# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief - December 6-10, 2004

**Date:** December 10, 2004

**Highlights:**

This has been a very productive week for the FEL Upgrade. Highlights of
the week's accomplishments include:

-   establishment of strong lasing with the new EM wiggler and upgraded
    optical cavities at 2.8 microns

-   at press time cw lasing has been extended to 540 watts at 0.6 mA of
    drive current

-   our initial look at the optical loading with these conditions looks
    good; the power is too low to get an accurate loss measurement;
    however, we can estimate that the absorption losses within the
    optical coatings are less than 500 ppm which extrapolates to an
    optical distortion limit of 18 kW of output power if there were no
    other limits.

-   our initial look at THz loading on the cavity optics shows that the
    THz chicane is reducing the THz loading at the optic location by 5x
    or more

-   the initial lasing efficiency \~0.9kW/mA is close to our best
    obtained at 88 MeV

-   the THz beamline commissioning has progressed to the point where we
    have obtained the expected radiation pattern at the output end of
    the beamline in User Lab 3a, and after installation of our BNL
    loaned spectrometer and obtained our first THz spectra at this
    location

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-   we also obtained our first useful measurements of the electron bunch
    length at the end of the linac using our streak camera

-   today we spent time optimizing the beam match so that we could push
    the current (up to 5.8 mA cw so far without lasing)

**Management:**

Weekly meetings are now in place for the design engineering tasks for CM
development (Mondays at 400pm) and for UV chicane transport systems
(Wednesdays at 0900). Excellent progress was made this week towards
developing a baseline lattice drawing for the UV chicane as modified by
our experience with the 10 kW IR commissioning.

We prepared draft presentations for next week's UV FEL project status
review on Dec. 16^th^.

for our AFRL program manager. A draft agenda was prepared and circulated
for comments.

Henry Helvajian from Aerospace Corp will be attending the review to
update the status

on the Aerospace LMES end station for User Lab 4.

Next week will be a busy week for prospective users of the FEL Upgrade.
A contingent from NRL led by Phil Sprangle will be visiting on Tuesday
Dec. 14^th^ to discuss our joint plans for laser atmospheric propagation
experiments. On Friday, Dec .17^th^, Prof. Rox Anderson from Harvard
Mass General Hospital will be visiting along with several of our other
bioscience collaborators (from UVA, EVMS, ECU and CWM) to discuss plans
for experiments in 2005 and

a joint workshop.

We prepared a number of abstracts for two invited talks (one by Michelle
Shinn on industrial applications of FELs and one by Dave Douglass on the
design of large momentum beam transport systems) and several contributed
talks for next year's Particle Accelerator Conference to be held on May
16-20 in Knoxville TN. A list of the submitted abstracts is being
prepared for submission to our program offices for review.

Gwyn Williams presented a talk on the THz user facility at a Terahertz
Systems conference organized by Technology Training Corporation in
Arlington, VA. At this meeting we met with colleagues from several
agencies and companies and discussed potential collaborations.\
\
Alan Todd and Gwyn Williams held a THz project status review on December
8^th^ with our US Army program managers, Mark Cumo, Pete Howard and
Steve Bishop at the Army's Night Vision Lab in Fort Belvoir.  Last
year's work was summarized and a statement of work for FY05 and FY06 was
presented and agreed upon.  It calls for commissioning and spectroscopy
work in the laboratory at Jefferson Lab, and the purchase of some
components to allow a prototype compact source to be constructed.

**Beam Improvement Activities (Steve Benson):**

Last week we scanned the cavity length several times and could not find
any lasing.  Since the cavity length might have changed quite a bit in
the months since we last lased finding the correct length was like
finding a needle in a haystack.   In order to make the needle bigger we
installed a second high reflector to allow lasing with a low loss
resonator.  On Monday we scanned over a large range of cavity length and
finally found the correct cavity length.  Once this was done it was
quite easy to optimize the system and find lasing with the 90%
reflector.  The laser lased quite strongly with both mirror sets.\
On Tuesday we optimized the lasing by getting a good match to the
wiggler and optimizing the Rayleigh range.  The detuning curve length
stretched to 17 microns, which is close to the spreadsheet prediction of
20 micons.  We also improved the match after the wiggler to allow strong
pulsed lasing with low losses.  We then changed course and continued
calibrating the beam loss monitors.  These needed to be calibrated
before high current CW beam could be run. 

