# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- March 15-19, 2004

**Date:** March 19, 2004

**Highlights:**

During the FEL Upgrade commissioning activities this week the following
kept us very busy:\
(1) during the optimization of the FEL performance using our 2.7%
outcoupling optic we achieved both a new power output record (2.25 kW CW
and 3 kW pulsed at 6 microns) and a new record for the power of the
circulating laser beam in the optical cavity (84kW). This latter value
is very close to the stored circulating power we need to demonstrate
(100 kW) when we outcouple 10 kW with final outcouplers. (2) Our low
loss 6 micron  (8%) outcoupling optics that was installed last week was
found to be free of distortion but unfortunately was cut to the wrong
radius of curvature (14.5m vs 16m ) by the manufacturer. (It is being
returned for regrinding and recoating with a promised return to us in
\~3 weeks). (3) The octupole magnets that were installed in the 2^nd^
recirculation arc last weekend worked extremely well. After tuning the
energy recovery phase and the extraction dump bend, we were, by exciting
the octupoles and using sextupoles in two families to manage path length
and nonlinear dispersion, able to recover and compress the full beam
energy spread (over 8%, perhaps as much as 10%) generated when lasing
quite strongly, even when backing up the sharp end of the detuning curve
at high currents.

The power supply currents required in all elements is low enough (1.6 A
sextupoles, 5.5 A octupoles, at \~90 MeV) that operation at energies
provided by three modules (\>140 MeV)is quite likely possible with the
presently installed magnets and power supplies. (4) We completed the
initial commissioning of the 3rd cryomodule; all 8 cavities perform at
specifications above what is needed delivering a total energy gain of \>
85 MeV; (5) we had a successful final design review for the \"Laser
Microengineering Station\" that will be delivered to the FEL User Labs
next year by the Aerospace Corporation as a key component for the AFRL
funded UV FEL Upgrade program. (6) Today we held a design review for the
permanent magnet wiggler, 2 weeks ahead of schedule and approved the
manufacturing drawings for release.

**Commissioning Summary (by D.Douglas):**

1\. Monday, we touched up the linac-to-recirculator match using a WesCam
based quad scan, providing reasonably good beam quality through the
system. Results were consistent with those obtained from a cursory
multimonitor measurement down the linac and indicate the \"balanced
gradient\" injector configuration is yielding quite good emittances of
order 15 mm-mrad horizontal and 10 mm-mrad vertical.

2\. Dependence of momentum spread in injector was checked as a function
of buncher gradient. At recent gradient operating points of 3.4 MV/m,
the injected momentum spread was of order 100 keV; this leads to \~1%
full dp/p after acceleration, indicating an injected full bunch length
of \~8 psec. The bunch length at the wiggler for this setpoint is and
has been for some time around 750 fsec FWHM. If the accelerator is
compressing the injected beam properly, this is just about exactly what
is to be expected. As observed for some time, the beam momentum spread
is (charge independently) an asymmetric function of linac phase - the
quoted 1% is at the operating point of 10 degrees off crest, while at
-10 degrees off crest, it grows to \~2%

If the buncher gradient is reduced to \~3 MV/m, the injected momentum
spread drops to 50 keV. One would then anticipate the bunch length at
injection would grow, but that it would, as a consequence of the smaller
momentum spread, decrease after compression to 400 fsec FWHM or so.
HOWEVER, we observe that the final momentum spread DECREASES to 2/3% to
3/4% or so (indicating that the injected bunch length has fallen to 6
psec full), and the bunch length after compression INCREASES to 1 psec
FWHM. Morever, the factor of 2 asymmetry in final momentum spread around
crest persists.

