# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn/gpw

**Subject:** FEL Upgrade Project Weekly Brief - January 22-26, 2007

**Date:** January 26, 2007

**Highlights:**

Highlights of the week included supporting a significant Users workshop
for FSU in the development of their proposal to NSF for an FEL at the
National High Magnetic Field Lab. We observed excessive mirror coating
absorption which had changed since initial installation of the 900 nm
mirror. The cause of this coating change is still under investigation.
Progress is also reported below on the power couplers for the AES
cryomodule presently under assembly. We devoted significant effort to
our planning for the next 18 months directed toward the priorities
developed by ONR as a result of our annual review last week.

**Management:**

The primary management effort this week was devoted to program planning
for the next 18 months. This included estimating cost and schedule for
the ONR prioritized task list produced from our annual review. These
will be assembled for discussions with ONR next week

This week we hosted a workshop on the use of Free Electron Lasers and
High Magnetic Fields for research.  The meeting was NSF sponsored and in
collaboration with colleagues from the National High Magnetic Field
Lab.  Many of the speakers have used other FEL facilities, and very high
levels of discovery class science were presented along with
opportunities afforded by a new FEL/THz facility proposed for FSU.  The
discussions and input will help us develop our own facility as well as
plan one at FSU.

This week we set up a video conference with our Site Office and several
other national labs including Argonne, Fermi, Brookhaven and Berkeley to
present our incident report, and our actions to raise awareness of
hazards associated with exclusion areas, and to invite suggestions.  It
was an extremely useful meeting with many useful comments and general
agreement with our course of action.

**Operations:**

As of the end of last week\'s operations we had tuned up the electron
beam and gotten quite good performance from the laser in  pulsed mode. 
The measured gain was about 50% compared to a prediction of 60%.  The
measured efficiency was close to the predicted value of 1.2%.  When
running CW however the efficiency fell off rapidly with duty cycle.  The
power seemed to be clamped at about 600 W when running at 900 nm.  We
could get higher power (about 800 W) when detuned to about 960 nm. 
There were clear indications that the efficiency droop was due to mirror
heating.  The mirror that seemed to be absorbing more was the high
reflector (HR).  When running pulsed beam we could also see evidence of
mirror vibration.   The gain was a bit lower than expected due to some
astigmatism in the HR.

On Monday we set up diagnostics to allow us to monitor mirror vibrations
and try to reduce them using the flow rate or motor speed of the mirror
chiller.  We determined that the mirror oscillations were dominated by a
yaw oscillation in the high reflector. They did not change with water
flow or motor speed.  We also looked at the efficiency vs. wavelength.
It seemed to highest for a wavelength of about 950 nm.  The laser showed
quite a bit of mode hopping when the mode was centered on the electron
beam.  By moving the electron beam down a fraction of a mm we could get
the laser to lase on a TEM01  mode, which seemed quite a bit more stable
and provided better  efficiency.

On Tuesday we ran pulsed with the 2.8 micron high reflector. 
This mirror has good reflectivity at 930 nm but has almost
no astigmatism.  The mode movement with the 900 nm HR was along
a diagonal.  Simultaneous vibration of both the yaw and pitch axes
at the same frequency is highly unlikely so our hypothesis was that
the diagonal movement came from the astigmatism in the 900 nm HR. 
With the 2.8 micron HR the movement was at the same frequency but
was completely horizontal.  This verified our hypothesis that the
mirror vibration was in yaw but the astigmatism rotated the oscillation
to be along a diagonal.  We then switched back to the 900 nm HR
and measured the cavity losses and gain vs. wavelength.  The
gain increased as the wavelength increased as expected, but the
gain dropped more than expected near the short wavelength end of
the coating and increased more than expected at the long wavelength
end of the coating.  We have seen this behavior with the 1.06
micron mirror set as well but do not yet understand it.  We checked the
high power performance and found that we could get about 500 W at 900
nm and up to 640 W at 950 nm.

On Wednesday we planned to carefully measure the mirror absorption vs.
wavelength but were stymied by a leak that developed in the
water cooling line for the 900 nm HR. Since we could not lase with
this leak we switched to providing beam for THz imaging experiments.

On Thursday we went into the HR can and changed out the 900 nm HR.   We
pumped down the can and today are providing beam for THz imaging and are
looking at the injector performance with high charge and low cryounit
voltage.

**WBS 4 (Injector):**

The photocathode gun has delivered 36 Coulombs and 7 hours of CW beam
time during the week. The power request from the drive laser is around
45%.

D. Bullard continued working on the mock-up wiggler chamber and figured
out a way to solder the Cu water cooling tubes without warping the
chamber. The machine shop added two ports to the chamber for
characterization of the sputter coating during the test. The gun test
assembly was moved to out of the way so that plant engineering crew
could install the hydraulic arm to operating the Gun Test Stand
shielding sliding door. We worked on the draft for the Physics Today
article on high brightness electron sources.

Mounted a new wafer in our test chamber and today we are heating it just
below the threshold for surface roughness, and soaking at this
temperature for three hours to see if time has any effect on increasing
surface roughens due to Arsenic evaporation.

