# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - April 24-28, 2006

**Date:** April 28, 2006

**Highlights:**

We continued our studies of efficiency-limiting effects in the FEL this
week while lasing high power at 1.6 and 2.8 microns.   We had many hours
of running at high currents (4.5 mA) with only half of the drive
electronics of the HVPS activated in hopes of uncovering a correlation
between HVPS loading fluctuations and efficiency loss.   The new shields
protecting the edge of the mirrors and their mountings performed very
well; the cavity was quite stable in its operation and required only
modest intervention to keep the output optimized.  The efficiency loss
symptoms remained qualitatively the same, however; efficiencies of \>
1.5 kW/mA at low duty factor were achieved dropping to below 1 kW/mA as
the average current increases.  We did measurements on physical jitter
of the beam in the wiggler and changes in optical mode size as a
function of power.  We also are able to show that the FEL produces
higher efficiency when the optical cavity mode is steered far
off-axis.  The reason for this behavior is unknown at the present time;
we continue our investigation.  

**Management:**

We continued support of work on converting Jefferson Lab\'s management
over to the new contracting organization, Jefferson Science Associates. 
Many of the FEL staff have acquired their new site badges.  Others will
need to do this in the coming week. 

 

We congratulate two of the FEL staff on achieving their 20 year
anniversary with Jefferson Lab: Dave Douglas and George Biallas.  They
were honored at a ceremony on Thursday. This elite group has been here
since the early founding of the laboratory.

 

We also want to thank our Administrative Assistants, Donna Gilchrist and
Linda Williams, for their continued support.  They are the ones who
really keep this show going so it is a pleasure to have the opportunity
to recognize them!  

User Interactions:\
\
This week on Monday we were visited by Brian Gahan, the Manager of E&P
Technology Development at the Exploration Production Center of the Gas
Technology Institute in Des Plaines, IL.  And by Bill Gwilliam of DOE\'s
National Energy Technology Laboratory - the only national lab devoted to
fossil energy technology.  The discussions focused on possible novel
laser-rock interactions that might facilitate horizontal drilling.

On Tuesday Mike Murphy, the Scientific Director of the Human
Effectiveness Directorate at Brooks City-Base in San Antonio, came to
look at our THz laboratory.  He is setting up a group under the
leadership of Jill McQuade, to study THz-tissue interactions.  Under
present rules, our THz source is considered Class 4, which would make it
undeployable.  The aim of the Brooks program would be to learn how to
establish safe limits based on real data.\
\
On Friday we were visited by Beverly Brown, who works in Strategic
Technologies at the National Air & Space Intelligence Center at
Wright-Patterson.  She is interested in learning about possible FEL
applications, and also in learning more about synchrotron x-ray
lithography from a national strategic perspective.

**Operations:**

This week focused on studying the reduction in efficiency with increases
in electron beam current.  After recesiating we came up and started
setting up the spontaneous radiation beam to a detector in User Lab 1 to
look at position or phase jitter on the electron beam in the wiggler. 
We noticed that the quantum efficiency dropped rapidly but could find no
cause.  We decided to do a drive laser alignment to check for losses in
the drive laser power.  None was found but a shield put in place for
leak checking was found on the light box output that might have led to
increased halo in the gun region, thus reducing the cathode lifetime. 
The QE was not so bad that we could not run however so we ran on Tuesday
looking at the image of the electron beam in User Lab 1.  We found that
the electron beam did move when the current increased but the position
jitter did  not increase.  Both these are valuable pieces of information
for running high current.  We had always assumed that the electron
beam did not move in the wiggler when the current increased.  Now that
we know that it does we can optimize the electron beam position at
high current.  We also took more BCM phase jitter data in both
the wiggler insertion and the injector.  This was all done with one
stack on the high voltage power supply. The system looked as noisy as
it did with two stacks.\
On Wednesday we recesiated again, did a full injector setup, phased the
linac, and checked the longitudinal phase space.  We then optimized the
laser at 1.6 microns using the old output coupler.   This was the
precursor to a high power lasing attempt with the new OC at 1.6
microns.  On Thursday we tried to get high power with the new OC and
found the laser performance as good as usual for pulsed lasing  (close
to 2 kW/mA) but was very poor with CW beam.  We looked at
the performance with respect to electron beam steering and optical
mode steering.  We found that the efficiency could increase if the
optical mode was steered off horizontally or down.  It went down if the
mode steered up.  This is all very puzzling.  We can now send the beam
at high power to the end-of-line dump.  This was used to look at
the spot shape vs. power.  No large change was seen.  The
end-of-line power meter is faster than our 50 kW power meters and proved
very useful in tuning the laser.\
On Friday we optimized the laser at 2.8 microns, found the usual stellar
performance at low current and lased at moderate power (4 kW) with 3.5
mA of current with one and two stacks.  No difference was seen.  We then
ran up to 6 mA but the electron beam seemed to have more noise on it
than typical.  The new shield on the output coupler worked very nicely,
providing very stable operation at 5 kW with the exception of the cavity
length.  This might be due to the overall heating of the mirror
cassettes by scattered light.  The absorbed light in the shield can
exceed 100 W, which can move the cavity mirrors by 10s of microns over
the course of an hour of lasing at  high power.

