# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - April 10-14, 2006

**Date:** April 14, 2006

**Highlights:**

This was a productive week for high power operation of the FEL Upgrade.
We obtained a new world's record of **4.2 kW for cw operation at 1.6
microns**. The control room data copied below shows that we were able to
hold the machine at cw operations for an hour so that we could collect
data on mirror heating, scattered light and long term stability. The FEL
delivered over 10 MJ during this period. Today we are studying high
power cw operation at 2.8 microns, with the output power peaking at 6
kW.

In both high power runs we are investigating various near-term
experimental limits to the power that vary and interplay between: heat
losses in the optics, pressure rises in the optical cavity due to gas
desorption from stray light, and some beam loading effects in the
accelerator. Details are given in the Operations Section below.

On evening shifts we devoted another shift to the low-level rf control
system development and a production run for NASA for carbon nanotubes.

Quite a bit of press attention was generated for our colleague, Dr. Rox
Anderson at Harvard's Wellman Center for Photomedicine and the FEL team
with a joint press release on the recent "fat burning" experiments using
the FEL at 1.2 and 1.7 microns for differential heating of fat tissues.
The press release can be found at:
http://www.jlab.org/news/articles/2006/zap.html

At our press time, the release has been picked up by more than 25 media
outlets.

**Management**

On Monday, April 10^th^, we were visited by the Dr. Janet Fender, the
Chief Scientist of the USAF Air Combat Command, along with 7 members of
her staff. To keep up our

Joint Services activities, we were joined during the USAF visit by our
ONR Program Manager, Quentin Saulter.

We took advantage of Mr. Saulter's visit to have a brief review of
project cost-performance goals for the second half of FY06 with the
Project Management Office and the JSO contracting officer.

As of press time we are working with our colleagues at EVMS to complete
our submission of a proposal to NIH for an Interdisciplinary Research
Center based on bioscience applications of the FEL. The proposal is
being submitted by a team from nine institutions led by EVMS and three
other medical schools (Harvard, GWU and UVA).

> Two hours of running; most recent time to the left.  The dark blue
> trace is beam current at 10 mA full scale showing \> 5 mA current for
> over an hour.  Red is FEL Power, 10 kW full scale showing  4 kW for
> more than 1 hour at 1.6 microns.

 

![](media/image1.jpeg){width="3.6868055555555554in"
height="1.2055555555555555in"}

> The strip below shows another 2+ hour run.  We brought it up to 6 kW
> (far right) then tried several mirror settings (center) before
> settling in on a stable setup (left) that ran \~6 kW for an extended
> period (\~ 20 min).  Laser power is red, 10 kW full scale.  The blue
> is beam current 10 mA full scale.  The limit in this case was
> ultimately a vacuum trip in the beamline;  the gas pressure gradually
> increased to the trip point due to heating from scattered light in the
> optical cavity.  

![](media/image2.jpeg){width="3.6972222222222224in"
height="1.5034722222222223in"}

**Operations:**\
After a cesiation on Monday we came up and tried out the new 1.6 micron
mirror.  It lased quite well, producing 1 kW with 0.6 mA of current.  On
Tuesday we decided to try a power push.  When we ran in January with the
original 1.6 microns mirror we could get 3.2 kW with 2.6 mA of current. 
We were limited at that time by some RF limit so  we could not push
higher.  Recently the power was limited by mirror absorption that
started after running a while.  With the new mirror we could run  high
current and get higher power.  Indeed we got up to 4.2 kW at 5 mA of
current.  When we tried vernier mode we found that the efficiency stayed
constant up to about 3 mA and then rapidly went down.  This was not due
to mirror absorption but due to some other factor.  There was no
evidence of strong mirror heating.  We lased at over 3.5 kW for almost
an hour and the cavity was very stable and the cavity length did not
lengthen at all.  When we went off the long end of the detuning curve
and back the power just came  up to full power and did not overshoot as
it does when strong mirror  heating is present.  Nevertheless the
efficiency at high current was very low.  It was 1.5 kW/mA at low
current and less than 0.5 kW at 6 mA.  We then tried to reproduce the
setup from January and were still able to run 3.1 kW at 2.6 mA.\
On Wednesday we looked at the electron beam at high current to see if 
we could find some source of the efficiency falloff.  A lot of data  was
taken.  One thing that looked suspicious is a phase noise source at 38
kHz that grows with current.  This is a very bad frequency and could
reduce efficiency greatly.  We also looked at THz mirror heating and
found that the old and new mirrors have the same THz  absorption of 2
W/mA.   Finally we switched back to the old 1.6 micron mirrors and
delivered beam to the nanotube group for the evening.\
Thursday we recesiated again on Thursday and did some other  maintenance
work.  We came back up and verified that the 1.6 micron lasing was
reproduced.  Friday is a power push at 2.8 microns.  We  did get back up
to 6 kW repeatably and frequently but the scattered  light absorption
kept steering the output coupler so it was  impossible to tune.

