# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - October 9-13, 2006

**Date:** October 13, 2006

**Highlights:**

We are continuing tests of our first cryocooled (50k) optical cavity
mirror. In the last two weekly reports, we noted that we have
successfully lased at cw output powers of 1 kW, but an observed vertical
oscillation of the optical mode was preventing us from pushing the power
higher.  We were initially stumped at the source of this motion because
the optical diagnostic system for the optical cavity was telling us that
the mirror motion was negligible. However, after some excellent
detective work this week we determined that the mirror was indeed
oscillating at a 150Hz rate and the driving term appeared to be the
helium gas flow in the cooling loop.  While we continued other FEL
measurements this week, the optics group designed a damping system for
the mirror and it is being installed today. We are also shielding
temperature sensors on the cryocooled mirror assembly which should also
improve the performance of the mirror at higher powers. (Details are
given in the Operations and Optics Sections below).\
\
Prior to the end of week shutdown for the above alterations, we spent
some time improving the \"match\" of the accelerator to the wiggler
section and into the recirculator, conditioned the driver accelerator
for high current operation (up to 6.5mA), and ran\
a shift of THz operations.  During the latter run a very important set
of THz absorption  measurements was obtained for materials of interest
to defense applications. These measurements set guidelines for the
development of THz imaging devices\
and other spectroscopic applications.\
\
Next week we should be bringing the FEL back on line late on Monday or
Tuesday depending on the cool-down of the cryomirror assembly.\
\
**Management:**

We completed our Navy project financial reports for the month
of September and forwarded them to the DOE and ONR project offices.\
\
On Monday, Oct. 16th, the FEL project management team has a meeting
scheduled with our ONR and NAVSEA program managers to confirm the
technical scope, budget and schedule for the FY07 portion of our 3 year
(FY05-07) MOA with the Navy.  We will also discuss the preliminary
agenda for our annual program review which is scheduled for Jan. 17-19,
2007 at Jefferson Lab. Final arrangements and a briefing book were
prepared for the Monday meeting.\
\
Our collaborators at EVMS and the NIH proposal coordinating team held a
conference call with the NIH program manager who oversees the
interdisciplinary research center proposals in NIH\'s National Center
for Research Resources (NCRR). Our pre-proposal was given excellent
marks on the proposed science, but it did not make the cut from the 300
submitted applications to a set of 16 that will go forward to full
proposals. We were given good advice on moving forward with a different
collection of revised proposals from the collaboration for submission
in 2007.  A revised program plan is being prepared by the coordinating
committee that will be distributed next week to the full collaboration.\
\
JTO Programs:\
We began assembling the monthly report for the JTO on our two current
programs: the Advanced Deformable Mirror and the Short Rayleigh Range
FEL Operation activities.\
\
Florida State:\
We were able this week to perform PARMELA modeling of the injector at
Rossendorf as an ersatz version of the injector we intend to build at
FSU.  Most operational parameters of the injector were confirmed as
agreeing with published measurements from that system and we are able to
determine sensitivities to parameter variations from the baseline.  We
also established a WBS of the FEL to level three for purposes of costing
the system for the NSF proposal and verifying completeness of the
design.   A conference call was held to discuss procurement strategy and
options.   Further discussions with the Florida State U. procurement
office will be required before a definitive plan can be adopted.\
\
Michelle Shinn attended the annual meeting of the Optical Society of
America (OSA) to present an invited paper entitled, \"High average power
optical systems for the Jefferson Lab FEL\".  She also was given a tour
of the DOE-funded Laboratory for Laser Energetics, with the specific
goal to learn more about their facilities and techniques for optics
deposition and metrology.

