# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - March 13-17, 2006

**Date:** March 17, 2006

**Highlights:**

We took extensive high power running data to characterize performance to
determine the source of our falloff in FEL efficiency at high duty
cycle. In the process we eliminated a couple of possible culprits but
have not yet determined what the cause is.  One effect which has been
clearly eliminated as a possible bad actor is ion trapping.  Mirror
absorption was also studied as a function of wavelength.  We found the
HR has lowest loss less than 1.6 microns and the OC prefers wavelengths
greater than 1.65 microns because of greater outcoupling.

We performed the first autocorrelation data on the FEL pulse itself,
measuring 160 fs FWHM on the 1.6 micron FEL output pulse. This is an
impressively short optical pulse which agrees with our expectations.

We provided two nights of nanotube runs for NASA at \~ 1 kW of 1.6
microns producing lots of tubes at high efficiencies.

We also spent one evening running tests of an advanced rf control
algorithm that will benefit our FEL program, Cornell University's ERL
project and and the CEBAF 12 GeV Upgrade project.

A cathode re-cesiation was performed on Monday and we will require
another one this coming Monday.  We believe this shortened life is due
to a small leak in the injector region so we are leak checking Friday
afternoon and will fix the problem over the weekend then perform a heat
cleaning of the wafer before re-making the cathode surface.

Finally, we are particularly proud of some detective work this week on
an intermittent problem in the electron beam transport system. Our
master electron optician, Dave Douglas, predicted that an intermittent
occurrence of poor transport of the recovered electron beam had to be
due to a magnet problem in the vicinity of the large dipole magnet in
the recirculation arc. On Monday, our master magnet builder, George
Biallas localized the problem during an observed power test of the
magnet string, and came-up with a real-time fix to a partially
delaminated layer on the pole piece of the path length corrector trim
magnet embedded in the large 180 degree recirculation dipole.

Next week we will continue the high power performance tests during the
day shift and have planned a four second shifts for diagnostic
measurements and nanotube user runs.

**Management:**

Gwyn Williams spent most of the week at the APS meeting in Baltimore
while Fred and George each separately spent two days there attending
various workshops and meetings. This included a User Meeting for the
National High Magnetic Field Lab to discuss the specifications and get
feedback on their proposed NSF FEL facility. Fred was honored to be on a
joint APS-AIP panel on "The changing dynamics of industrial research as
a consequence of global trends" 

with chief research officers from IBM, GE, Intel, Shell, Kodak, Dow, GM,
Exxon/Mobil, Micron and Xerox. He gave the national lab perspective on
this issue along with the Deputy Director NIST.

On Friday we hosted visits to the FEL by Air Force Maj. Gen Stanley
Gorenc, Colonel Richard Rankin and Colonel John Corghan.   

The project monthly financial reports for February were distributed to
the DOE and ONR program offices.

# Florida State FEL Design:

Under the auspices of our collaboration with FSU, we have developed a
proposed layout of the NHMFL FEL to analyze for performance.  In
presentations to the Users at the APS meeting in Baltimore we were
encouraged to provide better flexibility in providing simultaneous
broadband Terahertz simultaneous with the FEL lasing and to try to
extend the broadband performance capability of the proposed thermionic
injector.  

Several schemes for outcoupling the FIR overmoded waveguide were studied
and we will carry forward further analysis on two of the most promising
approaches using the GLAD physical optics code.  So far the outcoupling
efficiency appears to be limited to around 50% in both a hole and edge
scraper case. 

We also provided an update of the facility layout to FSU, developed a
set of power and cooling water specifications, and calculated the beam
dump shielding requirements.  This information is required by April 1
for the cost estimates provided by the team to the university to defend
the building construction budget.