Some details on our progress with beam phase noise measurements (thanks
to Pavel) that are giving us a hint of system changes as we push up the
current:

On Wednesday we took some time and systematically measured the phase
noise and the raw beam spectrum on the 4F03 BCM, which is right upstream
of the wiggler. We have taken the data as a function of the beam average
current running at 37.425 MHz micro-pulse frequency at the average beam
current from 1 mA through 4.5 mA. We have measured the phase noise with
the help of the Agilent Signal Source Analyzer (SSA). The measurement
made at the average current of 3.5 mA is shown at Fig. 1.

![](media/image3.png){width="6.0in" height="4.5in"}

*Figure 1. Phase noise spectrum measured at 3.5 mA*

We do see that the RMS phase jitter increase with the average current.
Fig. 2 shows the measured dependence of the RMS phase noise as the
function of the beam current. We would need to repeat the measurements
at 4.5 mA to be confident in this point. Measuring the raw spectrum of
the same BCM we see the side bands at the same frequencies as the phase
noise spectrum shows.

The next step in the investigation whether this sort of noise and
modulations influence the FEL efficiency we want to measure noise
spectrum of the FEL power and to see if in that spectrum we would see
the same modulations. We also are thinking of doing the phase noise
measurements using another BCM installed in the injector, that could
give us an idea what of the modulations we see are coming from the
injector and what are picked up by the beam down stream of the injector.
We also need to do some calculations to understand how different values
of the RMS phase noise and its spectrum would affect efficiency of the
FEL.

![](media/image4.png){width="5.999305555555556in"
height="4.155555555555556in"}

*Figure 2. RMS phase noise as a function of current*

**WBS 4 (Injector):**

We re-cesiated the cathode on Thursday morning. The cathode delivered
141 C and close to 30 hours of CW and pulse beam for FEL ops in only
three days. This is almost three times higher than the charge delivered
in four days of operation after the previous re-cesiation done on Monday
of last week. Still the cathode lifetime is not long enough due to a
small leak in the beamline between the gun and the cryounit. We tried to
find the leak using Neon because the He background is too high, but we
could not get a compressed Neon bottle so Kevin Jordan kindly got us
three glass flasks filled with Neon. Unfortunately the glass flasks did
not have high enough pressure and we could not see anything in the RGA.
We will continue leak checking as soon as the compressed Neon bottle
arrives.\
\
With the kind help of Marcy Stutzman and Phil Adderley from the CEBAF
Source Group we activated a GaAs sample, partially coated with aluminum
oxide by ODU into a photocathode. This effort is to eliminate chemical
anodization of the GaAs wafers that reduces the maximum potential QE
when the wafer is cleaned with solvents after anodization. A non-contact
mask was used to deposit the alumina in a e-beam deposition chamber.
Previous samples showed that the coating leaked under the mask
essentially giving null QE. This time we obtained 3% QE and the map
shows the shape of the mask, although the transition between bare and
coated is not as sharp as one would desire. We will keep testing samples
coated with a smaller gap between sample and mask.\
\
On Tuesday we hosted a safety visit from the DOE site office in our
injector lab in the ARC building.

**WBS 6 (RF Systems)**

This week was a relatively quiet week for RF. We provided assistance in
the control room during high current runs and found that the RF, with
the recent parameter adjustments seems to run stably during high current
operations. One of the issues that we are pursuing is the concept of
energy shifts as a function of beam current. The energy shifts being
investigated include droop due to the fact that the control loops are
finite gain proportional loops which will always have at least a minimal
steady state error, non-DC components of energy shift due to phenomena
such as phase and amplitude loop oscillations and finally due to first
pass -- second pass beam interactions (through the cavity gradients) due
to lasing induced changes in the energy distribution. Preliminary data
indicates some energy droop, possibly due to phase shifts -- possibly
due to gradient droop, within the injector that starts at about 3 mA.
Further investigation is necessary to determine if this is a real energy
shift or due to some artifact in the beam position monitoring system.