**Operations:**

As of last Friday we were still trying to figure out why the FEL optical
mode was oscillating.  We measured the electron beam position at several
locations and found very small beam motion at 29 Hz but it was not
enough to explain the mode movement.  We also found that the mode, when
aligned with our alignment HeNe, was not centered on the mirror.  Either
the mirror was mis-centered or the HeNe was mis-aligned.  We checked the
HeNe with respect to the quad-centered electron beam position and found
that the HeNe was mis-aligned.  When the electron beam was carefully
quad-centered and the optical mode was centered on top of the electron
beam we found that the optical mode was centered on the mirror.  This
did not affect the mode oscillation at all however.\
  Since neither the OCMMS camera nor the center wiggler viewer camera
showed any mirror movement we concluded that either the mirror was not
vibrating or it was vibrating at some harmonic of 30 Hz.  If the mirror
vibrates at a harmonic of its frame rate you don\'t see any motion in
the image.  We then worked on getting the position sensitive detector
(PSD) in the OCMMS working and found a 150 Hz oscillation.  This
frequency is consistent with the observation that the mode moves slowly
when the macropulse repetition rate is 30 Hz but breathes when the
macropulse rep. rate is 60 Hz.  The macropulse repetition rate must be a
sub-harmonic of the oscillation frequency to move slowly.\
  Once we knew that the mirror was oscillating, the work turned to
trying to find its source.  We tried lowering the He flow on the mirror
and looking at the response both on the OCMMS and on an accelerometer on
the mirror can.  The accelerometer signal was dominated by a signal at
about 450 Hz, which is close to the turbine frequency for the CHL. 
There was also a 150 Hz signal but it did not vary much in amplitude as
the He flow was varied.  The 450 Hz signal declined rapidly with flow. 
We eventually looked at the piezo pusher output and found that the
mirror vibration could be seen on that.  When the flow was shut off the
150 Hz vibration went away, indicating that the flow was the source of
the vibration.\
  Even with the oscillation it was possible to better optimize the match
to the FEL.  We optimized this and found an exhaust energy spread that
is comparable to that of the room-temperature output coupler.  The
average efficiency is lower however since the laser is not lasing well
when the optical mode is mis-aligned.\
  After analyzing the vibration, decided that damping the mode would be
the best approach for remediation.  While developing the damping
mechanism we worked on operator training.\
  Though we pushed the power up a little bit this week, it was limited
by mirror steering in the cryomirror.  We decided that this was due to
absorption of the IR light in the Kapton tape holding the diagnostic
temperature diodes in place.  The power absorbed in the cryo-systems is
quite large. These sensors will be shielded during our intervention into
the optical cavity system today.\
  We also worked on improving the match.  With the electron quad
centered we were able to optimize the match to the FEL.  The detuning
curve length was as long as 8 microns, which is quite long for this
wavelength.  The measured gain is 63% assuming 20% losses but we could
not measure the losses due to ringing in the measurement circuit.  We
saw up to 1 kW of light downstairs with this configuration.\
  Since the power was limited by mirror steering we decided to try
pushing the current.  With some coupler conditioning we were able to
push the current up to 6.5 mA for the first time in several months.  The
system ran quite stably at 6 mA.  With some more conditioning we should
be able to push this higher.\
  Though we are not yet able to deliver the FEL light upstairs we are
able to transport light for THz users.  We recertified the lab on
Tuesday and ran beam for THz users on Wednesday afternoon.  The system
ran stably for hours at 1.25 mA with full CW or with lower duty cycle to
lower the average power on the THz detectors.\
  We shut down early to work on damping the mirror vibrations in the
cryomirror and to realign the HeNe lasers used to align the optical
cavity.  The optical mode should be well aligned with the electron beam,
which should make setup much easier.\
  Finally we spent some time this week training more staff in setting up
the accelerator and doing a \"miniphase\" procedure, which is used daily
to set the phases in the accelerator and bring it back to where it was
last phased using the injector phasing procedure.  As usual, exposing
more people to the procedure produced valuable suggestions on how to
improve it.

**WBS 4 (Injector):**

This week the photocathode gun delivered up to 6.3 mA for tens of
minutes and 5+ mA for a few hours of CW beam. On Thursday the
photocathode was re-cesiated and delivered 82 Coulombs and 63 hours for
FEL ops since the last re-cesiation back in September 19th. Polishing of
the molybdenum anode plate for the new gun resumed this week and a new
anode for the field emission suppression film testing chamber is also
being polished out. The vacuum chamber for the new gun was cleaned up in
preparation for vacuum firing.\
\
We want to welcome Deepesh Koppunuru, a graduate student from ODU who
will be working on his M. Sc. degree extending the PARMELA model from
the injector to the wiggler to study longitudinal space charge effects.