**Operations:**

We began the week with a recesiation.  After getting the gun back up we
did some injector studies to try to find the reason for the miniphase
procedure walking off with time.  We have found that the phases
determined through the miniphase procedure gradually walk away from the
values determined via injector phasing.  If one reloads the original
injector phases one finds that they work just fine, indicating that the
procedure is walking away and not the machine.   After trying various
starting points we found that the miniphase procedure is quite robust
except for changes due to steering drifts.   We tested this later in the
week when the phases drifted off.  We recentered in the injector quads
and cryounit and the original injector phases came back.\
With the user runs of last week out of the way we turned our attention
to figuring out why the efficiency dropped off when running high
current.  We tried running 2.5 mA beam pulsed with increasingly long
macropulses.  We found that the efficiency fell off monotonically as the
duty cycle increased, going from close to 2 kW/mA at low duty cycle to
about 1 kW/mA at CW.  We repeated this with 1 mA of beam at half the
micropulse repetition rate and found similar  behaviour.  After looking
at the mirrors we found that the loss in the output coupler was higher
than it had been previously.  We then decided to scan the wavelength and
look at losses in the mirrors.  We found that the losses in the output
coupler reduced as the wavelength shifts long with respect to the design
wavelength.  This is mainly due to increased output coupling so that the
power incident on the mirror decreases as the wavelength increases. 
Unfortunately the  losses on the high reflector go up as the wavelength
increases due to the increase in the light reaching the silicon
substrate.  Some slightly longer wavelength might have a bit better
performance than 1.6 microns.  As part of the power vs. duty cycle tests
we looked at the detuning curve length at 2.5 mA and with pulsed beam. 
They were the same length, indicating that electron beam jitter or
longitudinal degradation cannot be the cause of the efficiency reduction
with current.

The efficiency parametric studies also seem to lay to rest the spectre
of ion trapping as a possible explanation for efficiency loss. If ions
were responsible, the onset would be harder and at higher pulse length.
Given that we see rolloff at \"lower\" duty cycles like 30-50%, with
very large clearing gaps, its unlikely to be ion effects ( for any
reasonable mass ion ie, CO+ ,that we might observe in the residual
gas).\
\
As a side-light, we have also pretty much eliminated RF phase jitter as
a source of efficiency rolloff, in two tests - 1) BCM observation of
arrival phase during long pulse testing (dead-on and steady, even when
we were slamming the RF drive system at 30 Hz/10 msec and out to 60
Hz/15 msec) and by cross-checking the detuning curve length under pulsed
& CW conditions and verifying it was unchanged. Therefore, not ions, not
RF jitter, so far as we can see.

We are setting up today to measure the power vs. time for long pulses to
get the time scale for efficiency reduction at high current.

**WBS 4 (Injector):**

On Monday we re-cesiated the cathode, only one week after the previous
re-cesiation. The QE is decreasing rapidly because a minor vacuum leak
around the lightbox region. The QE dropped 1% in a few hours after
Monday\'s re-cesiation. The cathode has delivered 22 hours and 15 C of
pulse beam and 11 hours and 55 C of CW for FEL ops during this week. We
will investigate the leak today at the end of the shift and make a new
cathode on Monday.

Marcy Stutzman from the Source Group kindly anodized a German Matek GaAs
wafer for us to keep on the shelf and ready for installation when
needed. Previously Marcy helped us test this material in one of the
cathode activation chambers achieving 12% QE.\
\
A series of PARMELA runs of the injector model are in progress to
compare with data previously taken by the ops team as a function of
drive laser phase.

**WBS 8 (Instrumentation):**

Much of the week was spent transitioning to getting the balance of the
user labs (particularly Micro-machining AF Lab) ready for operations.
This includes installation of the new Laser Personnel Safety System
(LPSS) box and its connections into the LPSS master. The holes have been
drilled for all the needed cables/lights for the door. The cable for the
door lights has also been run along. The mag lock has also been
installed on the door and is ready to be wired/run where it needs to. A
beacon needs to be installed on the outside facing wall and the needed
wires will be run through that. We also ran for THz and Nanotubes
Wednesday and Thursday night. These experiments went very smoothly with
little or no interruption of delivered beam.\
    We are still working on the FPGA programming. The interface bridge
between the Cyclone and Coldfire bus have been coded and compiled. The
configuration of memory block in FPGA has been finished. So far, the
functional modules, ADC controlling and sampling, memory, and ColdFire
bus interface have been done.  The configuration of General Purpose I/O
Card is on the way. This work is crucial to many of the new systems we
are bringing on line with the embedded IOCs.\
    The Epics-2-Devlore application was updated to send a trigger in
between the normal one minute data requests. This tells the database
that new data has come in and it needs to update the associated web
pages.  The Injector phasing script was upgraded to use BPM averaging to
mask any jitter in the read-backs.  Testing on Monday will determine if
the averaging helps reduce the number of steering iterations, resulting
in faster run times.  The IR wiggler software is being revised for
increased robustness.  The wiggler controller will produce an error if
it\'s processing a move and receives another move instruction before
it\'s finished.

