**MEMORANDUM**

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- June 20-27, 2003

**Date:** June 27, 2003

**Highlights:**

We completed the first stage of operations and characterization of the
FEL in pulsed mode this week, taking a lot of performance data. In
virtually every test we performed, the laser acted as our models
predicted. We had substantial gain over the 5.5 to 6.6 microns tuning
range of the mirrors and could easily shift wavelength by simply
adjusting the wiggler strength. The details of the electron beam
transport were generally understandable and tracked what our spreadsheet
projected. The quantum efficiency of the cathode remained high during
the run and the gun behaved well. We got data on the power output versus
dispersion in the optical klystron, versus Rayleigh range of the optical
cavity, and versus wavelength. We confirmed the energy acceptance of the
initial arc is at least 10% and got an image of the optical mode.

On Wednesday we shut down and prepared for delivery of the final 180
degree bend magnet which occurred Thursday morning when 26,000 pounds of
iron and copper were successfully set into position. (We're told an
aircraft carrier anchor is only 22,000 pounds.) This will be hooked up
to water and power and (with a few other elements such as the full power
beam dump) brought into operation over the next 2 ½ weeks. We expect to
restart beam operations in mid-July with recirculation of the electron
beam first on our agenda.

![](media/image1.jpeg){width="4.052777777777778in"
height="3.0395833333333333in"}

**Commissioning:**

After achieving lasing last week we switched to collecting data on the
laser and accelerator operation and comparison to theory and
simulations. Friday Dave Douglas carried out a series of difference
orbit measurements to characterize the quadrupole strengths and the
diagnostics.  Three beam position monitors were found to be giving
unrealistic readings.  These will be checked during the down.  One
quadrupole was found to be wired backwards and was fixed.  This allowed
us to get a better match to the FEL and to reduce betatron oscillations
in the machine and eliminate losses in the beam transport.  We also
found that we were losing beam in the injector cryounit for high charge
operation.  Carlos Hernandez found a new focusing solution that allowed
us to transport up to 70 pC with no sign of loss.

On Monday we used the new injector setup and the new matching solution
derived from the difference orbit data to get a good match to the
wiggler. We also shorted out the conditioning resistor and eliminated
most of the wavelength slew we saw last week.  Finally we started laser
studies with the new optimized configurations.  We took some data
without a good nitrogen purge to map out the water absorption lines in
the region.  We took data on the laser spectrum vs. cavity length and
tuned the laser over the range of the cavity mirrors (5.4 to 6.6
microns).  Finally we tuned the laser to 18 microns resonant wavelength
and achieved third harmonic lasing at 5.8 microns. 

On Tuesday we continued lasing studies and took data of the laser gain
vs. mirror steering.  We found that, for an optimum Rayleigh range, the
gain went down by a factor of two for an 18 microradian mirror tilt. 
This tilt will rotate the cold cavity mode by half of the mode
divergence angle.  Our usual criterion is that the cold cavity mode
rotates by less than 1/6 of the mode divergence leading to a very small
reduction in gain.  We also adjusted the radius of curvature of the high
reflector and measured the mirror sensitivity and power as a function of
the radius of curvature.  This data is still being reduced.  We will
report on it in a future weekly.  We imaged the laser spot on a Pyrocam
detector and found it to be a nice Gaussian spot. Finally we measured
the laser characteristics as a function of the optical klystron
dispersion section strength.  As expected, the laser power varied in a
cycloidal fashion with dips at the half integral values of dispersion.
The spectrum varied with the dispersion and, near the dip, we would
establish bi-stable lasing where two peaks in the gain curve had the
same gain to loss ratio.  The wavelength would then randomly jump from
one peak to the other.\
After shutting down we ran some pulse propagation simulations of the
laser in its wiggler configuration (minimum dispersion strength) and
compared the results with the experiment.  The power vs. cavity length
detuning is shown in the attached figure.  The maximum power is slightly
over 200 W, in agreement with the experiment.  The length of the
detuning curve is 8\
microns.  In the experiment it varied from 8 to 10 microns.  The turn-on
time was 35 microseconds, compared to 40 microseconds in the experiment.
This is surprisingly good agreement since the simulation does not
include the turn-on transient in the electron beam energy.  This good
agreement bodes well for achieving 10 kW since the same pulse
propagation code predicts over 10 kW of power for our nominal 10 mA beam
settings with 10%\
output coupling at 10 microns (so many 10s!). 

![](media/image2.png){width="5.46875in" height="2.8854166666666665in"}

**Management:**

Gwyn Williams and George Neil presented an overview of the FEL program
for our annual DOE Science and Technology Review.

Fred Dylla presented the plenary talk at the 8^th^ European Vacuum
Congress in Germany where he got to announce first lasing of the IR
Upgrade.