e-Beam notes:

1.  After injector and linac phasing we checked the longitudinal tuning
    and matching of the accelerator using the phase transfer function
    (m55) system. Two interesting observations resulted. First, the best
    bunch compression at the wiggler did not result from \"nominal\"
    operation of the injection-to-wiggler transport, but rather for a
    modest deviation from this configuration. This suggests that the
    beam is either being injected with some phase-energy slew (not
    likely, as the injected momentum spread is modest) or is undergoing
    some phase-energy chirping in addition to that imposed by the RF
    during acceleration. This could be due, e.g., to longitudinal space
    charge. Secondly, after some tuning, the accelerator exhibited up to
    26 degrees of injected phase acceptance; the beam centroid could be
    slewed from the crest of the RF waveform to 26 degrees ahead of
    crest during high power pulsed operation. This corresponds to \~9%
    change in centroid energy. Given that the beam full momentum spread
    is maybe 4% at 26 degrees off crest, the accelerator as a whole
    (including the comparatively low-acceptance linac-to-wiggler
    transport) is thus willing to eat over 10% momentum bandwidth and
    deliver it to the dump.

2.  The system is now running well enough that Steve Benson was
    overheard muttering to himself - as he tuned up the machine at the
    eighth subharmonic of the cavity fundamental (that\'s right -
    EIGHTH, yes, 16 mirror bounces between every electron bunch) - that
    the system was lasing far too well to tune without some difficulty;
    it just wasn\'t sensitive enough to small changes in the usual
    parameters. To his credit, he immediately declined an offer to make
    it run considerably worse\...

**WBS 4 (Injector):**

The photocathode delivered 72 Coulombs on Monday and Tuesday after last
Friday\'s re-cesiation, but on Wednesday we had to re-cesiate again for
the fifth time since we made a cathode on March 20, 2006. Since
Wednesday\'s re-cesiation the cathode has delivered 52 Coulombs as of
Friday morning, and a total of 581 Coulombs since March 20, 2006. The
lifetime is certainly shorter than back in November-December 2005. The
vacuum in the beamline downstream of the gun is not as good as it was
back then, therefore we will activate over the weekend one of the NEG
pumps in the beamline expecting to improve the vacuum conditions.

**WBS 6 (RF Systems)**

It was a pretty quiet week.  Of note is that the system control loop
parameters seem to be stable.  None of the loop parameters needed to be
adjusted this week.  The only problems were a few cavities that did not
come up without manual tuning intervention.  This occurred after the
systems were turned off for a few days.  This matter will be further
investigated in the near term future.

A meeting is scheduled for Friday to determine the proper actions to
take regarding modifications to the buncher cavity temperature control
system.\
\
A requirements document for a 1 kW 748 MHz solid state amplifier was
finalized so that the procurement process could move forward.  The
intent is to use this amplifier along with stub tuners and iris plates
to do the initial testing of the AES cryomodule.  In the long run it
will be the driver amplifier for a IOT based high power amplifier.

**WBS 8 (Instrumentation):**

The automated miniphase procedure is being prepared for first release
early next week.  Automation of a full linac phasing is going to be
tested next week.  A new RF software utility is being added to zero the
absolute positions of the cavity tuners to better help manual tuning
when necessary.  Also some assistance is being given to A. Grippo with
new mirror controls.  The remote code image for Coldfire upload was
tested on the FEL Control Lan network. The code was stored in the remote
Linux server and the upload commands were executed from that server. The
reboot command, which remotely resets the Coldfire IOC, can also be
executed on any machine via channel access.  This allows for the
application code to be modified and updated without removing the
hardware or interfering with its current process.

    Time this week was spent on preparing Lab 3A for operation of the
LPSS system.  The proper conduit has been installed for the needed
cabling.  Further installation continued with wiring of the limit
switches for the doors on the lab and ID\'ing the mag lock cables.  The
needed 12V supply for the card reader was also installed in the box but
has not yet been wired. The relays have been labeled and wiring will
begin to take place now as soon as possible.  The old LPSS has been
decommissioned for transference of the many reusable parts.

