# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- February 23-27, 2004

**Date:** February 27, 2004

**Highlights:**

We made progress this week on three fronts with FEL upgrade
commissioning: (1) furthering our understanding of the rf phase dynamics
of the driver accelerator; (2) improving our laser calorimetry set-up so
that we can measure the absorption loss in vendor supplied optical
coatings before we procure the final optics; and (3) continued set-up
and initial check-out of the rf systems for the third cryomodule. In
addition, we completed the negotiations with our preferred vendor for
the short wavelength (1-3micron) permanent magnet wiggler and expect to
place the contract by Monday.

**NOTE: HOLD THESE DATES:**

The 2004 FEL Users/Laser Processing Consortium Workshop will be held at
Jefferson Lab on March 10-11, 2004. The meeting agenda, registration and
local arrangement information are posted on our website:
<http://www.jlab.org/intralab/calendar/archive04/FELusers-LPC/>.

**Commissioning Summary:**

Monday was a planned maintenance day.  On Tuesday we merged the
optimized configuration at 15 degree off crest with the previous 10
degree off-crest setup and attempted high power from the 6 micron
laser.  Unfortunately our cathode quantum efficiency had degraded to the
point where we could not operate at more than 5 mA at 75 MHz.  We ran
for several hours at 5 mA and tried to optimize the lasing.  We found
that we were efficiency limited, i.e. beam loss trips at the dump
limited our power.  The trips are due to non-linear dispersion induced
by having only one family of sextupoles in the second arc.  On Wednesday
and Thursday we provided CW low power beam to the optics group for
calorimetry studies and studied the behavior of the linac vs. phasing. 
Phasing measurements last week exhibit curious discrepancies that we do
not yet understand.  We came up with several possible explanations and
tested them but found no smoking guns.  On Friday we split the
sextupoles in the second arc, recesiated the photocathode which raised
the quantum efficiency to 4%, and successfully brought the beam back up
for continued studies.  At this point we are convinced that there is
some kinematic effect that is preventing us from getting shorter bunches
at the wiggler.  We are still trying the find the culprit.  We do know
that the bunch length is independent of charge so the problem is not
collective effects. (Note: this is a very good finding.) We also have
measurements of bunching using each cavity that show no bad actors (also
an important result). Linac field non-uniformities could explain the
dependence of the bunch length on the transverse match and we plan
measurements and analysis to check this hypothesis.

**Management:**

 We completed the negotiations with the preferred vendor for the
permanent magnet wiggler for short wavelength operation (1-3 microns)
and forwarded this package to our DOE Contracting Officer for approval.

The draft revision to our current ONR-DOE Memorandum of Agreement for
FY04 FEL activities that was completed last week was approved by our DOE
Contracting Officer and forwarded to ONR for their review and action.

We completed a draft Interagency Agreement and Statement of Work for the
FY04 activities for the UV FEL Upgrade project funded through the AFRL.
This draft was forwarded to our DOE Contracting Officer on Wednesday for
his review and transmittal to our AFRL Program Officer.

F. Dylla represented the Lab at the Virginia Research and Advisory
Commission's (VRTAC)

Capital Hill Day presentations to the Virginia Congressional Delegation
on Wednesday.

M. Shinn presented progress reports on our JTO funded activities at a
JTO contractor's meeting this week in Albuquerque.

**WBS 4 (Injector):**

Today we performed a cathode recesiation. The QE increased from 0.24% to
4%.

Again we are pleased to report that the recesiation and subsequent high
voltage conditioning

of the photogun is a very reproducible and painless process compared to
model 1 of the gun.

**WBS 6 (RF)**

All RF systems are all operational. The Buncher faulted several days at
startup for the day. After a reset, it operated properly for the rest of
the day. The control valve is being replaced today. A power splitter was
installed in zone 2 cavity 5 for Beam Break Up (BBU) studies and the
download files updated. Updated EPICS software is being installed and
will be checked today. This software is already running on FL03.

Zone 3 - Work is progressing on the commissioning of this new
cryomodule. Several interlocks still need wiring to be corrected before
any RF can be run into the cavities.

