# MEMORANDUM 

**To:** Distribution

**From:** George R. Neil

**Subject:** FEL Upgrade Project Weekly Brief -- November 26-30, 2007

**Date:** November 30, 2007

**Highlights:**

We've moved very close to being able to high voltage process the gun. We
now have shielding blocks in place and will have some temporary
interlocks to use for personnel safety while the real PSS system is
completed. We expect to be ready to put HV on the gun around next
Wednesday.

**Management:**

Significant planning effort at the lab is underway to handle
eventualities for a Continuing Resolution of funding for this year.
While the FEL funding is not directly affected there are significant
operational and manpower changes that will impact the FEL program. We
have been supporting those plans.

We have also been finishing our move into new offices.

**Beam Physics:**

We have concocted an arc design that imposes no parasitic compressions
and seems to have not unreasonable (in stark contrast with
\"reasonable\") behavior. Simulation with a mindlessly simple CSR model
(that in DIMAD, as documented in JLAB-TN-98-012, available at
<http://www.jlab.org/~douglas/FEL/technote/JLABTN98012.pdf>) suggests
that appropriate tweaks of the longitudinal match allow compression of a
1.35 nC bunch without horrific results. Examination of this solution -
established at 11:33 pm last night - will continue and results reported
later.

**Injector:**

GTS

Early in the week the corona shield was assembled and attached to the
gun corona ring. The new design with the stiffening ring and guiding
pins works marvelously. The running resistor assembly was mounted and
attached to the HVPS after having to slide the high voltage stack by 1/4
inch to align it with the drive probe. We also mounted the conditioning
resistor and lines up correctly with the gun and the running resistor
assembly. While attempting to attach the SF6 aluminum can to both the
HVPS and the gun, we discovered that it would not fit. We found a
solution by modifying the bellows and Survey and Alignment crew
responded promptly to re-align the gun and the tank. The bellows is now
being modified at the JLab machine shop and we expect to fully assemble
the SF6 tanks by Monday. The gun test stand beam line vacuum bake was a
success as reported by the residual gas analyzer. Unfortunately, while
re-aligning the gun, a leak opened in the beam line but it will not
interfere with high voltage operations. A Temporary Operational Safety
Procedure has been written and is being reviewed by the FEL team. This
procedure needs to be implemented in order to start gun high voltage
operations next week before the Gun Test Stand Personnel Safety System
is fully operational.

GTS HVPS -- The Running/Conditioning Resistor mechanism and Probe were
installed in the GTS this week.  The positions of the Driver &
Measurement stacks were adjusted to accommodate the transmission line
dimensions.  The Control Box to drive the mechanism was connected and is
fully operational.  The FEL bellows was moved and installed in the GTS
to accommodate the distance between the HVPS tank and the Gun tank.  An
additional set of mounting holes is being drilled in the flange of the
old FEL bellows in order to mate with the GTS Gun tank flange.  Gasket
material was provided and Gaskets were made for the FEL bellows.

**RF:**

RF -- Except for the Buncher, all RF systems were operated for 1 hour in
High Voltage (HV) this week.  By operating the RF klystrons in HV, the
vacuum of the klystrons is preserved.  Since the Buncher cavity is
prepared for a beam line bake cycle, the cavity does not have LCW going
to it.  This LCW flow sensor is summed with the LCW flow to the klystron
and thus this interlock prevents operation of the Buncher klystron.  If
the beam bake cycle is delayed, this interlock will temporarily be
masked for next month's HV vacuum conditioning.

 Work continues on commissioning the LLRF to advance drive laser systems
as well as understanding the M55/56 system which modulates the injector
phase with respect to the remainder of the machine and how it interacts
with the new drive laser.