  The cooling plate for the Sextupole Reversing Switch has been
completed and tested successfully.  The previous versions of the plate
were allowing temperatures approaching 200 degrees Fahrenheit.  This
becomes dangerous because the high power MOSFETs are then close to their
thermal break point.  By modifying the plate we\'ve gotten the
temperature down to 110 degrees Fahrenheit at the peak current.  This
switch has been successfully cycled through the hysteresis curve and
also has been running for several days now.  The majority of the parts
need to complete this project have been order/acquired this week.  We
have enough parts to complete four switches that can be installed on our
existing sextupole magnets.  This will accomplish two things for us, a
better supply for crucial magnets and field time with the system to
understand its operations.

    In order to keep other projects that we have in our queue moving
forward, numerous parts have been ordered.  The remaining parts for the
4-Ch Silicon Diode Boards and the 4-Ch RTD temperature monitoring boards
have been ordered.  Also the parts for the GC Magnet Control Cards have
also been ordered.  We continue to develop our General Purpose Single
Board IOC.  Currently we are trying to develop a carrier card for the
Arcturus Coldfire card that can be implemented into various systems by
utilizing a common pinout and predefined dimension.  The progress on the
General Purpose I/O card is moving forward as well.

    During the start of operations last Friday the drive laser pulse
control system produced a CW-like, long pulse which was not requested by
the operator after a trip-recovery while the machine was in an
unprotected state. Although no machine damage occurred, it was worrisome
to Ops and it prompted us to acknowledge a common mode of running that
we regularly engage in where we have disarmed the BLMs and rely entirely
on the EPICS software and the sensibilities of the operator to keep the
machine safe. Ops has good reason to do this. Namely, the MPS is too
restrictive for the mini-phase procedure to be carried out efficiently.
As a result, the BLMs are masked to eliminate false trips. We have
proposed adding a new beam-current interlock based on the alignment mode
permit chassis to provide this mode of machine protection while allowing
the BLMs to be masked. It has not been decided whether-or-not this is
worthwhile, because confidence is still high in the steady state
performance of the DLPC and this error (allowing the long pulse) could
have been avoided if Ops were to be extra vigilant before opening the
operator shutter. Regardless of whether we add the new MPS interlock,
the DLPC EPICS application is being updated with a new version which
will be more reliable.

     The analog-to-digital converter channels into EPICS have been
modified. A channel from the VMIC3122 card in iocfel7 has been provided
to User Lab 3 for use by the optics group. We are working on a way to
send this data right from EPICS to users via email, so that a set of
time-stamped data can be collected on-the-fly without having to develop
a specific EPICS application. 

     We are preparing for the FEL Operations Review set for Tuesday,
March 28, 2006. Our tools and methods of planning activities
and addressing configuration control are being reviewed.\
     In addition to the FEL performance graph in Devlore, we have added
a set of machine status parameters that allow users to know what the
operating status of the machine is without having to access the control
system proper.\
(See the figure below for a screen-shot of what is being provided)\
\
![](media/image1.png){width="6.5993055555555555in"
height="1.0090277777777779in"}

Also, the real-time graph of FEL performance data has been updated to
display mark the peaks in the data which tell the viewer what the
maximum beam current and FEL power have been in the last 8 hours.
Currently there are 3 separate FEL power curves to correspond with the
three optical beam dumps that exist. We have implemented a \"muxed\"
process variable in EPICS to consolidate this reading. The result will
be just one FEL power curve on the graph. This will go into effect the
next time we reboot iocfel9.

**WBS 9 (Beam Transport):**

# *UV Line*

• With the glint of some funds availability on the horizon, we are
starting to re-address the issues of QX quadrupole powering with the new
style, Trim Card II and how the field maps from STI Optronics can be
utilized. 

# IR Machine Re-commissioning and Operations

• I am waiting for an opportunity to go into CEBAF on a down day and
obtain more coils and poles to create the second Octupole for halo
handling.