Michelle Shinn presented a talk on our FEL at the National Space Missile
Materials Symposium in San Diego, CA

Gwyn Williams attended the first Research Summit for the Virginia
Institute for\
Defense and Homeland Security in Washington.

**WBS 4 (Injector):**

This week concluded a very successful first period of gun operations.
The gun delivered 350 kV, 60 pC beam for FEL commissioning and
operations starting on May 7th with the first cathode (0.65% initial
QE). Since then, we made two more cathodes with improved QE each time
(4.6 and 5 percent, respectively). The last cathode delivered about 80
hr of beam showing a 40% drop in its QE over that time. On Monday the
cathode was HV conditioned to 360 kV for about one hour, with no
evidence at all of field emission. Then the conditioning resistor was
shorted out with the running resistor to continue beam operations at 350
kV.\
We discovered that the PARMELA simulations used currently for the
injector do not consider the overlapping fields of the gun and the
solenoid. We are working on modifying the code to address this problem.\
\
Continuing with field emission suppression tests, a 6-inch electrode has
been implanted and SiO2 coated in the W&M ion-source plasma implantation
chamber using an extended 9-hr long process. The electrode will be
tested later in our Field Emission Test Chamber.

**WBS 5 (SRF):**

FEL3 Cryomodule status:\
All the FEL3 cavity helium vessels have had the holes cut in them for
enlarging the stub-up. The process went smoothly and the machine shop
was apparently successful in controlling the chips to avoid
contaminating the helium vessels. Kudos to Dave McCay and the machinists
for performing this delicate task quickly and precisely. The transition
saddle pieces are due in today or tomorrow and weld tests will be done
on spare parts first before finishing the helium vessels. The waveguides
have all been fired to 400C and show no new blistering. Some have areas
around the flange and knife edge where the plating is missing and on one
or two these extend into the waveguide. John and Larry are developing a
local repair using brush plating. All the chemicals are now in hand for
the nickel strike and the copper. Robbie Hicks and Larry have a proposal
for the thermal syphon and a there will be a meeting on Tuesday this
week to review it. The scheme uses a flattened tube joined directly to
the waveguide by a low temperature braze or solder. There may be a
conflict for the furnace and for Tom Elliott\'s time as he is also
working on the warm windows for the FEL quarter cryomodule and the FEL
mirrors for Michelle Shinn.\
The heat station for the waveguide should be capable of syphoning off
1-2W at 2K, ultimately limited by boiling in the tubing. This has never
been used at JLab before and there was some discussion about whether it
could cause any instabilities in the module if boiling does occur.
Unfortunately we have no easy way of testing the syphon under realistic
conditions before building the module. We discussed the possibility of
being able to go in and valve off or otherwise disable the syphon if it
proved problematic.

**WBS 6 (RF):**

The machine was run only 2 days this week prior to the shut down for the
GY installation.  The RF systems ran very well.  Several IOC\'s crashed
when their AC power connections were \"disturbed\" and a network adapter
failed.\
3dB attenuators were added to the probe legs of all cold cavities and
the download coefficients changes.  This is to ensure the "Gradient
Measured" diodes in the RF Control Modules are not overdriven.\
The brazing and assembly of the warm window for Quarter cavity 3 was
given a higher priority.  This window will be needed for high power FEL
operations, but probably not for e-beam currents of less than 4 mA.\
A procedure is being written for the LOTO (Lock-out/Tag-out) of the
linac Cathode Power Supplies (CPS) and their High Power Amplifiers
(HPS).  Nearly the entire EES group has been trained on LOTO of these
units.  This written procedure will document LOTO for the linacs and be
available electronically for reference and training.

