# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - October 16-20, 2006

**Date:** October 20, 2006

**Highlights:**

This was a very important week for the FEL Upgrade. As readers of this
weekly report have observed, as we push the performance of the FEL to
higher powers and shorter wavelengths, the usual limit to laser output
power has been due to figure distortion of the optical cavity mirror(s)
as the heat absorption in the mirror rose with increasing incident
power. Several weeks ago we began our first in-situ test of a
cryo-cooled (to 50K) outcoupler mirror. At the end of last week, we made
some modifications to the mirror assembly to damp out vibrations induced
by the He coolant and to shield scattered light on the coolant
temperature monitors. We are pleased to report that these modifications
by the Optics Group were successful and our initial lasing tests in this
configuration are very encouraging. Shortly after bringing the FEL back
on-line on Tuesday, the FEL system was aligned and achieved a very
respectable lasing efficiency of 1.7 kW/mA in pulsed mode. This
efficiency was maintained as we pushed the current up to 2.8mA,
obtaining \*5.0 kW of cw output power at 1.6 microns\*. At this power
level we could get our first accurate look at power absorption in the
mirror and it is low (\~10 W), giving us preliminary indication that
this mirror technology is performing nearly as predicted. Further work
had to be delayed until today because of a minor pressure burst in the
photogun which necessitated a heat clean cycle. We are pleased to note
that this photogun continues to recycle back to \~5% QE performance
levels despite 2 and ½ years of dedicated use. As this note is written
we have achieved 7.6 kW output at 1.6 microns and the mirrors do not
appear to be a limitation to further increases. Most importantly, the
efficiency does not seem to fall as we raise the current. We thank the
Optics, Photogun, and FEL Performance teams for their significant (and
successful!) efforts this past week.

**Management:**

The FEL program management met with our program our DOE and ONR and
NAVSEA program managers at ONR on Monday to discuss the FY06 results,
plan the annual review (scheduled for Jan. 17-19, 2007) and work through
a number of budget and administrative issues for FY07. Following this
meeting, ONR approved our latest change request (#06-016), which brings
the recent (FY06)and projected (FY07) scope of the project in line with
the available budget. In addition, we continued our discussions on the
tentative agenda for the upcoming annual review with the panel chair
(Prof. Bill Colson from NPS) based on the feedback we received at
Monday's meeting from ONR.\
\
The monthly reports for September for our two JTO projects were
completed and distributed to JTO. We currently have two funded JTO
projects: (1) the effects of short Rayleigh ranges of FEL performance
and (2) the development of deformable out-coupler mirror.

**Operations:**

The laser performance last week was limited by vibrations in the
cryomirror. Damping brackets were installed on the helium cooling tubes
last Friday and we cooled down the mirror on Monday. We also recesiated
and HV processed the cathode on Monday. We came up late on Monday and
only got an opportunity to miniphase. On Tuesday we tuned up the
accelerator and aligned the electron beam along the new alignment HeNe
beam and scanned the length. The laser lased well immediately and the
mode was well centered on the mirrors. The mode was no longer
oscillating so it was possible to easily optimize the laser and raise
the heater current until we had quite a short Rayleigh range. We then
slowly raised the duty cycle from 6% up to 100%. The efficiency stayed
constant at about 1.6 kW/mA. A bit more optimization raised this to 1.8
kW/mA. We then switched to 9.4 MHz and got 2.2 kW with 1.25 mA beam. We
switched to vernier mode and set up 2.8 mA beam.

This produced 5 kW of beam. The efficiency started to fall off for
higher currents but we got up to 5.8 kW at 4.7 mA. We pushed the current
up higher but had an arc in the gun region that lowered our quantum
efficiency to unusable levels.

Wednesday was spent doing a heat clean and making a new cathode. We
discovered a helium leak in the output coupler can. It was sufficiently
small that we could keep the pressure low by adding a turbo pump.

On Thursday we HV processed the new cathode, optimized the laser and
ramped the current up to 5 mA. We had a hard time optimizing the system
and could only get 4.5 kW. On Friday we set up the FEL more carefully
and tried to fiducialize the beam at low current. We could not get quite
as high an efficiency as on Tuesday. We could only get 1.5 kW/mA. This
was fine up to 3.3 mA however. At higher current we found that the beam
brightness fell off and we tried various methods to retune the machine
at high current to improve the peak current. This was partially
successful and we have managed to obtain up to 7.5 kW CW power with 6 mA
of beam. The optical cavity has not been a limit with this much power.
The optical mode does not seem to change much as the power is raised
from 1 to 7 kW. In fact the 7.5 kW was obtained with the same heater
setting on the output coupler as at low power. The electron beam has
various phase and energy drifts vs. current that have to be compensated.
We are still learning the best way to compensate these. We seem to be
able to run 6 mA without much trouble but get waveguide trips in the
injector cryounit for higher currents. We probably need to do a bit more
coupler conditioning before our next power run. At this point the
electron beam seems to be the limiting factor in raising the laser
power.