• I continued working on a new halo monitoring viewer unit for Kevin
Jordan that will be able to characterize the halo in beam in the 5F
return region.

• We completed the Temporary Operational Safety Procedure that allowed
us to observe magnet strings in the vault and during hysteresis cycling.
We were able to observe on Tuesday a delaminating pole surface shim in
the 5F GY Dipole. As the magnet ramps to full field, at the entrance to
the magnet, the rear edge of the Purcell-Gap-forming steel shim bends
upwards by 1/4 inch, stopping as it touches the chamber.  This shim is
part of a differentiated mini pole that is associated with the GG Path
length correctors that are built into the entrance and exit of the
magnet. We wedged shims in between the chamber and the pole shim to keep
the shim in position.  No other problems were found in any of the other
dipole strings.

• At week's end, we are working on adapting the still as yet uninstalled
GW dipoles to a quick experiment that uses laser light to look for a
pseudoscalar particle that couples to photons

for Hampton University's experiment.

**WBS 11 (Optics):**

#### FEL mirrors

This week we operated for long periods of time at high power.  In
particular, we collected valuable data on the drop in efficiency with
duty factor at two pulse repetition frequencies (PRF).  It\'s being
analyzed and results will be reported next week.  Taking this data
highlighted a long-standing problem with the OC assembly\'s *in-vacuo*
thermometry.  We are making preparations to replace it within a few
weeks.  We also plan to take some more data on the installed 1.06 micron
outcoupler with the tantala/silica dielectric coating, then replace it
with a hafnia/silica version.  We saw indications that the level of the
UV-induced IR absorption has a PRF dependence, and we would like to get
more data on this before removing it.  Good progress was made with the
QA effort on the spare mirror substrates.  We received a HR coated
witness sample from Colo. State Univ.  We will put it through a battery
of tests to see how it fares, compared to our commercial vendors.

#### Other Activities

After the weekly was sent out last Friday, we ran until evening for
the Mass. General Hospital (MGH) user run.  They left tired and happy. 
Joe Gubeli led the Optics Groups efforts and worked side-by-side with
the team from MGH, which helped immensely.  We also delivered beam two
evenings this week to support the NASA effort and provide cw beam to
help commission the FROG setup, which will determine pulse shape.  The
first result, an autocorrelation, is showing a best fit Gaussian pulse
length with corresponding FWHM value of 165 fs.  The spectra we are
measuring are appropriately wide, assuming a Gaussian pulse shape.  Next
week we\'ll attempt to commission the pulse shape capability to the
analyses.  The electronics for the asynchronous optical sampling (aka,
the PLL) is locking.  Initial measurements with the noise
analyzer suggest timing jitters about a factor of 2-3 higher than we\'d
like (but still less than 1 ps), so we will be looking into how to lower
the phase noise.   We rebuilt one of the pump carts and it will be
checked out later today. The material for new OC mirror frames was
submitted to the Machine Shop.  We will receive the first parts late
next week.  We are setting up equipment to measure the amount of
transient infrared reflectivity induced in a silicon mirror when it is
irradiated by the output of an amplified Ti:sapphire laser.  This is in
support of our collaboration with Prof. M. Kelley of the College of
William and Mary.  We provided budgetary information on mirror
fabrication and coating costs to the MGH PI.  They are willing to
purchase mirrors for us to use in support of their experiments.

**Terahertz:**

This week, a significant amount of time was spent on completing the
alignment of the THz beam through the transport line as well as out onto
the optical table in the lab.  Previous hurried attempts to align the
beam had proven to be inadequate and it was felt that a concerted effort
requiring both extended access to the vault as well as some dedicated
accelerator operations were necessary.  We had both of these this week
and we now have good THz beam propagation through the transport line
with the beam entering the hutch parallel to the table and collinear
with the alignment laser in the hutch.  The optics have also been
changed to all reflecting optics to eliminate any possible dispersive
effects.

Following the beamline alignment procedure, the beam was aligned into
the spectrometer and noise measurements were performed to compare with
similar measurements taken on the NSLS at Brookhaven.  These data will
provide information on the signal to noise characteristics of an energy
recovery system compared to those of a storage ring.