Much of Wednesday was spent in completing this task. We also took time
to test out some emittance measurement software.  One method yielded a
value of 8 mm-mrad for the horizontal emittance.  This is slightly
smaller than the PARMELA value.  Multi-monitor measurements yielded a
slightly larger value of 8-12 mm-mrad, very close to the PARMELA value.\
Thursday was quite productive.  We started running CW to the high-power
power meter.  We obtained up to 540 W with 0.6 mA.  The power was
limited by electron losses at the dump.  Indications are that the loss
is due to a high energy tail developed when we lase hard.  We saw
similar problems with a high Q resonator on the optical klystron.  We
decided that, since the losses were not a problem when the laser was
off, that we would try to push the CW current.  At approximately 2.4 mA
we started getting beam loss monitor trips in the reinjection
telescope.  We finally realized that this was a BBU problem.  We then
ran with 2.2 mA beam at 18.7 MHz in order to check out THz mirror
heating.  There was some evidence of heating at the threshold of our
measurement resolution but the absorption was very small compared to
previous work at 88 MeV despite a shorter bunch length.\
During all the work described above we worked on getting some of our
bunch length diagnostics on line.  We steered up the optical transport
to our new streak camera and measured the bunch length in the linac. 
The measured value of 10  psec is slightly longer than PARMELA
predictions.  We now have the capability of measuring the bunch length
during CW operations.  We also aligned the THz beamline into User Lab 3
and saw a strong signal on a pyroelectric camera in lab 3.  The
intensity of the image varied with the Arc1 trim quad settings as
expected.\
Today we will concentrate on raising the BBU threshold so that high
current operation is possible and will try to further reduce losses at
the dump due to the high energy tail.  Results so far are quite
encouraging.  The gain is so high that we might want to have more output
coupling for this wavelength.

**WBS 4 (Injector):**

The cathode made on October 11, 2004 has delivered 40 Coulombs for pulse
and CW operation at 135pC/bunch. This cathode will be re-cesiated for
the first time next week on Monday. Despite the relatively low delivered
charge (we expect to draw more charge by the end of FEL operations
today), two months is a new record for the cathode lifetime between
re-cesiations. The improved lifetime is a direct result from fixing a
very small leak in the gun gate valve flange.\
\
Field emission suppression on test electrodes continued this week. The
test tube coated in the College of William and Mary\'s ion-source plasma
implant system was mounted in the high voltage test stand chamber,
pumped down and baked. The system is ready for high voltage. Tests will
start this afternoon.

Gun HVPS - Fully operational.  Gathering data for the next generation
Gun HVPS with increased current capacity.

**WBS 6 (RF):**

RF - Measurements were made this week to determine the new cavity Q
values for zone FL02.  When the FEL RF is turned on, part of this heat
load is not compensated completely.  The new Q values should correct
this.  During the past few months the FEL has presented an increasing
heat load to CHL.  The shield vacuum was found degraded on cryomodule
FL02.  The addition of a vacuum pump to FL02 corrected this problem.  In
the process of finalizing the specifications for the RF power amplifiers
and their HVPS for a new Injector Test Stand.

 