**WBS 5 SRF:**

High current cavity update:\
    We passed another milestone last week with the arrival of the  dies
for the 750 MHz Ampere-class cavity prototype. We pressed an  aluminum
half-cell just to check them out and will be pressing a  copper single
cell shortly to check the cell profile and frequency.  This is the first
real hardware for the next generation MW class  machine. Up until now we
have been working with scale models and  simulations. If the copper
model is acceptable we will move on to the  750 MHz niobium cavities,
culminating later this year in a full-size  5-cell model that will be
the largest cavity ever produced by JLab.

![](media/image1.jpeg){width="2.672222222222222in"
height="2.004166666666667in"}

**WBS 6 (RF):**

RF -- Except for the Drive Laser pick-off, the installation of the Phase
Monitoring system's hardware and software are complete and the debug
process has started.  The Drive Laser pick-off requires the design and
fabrication of a high gain, high frequency photodiode and amplifier.  It
will be several weeks before it will be ready to install.

 The first try to automate high peak power pulse processing the power
couplers for the Quarter cryomodule was done this week with promising
results for one cavity.  The waveguide vacuum signal will need to be
"cleaned up", but the technique looks promising.

 The user interface to Upload/Download the various RF parameters was
rolled back to an older more convenient version.  The new version is
still available if needed for changing a few parameters on multiple RF
Control Modules.

 

**WBS 8 (Instrumentation):**

Time was spent this week involved in FEL operations.  Specifically, we
focused on training to perform the \"mini-phase\" procedure.  While
performing the mini-phase procedure we took the time to complete a rough
draft procedure that is currently being reviewed and assist with the
training.  In addition to this general training we were able to observe
first hand a bug in the pulse controller EPICS software which has been
fixed and will be implemented in the next reboot of iocfel10.  Also
discovered this week were some minor changes required to the User Lab
EPICS screens, specifically, the Alignment mode readback message from
the permit chassis.  These too will be fixed in the reboots scheduled
for this week-end.\
   In preparation for further system documentation, an FEL System Layout
Template has been crated in Auto CAD, this template will be used to keep
track of the locations of system components.  Additional refinements
have been made to the EPICS-to-Devlore data bridge tools. We now have
system performance data being achieved for online retrieval for the RF
systems, the drive laser system, the BPMs, the LPSS and the Vacuum
system in addition to the regular FEL OPs data.  The vacuum pvs that
were being reported to devlore were being truncated, this was fixed. 
EPICS bit values are now transmitting correctly.  We are modifying the
embedded BPM process variables to make them consistent with the other
signal names and to ease the EPICS to devlore process for the BPMs.\
   In response to the recent network security audit that was conducted
at JLab, a cyber security task team has been formed to carry out the
necessary actions which are required to meet the new laboratory network
security standards.  Time was spent this week with the CC folks in
preparations for the actions with will affect the FEL.\
   We also took advantage of the cryo mirror test to upgrade the LPSS in
User Lab 2. The new system is operational in Labs 2 & 4, with Lab 5 not
far behind. Lab 5 will be used to commission the new LPSS master, this
work will be done during the Dec/Jan down. A decision will be made next
week on when to bring lab 1 offline and convert it over. The time to
convert is \~ 2 weeks.\
   Work continues on the revision of the Single Board IOC Module and the
software refinements.  There are several routing issues that are being
taken care of and then this PCB will be ready to go for the next
revision.  The necessary parts for this revision have been ordered to
complete 20 pieces.  We\'ve also ordered parts for the General Purpose
I/O card, which came in this week.  The first board is being piecewise
assembled and tested.  So far the board has passed initial power
testing.  The dimensions of the PCB were all verified to be correct. 
The front panel for this PCB has arrived and the dimensions of this were
all verified to be correct.  The front panels are ready for a production
order, provided we don\'t decide to alter the PCB.  The first Beam
Viewer Control Card has been completely populated, power tested, and
successfully programmed.  The front panel for this board has arrived and
verified to be ready for the production run.  We assisted in moving,
numerous tables, machining equipment, and various parts over from the
test lab.\
   Effort continues in Lab 5 for the LPSS installation.  The door
interlocks, magnetic locks, access indicators, crash and exit switches
have all been connected and tested for function. Cabling for the smoke
alarm, dump klixon, mirror cassette and shutter control have also been
installed.  Additional various items were also ordered to continue LPSS
installation for the other User Labs\
   A major amount of time this week was spent constructing the air
cylinders needed to support the optics group.  A total of nine
assemblies and have been\
tested and are ready for use.  Most still need the air fittings that are
placed on the MAC valves but those will be put on as needed.\
   THz work also resumed this week.  Testing began on transmission
through various materials and basic set up after the construction group
built and installed a floating table inside the hutch.  The linear stage
is setup in the hutch and is ready for imaging.  Mike Kelley\'s Excimer
Lamp was moved from Lab 1 and mounted in Lab 5.  The power supplies and
all associated equipment were also moved.