**WBS 5 (SRF):**

\*100 mA AES/FEL Status\*\...

-   Fundamental Power Couplers (FPC)\
    Assembly for low level rf measurement has begun and some mechanical
    issues are been discovered.  The threaded connection between the
    inner conductor extension and the window is binding during
    assembly/disassembly.  We are having daily conference calls with AES
    and the FPC vendor to remedy this problem.  We also have been
    meeting with in-house rf experts to review the Cu plating of the
    components with regard to assembly and operation.

-   Beam pipes

> The warm-to-cold beam pipe assemblies have begun to go together this
> week.  The upstream assembly is complete.  The downstream assembly has
> proceeded to the point where it is in for brazing of the Cu cooling
> line for the \'halo\' absorber.  The brazing of the second HOM feed
> through is also ongoing, we expect to have it leak checked and ready
> for cavity measurements next week.

-   Other

> We need to add the activity of mounting the FPC\'s to the cavities in
> order to verify the \'real\' coupling, since this has never been done.
>
> Procurements will go out next week for the remainder of the assembly
> tooling and components.

**WBS 6 (RF):**

The RF system is fully operational.  There was no time lost this week
due to RF problems.   The Injector Phase Monitoring system indicated the
Buncher and the 2 Quarter cavities drifted less than 0.8 degrees of
phase over the last 3 days.  The Drive Laser sensor is not yet installed
for the phase monitor.

 

Gun HVPS - The Gun HVPS is fully operational.  It just runs without any
faults.

**WBS 8 (Instrumentation):**

Resumed Laser Personnel Safety effort in User Lab 6 and the Optical
Control Room. Lab 6 is being prepared to run the ODU FORT experiment
which includes integrating the new mirror cassette and into the LPSS. 
With the internals of the mirror cassette removed and no shutters, it
had not been able to accept FEL light. The mirror cassette, all
necessary interlocks (harmonic blocking filter, User Lab shutter and
hutch interlocks) installed, wired in and made operational. We hope to
certify this lab for FEL operation late next week. Work continues on the
new LPSS master in the OCR and the first set of hardware for the Laser
Safety goggle RFID system has been placed on order. The test parrot
board being used for the PA system malfunctioned; it turned out it was
because the microphone ground and the power ground had to be isolated. 
The circuit was modified to accomplish this, and a filter is being
designed to eliminate the static on the recording.  Once the filter is
finished, the boards will be mass-assembled for the lab lpss boxes. The
user lab doors are in various states of disrepair. Last week we had to
rebuild the latch and strike pieces on Lab 1 because the LPSS maglock
would not engage, this week Lab 6 failed because the closer does not
work properly. We need support from plant services for these chronic
problems!\
   The conclusion of our studies on the drive laser\'s E.O. cell
performance have shown us that we need to replace the 1st E.O. cell
(EO2) which was purchased in 2002. The bias voltage data plotted from
Dec. 2006 versus Jan 2007 clearly shows steady-state instability that
OPs have been reporting. In addition to determining the cause of the EO
cell bias drift, we have directed our attention to implementing an
\"auto-bias\" system. After discussing the system requirements with the
optics folks, we all agreed that an important first step in designing
the system is to include temperature sensors on the EO cells. To that
end, we installed a two channel OMEGA-RTD chassis into the clean room
rack (FL06B01) and installed to RTDs onto the EO cells. The analog
voltage readback have been cabled into the zone 1 racks (FL01B10) and a
new patch panel has been fabricated to break out the previously
unavailable ADC inputs to EPICS. MEDM screens have been created and all
of the new cables, chassis and panels have been documented and entered
into the database. The EPICS signals will be calibrated early next week
and we will then begin collecting EO cell heating information into the
system performance archiver along with all of the other system-specific
signals.      The final touches went on the new Wiggler control
software.  The new database, screens, and scripts for the upgraded
software were also written.  Accommodations were made for the flaky
communication that we experience right now.  The new communication
handling should prevent errors from developing as well as fix them if
they happen to occur.  This software will be installed during the
upcoming down.\
   The Aerotech linear translation stage was installed in Lab 3 and is
being used in support of the THz experiment. This week we have also
designed a water-cooled transverse beam stop for use in lab-1. The
design includes a removable reticle plate for easier transverse
alignment, and incorporates water cooling for maximum heat dissipation.
The motion of the stop will be powered with an air cylinder.\
   We have also made progress on a few electrical safety items; a
commercial phase rotation meter has been connected to the standard
Hubble twist-lock 208VAC/3Phase plug. This allows anyone to plug this in
to check that all three phases have power and the outlet have been wired
for correct rotation, of the first 6 outlets check two had incorrect
phase rotation. The motivation was the time lost when the FORT laser
would not come on; this was due to incorrect phase rotation - the system
was protected against the water pump motor spinning backwards.