**WBS 8 (Instrumentation):**

Initial (parasitic!)studies are progressing to search for evidence of
the elusive (thankfully) Killer Modes. Last week the HOMs (first two
passbands) of the third cryomodule (Zone 3) were measured in all eight
7-cell cavities. The frequencies and quality factors are in general, in
agreement with the copper model measurements. This week exploratory beam
transfer function measurements in energy recovering mode were carried
out. This experimental setup can - and has been - used to extrapolate
and determine the threshold current due to beam breakup. A single
vertical stripline of BPM 0F06 (a spare test BPM) in the injector region
was used to impart transverse momentum to the beam with the modulating
frequency of 1887 MHz, one of the dominant HOMs in Zone 2, Cavity 5. A
\~9 W amplifier was driven by a network analyzer at the proper frequency
sweeping the frequency across the HOM frequency, and the resulting
resonance curve from the field probe of cavity 5 was observed. The
signal was -80dBm down, far to small to affect the beam but observable. 
The width of the observed resonance is consistent with the previously
measured quality factor of 4E5. We ran several currents (1, 2, and 3 mA)
and saw the amplitude of the excitation increasing with current. These
data runs take just a few minutes each and will tag along on the end of
other test plans as the schedule permits. These studies provide the
required training for the people involved and refine the equipment
needed beam studies on the third module.\
    All IOC channels for the frame grabber have been updated and tested
on the operational IOC(iocfel4). Progress is being made on configuration
on the new machine. The capture card has been installed and tested. The
framegrabber backbone processes are installed and tested along with the
software support that they need. The Tcl scripts are in the process of
being modified to fix network changes that occurred for the move. The
interlace problem has been fixed. Interpolation of the image has allowed
the Gaussian fit to perform to standards for the measurement of beam
height. The CA routines have been altered to insure a timely delivery of
readbacks and histograms. New HP monitor is installed for control room
replacement. The Dell\
monitor is in short term use for the xterm in the gallery. Another new
Dell was slated for a replacement for Sally (we are the Sally Hemming
FEL facility after all) but it died even before it was loaded with
software.\
    The dump current monitor was replaced (again), there is a path to
ground of 425 ohms. This may be internal to the dump on the ceramic gap,
external on the supporting \"V\" blocks & mica insulation, or the two
rubber hoses that supply the dump cooling water. The dump is now
terminated to 50 ohms, gain was added in the tunnel buffer box & all is
back on line and calibrated.\
    Upon completion of the wiring for the motor, the SF6 filter is now
operational. Some tests might need to be done to ensure the filter is
working properly, however, the motor has been tested and is ready for
use.  Fabrication of one Beam line Gate Valve Controller test box was
completed and another has begun. Fabrication of a Geiger Mueller Tube
Analog Read back Breakout box was completed and installed for third
cryomodule commissioning.\
    Two Keithly Multimeters were installed and configured for GPIB to
monitor the RTD outputs in the OC Optical Cavity. Another 8 channel
stepper motor chassis was commissioned for the HR and OC I&C centers. 
This brings to 40 the total number of stepper motor channels either in
use or available in the FEL. With the splitting of the Sextapole lite
magnets at MSF5F02 and MSF5F03, 2 additional Dansfysik 896 power
supplies were commissioned for MSF5F00 and MSF5F04. Replaced and
calibrated the IDC1G03 dump monitor in the FEL Vault.    

**WBS 9 (Beam Transport):**

***Sextupole (SF)***

• Procurement on hold pending 10 kW efforts\
Sextupole Lite Re-coiling to obtain higher field\
•  Next on the priority list\
***Octupole (OT)\
***•   The magnet test group is assembling the magnets.

***Wiggler\
***[Electromagnetic Wiggler for 2.8µ\
]{.underline}• We visited a potential vendor early in the week, getting
good feedback from both a water jet

vendor and a brazing vendor for the cooling plates.  The cooling plate
design was split in half

for handling and brazing concerns. Water jet cutting will be very fast
for all of these style

parts, in the order of one week total.\
• Almost the entire design is now fleshed out. It is compatible with the
Optical Klystron Stand

and the proposed beam tube.\
• At weeks end, bids are due for a contract to make the long lead items
and the details of the

remaining parts are being finished rapidly. Long lead material is on
order and the steel is

ready for shipment.\
***Scraper Outcoupler\
***• The design is complete and parts are on order and in fabrication.

**WBS 11 (Optics):**

We used part of last Friday and all of Monday to add more beam
diagnostics to the new beam line running to the portable vacuum
calorimeter.  These diagnostics are 1) a Coherent ModeMaster, for
determining the beam position, size, and collimation when we run the
power up to the required 50 W, and 2) A Pyrocam pyroelectric camera on a
translation stage.  This will be used initially to confirm what the
ModeMaster is determining, and provide 3D profiling of the sampled input
beam.

On Tuesday, we operated the calorimeter with the new diagnostics.  The
sample of interest is the new AR-coated sample the vendor that\'s
coating our 6 um outcouplers sent us. We saw clear evidence of thermal
blooming (see blooming.jpg), so it appears we didn\'t have a
sufficiently airtight enclosure to prevent air (with water vapor) from
leaking back in.  We then used the rest of Tuesday to run the power up
and to work on the calibration of the diagnostic beam dump.  We were a
bit stymied in efforts to push past \~ 1.3 kW, in part this was due to
the fact the photocathode QE had degraded to the point that we couldn\'t
run more than \~ 50 pC/bunch.  Since the charge was low, we used
Wednesday to try the calorimeter again, this time using the FEL\'s
tunability to move to 5.5 um where the attenuation is less (see No
blooming.jpg).  This gave us almost the same profile as seen when
pulsed.  However, it appeared we were mis-steered on the sample, and had
scattered light giving us problems.  Three data runs collected on
Thursday all had this problem, so today (Friday) we took advantage of
the downtime to change the geometry of the beam stop to reduce the
affect of backscattered light, placed additional monitoring cameras and
installed a power meter with a pop in mirror so we can monitor the power
delivered to the input of the calorimeter.  We will get data late today,
and possibly on Saturday to check everything and get a good calorimetry
run. 

 

In addition to the activities detailed above, the alignment of the
ultrafast laser to the THz port for the injector was optimized, and is
ready for use.  Updating our other 10 kw activities, the optical scraper
is being fabricated.  The design of the cryomirror mount and transfer
lines is in process.  A PR for more molybdenum for mirror holders was
signed off, and fabrication of same with the moly we have in stock
proceeds. 

Michelle Shinn gave a presentation to the JTO in Albuquerque on the
laser damage work they are funding.

***Cryogenic Mirror for 6µm\
***• We have agreed on a design that allows the mirror to float within a
thin wall tube of a material

that almost matches the coefficient of thermal contraction of the
mirror. There seems to be

enough resilience to not break the bond between mirror and holder during
any mismatch in

contractions. Details are being drawn.\
• We reviewed the cooling fluid piping scheme. We decided to take the
fluids out via the

bottom flange that now admits the cooling water.\
• We are starting work on a sub cooled liquid argon/ liquid nitrogen
pumping system.

> **Blooming** **No Blooming**

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