**WBS 8 (Instrumentation):**

With the pulsed FEL run behind us the push is on to complete the machine
and shoot for the \>10kW. The recirculation dump viewer flag was coated
with phosphor by the Diagnostics group at DESY, a very special thanks to
Kay Wittenburg and the folks from the MDI group at DESY! This flag is
crucial to properly setting up the electron beam raster power supply at
the 100 kW beam dump.\
A number of viewers were found to be out of alignment during the past
run, these are being reworked. The list is as follows; ITV0F04 -
special - removed and being fixed , ITV1F03 - 5mm hole - ready for
install ITV1F04 - 5mm hole - Tony working on it 2F06 - Silicon - Ready
for install 3F12 - Silicon - Tony working on it 4F00A - Standard - ready
for install 4F03 - Standard (Happek) ready for install, ITV1G03 - ready
for install, ITV 1G02 - Mirror - viewer removed, waiting on rework and
mirror. A new camera test fixture has been built to service the viewer
Cameras. Thanks to all for their help and attention.\
    Work has begun fabricating the floating 2 A power supplies for the
QT in both Arcs. These must float on the 75 volt Danfisik bipolar power
supply to give vertical correction in and out of the GY/DY magnets. The
requirement is that it not shunt any current through the magnets
hysteresis cycle. The SF6 bag was tied back out of the way and the
tables were removed in order to make room for the GY magnet
installation.  The DY magnets that were behind the beam line for extra
load were removed and the water taps capped.  The steel shielding around
the temporary dump was removed and stacked on pallets so it can be
reinstalled around the 1G Dump.\
   We installed the HR & OC Analog Breakout Chassis in the Control
Room.  The cables for this chassis were pulled to the HR and OC. 
Progress is being made on the Analog Breakout Buffer board, the
schematic is being completed by EECAD and the PCB layout has begun. 
Once the layout is completed and reviewed, boards will ordered. A system
print has been sketched and will be turned into EECAD once rack
designations are decided upon.  The chassis that will be installed at
the OC and HR I&C centers downstairs are in the design phase and early
fabrication stages. The back panel as well as the front panels have been
completed and the wiring layouts are in progress.\
     A test power supply was assembled for the Golay Cell out of the
Happek Device.  Alignment of the mirrors and the Golay continued in
order to get the spare Happek operational.\
   Progress was made towards getting the frame grabber project into
EPICS.  EPICS is now installed on a Linux box awaiting a test process to
ensure proper configuration.  Documentation for the Input Output
Controller\'s (IOC) Record Reference Manual is being analyzed to aid in
understanding the system. The Beam Current Monitor cavity electronics
have been bench tested and are going thru EECAD for a PCB layout.\
   The HVPS Control Chassis Cable Diagram was signed off and turned in
to EECAD.  Also the drawings on the wiring and cabling of the Optical
Cans at the HR & OC have been signed off and turned in. A fifth stepper
motor chassis has been completed and is awaiting commissioning.\
The new Charge/Dump Current Monitor Buffer Driver was successfully
\"bench checked\".  It will be tested with the chassis as soon as
modifications for ICM0F06 are completed.  Two additional boards were
stuffed, and another box built and wired.  The second box still needs to
be tested.\
    EH&S conducted the required quarterly safety inspection of the FEL.
Unofficial results have been e-mailed to affected parties.  Official
tracking system entries should be completed today. \
The new Automatic External Defibrillator has been installed on the wall
across from the Control Room and 9 staff members were trained in its
use, in conjunction with CPR.  Additional training classes are scheduled
for July and August.

**WBS 9 (Transport):\
**

Dipoles\
\
Injector Dipoles (DU/DV)\
We continued to get different field integrals out of the 5F GV that we
had pulled from the machine in order to get additional measurement data
at other levels of excitation.   We thought a calibration of our
measurement system would remove any doubt about its integrity. In order
to calibrate, we removed a second GV dipole from the machine (from the
extraction string, which is not being used).  Its magnetic properties
repeated exactly.  This convinces us that our measurement system is
qualified and we will just adjust the field clamps on the 5F GV to bring
it into specification.  This will ignore the whole episode of that
magnet being in spec. and then going out of spec.\
\
Arc 180 Degree Dipoles (GY)\
The first GY was installed in the machine.  It is now being aligned.
Cooling water and power are the next connections.\
\
Multipoles:\
\
Trim Quadrupole (QT) / Vertical Arc Corrector (GC)\
We removed one of the combination trim quads and vertical corrector
magnets from the second arc to take some data on how the corrector field
affects the quad and vice versa.  We had only taken data as isolated
quadrupole of dipole corrector in earlier measurements.\
\
Octupole (OT)\
Procurement has to re-affirm the prices for the cores obtained during
the last procurement cycle and place the order.

**WBS 11 (Optics):**

The first part of this week was devoted to obtaining high-quality data
on the laser.  This included amplitude stability, cavity loss,
sensitivity to changes in mirror radius of curvature and tip and tilt. 
A summary is presented elsewhere.  This data is being used to help plan
for operations at high average power.  The latter part of the week was
spent with preparations for adding the high average power optics.  Our
plan is to install the HR mirror next week, and the OC mirror the week
after.  We are also interacting a great deal with I&C group to ensure
they have what they need to make some of the temporary signal cabling
and controls for valves and viewers more permanent.

 We are also incorporating changes to software screens for the FEL
mirrors to automatically remove backlash, and to make them more usable
for non-experts.  There are also changes being made to the diagnostics
hardware.  We are preparing to exchange a shutter for the second EO
modulator in order to raise the available laser power to the gun.  Some
work still needs to be done to create a low-cost pre-trigger for the
shutter, but we have a system we will begin working with during the
shutdown.

 Early in the week, Michelle Shinn, along with Dr. Frank Livingston from
The Aerospace Corp gave a tutorial entitled, \"High Average Power Free
Electron Lasers For Material Processing\" at the National Space Missile
Materials Symposium in San Diego, CA.