One of the things that we determined when we were making these
measurements is that the drive laser phase noise is a factor of 5 worse
in the drive laser enclosure than it was measured to be in Lab 6, which
is where it was initially commissioned. The noise is broadband centered
around 100 Hz. The RMS value of 1.2d or approximately 3d peak to peak
was too large for proper machine operation. A hypothesis was put forward
that the issue was some type of microphonic vibrations. As an initial
experiment the fans in the drive laser room were turned off and the
measurement was repeated. This reduced the RMS noise by a factor of 3.
There are a number of simple modifications that can be done to better
isolate the drive laser system that should reduce the phase noise to an
acceptable level.

![](media/image1.png){width="5.626388888888889in"
height="3.564583333333333in"}

Experiments were also done using a step recovery diode rather than the
drive laser to ensure that the measurement techniques were adequate and
that the phase noise for the low level RF (LLRF) system coupled with the
new divide by 20 chassis were acceptable. The phase noise was measured
to be less than 0.1degree peak to peak.

We started work in tuning the LLRF control loops as well as
understanding the Advance Drive Laser Control loops so that we can
reduce the coupling between the loop within a loop system while
maintaining the appropriate level of phase control and stability. We are
seeking advice from on-site experts outside of the FEL Division in order
to better understand the systems and complete the work in a timely
manner. This work should continue for the next several weeks.

**Instrumentation and Controls:**

There has been good progress with the GTS HVPS Controller this week. 
We\'ve managed to get all of the cables terminated, labeled, and dressed
into the control rack properly.  The Glassman Remote I/O Unit was
relocated to the upstairs control racks with the associated cables.  The
location for the controller has also been designated and the EPICS I/O
routed for that crate.  The first cut at the controller software has
been developed and will be tested with the chassis once it is
installed.  To adequately interlock the HVPS with the vacuum system, the
analog card is being set for the OK window of the gun vacuum.  This
modification and some testing with the digital portion of the controller
are still ongoing.  Once this is completed the chassis will be installed
and the complete system testing will commence.

The Vacuum cables for the Ion Pumps in the GTS were terminated upstairs
and routed to there locations in the vault.  The one pump that will
instrument the gun is terminated into an interface box that will allow
the bake cables to connect to the pump upstairs.  The I/O cable from the
Varian Power Supply has been installed for communication with the HVPS
controller.  This will be terminated today and checked out.

The electrical work in the Drive Laser hutch has been completed with the
exception of the HEPA filter.  Several lights and outlets were installed
in the hutch with switches outside of the enclosure.  The feed for the
HEPA filter is ready and it will only require connections once it is
installed.  Several feedthrough holes with approved elbows were
installed in the hutch for control cables and other miscellaneous items
that need to be routed inside the hutch.  Other electrical work included
the wiring of the PLD apparatus in User Lab 5.   

The breadboard above the gun for the GTS beam transport will be
comprised of at least a 10\"x10\" board allowing change of angle to
align with the board inside the hutch.  We designed the drive laser
enclosure to fit inside the current GTS hutch, and are awaiting a quote
from the manufacturer.  We accurately measured and designed the lens
disk and limit switch bracket for an attenuator. The drawings were sent
to the machine shop for fabricating three more sets. The small aluminum
beam transport enclosures have arrived from machine shop and were
assembled.  We fixed the GTS drive laser hutch to the table
using modified brackets.

We made good progress evaluating and \"cleaning\" the M55 measurements
system.\
Particularly we have tested the comparability of the M55 system phase
modulator with the new drive laser. Thus, we have modulated the phase of
the RF reference used be drive laser to sync to the accelerator RF and
made sure that the drive laser keeps the mod lock and does not show any
significant change in its phase noise. We have tested the response of
the drive laser to the modulation frequency and the linearity of the
response to the modulation amplitude at one fixed frequency. We still
have to do some data analysis, however it is clear that the new drive
laser will work with the M55 system.

**Beam Transport:**

Improvements and Upgrades

 *Gun Test Stand (GTS*)

•  The SF6 Tank system was installed in on the GTS Gun.  We ran into a
problem with the length of the bellows.  We substituted the existing
bellows that adapts the tank to the High Voltage Power Supply Tank.  At
week's end we are adapting the similarly rated bellows from the FEL gun
to the GTS use.

