# MEMORANDUM 

**To:** Distribution

**From:** Steve Benson for George R. Neil

**Subject:** FEL Upgrade Project Weekly Brief -- September 3-7, 2007

**Date:** September 7, 2007

**Highlights:**

The GTS gun vacuum was completed, and the configuration of a diagnostic
test beamline was established.  Components for this are now in the test
cave.\
\
The advanced drive laser was moved into position and prepared for
operations next week so that it could be inspected by the lab laser
safety officer.

**Management:**

George Neil attended the IRMMW-THz Conference in Cardiff this week.
George is a member

of the International Organizing Committee.\
\
The BIGLight report for Florida State University was delivered as
scheduled.

**Beam Physics**

CSR simulations of BigLight continue. Tennant has resolved various
questions about how elegant models RF cavities (other than
\"elegantly\") and simulated MIR operation. Agreement with DIMAD is
good, though elegant\'s significantly more sophisticated model captures
details absent in DIMAD and predicts somewhat (though not excessively)
larger impact from CSR. The evaluation is ongoing, with NIR operation
next in the queue.

We have encountered an amusing and here-to-fore neglected longitudinal
aberration in the course of these studies. Turns out that magnetic
compression with curvature correction will unavoidably distort the
longitudinal phase space in a manner not dissimilar to the distortion
induced on a torso by a corset (Figure 1). We\'re pondering the
implications, and are particularly interested in the impact of the
inhomogeneity on processes like CSR.

![](media/image1.png){width="2.042361111111111in"
height="2.5527777777777776in"}

![](media/image2.png){width="3.6881944444444446in"
height="2.4069444444444446in"}

Figure 1a: A corset Figure 1b: A corseted longitudinal phase space

**Injector:**

We made a lot of progress this week in the final stages of the GTS gun
assembly. On Tuesday morning we fixed the gun cart allowing us to attach
the gun ceramic stack to the vacuum chamber. The previous week we
discovered that the cart hit the gun stand frame leaving a 7/8-inch gap
between flanges. Later in the week, the RGA, the extractor gauge and the
NEG pumps were installed. On Thursday, the gate valve was bolted down
after sliding the Solenoid into position. The valve is just downstream
from the solenoid and traps it, so the solenoid had to be installed
first. The NF3 tank was cleaned, welded, mounted to the stand and
connected to the vacuum chamber leak valve. On Thursday evening we
mounted a brand new wafer, but it broke after being pressed down with
the tantalum cup. The wafer was a few microns wider than its base on the
stalk. Early this morning we mounted a wafer with the proper diameter,
but unfortunately it broke when it was inserted into its final position.
We know why and we will fix our procedure. We also discovered that the
layout for the gun stand floor anchors on the floor was based on a
drawing showing the FEL upgrade vacuum chamber, which is shorter than
the GTS one. Since the entire gun assembly will have to be shifted along
its axis by about 6 inches because of this, we will wait until the gun
is in its final position to replace the wafer. The concept of the
lightbox integrated into the gun chamber works marvelously.

One can see the wafer perfectly and effortlessly through the various
viewports, The oven panels are being modified for the electrical heaters
to be used from now on to bake the gun assembly.

**Instrumentation and Controls:**

We spent time this week checking out systems and gathering parts for the
GTS.  The beam line hardware has all been gathered in the GTS vault. The
two beam viewers required for the beam line have been assembled and are
ready for cleaning and then installation.  One of these is fitted with a
Cromox flag and the other has a phosphor coated flag.  This hardware
will be installed on girders as soon as they are ready.  Work also
continues on the GTS SF~6~ gas transfer system.  The SF~6~ system was
leak checked successfully after repairing one leaking joint.  The
compressor\'s inlet and outlet ports have also been attached to the
system with hose.  The control box is currently being designed and
wired.  The front panel is nearing completion.  Once this is ready for
installation, the interconnects between the PLC and the hardware can be
established.  The cables coming into the control box were all labeled,
stripped, and cut to the proper length.  We\'ve also modified one GTS
instrument cabinet to prepare for safe routing of instrument connections
between the ceiling and floor level of GTS area.  As a group we also
provided support for transport and mounting of wafer in the GTS
Gun.      