As noted previously, various compression checks (compressing using
energy slew from all cavities, single modules, and single cavities) have
failed to isolate any cause for either the longish bunch or the
asymmetry. On Thursday, we therefore performed an \"anticavity\" check
on the first module, in which compression was performed by the whole
linac, but with individual cavities turned off in sequence. Data is
undergoing evaluation, and the tests will be extended to the 2nd module
during operation today. We found, during the tests, that turning off the
third cavity in the first module (\"#6\" - the third seen by the beam)
reduced the bunch length from 1 psec to 750 fsec. Turning off the first
cavity (\"#8\") reduced it from 1 psec to 850 fsec. It was not possible
to turn of the 2nd cavity (\"#7\"). Yes, you read it right. The module
WOULD NOT RUN without power to the 2nd cavity. It crashed to filaments
when the cavity was off. If individual cavities were powered up, it
crashed to filaments when the third cavity (#6) was powered up. We
(well, okay, Richard Walker) found that it could be run with power on
with a zero gradient setpoint. In this condition, the measured gradient
was \~1.7 MV/m, unless the gradient set points of the adjacent cavities
were zeroed, in which case the measured gradient reduced to 0.4 MV/m.
With all cavities but the second at nominal gradient and the second at
zero gradient set, we found the Happek device gave, at one point, a
totally unphysical, antisymmetric bunch length scan and the BPM system
said the energy was drifting around, even though the beam exhibited
little or no motion on either the 2F08 synchrotron light monitor or the
Happek viewer. Hmm. Very weird, right? But these weird results probably
provide the clues we need to solve this bunch length mystery.

Thursday was further subsumed with difference orbit measurements
(delayed by 21/2 hrs due to an unauthorized cable movement!).
Preliminary review of a bit of the data suggests this particularly mind
numbing activity may have actually proven useful, inasmuch as the early
reaction to the data suggests the machine is reasonable well decoupled.
\"Well\", you ask, \"why not? Didn\'t you claim you decoupled the
machine six months ago?". Yes, we did.

And we have no reason to believe the skew quad at the front end of the
linac, which corrects the skew quad RF field in the HOM couplers of the
first 4 cavities, has suddenly decided to get three times stronger.
However, when the \"balanced gradient\" injector was set up, Steve
Benson attempted to alleviate obvious H/V coupling in the beam not only
by adjusting the single injector skew, but by using the 1st linac skew
as well. It is, after all, BEFORE injection, is it not? He found that a
reduction of the first skew quad in the linac by about 70% in strength
(from 99 g to 29 g gradient integral) would give nice, upright spots
through the system. So we\'ve run this way, deciding to eat the orbit
coupling that this change should produce.

Except that it DIDN\'T PRODUCE orbit coupling - the orbit is fairly well
decoupled, despite the fact the quad was set, some time back, by
decoupling the orbits. Enquiring minds want to know: \"what changed\" -
either the magnet did, or something in RF did. Given the above whiney
discussion about the RF in the front of the 1st module, the reader may
hazard a guess as to this author\'s opinion.

Investigation of this mystery continues. It is rumored a castle full of
French knights has both the Holy Grail and a piece of the True Cross. We
will journey to this site as soon as the swallows (either European or
African) bring us new coconuts.

3\. Suspicious that we were recovering too close to trough, and thereby
creating a noncompressed high energy tail at the dump, egged on Monday
by the encouragement of one Mr. Benson, we increased the extraction buss
excitation and backed the energy recovery phase away from trough,
raising the energy of the exhaust beam and bending the highest energy
components harder around the corner into the dump. The exhaust energy
went up to about 9.7 MeV (injection buss = 9.2 MeV/c = 10600 g-cm,
extraction buss now at

11200 g-cm) - so the energy recovery is \"incomplete\" to the tune of
1/2 kW/mA overall, or about 30 W/cavity/mA additional forward power
consumption - well within the available klystron output and probably
smaller than you would measure if you weren\'t either really interested
or being really anal. This configuration moves the high energy tail of
the beam fully into the trough (rather than drooping across it) so that
there is \"leverage\" for the RF waveform to compress it. This also
provides better coupling to the momentum compaction, which was
immediately seized upon, during high power FEL operation, as an
opportunity to use the recently installed octupoles. With the core beam
compressing more completely into the dump, the tails were treated to a
healthy does of octupolar field - modifying the lattice and linac
longitudinal torsion and alleviating beam loss after energy recovery
while lasing strongly.

As a consequence of the dump resteer/path-length-linac recovery
rephase/octupole activation (or perhaps only purely coincidentally,
though the machine DOES trip off if you fiddle the octupoles too far
away from their set point - for example, if you turn them off, and Alex
Dragt always used to say that \"a little octupole is a good thing\"), it
became possible to run high current while lasing quite hard. Thus for
example, Photonmeister Benson could generate 3 kW of light during 5 mA
in 1 msec long trains of pulsed beam at 60 Hz whilst surviving both RF
and FEL turnon transients; similarly he could back UP the steep side of
the FEL detuning curve running high CW current - a stunt that worked in
the Demo only on very, very good days of operation. The reader is
referred to statements by knowledgeable parties for the details.