**WBS 11 (Optics):**

**Cavity Optics**

We began this week by attempting to quantify the apparent optical cavity
mode motion. We routed several of the cavity mirror PZTs up to the
control room to monitor mirror vibration. These PZTs are located on the
ends of each mirror's pitch and yaw actuator. They provide and excellent
tool to measure resonant vibrations. It was clear that the major source
of vibration was from the high-reflector yaw plate. By changing the
frequency of the FEL and using it much like a strobe light we where able
to confirm the culprit vibration frequency of 23Hz. It was also noted
that 900nm HR had developed some scatter sites. We opened the HR vessel
up on Thursday to fix a water leak that materialized on Wednesday. We
took this opportunity to replace the 900nm optic with a spare. The lead
strap was relocated on the yaw plate. This reduced the 23Hz oscillation
from \~-60dbV to \~-75dbV. The operations crew had noted previously that
the pitch mirror motion response of a few of the HR mirrors were
sluggish. It was difficult to verify on the wiggler viewers if the
change we made improved response. This will be checked when we next run
the FEL. The new scatter light shield design for the cavity mirrors are
in the machine shop for fabrication. These new shields will hide the
mirror mount fix clips from the FEL for the two and three inch mirror
positions. Another shield is currently under design for the front of the
inner vessel.

**Optical Transport/User Labs**

Work began this week on assembling the parts needed to begin
installation of the mirror cassette assembly in Lab 5. The stage for the
mirror cassette has been cleaned and is in the process of being put back
together. We hope to install the mirror mounts to the stage by the end
of today. The Lab 6 installation is almost complete. A new lab shutter
was installed that was shorter and the harmonic blocking filter was
modified so the assembly can fit in the back of the room without the
need for moving the laser table. The stand that supports the blocking
filter and shutter has been modified so the door on the back of the
hutch can be opened.

An inventory of the electrical needs for the internals of optical
transport vacuum cans is complete.  We continue to search for a suitable
stage to complete the mirror cassette assemblies for Labs 3 and 3a. The
stages used in the other labs are discontinued and a direct replacement
is not available. We are looking into various stepper motor stall
detection methods. The Optical Beam Position Monitor (OBPM) uses a
stepper motor to spin a wire across the FEL beam for position
monitoring. It is important to know if the motor stalls so that it can
be removed from the beam path.

**Advanced Drive Laser**

We are trying to finalize the remote control for the advanced drive
laser with the laser running. We investigated the Antares laser EO cell,
reported by operation crew last week to be faulty. We found that one old
cell is indeed not functioning properly. Further observation shows that
the pulse could be stabilized with some additional optimization. We are
also checking to see if the thermal behavior of the EO cell is related
to the laser power dissipation by monitoring the cell temperature. The
design of a beam shaper is completed. We helped gun group with the QE
monitoring system. We contacted vendors and submitted procurement
requests for several important parts for the current drive laser.

# FEL Mirror Development

As discussed in the Operations section, we started lasing on the
newly-installed 900 nm mirror set last week.  In situ loss measurements
were very intriguing.  High power lasing (ca 1kW) was possible through
Thursday of last week.  Loss in the HR was very low, essentially in the
noise.  By Friday we could only make \~ 550 W and the loss in the HR was
\~ 0.25%.  The loss in the OC was essentially constant.  This is the
first time we\'ve seen a fast aging event, and also the first time,
since switching to ion-beam sputtered coatings, to have the HR be the
optic that was exhibiting high loss.  We didn\'t see any tell-tale signs
of damage while lasing.  This week, we carefully measured the loss
(through cavity ringdown) as a function of laser wavelength (from 845 -
988 nm), then compared the loss with the loss expected from the
vendor\'s measured reflectivity.    What we found was good agreement on
the long wavelength (\> 950 nm) side of the tuning curve, but
progressively poor agreement on the short wavelength side.  This
suggests there is a reduction in the reflectivity or an increase in the
absorption in the near IR.  Since laser operations terminated with a
small water leak, the optic was pulled for inspection and testing
outside the cavity.  The results are hard to explain on the basis of
some sort of harmonic absorption, since the OC mirror intercepts more
harmonics than the HR.  Thus, we are also questioning the vendor as to
the pedigree of their target, and whether there was anything out of the
ordinary during the coating run.

**Terahertz:**

On Monday and Tuesday of this week, we held the FEL/HMF Workshop, which
was dedicated to providing guidance for the design of the FEL planned
for construction at Florida State University.  That FEL will be
dedicated to research making measurements under the conditions of high
magnetic fields and will likely incorporate two THz sources, one that
will be a narrowband undulator based source, and a broadband bending
magnet source.  Participants from many institutions contributed during
the presentations and focus discussions and good progress was made in
developing the FEL design for FSU.\
\
Following the workshop, we setup for performing the real-time THz
imaging tests again.  This time, we are using a camera that is on loan
from Indigo Systems.  We were assisted by one of our collaborators,
Susan Cox from Los Alamos National Lab, who was here as a contributor
for the FEL/HMF Workshop.  For our first set of tests, we reproduced the
tests that we had done previously with the group from the University of
Delaware.  These were all successful, and we were able to achieve images
similar to before, though our field of view is smaller due to the higher
f/# of the optics used for imaging onto the detector array.  Today, we
are continuing those tests for another short run and will attempt to
improve the imaging resolution.