One other thing we found this week is that the wiggler viewers move when
the wiggler vacuum chamber heats up due to resistive wall heating. We
think we know a way to prevent this and will try it next week. This
explains some of the drifts we have seen in the optical cavity. They are
not real but are changes in the fiducial used to align the cavity.

**WBS 4 (Injector):**

On Wednesday we activated the GaAs wafer into a photocathode for the 8th
time since May 25, 2004. The last time the wafer was activated into a
photocathode was on August 5, 2006, delivering 292 Coulombs, 166 hours
of pulse and 56 hours of CW beam for FEL ops with four re-cesiations
since then. Usually the photocathode is re-cesiated an average of six
times between activations. However, this time the photocathode
experienced an event that resulted in almost complete QE loss due to a
second divot on the surface while delivering over 5 mA CW beam for FEL
ops. The last time a similar, more violent event like this happened was
in December 17, 2004. The QE after this week\'s activation was about
4.8% and the photocathode delivered 5 mA CW and close to 30 C in just
one day of operations.\
\
Drawings for a modified gun corona shield and attachment of a DC motor
to drive the cathode stalk are under development. The torque needed to
drive the stalk was measured by D. Bullard this week for input to the DC
motor vendor. The intent of this work is to shorten the re-cesiation
time from over 3 hours to 30 minutes by avoiding the need to open the
SF6 tank.

**WBS 8 (Instrumentation):**

The conversion of Lab 2\'s Laser Safety System is complete. On Friday
morning the comprehensive certification was successfully completed. Lab
1 has been taken off line to convert it to the new system. Work
continues on the LPSS installation in Lab 5. This effort is necessary
since the original Programmable Logic Controller resources have long
since been used up. Key features of the new system is the ability to
easily move between operation with the FEL or a local class 4 laser as
well as many added status bits available through EPICS.\
    We\'ve completed the layout of the Single Board IOC Module
revisions.  The quote has been received from the board house and we will
order this early next week.  Some of the parts that are needed for the
revision have arrived and have been placed with the assembly kit.  Other
parts have arrived for the 3U General Purpose I/O card.  We continue to
piecewise assemble this card and test each individual portion of the
board.  We\'ve also completed the Beam Viewer Backplane revisions and
submitted that board for quotes as well.  The prototype crate back panel
has also been revised and is ready for the production level order.  This
order will be placed with the production level order for both the Beam
Viewer Control card front panel and the RTD front panel.  Both of these
panels were completed this week and are ready to go.  Some time has also
been spent this week looking into the next revision of the BPM
electronics, specifically the bandwidth requirements for the RF front
end.\
    In an effort to satisfy OPs request to eliminate unnecessary
switching of the injector shorting switch, we\'ve designed a chassis to
manually control this and eliminate the EPICS control.  The chassis has
been 90% completed and will be finished when operations gives approval
to mess with the shorting switch.  The new chassis requires a key switch
in the correct position and then a manual push button to move the
switch.  This gives us the ability to lock the shorting switch in place
and only allow movement when the key is inserted and turned.\
    Work was done with Steve Benson to investigate some jumps in the
wiggler\'s motor resolvers.  These jumps cause the firmware to \"hunt\"
for the proper gap much longer than it should.  This is not a problem
until we do more machine running while performing wiggler scans.  The
wiggler\'s EPICS software was updated to change the minimum gaps
allowances.  A newer version of the software is being worked out to
provide more readbacks and better error recovery.\
    Jianxun Yan has written a paper and presentation slides for the
PCaPac workshop hosted here at JLab next week. Progress has also been
made on programming the code that allows the FGPA to communicate with
the ADC on BPM electronics.\
    Another highlight is the arrival of the new laser printer for the
FEL control room. We have nursed the old one along for far too long. The
new one has a built-in fax so we can also drop kick the POS inkjet fax
machine (that is always running out of ink). Thank You.

**WBS 9 (Beam Transport):**

#### IR Machine Operations

•  I installed a series of improvements to the OCMM mounting that may
allow at least the downstream OCMM to operate despite the Wiggler Beam
Tube's beam heating and subsequent growth. The philosophy is to anchor
the downstream OCMM and let the upstream end go through a controlled
slide. I substituted a vee anchor on the inboard end of the down stream
OCMM Cross for its original horizontal slide surface.  At week's end
this Cross was seen to move horizontally only 0.001 inch at 5 mA beam
current.  At the Upstream end, I added brass bumper restraints to
stabilize that OCMM Cross' lateral X position while I released the
restraining band clamps and added a more slippery Kapton interface to
the two horizontal sliding surfaces of its mount. I also decoupled the
remainder of the attached OCMM optical chamber from its floor level
support along the beam axis by replacing the simple shim interface with
a cross roller linear bearing.  Some dial gauges were set at the
Downstream Cross and the Optical Chamber at floor level to monitor the
status of the improvement.  The Cross' gauge read as above, but
unfortunately the TV Camera on the gauge for the Optical Chamber yields
no image.  However, at week's end, high currents correlate with the
OCMMs laser beam drifting beyond sight.  We have not solved this problem
yet. 

