/fel> is now our one-stop-shop for access to:
system documentation, cable lists, maintenance & commissioning
priorities, performance logs, IOC reboot call lists, operations &
check-out procedures as well as all other support notes and archived
information for the I&C systems. As the system is still new, we are
engaged in the data entry process, but current group activities and
system configurations changes are being documented with this tool.

**WBS 11 (Optics):**

This week was devoted to operating the optical cavity hardware \"for
real\".  Given the uncertainty in cavity length, we had to scan the
cavity length well beyond the 1mm range we had anticipated ever
needing, and found that we had some drifts in the HR mirror
orientation to contend with.  That made attempts to lase more
difficult.  When the laser did not start after repeated length scans, we
opted to look for changes in the spontaneous emission output, and found
out diagnostics were unable to detect them.  We setup a simpler
arrangement, and this worked well.  The failure of the cavity to show
the expected length enhancement could arise from several factors, one
being the astigmatism in the high reflector making it harder to align
with HeNe lasers.  The accumulation of several factors made it clear
that we interrupt operations in order to a) allow Survey and Alignment
to perform another determination of the cavity length, b) replace the HR
mirror with a spare, and c) incrementally add some extra functionality
to the optical diagnostics.  This is all occurring as of this writing. 
We also found that with use, one of the ultraviewers was becoming very
air pressure dependent.  We instituted a temporary fix by adding a
pressure regulator, and will incorporate a better fix in the next few
weeks.

The drive laser performed flawlessly, however it is clear we need more
power.  We are going to install the fast shutter as a replacement for
the second EO cell, this will raise the throughput by

\~20-25%.