**WBS 8 (Instrumentation):**

   An order for additional duct tape and ty-raps has been placed to
accommodate schedule and support issues. In spite of that we\'ve had a
great week of lasing and with first results from both the streak camera
and the THz beam line, and even seeing BBU for the first time in a
number of weeks. The streak camera and the THz spectrum are two unique
and complimentary beam diagnostics that sort out machine reproducibility
issues. We have seen the sync light and THz with visible cameras, IR
cameras, the PyroCamIII, and taken a spectra with the Nicollet FTIR.
Hat\'s off to Shukui for installing and commissioning the streak camera
and beam line to the 2F sync light port. A design is nearly complete for
a ladder/fork that will be used to investigate halo in the injector
line. This is mounted to a stepper controlled 6 inch linear vacuum
feedthru for inserting into the beam tube. Progress also continues on
the new multislit emittance measurement device. The new multislit will
have 3 actuators so each of the horizontal and vertical slit arrays can
be independently aligned. The slit will be EDM wire cut out of Niobium.\
   In  preparation for bringing high power beam around the accelerator
much time was spent in support of Operation to calibrate the Beam Loss
Monitors. All BLM\'s were verified for Ops with low current, low duty
cycle (2.5 microAmps average current) beam.  The new loss monitors that
were installed or relocated during the summer installation period
(ILM4F03, 4F09, 5F10, 5F10A and 5F10B)  were mapped and HV defaults set.
Also new photos have been taken for each BLM head and made available
from the devlore database. These photos are useful tools to provide
insight about what area of the machine the system would be sensitive
to.\
   The charge/dump current monitoring chassis was serviced early this
week to provide EPICS read back of the charge/bunch from the IDC0F06
current cavity. Although we fixed the EPICS read back, we found that the
RF circuit that makes the measurement must always be recalibrated after
servicing the chassis. As a result, we have moved the charge/bunch
measurement out of the chassis. This chassis will be upgraded soon with
a revised PCB that will greatly improve the reliability and accuracy of
the measurements. A service request was put in to address this log:
(see) Tuesday November 10, 12:43:48
([http://laser.jlab.org](http://laser.jlab.org/))\
   During the course of the week, we added four new signals to the video
channel line-up. These include: a camera in the clean room that monitors
a scope showing the relative phase of the buncher and the drive laser, a
PC-to-video converter for the streak camera, a PC-to-video converter for
the THz equipment in user lab 3 and we took some time to get the EPICS
machine status page back online in the web-based video server. We are
running woefully short of outputs on the existing video switcher system.
We have 128 inputs and 16 outputs that feed nearly 100 monitors
including the web servers. The new system has 256 inputs and 32
outputs - the same as the Analog monitoring system which has worked
flawlessly since it was upgraded a few months back. We need to get this
system redone within the next quarter. The chassis are built; it\'s just
one more thing that takes manpower and operations downtime to complete.\
    An interface chassis for EPICS  control of the optics group\'s
proportional valves is being constructed. The design for this has been
completed and 75% of assembly is accomplished. The documentation process
for this chassis has also begun. Currently the proportional valve for
the Optical Dump has been configured to operate off of a temporary
variable power supply that is located within the control room. This
provides the control to vary the flow rate for the cooling of the
Optical Dump.  Installation of the LPSS system in Lab 3 has resumed, and
all cabling required for the new door system  interlocks has been
installed.  Installation of the control system for the Nanotube table
has begun in Lab 1, and will be completed on Monday during the
maintenance down. To support the additional Stepper Motor chassis\'s
that have been fabricated a stepper motor test station in lab 5 was
implemented\
   The Photo Cathode Gun Coulomb Archiver was not recording any coulomb
usage for the time that is was logging, this was traced to a
communication problem with a CAN interface. The CAN interface was
powered down at some point and when restored the communication did not
correctly come back up. The archiver is now properly recording the gun
data. The new Laser Personnel Safety System (LPSS) was successfully
tested with a multiple reader configuration, simulating correct handling
of multiple user labs. User lab 3 will be fitted with a new card reader
on Monday. The WesCam calibrations were updated to provide the correct
scaling for the beam viewers in the machine. The beam profile
information computed by the WesCam is producing data that is in
agreement with the machine models. The resulting emittance measurements
are also in the expected range of the current machine setup.\
   The following drawings were completed this week: Dump Water Skid
Interface Box Assembly & Fabrication drawings, Can Bus Printed Circuit
Board - all,  and Synchrotron Light Monitor Attenuator Box
Fabrication.   The Optical Heater Control Chassis Wiring diagram is
being updated as is the schematic drawing for the Charge Totalizer PCB.
All but two components for the new Beam Current Monitor printed circuit
board (PCB) are in house.  As soon as they are received, it will be
populated and tested.  Development continues on a major update to our
web site that will be made over the holiday shut-down. An order was
placed for the 12 kilowatt power supplies for final sextupoles that will
incorporated in the FEL Upgrade.