* *

*SF Sextupole Magnet Testing            *

•  Magnet Test advanced to the 6th magnet on the test stand.

\*  The process of getting at least the First Arc's Sextupoles into
place has started.  We are running into a glitch with availability of 2
more Power Supply Control Boards.  We will work that next week. 

**High Power Optics Risk Reduction:**

Analysis of the thermographic data as described in the literature gave
an unanticipated result, so I am working back through the theory to
figure out what is incorrect.   A teleconference with the co-PI's of the
MRI grant on improved FEL optics generated interest in analysis of
optics in our possession, and I am collecting them for shipment later
today or Monday.  Several of the FEL TAWG members contacted me with
questions about the FEL beam quality, and I answered those -- this is
leading into a presentation next Monday while our visitors from ONR are
here.

**Optics:**

For the last week and a half we have worked primarily on prepared the
high reflector end of the FEL cavity.  In order to fix the leak in the
deformable mirror assembly (DMA) the optics were debonded to eliminate
possible damage. The DMA's were made leak tight by using a vacuum
compatible solder alloy around the cooling tubes. The optics were bonded
to the leak checked DMAs, checked for figure and installed in the
cavity. Currently the high reflector vacuum vessel is pumping down and
we should be able to turn the ion pumps back on by the end of the day.
The output coupler optic (OC) have been brazed to their frames and
checked for deformation. All have passed and we will start the OC
installation next week. We are completing the air line supplies to the
harmonic block filter and beam stops in the user labs. The last low
power cassette for lab 3 is now reassembled, the wiring for the stepper
motor and the limit switch are being worked on. We have started to work
on the Optical Beam Position Monitor (OBPM) system again. We will
replace the stainless steel "flying" wire with one made from tungsten to
reduce the wire diameter and, therefore, the amount of scattered light.
The OBPM CPU board was reworked to reduce some noise issues. The timing
routines for the Optical Cavity Mirror Metrology System (OCMMS) CPU
board was also worked on.

**Lasers and Optical Diagnostics:**\
Our work this week for GTS has primarily focused to the optical
diagnostics and transport since the laser system is ready for
installation. There has been lot of interaction with other lab staff in
the crowded GTS cave as we are making progress. Most of the electrical
work for the laser hutch is already done and we are waiting for the PSS
to be installed and certified. We also ran the Advanced Drive Laser for
RF group to finish a scheduled phase modulation measurement. This
yielded useful data and provided important information about using drive
laser to tune accelerator system for the coming machine operation. We
saw low frequency phase noise contribution from the environment which
was not seen when the laser was fully characterized in a different room.
This needs further observation. The seed laser for the PLD project that
did not lase during certification in its present lab. This laser has not
been run ever since its relocation several months ago. An alignment is
necessary and will be done when the schedule permits.

**Terahertz:**

Much of this week was spent monitoring the warm-up of the 16 T
superconducting magnet system. Following a complete LN~2~ fill and 2/3
of a LHe fill on November 19^th^, the magnet was still at 4.4 K when
this report was submitted. We expect the remaining LHe to boil off this
weekend. The hold time for the cryogens in the system appears to be very
good when the magnet is idle, although we will need more time running
the system to see what our boil off rate is when ramping the field.

Also this week, we have begun preparing a report of the magneto-optical
work done at Brookhaven. There is a large amount of data to be sorted
and analyzed. While there were some problems with reproducibility in the
system between the two runs, we are attempting to extract as much as
possible from the various data runs to glean what we can from this work.

Finally, Mike Klopf has been invited to present a talk on coherent and
incoherent synchrotron in the THz and sub-THz region of the spectrum and
how it may relate to similar spectral features observed in solar flares.
The talk will be presented at a workshop to plan the mission for a solar
observatory satellite.

**Word version:** [FEL Upgrade Project Weekly Brief -- November 26-30,
2007](http://www.jlab.org/accel/fel/documentation/07weeklybriefs/upgradeweek07nov26-30.doc)