MEMORANDUM

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- March 29-April 2, 2004

**Date:** April 2, 2004

**Highlights:**

During this week\'s efforts for commissioning of the FEL Upgrade, we
summarize the following accomplishments:

\(1\) The rf commissioning of the of the new 3rd cryomodule was
completed this week on schedule. The results are excellent. The module
delivered a gradient of 82 MV with operation on its standard (Zone 4) rf
power and control system. The system has had two extended periods (6-12
hrs) of operation without rf trips. Based on last week\'s measurements
on individual cavities, the maximum gradient for this module is in the
range of 86MV. This is the highest gradient module ever produced by
JLab.

\(2\) We received new low loss optics for the high power tests at 6
microns. Both optics meet our critical ROC (16m) and surface roughness
specs. One of the optics is being installed in the outcoupler tomorrow.
We are also installing a 10 micron scraper outcoupler.

\(3\) Modifications of the photogun high voltage power supply were
completed and tested with pulsed beam operation up to 7 mA that showed
we had eliminated previous noise and ringing problems with this supply.
It successfully demonstrated more that 10 mA supply current.

\(4\) We did a further test of the injector performance and comparison
to theory and believe we now have successfully correlated that
performance against bunch length at injection. See the commissioning
results below for details.

**Commissioning Summary (**Steve Benson and Dave Douglas**):**

This week was again a short week from a commissioning standpoint. We had
three main goals. The first was to test out the machine modifications
made during the Friday and Monday downs. The second was to see if the
efficiency at 10 microns was still over 1 kW/mA. The third was to test
out our hypothesis concerning space charge as the cause of the energy
asymmetry in the linac.

Checkout of the maintenance changes went well. Cavity 5 in zone 4
appeared to be quite stable with the replacement of its RF control
module. Small changes in the GU shunts were needed due to a change in
the hysteresis procedure. The biggest success was the gun high voltage
power supply (HVPS). We were able to run high pulsed current with two
stacks active with acceptable voltage noise. This allowed us to run the
highest pulsed current to date---over 7 mA. This was done at 37.425 Mhz
so the charge was close to 190 pC. The pressure in the gun region did
not increase very much so the injector setup is good. We saw some BLM
activity at one place known to have large beta function. At 5 mA pulsed
the voltage droop was around 1.3 kV, which is acceptable for pulsed
lasing. Previously we were unable to run over 1 mA pulsed with two
stacks of the HVPS working. The noise while running CW was also reduced
and we were able to run over 5 mA with no difficulty at all.

Operation at 10 microns did not go well. A shorted wire in the motor
that controls the pitch of the 10 micron output coupler kept us from
aligning the cavity with 10 micron mirrors. We therefore switched back
to 6 microns to test out the new injector setup described below. The
hypothesis that space charge leads to an asymmetry in the energy spread
vs. phase in the linac was tested by operating with an injector setup
that has a much longer bunch at the linac entrance. This is accomplished
by running the second injector cavity 10 degrees off crest instead of
the usual 20 degrees and lowering the buncher gradient until a minimum
energy is found in the injection chicane. As predicted, this produced a
bunch with a full length of 6 degrees (11 psec). The match to the linac
was found empirically but matched the PARMELA solution quite well. The
asymmetry was reduced from over 50% to under 20% when this was done,
also as predicted by PARMELA. The pulse length at the wiggler was
shortened to approximately 0.5 psec FWHM. We discovered while measuring
the pulse length that steering the electron beam at the Happek OTR makes
the Happek scans more symmetric. Unfortunately the energy spread was
larger than we would like. The rms energy spread was approximately 0.6%
and we would like it to be less than 0.5%. This is best dealt with by
operating closer to crest. We will check this out next week. Lasing with
this configuration was about as strong as the previous configuration but
the peak efficiency in the detuning curve was over 1 kW/mA. This is the
first time we have obtained such high efficiency at 6 microns. The
energy spread for this efficiency is still much higher than we would
expect. When we get the new output coupler installed we can see if this
persists for a resonator with lower Q. One difficulty with the new
injector setup is that the "ghost" pulses (those that are caused by
light leaking through the electro-optic cells) are quite long in the
linac.

In general we are now getting excellent agreement between PARMELA
predictions and machine behavior. The measured performance matches the
model in detail. Emittance growth from the linac entrance to the wiggler
is the present focus of our attention.