**Management:**

We held the final design review on March 18^th^ for the Laser
Microfabrication Station that is being designed and fabricated by the
Aerospace Corporation.  This station will be a very versatile laser
microengineering device that will be installed approximately 1 year from
now in User Lab 4 to take advantage of the output of the UV FEL Upgrade.
Its capabilities are unparalleled in the micro machining world with the
ability to microengineer 3D components with write speeds of 400 mm/sec
over a field of 0.5m.  We congratulate Henry Helvajian and his team at
Aerospace for a very successful review, which completes the final
milestone of this phase of the Aerospace subcontract.\
\
On Friday March 19th, we held a design review by teleconference with the
engineering team at STI, Inc. that is designing and building the short
wavelength, permanent magnet wiggler for the IR Upgrade. The review was
successful and the release of drawings for manufacturing was granted.\
\
On Wednesday, March 17th we conducted a tour and briefing for a
delegation from Hawaii that is interested in the economic development
aspects of a facility much like JLab\'s FEL facility.\
\
On March 16-17, SURA hosted a workshop on the applications of terahertz
radiation attended by a mixture of industry, government and university
representatives. 

**WBS 4 (Injector):**

After the fourth re-cesiation performed early Wednesday morning on the
January 27 cathode, the DC Photocathode Gun delivered up to 4.8 mA of CW
beam for high power lasing operations that set a new FEL power record.
The total charge delivered by the cathode at the end of the day was
about 40 Coulombs. After the re-cesiation the cathode QE increased from
0.5 to 3%.

The Gun HVPS will be modified to run both drive stacks under voltage
control, when there are several days available to make the changes and
test the new configuration disconnected from the Gun.

**WBS 6 (RF)**

All RF systems are operating normally, except for cavity 2-7 which trips
the zone back to Filaments when it is turned off.  The obvious masks
were turned on, but to no avail.  This problem is being pursued today.

 

The commissioning of the FL03 cryomodule is making good progress.  High
RF power has been run into all cavities to measure the maximum gradient
available, the various Q\'s, and the GMES probe calibration factors. 
Tasks for today and next week include:  checking the HOM filters in the
RF Control Module, installing the Y Heater cables, operating the RF
under EPICS control, checking the cavity piezoelectric tuners for
turn-on control, and running the zone at gradient for 8 continuous hours
for stability and tuner tracking tests. The results from all 8 cavities
are as follows:

Cavity        Max Gradient, MV/m        Qo e+9 \@12MV/m

1 17.1 8.2

2 15.0 6.4

3 17.1 8.2

4 17.1 8.1

5 12.0 7.0

6 15.1 6.4

7 15.0 7.9

8 15.3 7.4

 

Total 123.7 MV/m

Cavity Length 0.7 m

Max Accel. 86.6 MeV (Note :new RF Control Modules will be required to
control this maximum available acceleration.)

**WBS 8 (Instrumentation):**

Much of this week was spent in support of operations while the Air Force
design review was in session. Another week brings another \"fresh\" idea
from Michelle and George on an XPM (eXtreme Power Monitor) for the FEL.
This one is designed to fit into the same 4\" holder as the wasted CaFl
(near) Brewster window pickoff. We\'re mounting a grid of 100 micron
Tungsten wires on 1mm centers as another try on a high power pickoff. If
we burn up this one at least it will look pretty cool (HOT) before it
goes\...\
   Final connections to the Octupole magnets and power supplies were
completed, Dave seems, well, pleased\...\
   Effort continued on the design of the Pre-Amp chassis. This solves
the problem of sharing the Stanford Research pre-amp between the two
bunch length instruments. Continued fabrication of the 6th Stepper Motor
chassis. We\'re looking into raising the output of the mirror heater
chassis to increase the available ROC control. The new DC-DC converters
are in for the third cryomodule piezo tuners. These convert 0 to 12
volts to 0 to 200 volts (2.5mAmp) and are less than one cubic inch in
size. These PCB\'s are in the cue to be designed and laid out. Several
new color cameras have been built for various systems monitoring
applications in the FEL. Preparation for reinstallation of new SF6 pump
system has begun. Additional video links were installed in the Output
Coupler region.\
   Update on the Enterprise Linux Kernel: Red Hat has seemingly changed
some of the kernel source code that is the foundation of the memory
operations for this pci driver. The problem has been associated with the
now inability to perform memory copies equal to those of Red Hat 9. A
test program has been written to exploit this memory problem to prove
that it was not due to an error by the coder. Red Hat support is going
to be contacted to find the fix for the kernel. Anthony Cuffe is going
to help setup an account with the proper group to talk to at Red Hat.
Until Red Hat comes up with a solution, we will run Red Hat 9 as a
secondary kernel on the new machine. This will also provide a safe
rollback along with the continued testing and operation of the
framegrabber.\
   Configuration Control Database - A major update to the configuration
control database ([https://laser.jlab.org](https://laser.jlab.org/)) has
been posted. The majority of the changes made will not be immediately
evident (aside from the obvious formatting changes), however, the
required internal programming changes have been implemented to allow for
a seamless integration of FEL system status \[high-level(WBS) and
low-level (component) status\], equipment servicing and preventive
maintenance, item history, data archiving and operations scheduling as
well as the usual flogs. Currently, the workstations in the FEL control
room are running a \'very-old\' build of UNIX and are not capable of
running the system. This situation is being remedied by the computer
center.