**WBS 8 (Instrumentation):**

The week was one of hours at high power and multiple wavelengths. This
included a very successful run Wednesday night for the carbon
nanotubes.\
    While we were open yesterday for gun work several of the SEE BPM
Electronics front ends had additional attenuation added.  We found that
some of the electronics had to much gain and would go into saturation
when we ran currents higher than 3mA.  We added 6dB to the front ends of
the suspected electronic sets to raise the saturation point to 9mA. 
This altered the overall position readbacks that OPs saw until these
were rezeroed.  Once this rezeroing occurred the 6dB attenuation is
nulled out in the code so the position response isn\'t altered.  Also
this week another set of Embedded BPM Electronics was completed and
tested successfully.  We will use this set of electronics to test our
new version of code with the server in the FEL.\
    To prepare for the next machine maintenance period the Halo Monitor
Targets have been coated four times with the phosphor powder.  It looks
like two more coatings will be needed to have the targets ready for
installation.  Schematic work continues on the Single Board IOC.  All of
the major parts have been selected and placed in the drawing.  The next
big hurdle will be to keep the board size as minimized as possible so
that it will be applicable to VME crate applications, the Embedded BPMs,
etc.\
    When looking into how the thermal protection would work for the
Sextupole Reversing Switch it was found that we needed to take away all
power to the control card instead of just one rail or ground.  A relay
was added to the design so that when the thermal limit is hit the relay
will open and all power to the switch electronics will be removed and in
effect cutting off the switch.  The harnesses are being made up for this
portion of the assembly.\
    The User Lab Status Indicators have been fixed and fully checked-out
this week. This work had an impact on the Alignment mode system as well.
The alignment mode chassis supports two modes of alignment mode. These
are the two unique modes; 2Hz/250us mode and a 60Hz/10us. The mode is
defined by a logic bit that comes from the GPIO card in iocfel7 (this
item). This IO card is primarily used for driving the user lab status
indicators and was taken offline in order to service the user lab status
lamps. As a result, the alignment mode chassis was \'stuck\' in
60Hz/10us mode. This has been fixed, as well as the user lab status
lamps.\
    The automated Miniphase procedure is progressing.  An early draft of
the system is being posted today.  A script was written to save/restore
video switcher configurations.  This saves operators a lot of time when
changing shifts. A Procedure was written for adding new items into the
Devlore system as well as how to add data files to the database. This
procedure was used successfully by Carlos. He used it to add the cathode
recesiation SOP.\
    We are continuing to work towards defining a new numbering system
for technical drawings that will be generated by us as the FEL division.
Additionally, we are working out the details of how we go about doing
our own documentation using AutoCAD and Pcad for mechanical and
electrical documentation.\
Progress continues this week on the lab 4 Laser Personnel Safety System.
Cable was run for the card reader and a new card reader position was
decided on.\
In many areas of the FEL user facility, we have outgrown our installed
networking infrastructure. As a result, we need to do a survey of how we
are using the JLab network and begin planning what an upgrade will look
like. We are working with the CEBAF CC and Ace team to gather this
information. The FEL voice alert system ran all week with out any
problems. The system will continue to run silently until it has shown to
provide the right balance of timely information and minimal
intrusiveness.\
    Numerous assembly jobs also progressed; 1 AMS 4 Channel Analog
Buffer Board for buffer\
Box was populated, re-terminated RF Heliax in the drive laser clean
room, and populated 1 VCO Loop & 2 Buffer circuit boards for Tom
Powers.\
    The entire group worked a number of safety items including shin
guards on the magnet stand alignment fixtures.

**WBS 9 (Beam Transport):**

#### IR Machine Re-commissioning and Operations

•   I installed a number of brightly colored re-bar protection caps on
the bolts sticking out of the alignment cartridge caps of the UV Line
stands to minimize leg injury.  The students modified the caps to adapt
them to the service.  Thanks to Mike Bevins for the idea of using this
inexpensive commercial part for this safety duty.

•  I moved the Camera that is able to view delamination of the GW pole
tips to the second magnet of the Optical Chicane.  David Douglas thinks
he sees unusual steering from either the second or third magnet.  So
far, no delamination is detected.

•  On a more general note, I thought of a plan for using the beam
chamber (It is robust enough) to keep all the pole tip laminations of
the installed GWs in the glued down position without opening the
magnets.  It could be done at night with no effect except curing the
problem (which may have some beam steering implications.  I can
fabricate a set of bladders of copper or aluminum (no change in magnetic
properties) that would mimic the shape of the local beam chamber.  I
could insert them on both top and bottom of the chamber, evacuate them
and then fill them to less than 7 psi with epoxy.  Upon curing, the void
space would be filled and local de-lamination would be
precluded.  I would insert custom wedge sets on the side of the poles
where there is no chamber and the gap is accessible.  George Neil
approved the concept.