***Additional comments from Dave Douglas:***

The balanced injector configuration continues to supply excellent
injected transverse emittance: the observed value during the linac
optics work was around 10 mm-mrad in both planes.\
Last week\'s discussion of space charge evolved rapidly, and it became
apparent that Carlos\' PARMELA modeling, estimates, and machine behavior
were all quite consistent. The estimates suggested that space charge
effects should roll off rapidly with longer injected bunch length; this
was observed in further PARMELA simulations, which also gave immediate
guidance on how to achieve the longer injected bunch: simply operate
unit cavity 3 closer to crest. Steve revised the injector setup in this
fashion, and after a bit of optimization determined a) the
phase-asymmetry in final momentum spread was reduced, b) the trim
settings minimizing the bunch length were close in value to those giving
zero M55, and c) the bunch length was reduced to as low as 520 fsec. The
FEL lased like gangbusters at 6 microns with the shorter bunch,
generating a momentum spread so large that it was not immediately
measurable. The recirculator transported the beam back for energy
recovery without excessive loss; however, there were however losses at
the dump that we did not have time to alleviate before the shutdown for
hardware work.  Given the very large energy spread, it is entirely
possible we are back to the \"momentum spread exceeding longitudinal
aperture\" situation we had when running at 10 degrees off crest (this
setup was at 15 degrees). The losses may therefore possibly be managed
simply by raising the exhaust energy slightly and recovering the beam a
bit farther out of trough.

The loss may also be related to a separate issue associated with this
setup - that of loss of ghost pulses. With the altered injector, the
ghost pulse beam from the drive laser is apparently being longitudinally
manhandled and injected into the linac with very long bunch length. This
is apparent in PARMELA simulations, and manifests itself on the machine
as a ghost-pulse beam with extremely large (several percent) relative
momentum spread at full energy. The present arc 1 hardware configuration
is not intended to manage beams at momentum spreads over a couple
percent or so; for example, there is only a single sextupole family
presently installed so that nonlinear dispersion is not compensated. The
PARMELA simulations suggest that the ghost beam momentum spread will be
of order 6 1/2%, and the spread observed on the machine may approach
10%.\
Simulation and observation of nonlinear dispersion in the transport of
the wiggler show 1% momentum offset will yield an orbit with peaks of 1
mm at several backleg locations, and 2 mm just upstream of the wiggler
(see Figure below). As the offset magnitude is quadratic, at 3.25%
offset (half the spread predicted by PARMELA), the orbit will be
displaced a factor of 10 times farther - or \~1 cm at several points and
2 cm before the wiggler - so if PARMELA is right we have a beam that is
likely around 1 inch wide just upstream of the wiggler. If our
perception of the machine data is correct, the half-width is 5%, so the
beam is 25 times bigger than the 1% offset - or 25 mm (1 inch!) at
several locations in the backleg and 50 mm just upstream of the wiggler.
This should be a problem - and is. We were loss limited by trips not
only at the dump (which are likely manageable as noted above) but also
by losses at the wiggler. Furthermore, surveys following beam operations
indicated that the location of one of the early peaks in the backleg
(3F04, see Figure) was activated. We have yet to analyze where any beam
that makes it past these lost points ends up *after* the wiggler.\
It therefore appears that some combination in reduction of ghost pulse
intensity, management of the injected ghost pulse distribution, and
correction of arc 1 nonlinear dispersion is in order. These steps are
under discussion and analysis.\
![](media/image1.jpeg){width="6.497222222222222in"
height="1.4229166666666666in"}

**Management:**

We prepared an updated status report on our Feb. 6th 10 kW plan that we
will be forwarded to the Navy program office and the FEL steering
committee next week.

On Thursday and Friday of this week, we participated in DOE\'s annual
on-site review of JLab\'s operations and long term institutional plans
for DOE\'s Office of Science.

**WBS 4 (Injector):**

The DC Photocathode Gun continued to deliver 135pC beam for FEL ops
during the week

(including brief operation at 190pC).

PARMELA simulations of the injector and propagation through the first
accelerator cryomodule continued this week. A new setup for the injector
was tested in the simulations to produce a longer bunch at injection.
Later on that setup was successfully implemented on the machine.

Gun HVPS - The modifications to the Gun\'s HVPS proved to be
satisfactory this week for high current pulsed operations. This was not
possible with the original HVPS as the current controlled driver would
cause a large voltage spike(10 - 20 kV) on the Gun. The new
configuration experiences a small droop during the current pulse with
less than 1 kV recovery overshoot. The noise voltage at 350 kV is
approximately 70 v p-p.