**WBS 9 (Beam Transport):**

***Sextupole Lite Re-coiling to obtain higher field\
***• The drawings for the recoiling of the Sextupole Lites with 50% more
turns are being checked.\
***Octupole (OT)\
***• The magnets were installed last weekend. I believe it was wise to
fund their manufacture early in the fiscal year as we did. (See the
report from Operations for this week)\
***Wiggler\
***[Electromagnetic Wiggler for 2.8µ\
]{.underline}• Process Equipment Company (PECo) was successful in having
their water jet cutter subcontractor make two of the Type A conductors
(one of 6 style variations of conductor) using water jet cutting. This
effort is part of a separate prototyping effort to get a jump on
problems. The parts came out beautifully and their concerns that the 8
foot pieces may be difficult to handle were laid to rest.\
• They will now start water jet cutting the actual insulator sheets next
week since that have material. They prototyped earlier.\
• They started machining the core pieces.\
• They are tack brazing the cooling tubes to the prototype cooling
plates today in anticipation of proving out the full brazing by the
brazing subcontractor over the weekend or early next week.\
• Their comments today were: "For a project that started out (as
tenuous) as just a series of sketches, it looks like we can complete it
as anticipated. There don't seem to be any showstoppers. Both the
brazing subcontractor and the water jet subcontractor are very
enthusiastic about this project and are being a great help"\
• We obtained bids for making the ancillary parts in two batches by
local vendors. The order for one batch, the manifolds, was placed.\
• In the design area, we are down to the parts that make the wiggler
mate well with the existing wiggler strong back and generating the final
assembly from the 3-D model.\
• The shop is welding up a sample junction on the new beam chamber for
the 2.8 Micron Wigglers before they weld the final assembly. All the
full tube parts are ready for welding.\
***Sextupole (SF)\
***• Procurement on hold pending 10 kW efforts

**WBS 10 (Wiggler)**:\
STI Optronics continues to make excellent progress on the new permanent
magnet wiggler.  We had the mechanical design review today. This is two
weeks before the deadline originally agreed to.  No serious issues were
found.  The design looks simple, robust and well thought out.  The
approval for the first progress payment was made this week as well.  The
detailed end magnet design was completed this week and parts are being
ordered.  The effective number of periods is 30.6 according to B2E. 
Calculations by STI Optronics show a shorter length but they use a
different ideal model than we do.   The bottom line is that the wiggler
is about a long as one can get for the number of poles used.