    Progress has been made on the Analog I/O Distribution Module.  The
routing and layout was completed and the board has been reviewed by the
board house.  Currently we are working through several details before
the order is processed.  Some more details were clarified for the crate
wiring.  The cabling harness for the HVPS crate is making progress.  The
power supplies have all been mounted and the breakout connections are
beginning to be assembled.  Once the Analog card goes out we will push
forward with the revisions to the 3U General Purpose Processor Card.

    To test the operator shutter that will be used with the GTS Drive
Laser we setup a simulated control scheme to understand what signals are
required to operate this shutter properly.  After some simple test it
was found that the shutter is extremely easy to control and will perform
exactly as expected.  To get expert control of the device, remotely we
may have to develop a more complicated interface but from the
requirements so far there is no need or desire to have this
functionality.  Everything for the shutter is setup locally on the
control unit and then locked in.  Also we\'ve been working with the EES
group from CEBAF to get a temporary VME crate for testing since our new
crate is delayed for another month.  We would like to get this installed
as soon as possible to begin testing applications and shake out of
possible problems.

    Progress has been made with the LPSS system.  The controller link
communication has been established between Labs 3A and 4.  The other
labs are not currently communicating to each other.  This will be
established when it is an opportune time to recertify each of these
labs.  The memory mapping is continuing to take place within the master
as we are coming closer to developing the prototype user interface
screens.  These screens will be checked over and reviewed for
functionality and user friendliness.

    The PHP code written to input the FLOG on the wiki was tested and
debugged.  We are in the process of replacing the vacuum pump and valve
cables in lab 4 for the beam transport system.  Feet for the CO~2~ Lab
3a laser were designed and went out for quotes.  Further quotes are
still being sought after for the fabrication to get a reasonable cost.

**Electron Beam Transport:**

***Improvements and Upgrades***

*Gun Test Stand (GTS*)

• In building the SF~6~ Tank system, the JLab Shop continued welding the
shells and started welding the flanges of the main vessel.  A
consultant, hired by the EHS& Q Division, in addition to other duties,
performed to role of "Inspector" in observing our calculations and
documentation procedures.  He will audit the shop's operations next
week.    

• The Gun Solenoid was mounted on the gun. 

• A leak in the SF~6~ Gas Transfer System piping was repaired and work
continued on adding the relief valve, adding the connections to the High
Voltage Power Supply Tank and the outside bladder.  Connections to the
vacuum pump and the vane pump were started.  Wiring the controls to the
solenoid operated valves started.

• The elements of the beam line and dump for the GTS were defined.  A
number of vacuum parts were put into the Production Chem. Room for
cleaning and the shop completed an existing stand needing scallops in a
top plate.  The Cryogenics Group helped out with supplying some short
pieces of 10 inch diameter tubes which will act as the base extenders to
bring our standard 27 inch height stands to the 43 inch beam height of
the GTS.

*SF Sextupole Magnet Testing*

• Magnet Test started up the testing of the first sextupole with the new
power supply and controls on the Stepper Stand no that the solenoid test
is complete.

**Optics**

Work continues on the optical transport. We have had to vent a short
section in order to fix a small leak. The pressure in the transport is
slowly falling from the current low 1e-6 torr level. An RGA scan
indicates that the new components we installed are still outgassing
water. We will reevaluate the situation next week. We have started to
work on the Turning Mirror Cassette (TMC). TMC #1 is built up with the
excepting of the cooling tube harness. We have had some difficulty in
having the harness accommodate the full motion of the door slide. We are
looking at anchoring the end of the harness to the slide door instead of
the side of the vessel. We are awaiting the 3D drawings from the
Mechanical Group.

We have started testing the macor version of the 2^nd^ generation
deformable mirror assembly. Initial tests shows that we can change the
sagitta enough to warp a 16 meter radius of curvature optic to nearly 18
meters. Unfortunately, while dialing out the astigmatism, we broke the
macor back plate. A stainless steel version is in the machine shop for
fabrication.

**Lasers and Optical Diagnostics:**

This week we moved the old Antares laser out of the clean room
and replaced it with the Advanced Drive Laser (ADL). The Antares laser
has been serving FEL operation for over a decade and was the drive laser
for many FEL power records made here at the JLab FEL facility. The
installation of ADL will continue for the next a couple of weeks. We
will test the new control systems after the installation. In the mean
time, we will use Antares laser as a seed laser (with amplification to
higher energy) for the Gun Test Stand in the coming months.