•  At week's end it became evident the expansion of the Wiggler Beam
Chamber while attached rigidly to its non expanding strong back may
create a bi-metallic-strip-like effect, bowing the system down and
lowering the center wiggler viewer during high current operation.  This
effect may have given us a wrong indication in the past that the
electron beam was rising.  At week's end we are anticipating placing a
televised dial gauge to see the extent of the problem.

• I helped the Optics Group with making some dampeners out of lead and
some brackets that substantially reduced the vibration on the Output
Coupler Mirror.

***Improvements and Upgrades***

•   With regard to SF sextupole, the shop made the longer, more easily
removed pins to substitute for the pins at the two yoke separation
surfaces.  I am working on getting Neil Wilson's installation crew to
perform a disassembly and reassembly in Magnet Test to mimic the vault's
cramped conditions.  Then Magnet Test will be able to do their
repeatability test.

•  Facilities Management continues working on raising the capacity of
that Drive Laser Room's cooling unit.

•  Concerning work on the second wiggler chamber, the shop completed
straightening and removing any dimples.  It is ready for soldering on
cooling tubes and flashing a copper coating to the inside to minimize
beam heating.

• In work for the New Gun and injector Test Stand, McAllister Technical
Services continues design of our stalk retraction system.  A revised
plan has them promised start asking about clarifications to our
specification at the start of next week.  With regard to the contract to
Riggins Co. to bring the tank for the high Voltage Power Supply back to
operable condition with a "hydro" test, they are ordering parts so that
the operation goes smoothly once they take delivery of the tank.  They
will call for delivery of the tank to their facility when they are
ready.  With regard to vacuum bake-out of the new gun chamber, we got a
ballpark price from. Howmet Company, a local manufacturer, which was
somewhat higher than we expected.  We have sent the specifications to
other heat treaters for their prices to get a calibration.  I worked
with Ron Lassiter, the designer, to prepare the design and the drawings
of the aluminum shell that covers the apparatus at the high voltage end
of the ceramic stack for both the injector test stand gun and the new
upgrade to the gun in the vault.  Both devices incorporate the ability
to cesiate without transferring out the SF6 gas and opening the
enclosing tank.

*** Experimental program***

*Experiment to see a pseudoscalar particle that couples to photons*  

• I remain designing the parts to hold the new pole tips in the proper
location and restrain the Purcell gap pole sheet in the GW dipoles.

•  The temperature of the camera bottoms out at -29C with its
thermoelectric cooler running at maximum power.  This is 6 C higher than
required.  I am working on a refrigeration scheme to lower the camera's
ambient temperature. 

**WBS 11 (Optics):**

This week\'s exciting results with the cryomirror testing, is reported
in several places, so this section focuses on the performance.  With the
addition of better stray light shielding and removal of obvious sources
of absorption, the temperature rise in the helium gas used to cool the
cryomirror fell dramatically.  For similar output power (3.8 kW), the
temperature rise of the outlet He fell by 5X, indicating a more
reasonable 10W absorbed.  This is more power absorbed by \~ 3X than we
saw for a mirror with the same outcoupling (and the same coating run)
operated at room temperature.  The cause of this increased absorption is
under investigation.  Possibilities range from scattered light or the
tails of the mode being absorbed on the mirror frame, to color center
absorption.  However, the degree of absorption, assuming it scales
linearly in power, will not prevent us from reaching 10 kW.  Other
signatures of absorbed power, like large amounts of mirror steering,
have been greatly ameliorated as well. 

 

The new 1.06 micron outcoupler mirrors have nearly completed our
metrology suite.  So far, they are meeting or exceeding specs.  These
mirrors' coatings are especially good.

 

As part of the Navy\'s request for a plan to produce robust optics for
higher power FELs, a strawman cavity design is under development so the
irradiances and other optical parameters can be calculated.