    Assembled laser diode prototype for the fast detector in Lab 1.  To
support the phase noise measurements that were taken place in the FEL,
two cable installations took place.  The control cable and signal cable
for the fast detector in Lab 1 were routed to their respective
locations.  The video output of a \"Source-Signal Analyzer\" (SSA) has
been added to the video switcher channel line-up to assist with these
measurements in the FEL.  A \"wet\" EO cell is being sent to us for
evaluation from ConOptics.  We are preparing to test this EO cell.  A
\"retractable\" EO-cell mount is being addressed.\
    Based upon the recommendations of the JLab Mechanical Engineering
(ME) group, we have established a line of drawing numbers within their
drawing tree. The FEL group has been given complete control of the line
of drawings beginning with \"FEL001\". This enables us to deal directly
with Document Control (DCG) in the documentation process. Furthermore,
an automated system for providing and tracking drawings numbers is being
developed which will allow the whole process to be driven by the
designers.  We are continuing to develop an automated interface for the
DCG groups Oce tool which will allow us to access Oce resources from
within the configuration control database. A open-source tool called
\"cURL\" is being tested to make this possible.  An FEL machine photo
archive is being constructed. We have defined the scope of the project
and established a file naming convention that will allow the photo
archive to grow over time and enable a search-tool to be created for
users to quickly find drawings later.    A devlore folder was started
that will have all regions\' pictures added as taken.  The capability of
adding file attachments to flog entries remotely (from Devlore) is being
pursued. In doing so, we are preparing to install the PHP server-side
application to laser.jlab.org. This will mark the beginning of the move
away from Microsoft IIS and MSAccess as the configuration control
database scales to larger usage.  Work continues with both the JLab
computer center and the CEBAF ACE team to put together an accurate
overview of the currently installed network hardware in the FEL.\
    The Happek Insertable Lens Assembly Cover was installed, but there
was an issue with the Happek Assembly hitting the side.  Changes were
made and the cover was put back into place.  The AutoCAD drawing was
also updated to match the changes made.  The AutoCAD drawing for the
Synchrotron Light Monitor Enclosure was also updated.  Two new parts
were put into the assembly, which were added to the drawing also. The
pictures for the 1F and 5F regions were organized.  Painted copper
cooling plates for the 6kW reversing switches.  Completed the wiring
harnesses and beginning assembly. The new HALO monitor has been
assembled and is ready for leak checking prior to it\'s installation the
week of May 15.

    After doing some lead testing this week and finding residual lead on
a workstation, some time was devoted to cleaning up all the test
equipment and spare parts that had accumulated over the last couple of
weeks, and cleaning everything down to rid the area of lead.  One set of
shelves in the trailer was cleaned to make better use of existing
space.  Nineteen additional BarGards were fabricated for use on the
threaded rod on the UV magnet stands.

    With conferences coming up soon we\'ve dedicated some time to
developing papers and posters.  A paper and poster will be submitted to
the 2006 Beam Instrumentation Workshop next week on the Embedded BPM
system development.  With the FEL Conference coming up this summer an
abstract will be submitted about the Vernier control of FEL power in an
optical cavity with constant charge in the electron bunch.

The last seven weeks of data FEL operations data in Devlore has been
plotted. ![](media/image1.jpeg){width="5.205555555555556in"
height="3.214583333333333in"}

See figure 1 below:

**WBS 9 (Beam Transport):**

***IR Machine Re-commissioning and Operations***

• I moved the Camera that is able to view delamination of the GW pole
tips to the third magnet of the Optical Chicane.  No delamination was
seen in the upper pole of the third GW magnet of the optical chicane.

• 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.  The pole tip extenders for GW magnets need
just a shave of a corner to the pole tip blending piece and a new tapped
hole and counter bore in one end to install easily.  The shop is working
on it.

• 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.

•  Dano Oprisko is working on the technician portions the helium system
for the lower portion of the waveguides for the Cryomodule in Zone 3. 
We will install one unit on one waveguide to try it out, installing it
during the shut down starting on May 15.

• I started working on the gun for the new injector test stand.  I was
able to obtain a contact at Howmet Co. in Hampton, Virginia that may
allow us to perform the vacuum bake of the chamber locally. 

