# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief - January 30 -- February
3, 2006

**Date:** February 3, 2006

**Highlights:**

We continued pushing toward higher power by carefully improving the
e-beam match in the energy recovery.  As we were concentrating on
getting the match right we weren\'t focusing directly on getting a new
power record but did achieve a modest increase to 3.2 kW at 1.6 microns
and efficiencies as high as 1.6 kW/mA. (Note last week we reported
efficiencies of 1.3 kW/mA). The power limit was due to current limits
from BBU so we are now in the process of including the rotator in the
setup to suppress the instability as we demonstrated last year; this
activity will proceed through next week.   Interestingly, we proved that
lasing can suppress the BBU instability to some extent by raising the
current threshold (\~2x?).  We also achieved a record short bunch of 299
fs FWHM with good agreement between the bunch length derived from
the THz spectrum and from the "Happek" bunch length diagnostic.  Working
through these issues carefully has proven beneficial as the machine
performance has been significantly more stable and reproducible. We will
be continuing to identify and fix small hardware shortcomings that
affect machine performance.

 

**Management:**

HFD spent considerable time for the final week of preparation of the
presentation of the new lab management proposal to DOE next week in Oak
Ridge.

We hosted a visit from Mufit Tecimer of FSU who will be working with us
to design the  FIR FEL system for their NSF proposal.

We continued improving our safety procedures by developing some specific
responses to hazards identified during our last safety stand-down.  This
also included an update to the chapters of our lab ES&H Manual regarding
operation of the FEL for users.

**Operations:**

This week concentrated on raising the average beam current in
the accelerator and getting a well-optimized laser setup for testing. 
We could not get much higher than 2.5 mA early in the week due to losses
of second pass beam in the linac.  We were able to eventually get losses
down as the week progressed and could run at the 3.5 mA level  pulsed
with very small losses.  In parallel we optimized the laser and could
get 1.6 kW/mA at up to 1.25 mA reliably.  Operation at 2.5  mA had lower
efficiency of 1.25 kW/mA.  We were able to slightly raise the bar this
week and lased for a few minutes at 3.2 kW.  The reduction of efficiency
at 2.5 mA might be due to mirror heating or might be due to electron
beam degradation.  We worked to get our THz  spectrometer working so
that we could measure the longitudinal bunch  profile at the various
currents.  With this we could see whether the efficiency reduction was
due to the optics or the electron beam.

We also noticed during the week that the injector was slowly drifting 
in its longitudinal match.  Due to this, the bunch length from
the injector increased and the longitudinal phase space tilted. 
We compensated the tilt by changing the electron beam energy in the 
first arc(the lower energy goes through the sextupoles in a different 
location and produces a different M56).  The resulting energy
droop caused the FEL to drift longer in wavelength.  The mirrors
are designed to minimize losses at 1.6 microns so a drift to
longer wavelengths increases the loss.  This increase in mirror loss
was seen.  We have changed our setup techniques to try to minimize
this injector drift.  Every cloud has a silver lining however and
the bunch got shorter as the week progressed.  We had 300 fsec FWHM 
bunches, repeatably, at the end of the week.\
Once we had improved the match in the linac we found that the
current was now limited by a low BBU threshold.  Here we came upon
an interesting physics observation.  The BBU threshold depended
on whether or not we were lasing.  On Thursday we did a brief
experiment to verify this.  We could only run up to 1.6 mA before BBU
tripped us off with the laser off.  With the laser on we could run up to
2.5 mA.  If we then closed the laser shutter, the electron beam 
immediately shut off due to a BBU instability.  We are not sure of the
physics of this but it is very interesting.\
Since the current was still limited even with the laser on, we decided
to turn on the rotator.  This requires that the match into the backleg
region be very good and that the dispersion be properly suppressed in
the first arc.  We are working now on improving the match to the backleg
after minimizing the dispersion in the first arc.  This setup should
allow us to run up to 5 mA without tripping.   We will then have to
re-optimize the match to the wiggler and try to push the power.\
During second shift we did some work on the injector phasing script.  
This script, when finished should allow us to quickly check the phasing
of the injector and bring it back to where it was previously.  We only
do this every few weeks now because it takes too long. We can also use
the script to see if any one element is drifting.

 

**WBS 4 (Injector):**

The DC photocathode gun has delivered over 30 hours of pulse beam and
over 35 C, 5.5 hours and up to 3 mA of CW beam for FEL ops during the
week. Assembly and components gathering for the gun chamber NEG coating
sputtering system continued this week. We devoted about 12 hours of beam
time for testing, troubleshooting and fixing the injector phasing script
making significant progress. The script is about 2/3 complete.