We continued to work this week on providing support for the Cryo-cooled
OC mirror. As reminder, at the end of last week we vented the OC vessel
to accomplish two major tasks. First to add dampers to the mirror
pitch/yaw motion and second to manage the stray light. On Friday
afternoon we installed lead straps to both the pitch ring and to the yaw
plate. In conjunction to these straps we added supports to the He Lines.
Using the PZTs as vibration indicators we were able to see immediately
that we reduced the vibrations on the mirrors. We also removed the
temperature diodes and more importantly their associated wiring and
Kapton. These were believed to absorb stray light and cause mirror
'walking'. A copper sleeve was also added to the back of the optics to
funnel stray light from the optical mounts. All these tasks went well.
We had the OC vessel under a nitrogen purge during the weekend and
cooled down the optic on Monday. On Tuesday the FEL proved that the work
we accomplished was successful. The FEL was able to achieve high
efficiency with little if no mirror motion on the OC optic.
Unfortunately the work in the OC can, or perhaps the temperature cycle,
opened a small He leak. We added a turbo pump to the vessel that is
overcoming the leak rate. The addition of this pump did introduce
vibrations on the mirror that we attempted to mitigate this morning. We
should know the results this afternoon. We are still waiting for time on
the vacuum furnace to braze our 'warm' OC mirrors. We are preparing two
high reflector mirrors for bonding. These include a new 1.6 um mirror
with a metal under layer and a 1.2 um mirror. Work continues on the
Turning mirror cassettes. We are close to having a replacement for the
cooling chain assembly. The current chain assembly restricts the motion
of the mirror carriage. The Drive Laser EO cell driver was replaced.
This replacement made a distinct improvement to the micropulse structure
at 37MHz. Thanks to the I&C group for their assistance. The Advance
Drive Laser (ADL) is now fully optimized. Testing continues with pulse
synchronization and gating of the EO Cell with the ADL.

**Terahertz:**

Work on the THz program proceeded this week on several projects that did
not require beam time while beam operations were focused on setting up
the machine for high current operations and testing of the cryo-cooled
output coupler.  Early in the week, Wade Brock and I made more progress
on the software that we are writing to calculate the beam propagation.
We have tested parts of the software with real beam profile data taken
during the THz beam time last week.  We have also tested the software
more extensively with simulated Gaussian beam profile data.  We are
still working on the beam propagation calculation which should
eventually provide a means for determining the beam waist at the focal
plane when it cannot be directly measured.\
\
Revisions on the design for the magneto-optical experiments to be
conducted initially at the NSLS were finalized this week and we will
work on scheduling beam time once all of the necessary parts have been
machined.  Also, this week, it appears that I was able find a new
provider for additional pyroelectric detectors.  Currently, we only have
one pyroelectric detector that is shared for use on nearly all of our
THz measurements.  We need to dedicate one detector exclusively for
in-vacuum operation of the FTIR spectrometer.  Also, to bring the vacuum
systems online, risers for the newly completed "Ox Box" were submitted
to the machine shop to bring the optical axis of the box to the match
the optical axis of the other vacuum systems.\
\
This week we have had discussions with Kristina Woods from Carnegie
Mellon University regarding experiments on biological systems. This work
would further extend our experience in this area.  We are discussing the
experimental details to determine what equipment will be required and
the best procedure for performing these tests.

**FSU FEL Injector Design Study**

Fairly extensive designs studies of the possible injector for the FSU
FEL have been performed during the last three weeks. A system very
similar to the injector of the ELBE (at Rossendorf) was considered. The
system has a thermionic triode gun, two buncher cavities, and an
accelerating module made of two TESLA-type SRF cavities. A case with a
beam energy of 10 MeV at the end of the accelerator modules was
considered. The study includes systematic beam parameter calculations,
such as bunch length, longitudinal emittance, energy spread, transverse
emittance and the betatron functions as a function of the following
system settings:

a.  gradient of the first buncher, which is operated at 260 MHz,

b.  gradient of the second buncher, which operated at the frequency of
    the TESLA cavities, i.e., 1.3 GHz,

c.  phase of the first SRF cavity,

d.  phase of the second SRF cavity.

Also a comparison was made of two different cases of the SRF cavity 1
gradient, when the energy at the end of the cavity would be 5 MeV or 6
MeV.

Results of the simulations are the above mentioned dependences as well
as the particle distributions in the longitudinal and transverse phase
space at the end of the second SRF cavity for all the simulations. We
also have obtained very detailed information of the particles'
distribution evolution in phase space from the beginning of the system
to the end of the injector.

The simulations show that one can transport the beam from the gun to the
end of the injector with no significant growth of the transverse
emittance and, what is more important, that one can achieve a very good
longitudinal emittance. The simulations also show how to manipulate the
longitudinal phase space at the end of the injector, that is, cavity 1
can be used to adjust the bunch length whereas cavity two can be used to
adjust the energy spread without changing the bunch length. That is in
perfect agreement with the operational experience at ELBE.

Results of the simulations will be used as the input parameters for the
next steps in the design process, transport of the beam through the
linac to the FELs and incorporation of injection and extraction chicanes
as well as energy recovery arcs.