**WBS 5 (SRF)**

The rf commissioning of the of the new 3rd cryomodule was completed this
week on schedule. The results are excellent. The module delivered a
gradient of 82 MV with operation on its standard (Zone 4) rf power and
control system. It has had two extended periods (6-12 hrs) of operation
without rf trips. Based on last week\'s measurements on individual
cavities, the maximum gradient for this module is in the range of 86MV.
This is the highest gradient module ever produced by JLab.

**WBS 6 (RF)**

RF - All RF systems are operational. The klystron powering cavity 4-7
appears to be receiving more reflected power than is expected. The
calibration will be checked and if still a problem, the circulator will
be replaced. The commissioning of FL03 is nearly complete. The HOM
filters for the RF Control Modules are being adjusted. Measurements are
continuing on how best to guard against HOM\'s in the cavities. Stub
tuners on the HOM ports are being considered.

**WBS 8 (Instrumentation):**

The beam viewer ITV1F02 was slowed down to try to limit the induced
microphonics in the adjacent cryomodule. This viewer was tripping off
cavities when inserted and retracted. It did not posses a particularly
isochronal motion, but some of the cavities were none the less
splenetic.\
   Parts continue to arrive for the cryogenic mirror experiment. We
moved \"Big Bertha II\" into the vault for additional power. Bertha is a
480 to 208 volt transformer  and service panel which plugs into a
welding outlet to provide up to 200Amps of additional 120VAC power. This
will some the problem of lack of AC power near output coupler
precipitated from the lack of an optical transport line.\
   More RTD\'s were connected and configured for the Optical Dump that
was installed at the downstream OCCMS.  These will provide temperature
read backs for both the water inlet and outlet.  Additional 4 Wire
RTD\'s were connected and configured to provide temperature read backs
for the Scraper Dump water inlet and outlet.  All channels have been
tested and are displayed in EPICS.  Modifications to the Optical
Cavities Flow Chassis to allow for flow status of both the HR and OC
water flow are in progress.\
   The I&C Pre Amp Module (F0295) PCBs have been ordered and are
expected in early next week. The Analog I/O Board (F0190) REV_B PCBs for
the HVPS Control Chassis arrived from the vendor two will be assembled
and tested, one for operations and one for a spare. The new DSP board
that we contracted, the design and fabrication is complete and testing
has begun. The updated backplanes for the diode thermometry system has
been sent out for a 2 week turn. This incorporates the aforementioned
DSP as a controller.\
   Revisions of chassis panel drawings were received from EECAD.  They
will be added to the current Auto Cad library that is being constructed
of objects that are frequently used. Progress continued on the SF6 pump
system. Fabrication continued of the 32 channel Video Switcher chassis
after receiving the front and back panels from the vendor.    FEL
Allsave Data graphing - All of the GSETs and PSETs as well as many of
the magnets settings from the archived \"Allsave\" data for the machine
setups has been extracted and imported into a database. Once the data
was acquired, a page was written that shows a scatter-plot graph of the
channel values and the dates they were made. The page is accessible at:\
<http://laser.jlab.org/devlore/FELAllSaveData.asp>\
   Configuration Control Database - Searching, and editing tools are
being refined, a significant\
update will be posted over the weekend.

**WBS 9 (Beam Transport):**

***Sextupole Lite Re-coiling to obtain higher field***

• The order for the 48 new add-on coils was placed with New England
Technicoil for an incentivized delivery in 4 weeks.

***Wiggler***

###### Electromagnetic Wiggler for 2.8µ

• As part of the prototyping order, Process Equipment Company (PECo) is
doing a test on the cooling plate production method where they will
reverse the fabrication order. They like cutting the plates out entirely
by water jet and then brazing the tubes on. This reversal eliminates the
extra care the water jet vendor takes when a cooling tube is in place.
We avoided this earlier because potential distortion mismatch with the
pole tips. The developed tacking and brazing method may bypass any
distortion.

• PECo received cooling plate material and is anticipating receipt of
conductor material for full water jet production of all components next
week.

• PECo continues machining the core pieces.

• Ancillary fabricated parts are on order from local shops.

• In the design area, the assembly drawing is now taking shape and the
fastener parts list is good enough to order all fasteners.

• The shop is cleaning all the parts for new beam chamber for the 2.8
Micron Wigglers before they weld it.