**High Power Optics Risk Reduction**

In the area of optical modeling, the last two weeks have been spent on
incorporating the ability to do cold cavity simulations into the
user-friendly front end.  Recall that what is done in these simulations
is to input all the optical element\'s dimensions, reflectivities, and
positions, launch an initial Gaussian beam, and then on each pass, reset
the power to the same value, but let the mode evolve to a steady state. 
Heretofore, this modeling, which is working quite well, was done by
creating an input script, not the most user-friendly way to do this
work.  This added functionality, which we call looping, had some hiccups
due to a corrupted file.  Since refreshing the source code, it has run
without issues since yesterday.  We\'ll continue testing before
declaring the issue resolved.  We received the input electron beam and
wiggler parameters for the IR Upgrade FEL with the optical klystron.  We
will do a careful comparison, initially with Genesis, of the theoretical
behavior of the scraper-outcoupled FEL with the data taken and published
several years ago.

I drew up a proposed plan for the optics to be installed for when we
start back up.  The choice is based on funded programs, which dovetails
nicely with our desire to advance the state-of-the-art in both FEL and
optical physics.  It has been discussed informally, and a proposal to
the staff goes out this afternoon. 

A paper on the FEL facility, given as an invited talk for me by Steve
Benson at the FEL 2007 conference, is going through the publication
process.  Another is in preparation for presentation and publication at
the proceedings of the Boulder Damage Symposium.  Costing for the
optical subsystems for the FSU \"BigLight\" FEL was revisited and
updated.

**Terahertz:**\
Much of this week was spent assembling, converting, and analyzing the
data taken at BNL on both the magneto-optical and the spectroscopy
studies. A great deal was learned about the operation and performance of
Jiufeng Tu's (CCNY) magnet installed at U4IR. Numerous tests were run to
characterize the repeatability of the entire system throughout a data
run and to determine the limitations of the current configuration. One
concern is the spectral transmission of the quartz windows of the
magnet. These present a high frequency limitation with an absorption
resonance at about 130 cm^-1^ and then a complete cut off above 170
cm^-1^. Between 130 cm^-1^ and 170 cm^-1^, there is usable light, but
the signal is quite small, and the error in that region is roughly
5-10%. We may have to consider another window material for the magnet at
BNL, but this must be discussed with Jiufeng, Larry Carr (BNL), and our
colleagues at LANL. Transmission data for a doped InSb sample are shown
in the figure below. The false color depicts transmission. The cyclotron
resonance absorption feature can be seen clearly tracking to higher
frequencies as the magnetic field is ramped.

The spectroscopy measurements made at U12IR were also quite successful.
We have now measured the THz transmission of every laser goggle used at
the FEL. A report of these findings is being assembled and will be
presented soon to Steve Benson and Patty Hunt. These measurements will
determine which laser safety goggles also provide sufficient attenuation
of the THz beam in anticipation of FEL and other laser sources being
present in the THz lab.

Another important spectroscopy study was the transmission measurement of
several different optical components that make up the THz imaging
system. One critical element is the window which seals the vacuum of the
microbolometer array. The window material of the A­10 is Ge, while the
newer Photon core that we intend to purchase for future tests uses a Si
window. The Photon window is expected to provide better transmission
over the A-10, but this had not been measured. The transmission
measurements indeed show the Photon window providing superior
transmission in the THz region of the spectrum as seen in the figure
below.

![](media/image3.png){width="3.254861111111111in" height="2.90625in"}

Other optical elements, such as different THz lenses, band pass filters,
and polarizers were also measured during the spectroscopy study at BNL
and the data is being analyzed at this time. These data will be shared
and discussed with the component manufacturers as part of our ongoing
effort to assist in the development of THz imaging technology.

![](media/image4.wmf){width="4.240277777777778in"
height="3.178472222222222in"}

Transmission data for the windows sealing the Photon and A-10
microbolometer arrays. The Photon window (Si) provides significantly
better transmission than the A-10 window (Ge).

**Word version:** [FEL Upgrade Project Weekly Brief -- September 3-7,
2007](http://www.jlab.org/accel/fel/documentation/07weeklybriefs/upgradeweek07sep3-7.doc)