# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - October 2-6, 2006

**Date:** October 6, 2006

**Highlights:**

Our goal for this week was to continue testing of the cryomirror as we
tuned up the driver accelerator and the optical controls to push the
power output. Our plans were delayed until late yesterday by a problem
with a protective RF window on one of the FEL cryomodules. The details
are given in our report below (under RF).

As a summary, we are continuing to learn about how best to protect the
primary RF windows from potential small (air-to-vacuum) leaks. We
commend our SRF and RF teams for a very careful job that allowed us to
bring the cryomodule back on-line very quickly on Thursday. We also
subsequently brought the FEL back rapidly and re-commenced the
cryomirror test program. We obtained our first measurement of FEL gain
with this optical cavity set-up: 55%-a very good number that meets our
expectations. Our measured FEL efficiency is still low with this set-up,
approximately 700 Ww/mA; however we seem to be loosing about a factor of
two with a vertical mode oscillation that is not coupled to any mirror
motion. This interesting problem is giving us good reason to check all
the system parameters at the limits of our installed diagnostics.

We are pleased to report that the FEL injector group successfully
installed the new FEL photogun assembly and put the assembly under
vacuum in the new injector/gun test stand area in the back of the FEL
Facility.

All of the hardware for the suite of planned instrumentation for the THz
beamline has been acquired so that spectroscopic measurements in the
FIR-THz range can be made in vacua (and unimpeded by air absorption) in
User Lab 3a.

**Management:**

Taking advantage of the machine down-time earlier in the week due to the
RF window problem, we spent Tuesday in a brainstorming session to
discuss our priorities for the present year and plan our resources
accordingly. We also distributed assignments for our annual report which
we plan to make available well before our scheduled annual review by ONR
on Jan. 17-19, 2007.

We started to prepare material for our next scheduled planning meeting
(Oct. 16^th^) with ONR and PMS-405.

**Operations:**

The week started with a down to fix a few problems found last week.  The
down was supposed to end in the morning but was extended when the CHL
crashed due to a power glitch.  We came up in the afternoon and
discovered a problem with the Rexolite window in zone 3 cavity 8.  This
is a protective window and not the front-line air-vacuum interface
window. The window problem is described in detail in the RF report. 

Recovering from the damage of this window failure took until Thursday
afternoon.  We then turned on and quickly re-established lasing.  We
optimized the lasing using the fast detector in the optical control room
and obtained turn-on times as short as 20 microseconds.  This
corresponds to about 55% gain.  We found an oscillation in the vertical
mode however that we could not explain.  The mirrors were not
vibrating.  The mirror angles were stable to better than 200 nrad.  With
this small an angular change the optical mode should have been quite
stable.  The electron beam also seemed stable and we could not steer the
optical mode by steering the electron beam.  At this point this is still
a mystery that we are exploring.  The efficiency was still low but part
of this is due to the mode oscillation.  Since the laser is only lasing
well about half of the time, the maximum efficiency with this present
set-up could be 2x higher.\
On Friday we did some experiments to find the source of the mode
oscillation.  We eliminated some candidates but did not find a smoking
gun.  We also ran for a while at about 600 W to look at mirror heating.
 We do find that the helium is heating by a few degrees.  It is not
clear at this point where the power is being absorbed.  We do now have
the capability to image the mode on the output coupler at 1.6 microns. 
This proved quite useful in optimizing the system.  We are continuing to
identify the cause of the mode oscillation and any other possible causes
for the low efficiency (we typically see about 700 W/mA right now).

**WBS 4 (Injector):**

This week we accomplished yet another milestone by moving the new gun
assembly (completed last week) to the Gun Test Stand and having it under
vacuum. We remind our readers that the Gun Test Stand will serve to do
beam studies at 500 kV and high charge (1-2 nC) bunches. Many thanks to
Don Bullard (left) and Nimel Theodore (right) for their great effort in
making this happen.\
\
![](media/image1.jpeg){width="4.166666666666667in" height="3.125in"}

On Thursday one of the electrodes coated with the graded field emission
suppression film was removed to discover that the film was damaged
during the last high voltage test when the gap was closed down in an
attempt to condition beyond 32 MV/m. The film was successfully
conditioned up to 27 MV/m with no detectable current or vacuum rise. The
highest gradient in the gun is 12.4 MV/m at 500 kV. A new anode plate
will be polished before resuming the testing with a second coated
electrode. Understanding the coated electrodes behavior will be very
valuable when we condition the new gun assembly.

The Phase Monitoring system for the Injector is installed and ready for
commissioning, except for the photodiode pick-off for the Drive Laser. 
It will be an additional 2-3 weeks before it is ready.  This will give
us time to debug and test the balance of the system.