**WBS 11 (Optics):**

Plan A is to obtain low loss 6 um optics.\
 \
2 figured Si mirrors that will be used for HRs were sent to the vendor
that produced a very low (200 ppm) HR coating.  More OC mirror
substrates, using the lowest loss material available, are at the vendor
for fabrication.\
 \
Our three Plan B\'s had the following progress:\
 \
Scraper outcoupler:  The first article was brazed Thursday evening, and
sent to the vendor for final machining.  It will return early next week
for polishing, the final brazing of the cooling tubes, and then
coating.  George Biallas reported: \"Cooling tubes and their standoffs
are made and in assembly. Don Bullard made a mock--up of the scraper
piece that he will use next week to prove out the "rolling' of the flex
for cooling that are in the chamber beneath the beam line. These are the
features that allow the scraper to withdraw from the beam-line, allowing
lasing at shorter wavelengths to continue.\"  

 

***Cryomirror:\
***A holder that would thermally isolate a 3\" mirror in a 4\" holder
was designed and is in fab.  This will be pressed into service if need
be. \
 \
George Biallas had this report:\
The major parts for the flexible mirror mount and its hose assemblies
continue in fabrication. The parts that will allow cryo cooling of the
existing mirrors within the OC Can are done.\
• A mock up part that resembles the membrane-like .015 thick tube that
is the heart of the flexible mirror mount design was made by wire
Electro Discharge Machining. The brittle molybdenum shattered far too
easily to be a reliable part. However, the parallel manufacture of a
niobium substitute by deep drawing is going well. The part was made, but
has .002\" wrinkles on part of the surface. We will work on eliminating
the wrinkles for a short time more, but they may be OK as is.\
• The work on the cryo cooling liquid flow advanced. Two methods are
being pursued in parallel, Dewar pressurization and pumped. Consulting
the charts for nitrogen at cryogenic temperatures shows that flows of
0.09 GPM of liquid nitrogen would remove the 50 watts anticipated
without boiling. Initial look at the state of flow indicates it is
between laminar and turbulent through the mirror cooling assembly.
Calculations continue to make this aspect more clear.\
• The pressurization of a Dewar of liquid nitrogen to supply fluid is
simple. It may drift up in temperature to the high 80's K during
pressurization and the flow is lost. The pumped circulation loop system
would maintain the 77 K but is more complicated. We have received the
circulation system's pump and it is being modified for cryo service. We
are ordering valves and working on the transfer lines for both systems.\
\
2.8 micron mirrors:  In fabrication.  We expect mirror holders to mount
them in from the vendor in about a week.\
\
Other activities:  As mentioned in the commissioning summary, when we
failed to lase after achieving gain \~ 3X the outcoupling, we used part
of a swing shift to do some in situ checks of the ROC with the OCMMS. 
Measurements showed an ROC of 14.5 m, deviating substantially from the
16.0m spec., despite the manufacturer's certification. A reanalysis of
the ROC determination of the test plate images turned up a flaw in the
calculation we used during our in-coming QA inspection, which resulted
in almost perfect agreement with the in situ determination.  The optics
have been returned to the fabricating subcontractor, who promises quick
turnaround, then they will be recoated and returned to us.  We returned
to our old standby, the OC with low (\~ 2.7%), and lased at sufficiently
high power to calibrate our various laser power probes.  Agreement was
within 10% (at worst) for all three.  We also had an IR camera that
imaged one of the power probe surfaces, which gave us a sense of the
beam size as a function of lasing.  When we ran higher amounts of power
(\> 1.5 kW), we noted a slight decreasing in beam size, consistent with
the predicted thermal lensing.  After this excursion we saw patterns
consistent with the pickoff window cleaving, so we withdrew it and
continued lasing, achieving 2.25 kW at 5.7-5.8 um.  The new version of
the diagnostic beam dump worked as advertised, with much faster response
than the prototype, but it was still a bit slow to optimize the laser
with.  Today we connected the 4 RTDs that are embedded in the wall of
the dump, these should give us the response (and position information)
we want.  We discovered the pickoff window had cleaved.  Our suspicion
is that we had missteered to the point that we hit one of the stainless
steel mounts, and that heated the optic unequally, causing sufficient
stress that it made the optic cleave.  We temporarily removed the PSD
(position sensitive detectors) in the OCMMS and replaced them with
cameras, so we can follow the evolution (and position) of the return
reflection over a larger range. 

 

We cleaned the cavity optics in the drive laser.  It still has lower
output than desired, so more work will need to be done.

 

**Terahertz Project:**

Shipments of beamline material continued to arrive.  This week both
Jefferson Lab and Advanced Energy Systems participated in a THz
applications symposium in Washington, DC, sponsored by the Southeastern
Universities Research Association (SURA), the company that manages
Jefferson Lab.  The meeting provided an opportunity to meet many of the
people involved with THz remote sensing, and to make them aware of our
facility and our source development work.  Several collaborations were
discussed including with DARPA.