**WBS 6 (RF):**

Last Saturday we reworked the connector backshells on cavities 2-4 and
2-6.  We did find some problems with the terminations and the ferrule
that is used to create a "flared" joint between the outer conductor of
the heliax and the connector backshell.  Once they were both corrected
the gradient stabilized for the two cavities.  We also checked the
tightness of the field probe connectors on zone 5 and found several that
were not as tight as they might have been.  Further investigation
indicated there is an interference problem between the N-connector and
the flange mounting bolts.  Unfortunately most brand\'s type-N
connectors have an outer shell that is to large.  We did find 8 pieces
which are small enough (by 0.040 inches) and installed them on Thursday
evening.  Unfortunately, they were not marked with vendor names.  We
have ordered samples from a number of manufacturers in hopes of finding
some more that work.  We continue to monitor the gradient stability of
the machine, reviewing the data daily.\
 \
The next target of our effort is the stability of the phase between the
drive laser and the buncher cavity.  Minor motion of the drive and
feedback cables (Heliax) from the laser drive electronics destroys the
injector setup.  We started the process of designing a system to
mechanically stabilize those cables as well as investigating the need
for thermal stabilization of this 30 foot run of cables.  We are also
going to investigate the buncher cavity temperature control system and
evaluate potential improvements.\
 \
RF systems stable this week with no failures.

**WBS 8 (Instrumentation):**

The mMajority of the time this week has been spent working in Ops on
nagging issues with diagnostics. Centering rings and additional narrow
band filters were added to a number of viewers (3F region) to reduce the
intensity of the OTR signal. One of the OD1 & 2 filter assemblies was
moved from a little used SLM port to the viewer at 3F12 (to David\'s
delight!). The main Operations software focus this week has been on the
high priority injector phasing \
script. Time on swing/owl shifts was devoted to testing and correcting
the script. The script\'s calibration issues of last week were quickly
understood and fixed. The equations for phasing cavity 3 were verified
and the things learned are being applied to the cavity 4 phasing routine
in preparation for testing. Further testing next week will help verify
the cavity 3 phasing and test the cavity 4 phasing routines.\
  Three more sets of embedded BPM electronics have been calibrated and
programmed.  These three are ready for installation when time from Ops
allows.  The necessary components for this installation are all being
gathered so any effect to machine operations will be minimal.  Setup 3
new BPM board (E3F10, E4F09, E5F07), including databases, device
drivers, sequencers, and calibration data. We are working on the
next version BPM board, which has the configuration of ADC-
FPGA-Coldfire.  The development tools and prototype have been set.  We
are also working on the configuration of the General Purpose Processor
Card, which will be versatile for several different
applications.  We further tested the Sextupole Reversing Switch this
week and made much  progress.  The switch has been successfully tested
with the actual power supply and magnet at 100 amps.  We noticed heating
with the high power MOSFETs and we are currently examining which
direction to go to eliminate the heat.   Two GC chassis are nearing
completion, with work still to be the documentation of the layout.  When
these are done, cards will be tested in each chassis.  When more
power supplies arrive, more chassis will be made. The front part of Lab
5  has been cleaned, along with a few odds and ends in Lab 3.  Also
in Lab 3, additional BNC cables were run from the workbench to
inside the hutch.\
   Documentation of the Beam Viewer crate PCBs is underway and
making good progress.  Currently the backplane schematic is
nearing completion, so the pin assignments for the processor and
control cards are being defined.  The control cards schematics are
also making forward progress.  In order for the crate to utilize
the preexisting MPS cards we are designated the crates to have
36 channels per crate.  This will alleviate space constraints for power
supplies and ensure the back panel field connections are manageable. The
processor card design is on-going, currently we have designated the pin
assignments and the major functionality of the card.  This card is
intended to be a general purpose processor card that can be installed in
any 3U crate and provide both Digital I/O as well as Analog I/O.  This
module\'s first implementation would be in the Beam Viewer crate but
there are current applications where it will be useful such as the
Charge/Dump Current Monitor Crate, 64-Ch Diode/RTD Crate, and Sextupole
Power Supply Control Crate. Also, the preliminary drawing for the new
charge totalizer board was completed; it is ready to be sent to EECAD
for check prints.\
  It was decided early this week that the ability to strip-tool
the slow-scale trends of the Low-level RF signals (like the GASKS)
and the integrated BLM signals would be worthwhile. Noting that
these signals are all currently available in our Analog Monitoring
System (AMS), we decided to dedicate some of the unused AMS outputs for
this purpose and to re-install an old-style 32 channel
True-RMS/Buffer crate from the old system which has the integration
circuit. So, this week, we have tested and installed the AMS crate and
its power supply into FL02B11 and updated iocfel8 (AMS control ioc) to
include a 64 ch. ADC which will read the signals into EPICS. All that
remains now is the cabling, which is currently making progress.\
  We have also made more progress on the new rapid scan THz bench
(FT-IR). Although the reference laser was aligned and the FT-IR
was running as of last week, the reference laser was not well secured
in its position and its mounting plate needed to be addressed. This
was done and now the reference laser\'s critical alignment is secured.\
  Also, we are pleased to show the latest results of our
coherence calculations for 10\^6 particles which have been performed
over a high resolution spectrum of wavelengths. Although the supplied
particle distribution is not realistic, the simulation does clearly show
the statistical mechanism which predicts the coherent harmonics in
the area of the spectrum which is typically incoherent based on
the potentially complex structure of the particle distribution. For
this  simulation, we used a sinusoidally modulated gaussian
distribution with 10\^6 particles. See the Figure below.