**WBS 11 (Optics):**

Much of this week\'s results on our primary activity, cryomirror
testing, is reported elsewhere.  This section focuses on the causes of
the absorbed power mentioned in last week\'s report.  Analysis of the
data collected on Oct. 5&6 (calorimetric, as well as mirror steering and
cavity length adjustments) indicated that the power absorbed was not due
to the mirror, but in other components.  More specifically, photographs
of the installation as well as data that showed as much temperature rise
on the input temperature transducers as the output transducers all
pointed to the use of polyimide tape to secure the transducers as a
major source of this absorbed power.  This is being addressed as this
report was written.

 

Observations of a loss in power transmitted upstairs while operating
with the cryomirror led to speculation that since dn/dT was greatly
reduced, so was the index of refraction.  Such a reduction would change
the divergence from the laser, and hence cause loss.  However, a recent
(2002) conference proceedings report provided data that conclusively
showed the index of refraction changes very little (about 0.05%) upon
cooling to 50 K.  We now believe the loss of power is due to the
oscillating laser mode not being fully intercepted by the beam dump.

 

We received our new 1.06 micron outcoupler mirrors early this week and
they are going through metrology.  These mirror\'s coatings are
especially designed for operation not only in the NIR but also in the UV
(355 nm).  Laser measurements at this short wavelength will advance our
understanding of the performance limits of the FEL.

This week we characterized the OC cavity mirror motion. Using the
Optical Cavity Mirror Metrology's (OCMMS) Position Sensitive Detector
(PSD) we where able to see a \~150Hz oscillation. This frequency
correlates to the observed motion of the mode on the OC mirror. The
PZT's on the end of the cavity steering motors also provides a good
means to measure oscillations. One source of vibration appears to be the
He gas flowing through the hydro-formed bellows. Another source is the
He transfer pipe that was loosely resting on the floor of the vault.
Some experiments were conducted to test vibration mitigation schemes. We
plan on make the following changes: (1) Adding a lead strap from the
pitch ring to the yaw plate. This should provide a strong damper to the
vertical oscillation. (2) Replace the 'L' bracket that supports the He
lines closest to the mirror with a gusset bracket. (3) Add Macor
supports between the input and return tubes. (4) Wrap aluminum foil
around the bellows. (5) Remove the cool-down diodes. The diode wires are
believed to be the source of mirror steering through scattered light
absorption. (6) Adding a copper sleeve to the back of the mirror to
direct the strongly diverging harmonic light away from the OC vessel. We
will also add a 1.6um camera to the HR mirror to observe the fundamental
mode on that optic. Monday afternoon the Advanced Drive Laser's
oscillator lost modelocking. The oscillator was fixed but the system has
not fully recovered. We expect the laser will be fully optimized by
early next week. We will continue with EO cell testing next week.

**Terahertz:**

On Wednesday, we had a ½ shift of dedicated beam time for THz studies,
which we took advantage of to take beam profile data to measure the
trajectory and divergence of the beam when it leaves the beamline. We
also used the beam time to make transmission measurements on materials
of interest for defense applications. These measurements were
particularly important for the development of THz sources and imaging
technologies

While our Spiricon Pyrocam III is not sensitive enough for the
transmission measurements described above, it does provide a means for
taking the beam profile data necessary for characterizing the beam
trajectory and divergence in the lab. Wade Brock is helping with this
for his senior thesis project and performed much of the work to setup
and align the linear stage used to control the camera position. Using
that system, we were able to measure the beam trajectory before
collimation and we are working on developing the software to analyze the
data.

We also made more progress in improving the lab and hutch setup to
accommodate the new vacuum systems that were moved onto the optical
table last week. We raised the table up to the proper height for the
optical axis of the vacuum boxes and had an electronics shelf built over
the table. The shelf allows all of the electronics, computers, and
controllers to be moved off of the table, and signal and control cables
no longer have to be routed across the table, allowing the entire work
surface to be used for testing.