**WBS 9 (Beam Transport):**

***Sextupole (SF)***

• We had a very good review of the requirements of the drawings and
contract at New England Techni-Coil.

***Replacement Chicane Dipoles GW)***

• We had a very good review of the requirements of the drawings and
contract at New England Techni-Coil.

***UV Line***

• The nomenclature for the UV Line layout was added to the layout and is
again being reviewed by David Douglas. From the present layout we can
see that there are ten varieties of girder/stand that we will have to
design and build, more than we thought from our initial views.

***Injector Multi-slit upgrade***

• Layout is continuing.

**WBS 10 (Wiggler):**

The W80 electromagnetic wiggler has performed nicely this week with the
turn-on time and cavity length detuning curve length showing very high
gain.  The excitation curves for the correctors were updated so that the
wiggler can now be scanned continuously.\
The RFQ for the wiggler gap mechanism was finalized and sent out for bid
this week.  All drawings are now signed off and available on the web.

**WBS 11 (Optics):**

Progress occurred in the following areas:

 

2.8 and 1.06 micron mirrors:\
 As discussed in the Beam Improvement section of this week\'s report, we
lased easily with both cavity configurations (high Q and lower Q) once
we found the correct cavity length.  This cavity length agrees well with
an estimate based on the change in the vault floor temperature, and
assuming a CTE of 1E-5 per deg C.  Once we began lasing more strongly on
the higher outcoupling set of mirrors we could make our first attempt at
mirror absorption loss measurements.  We did this yesterday.  The loss
is no worse than 500 ppm, which gives us considerable margin to make
high average power with these coatings.  We also noted a scattering
center on the HR mirror, and investigated that last night.  It appears
to be a damage spot on the coating.  It is not absorptive, so as long as
it doesn\'t increase in size we will continue running with it.  In the
meantime, we will prepare a spare HR assembly with one of our spares. 
Operation at higher cw current gave us an opportunity to see how
effective the THz chicane is at reducing the THz radiation produced by
the nearby dipole.  The value as of this writing is 0.8W/mA, well below
the \~ 5 W/mA we measured last Spring, when we were running at the same
energy.  Procurement of the 1.06 micron mirrors is holding to schedule,
we should have outcouplers next week, and HRs the week after that.  We
are readying holders to mount the OC mirrors.

 

Other activities:\
We are finding that the unanodized aluminum mirror mounts in the
collimator lite have reduced outgassing, but our pumping speed is still
too limited to bring the pressure into the required 10E-7 Torr range. 
We will add another pump on Monday.  Work continues on a number of
initiatives mentioned in the last few week\'s reports.  The PbSe array
for our spectrograph is now running on a local micro-PC (mini-ITX
platform), and providing data to EPICS.  This is an exciting development
for us, as some of our newer hardware isn\'t amenable to our controls
interface.  After a more thorough cleaning of the turning mirror
cassette (TMC) vacuum vessels, we began testing the internals for
contamination.  After many weeks of anticipation, the synchrotron light
from the first bend after the linac was delivered to the streak camera,
and we are beginning to make measurements.  We also assisted with
the installation and operation of the pyroelectric camera purchased for
the THz beamline.

UV FEL

 

We received the HeNe lasers and their collimation optics.  This will be
used for the alignment system.  We also received another fluid heater,
which will be used to speed degassing the polyimide-insulated wiring and
other hardware on the utilities chain.  In our weekly hardware meeting
we discussed vacuum requirements for the optical cavity and how to
achieve it.

**Terahertz Project:**

***Operations:\
***The optical system bringing the THz into Lab 3 was re-aligned by
first superposing the visible synchrotron radiation with the reverse
tangent alignment laser, then using the latter to re-align the mirrors. 
M4 required the most adjustment, as expected since it had been removed
and remounted since the first alignment.  The beam was then incident on
a pyrocam, and a clear signal and distribution of the THz light was
observed.\
\
The Nicolet Impact-400 Michelson interferometer was then installed and
aligned and with pulsed beam, a signal was observed on the detector.