**WBS 11 (Optics):**

#### FEL mirrors

This week we got to test out how the additional shielding for the
outcouplers worked.  At this writing we have lased at high power (\~ 3
kW at 1.6 microns, over 4 kW at 2.8 microns) and seen reduced heating of
peripheral components.  In particular, the dramatic steering of the 2.8
micron OC mirror has been greatly reduced.   While installing the new
mirror holders and shields last Friday, we noted that the high
conductivity vacuum paste that is used to heat sink one of our OC
mirrors had flowed, leaving some small gaps near the top of the cooling
frame.  We will need to make a new frame with a smaller gap, so that
surface tension/capillarity will keep the paste from flowing.  In the
meantime, we\'ll see if this flow trend continues, and for now plan to
braze mirrors into their holders, as we usually do.  Discussions with
the vendor coating our new 1.06 micron OC mirrors resolved the issue of
such low reflectivity at 633 nm that we couldn\'t align or do metrology
on the mirrors.  The slightly modified design will give us 5%
reflectance at

633 nm.  Coating should begin next week.

***Other Activities***

Tests to resolve the cause of the laser efficiency continue. We imaged
the wiggler spontaneous radiation and observed a small monotonic
movement with beam current.  At the low sampling frequency of a video
camera, no clear difference in jitter with current was detected.  We
then put the light on a position sensitive detector (PSD), which has a
much higher bandwidth.  Again, there was no discernable difference in
motion, at least in one plane, as the beam current was varied.  We then
concentrated on getting a phase noise measurement of this radiation. 
The signal level is quite low, about 200 microwatts total, so great care
must be taken to collect the light and get it onto a small detector.  By
week\'s end we had a weak signal but could not get a good phase noise
measurement.  Next week we\'ll use a shorter focal length lens and a
more sensitive detector.   While lasing at 1.6 microns this week, we
sent the high average power beam through the end-of-line window and on
into the beam dump.  We exceeded 1.7 kW, a new record for us.  In the
process, we also showed that our power meter and a NIST-traceable
calibrated power meter agree within 1-2%.  With the apertures in place,
we determined that the in-vacuo aperture, which intercepts scattered
fundamental and harmonic radiation, was getting warm.  The flange was
also getting warm, so we\'ve added more thermometry and an IR camera
looking back at the external shield and Brewster window flange to help
determine what is happening.  Progress continued on the advanced drive
laser integration, with the new harmonic conversion crystal heater and
controller ordered, as well as more optomechanics.  The cryocooled
mirror in-vessel loop design underwent another major revision, but we
feel much better about the product than the earlier design.  We met with
the Cryo Group on their portion of the hardware, which includes the He
distribution can and transfer line, and plan on having them install this
hardware the week of May 15.  The Machine Shop is pressing hard to give
us optical cavity mirror metrology (OCMMS) parts to install during that
down.  Our in-lab optical transport system hardware (Brewster window
transition, hutch shutter, & harmonic-blocking filter) is on track for
completion sometime in the latter part of May.

**Terahertz:**

Due to a lack of dedicated CW operations this week, it was not possible
to perform any imaging tests.  Some forward progress was made on
completing the characterization of the new spectrometer.  It was
confirmed that the spectrometer and the DAQ software performs reliably
and as expected when using a globar as the source, but there have still
been occurrences when the control system for the spectrometer becomes
confused and stops scanning properly when using the THz beam as the
source.  We are diligently trying to determine the cause for this in
order to provide a more reliable diagnostic to the accelerator
operations as well as for planned experiments in Lab 3.\
\
Another discovery that was made this week is that the amount of THz
power provided during high current CW operations is so large that the
crossed polarizers cannot provide sufficient attenuation to keep the
detector out of saturation.  This appeared to be the case for electron
beam currents above \~ 4 mA.  Too much power is certainly not a problem
though and we should have no problem reducing the signal on the detector
by moving it away from the beam focus or changing the collimation of the
input beam.\
\
In fact, regarding the collimation and focusing of the THz beam, Gwyn
Williams has performed new calculations to determine the beam
propagation following the collimating mirror that is being used and
several interesting features were discovered.  One is that there indeed
appears to be a change in the orientation and shape of the beam profile
as it propagates away from the collimating mirror.  The beam profile is
not symmetrical, but this is not surprising since the divergence of the
source is not symmetrical.  Another intriguing feature was the affect of
placing a small (5 mm) aperture at F3.  The beam profile then propagates
with a more symmetrical profile and good collimation.  This is expected
since the aperture acts as a new source point for the beam.  I have been
running more detailed calculations to better understand the beam
propagation, but will not have the results from that until next week.\
\
Finally, I met with Carl Zorn of the Detector and Imaging Group to
discuss possible imaging systems for use with the THz beam.  While he
did not have any experience with cameras sensitive beyond the near
infrared region of the spectrum, he was able to provide me with some
contacts of several camera designers that may be of some help.