***Sextupole (SF)***

• Procurement on hold pending 10 kW efforts

**WBS 10 (Wiggler)**:

The order for the 5.5 cm fixed-gap wiggler support structure was placed
Wednesday, March 31. Delivery is 5 weeks as per the schedule. Magnet and
pole clamps are arriving today, also on schedule. Everything else is
also on track and there have been no slips in the schedule. Lorrie
Sanborn calls suppliers frequently and they call her as well if they
note any manufacturing or schedule issues. For manufacturing changes STI
Optronics releases ADCN\'s (Advanced Document Change Notices which are
formal red-lines needing project, engineering, design, manufacturing and
QA approvals).

**WBS 11 (Optics):**

Plan A is to obtain low loss 6 um optics.\
We received the new substrates made with the lowest loss material
available.  They will undergo metrology today then be shipped out for
coating.  We also received the refigured and recoated 92% outcouplers. 
They passed all specifications.  One was readied for mounting and brazed
last night, but it appears our choice of temperature was a bit too low
(160 deg C vs 170 deg C), so the braze parially melted but did not wet
the optic.  We are rebrazing it and will install it this afternoon.  New
HRs are at the vendor.  We should receive them in about 1 week.

Our three Plan B\'s had the following progress:

Scraper outcoupler:  The first article was polished and passed flatness
(\</= 3 waves at HeNe, vs 10 waves spec) and roughness (measured 1 um PV
vs 1 um rms spec).  It was ion etched, then gold was sputtered onto the
surface.  As this report was being written it was having the
water-cooling tubes welded to it.  It will then be installed later this
afternoon.  Ancillary diagnostic hardware was mounted on a small
breadboard, and will be installed after the scraper installation is
complete.  Thanks to the I&C group for working late last Friday to run
another beamviewer cable, and for the new RTD readbacks for the extra
laser beam dumps.

***Cryomirror:***

The holder that will thermally isolate a 3\" mirror in a 4\" holder is
finished and ready for installation.  It should be ready next week.  Tom
Elliot's pump system is flowing liquid nitrogen at 8 psi, he feels the
very long coil of copper tube is causing excessive pressure drop, and is
trying a shorter length to raise the pressure. 

***George Biallas had this report:***

The cryogenic mirror mount design, as it has matured, exposes the entire
mirror edge to a thin niobium interface, backed with flowing cryogenic
fluids. This interface is a sleeve that extends beyond the mirror. This
construction provides radial flexibility that we believe may minimize
any distortion caused by differences in contraction during cool down.

• This assembly is ready for brazing early next week.

• Sam Morgan, the sheet metal fabricator for the SRF Institute was able
to find thicker niobium sheet in our stock pile and he fabricated .010
thick sleeves, on very short notice, to substitute for the .004 sleeves
we had formed last week. The thicker sleeve is judged to be suitably
robust.

• The work on the cryo cooling liquid flow continued to advance. Tom
Elliot's pump system was tested and most tubing, valves and controls are
on order.

2.8 micron mirrors:  In fabrication.  One each mirror holders for 2\"
and 3\" arrived this week and are being cleaned in preparation for
brazing.  more arrive next week.  We received  2\" Si HRs, the OCs will
arrive in about two weeks later.  3\" mirrors are in fab, expected in
mid May.

***Other activities:***

We replaced the drive laser rod for a good spare, and remounted the
second spare.  When we pulled the rod that had been in service for over
5600 hours of use, we saw that it was thinner and showed preferential
erosion along the crystallographic axes.  We needed to do a more
complete alignment than anticipated, but once done we are producing \>
16.5 W at our normal operating current, and in excess of 4 W of SHG, all
with low phase and amplitude noise.

Besides adding the scraper outcoupler (and the modifications it
entailed) and the new mirror, we also replaced all load springs
with less stiff versions.  This makes the motors respond better,
and should eliminate the failures we\'ve seen.  A motor that developed a
short at the casing, preventing us from lasing at 10 um, was replaced. 
The wiring in the HR vessel was changed to the new orientation that
makes it faster to work on the optics.  A cable tray was mounted on the
floor in the OC area to provide a better conduit for the signal cables
to the ever-growing number of diagnostics.

The noncontact profilometer that was purchased as part of our JTO
contract on laser damage studies was installed, and we are already
making use of it to check the ROC of mirrors that are hard for the laser
interferometer to measure, and the microroughness of the surfaces.

**Terahertz Project:**

The M1 and M2 mounts have been modified to accept the phytron actuators,
and are being shipped with the M3 and M4 modified mounts to Jefferson
Lab.  Fabrication of M1 is nearing completion.