![](media/image1.jpeg){width="6.55in" height="3.504166666666667in"}

Further work continues on trying to enhance the performance of
our existing CCD cameras through the use of advanced triggering, gen
lock and integration plug-in boards, with that, a test setup for the
new JAI Compact Mega pixel Progressive Scan Camera was also assembled \
this week. This new high resolution camera will be used at ITV0F04 to
analyze the multislit  image. The current problem is that the interlaced
image losses every other row one an image that is an array of rows.\
   The students spent time organizing and sorting out the video hardware
in the trailer. There has been an area cleared off on the shelves next
to one of the doors that will now be the area for video stuff. Nine area
cams were found to be good and are ready for use. A mound of power
supplies was shifted through and will begin to get tested tomorrow. An
old laptop was reassembled and resurrected this week, for use as a
portable test machine. Finally, in keeping with the importance of safety
at the lab, Lab 5 was organized to provide a safer working environment
and recover \"lost\" materials.\
\
**WBS 9 (Beam Transport):**

# *UV Line*

• No change in status

# *IR Machine Re-commissioning and Operations*

• We looked at the gaps between pole tips and beam chamber on the first
GW dipoles of the Debunching Chicane with the camera set. We observed no
closing gaps with magnet excitation. We will advance the camera to
others in the string as the vault becomes accessible.

• We will talk to the folks from the Advanced Measurement and
Diagnostics Branch of NASA today to find out if they have any more
suggestions on instrumenting GWs to pick up a delaminating pole tip.

• We verified that under certain conditions the GF correctors were not
set at their correct values during a BURT Restore of an arc's dipoles.
These correctors are built into the large dipoles and a turned off
during their hysteresis cycle. If the new current value of the arc
string is higher than the original value the hysteresis loop of the main
dipole string is tripped. Since the hysteresis loop grabs and stores
original GF settings before BURT places its new settings in place, and
then the cycle restores the original settings after the looping is
complete, the GF correctors are not set at the BURT Restore settings. I
will work with the software folks to correct this.

**WBS 11 (Optics):**

***FEL mirrors\
***When we shutdown at the end of the day last Friday, high power lasing
at 1.6 microns was stymied by vibration.  Our group concluded that the
cause was a partial blockage of the debris strainer in the chiller that
needed to be cleared.  Over the weekend we cleaned the strainer, which
indeed was blocked, and installed a filter to prevent this from
happening again.  Lasing on Monday showed that we had indeed fixed this
problem.  That impediment removed, we achieved yet another record level
of output at 1.6 microns. We also learned (once again) to keep the mode
centered on the mirrors or we create scattered light.  This is now
managed by the beam dumps near the wiggler, but it still causes beamline
pressure rises near the wiggler.  We also learned that the harmonic
power management implemented by the coatings has narrowed our tuning
range to \< +/- 50 nm about 1.6 microns.  If you exceed this, we see
increased HR heating.  This highlighted an issue the accelerator was
having with energy shifts, which should be ameliorated with work done
yesterday evening.  So far, at lower currents, the lasing efficiency
exceeds the spreadsheet predictions.  We are going to do a systematic
check of the lasing wavelength as a function of current to see if
that\'s the source of higher levels of HR heating.  Metrology on the
other sapphire samples is nearly complete and several sets go to a
commercial vendor for coating today.  We received many more silicon and
sapphire substrates that will be used for spares and to test new coating
formulations, and they were added to the growing number going through
QA.

#### Other Activities

We began some tests of optical transport system (OTS) this week.  Pulsed
beam at 1.6 microns was sent to the end-of-line (EOL) beam dump.  We
also looked at the spot sizes with the alignment HeNe.  The beam looks
fairly stable.  The spot size for the IR beam was close to prediction,
but we had no control over the focusing with the deformable mirror in
the collimator. This will need further investigation, but won\'t affect
beam operations at the 1 kW level.  The vendor for the high power
windows is having problems achieving the wavefront spec.  They asked for
a variance, but were told no, so we are still waiting for the parts. 
The optical beam position monitor (OBPM) reassembly was started.  Good
progress was made on the cryocooling of one of the outcoupler mirrors. 
The Cryo group ordered all the long lead items they identified and took
advantage of Thursday evening\'s shutdown to make measurements of the
lengths of transfer lines they will build.  The conceptual plan for
additions/modifications to the OC internals has gelled and is now moving
to the design phase.  TMC#1 nears completion.  We bid farewell to Dan
Oprisko, our contract mechanical technician for the last year.  Dan
participated in every facet of Optics Group activities, and his presence
will be sorely missed.  However, we hope to see him again soon working
with one of our user groups. 

**Terahertz:**

The terahertz beamline in Lab 3 has benefited from the continued gains
made on the FEL operations.  With more frequent runs of CW beam in the
accelerator, we have now been able to perform some verification of the
alignment of the mirrors in the THz beamline.  This has been possible
due to the improved reliability in the electron beam orbit through the
optical chicane.  We have also taken our first good THz spectrum.
Attempts to accurately measure the power were unsuccessful due to some
problems with the power meter, but the need to drastically attenuate the
beam for the spectrometer measurements indicated that the power was
quite high.