•  We continue working on using four uninstalled GW dipoles in a quick
experiment that uses laser light to look for a pseudoscalar particle
that couples to photons.  I designed the stationary and the movable
stand set for the GW dipoles.  Fabrication can proceed.

•   Don Bullard successfully installed the temperature stabilized
coaxial lines between the Drive Laser Room's divide by 40 unit and the
drive laser controller in the Gallery Rack.

•  The cover for the W55 (IR Line) Wiggler is in manufacture.

• I continued working on the design of the permanent cover for the
optical transport between the drive laser line and the entrance into the
Light Box in the injector.  Steve Benson approved the concept I
developed. 

**WBS 11 (Optics):**

#### FEL mirrors

This week we operated with the newly-installed 1.6 micron OC mirror. 
Its \"twin\" was left installed for comparison.  Operation at the time
of this report yielded a new power record at 1.6 microns (4.2 kW), as
well as extended (\~ 55 min) at powers above 3 kW.  So far, it looks
like the use of the paste (rather than metal braze) does not limit our
performance.  For essentially the same accelerator setup, the new mirror
has remarkably better performance than its twin.  Signatures of mirror
heating, like steering, cavity length drifts, and Rayleigh range change
are either absent or reduced in comparison to the well-used 1.6 micron
mirror.  We also collected images and calorimetry data that indicates
that the THz load is slightly to the right of center, as predicted.  A
value of 2W/mA was obtained.  We were also able to make a cleaner
measurement of the THz load on the mirror, so we can more appropriately
separate the THz contribution from that from the laser.  What we can\'t
do yet is separate laser power absorbed in the mirror from laser power
absorbed on the frame, in the paste, etc.  We know there is a
contribution, because FIR camera images show what appears to be the
mirror adapter rings, which are uncooled, above ambient.  Our new shield
adapters will be received from the Machine Shop early next week, and
should be ready for installation by week\'s end.

A new contract for coating was let and sapphire substrates are being
sent out for new coatings with low Zr-contaminated hafnia.  Our
collaborators at Colo. State Univ. sent us a batch of witness samples
with a 90%R/AR coating at 1.064 microns.  We will evaluate them relative
to our commercially-obtained coatings to see how they perform.

***Other Activities***

This week we did some experiments hoping to benefit our understanding of
the efficiency loss at higher currents.  An attempt was made to image
the third harmonic (at 533 nm) wiggler radiation.  We got an image, but
the high f/# \> 200, of the optical transport system (OTS) led to us
imaging the entire wiggler tube, so nothing immediately useful was
obtained.  We are assisting in phase noise/ amplitude noise measurements
as well.  Providing support for the NASA/CWM nanotube expt has become a
routine taking less than 30 min.  The drawing packages for the lab OTS
hardware was submitted to the Machine Shop, as were drawings for new
OCMMS beam dumps.  The lead engineer for the OCMMS fast-steering
hardware is meeting with the Machine Shop to gather up the hardware
built to date.  Our plan is to build that up and get it ready to install
to monitor the OC mirror.  Given a week without an anticipated OC entry,
the cryomirror project made good progress.  Over several meetings it was
determined that my desire to be able to move to other, water-cooled
mirrors while cryocooling wasn\'t viable in the short term.  Instead,
our intent is to be locked in at one wavelength, 1.06 microns, and look
at the performance with cryocooling vs water cooling.  This is a
wavelength a number of our users running this Summer can take advantage
of, so it isn\'t much of a restriction. 

**Terahertz:**

The new FTIR has now been aligned to THz beam in the hutch in Lab 3.
During evening beam operations this week, the system was fully
characterized and tested to provide the best possible spectral data for
accelerator diagnostics.  It has also been tested with two different
detectors, which both yielded good results.  Also during evening beam
operations, more imaging tests were conducted with the remaining beam
time.  There are problems with beam collimation that have not been
resolved and will continue to be addressed as beam time allows.\
\
During the vault access on Thursday, some adjustments were made to the
beamline mirrors to compensate for an apparent drift in the electron
orbit indicated by a shift in the position of the synchrotron light
image on the first diamond window.  The new alignment appears to be good
as indicated by the spectrum measured during beam operations on Friday.
 During high current operations on Friday, the THz beam had to be
aggressively attenuated to keep from saturating the detector, so there
is certainly no shortage of THz power being generated.\
\
Also, the final review of the undergraduate thesis of Amelia Greer from
William and Mary was completed this week.  She will address her final
edits and then present her thesis at the beginning of May.  Her work was
instrumental in helping Rich Evans with the configuration, testing, and
characterization of the new FTIR system, as well as in the commissioning
of the THz beamline.