**WBS 6 (RF):**

RF - Removed all Rexolite windows in zone 3 after the one in the
waveguide for cavity

8 burned out.  The waveguides for all cavities and the HOM filters for
cavity 8 were removed, cleaned, and reinstalled.  The ceramic windows on
the cryomodule were also cleaned.  An IR camera was then positioned to
view and measure the temperature of the ceramic windows for cavities 5,
7, and 8 under RF operations.  The results were consistent with the
original tests for these ceramic windows.  No abnormal heating or hot
spots were observed.  The balance of the RF system was then reassembled
and turned over for operations.

 

**WBS 8 (Instrumentation):**

By taking advantage of the downtime this week we checked the
three problematic BPMs that operations had noted.  One of the BPMs was
found to have some cabling issues within the CAMAC crate that caused it
to work but respond in an unusual way.  With EES\'s help we discovered
this problem and rechecked the system successfully.  The other two BPMs
checked out ok so further operations time with them will provide us with
some more data to check their operation.  Also while we were down we
took the time to survey the vault for magnetic hardware near the
beamline.  Several stainless steel support bars were found to be
magnetic and these were replaced.\
    This week some time has been spent to complete the revisions to the
Single Board IOC Prototype. The code was written this week for the
memory in FPGA and tested with address and data bus through the Coldfire
and FPGA bridge. The schematic has been modified to reflect the desired
changes and the layout is 50% complete.  The board package has also been
modified to a layout to eliminate interference with existing designs. 
We have also successfully programmed and calibrated five RTD boards. 
These boards are ready for installation once front panels have been
installed.  In an effort to move forward with the next revision to the
embedded BPM design, the projected amplifiers were tested for gain and
noise characteristics.  At first glance the gain for the amplifier is 12
dB and seems to be well within the spec of 1.5 dB Noise Figure.  With
this amplifier we can boost the signal strength without generating noise
and improve the VSWR of the system.\
    Significant effort was focused this week on the changes made to the
existing timing system and the testing of the new timing hardware and
software that will be installed soon. Software tools to monitor the
timing system\'s operational performance were made and are being used.
We are also happy to report that the viewer camera triggering
electronics and signal distribution card was finished this week. We will
install it as soon as a proper time is determined.\
    The settings on the new Kollmorgen drives were compared with the
drives in the existing IR wiggler controls.  The new drives were set
with the wrong peak current and brake settings.  These differences would
result in a wiggler taper.  The drive settings will be corrected and
scheduled for install during the next down.\
    Most of the student's time this week was spent preparing the
cylinders that are required for the optics department. Five (5) have
been completed and a total of 9 are needed. Work will also go on today
for the THz stepper motors. The rotary stage will be calibrated along
with the wiring of new adaptors so these stages can be used in the lab.
A home switch will also be wired in for these.\
    Lastly significant progress is being made on conversion to the new
Laser Personnel Safety System (LPSS). Lab 5 continues to progress and
lab 2 has been taken off line to convert to the new system.

**WBS 11 (Optics):**

The primary activity continues to be the cryomirror tests.  Work on the
accelerator, documented elsewhere, precluded further characterization
until yesterday evening and today.  As of this writing, we observe a
vertical oscillation of about +/- 6 mm on a 76 mm dia mirror.  The
frequency is about 29.8 Hz.  The cause of this motion is under debate,
the OCMMS beams for both mirrors show random motions of order 100 nrad. 
We are seeing a temperature rise of a few degrees in the return He that
is correlated with operation of the FEL, and will spend some time
determining if this is entirely due to absorption in the mirror, or due
to absorption of scattered laser light.

 

We anticipate receipt of new 1.06 micron outcoupler mirrors today, or at
the latest, Monday.  These mirror\'s coatings are especially designed
for operation not only in the NIR but also in the UV (355 nm).  Laser
measurements at this short wavelength will advance our understanding of
the performance limits of the accelerator/laser as a system.

We added a camera with response at 1.6um to view the fundamental mode on
the OC mirror. We have had four OC optics and their corresponding frames
ready for brazing since last week and are still waiting for time on the
vacuum furnace. It appears that we may get time on Monday. Work
continues on Turning Mirror Can (TMC) #1. While this assembly is near
90% complete, we are still having some problems with the cooling tube
chain. The chain hangs on both a limit switch and a port opening when
translating the slide.

**Terahertz:**

This week, we received the last vacuum chamber necessary to complete the
full "in vacuum" experimental setup that has been planned. The new "Ox
box" delivered this week provides the capability for measuring samples
both in reflection and transmission. A cryostat can be incorporated into
the system as well so that samples, such as superconductors can be
measured above and below the transition temperature. All of the vacuum
chambers will be moved onto the optical table today and we will begin
aligning the chambers to the beamline and assembling all of the vacuum
interconnection fittings. All of this equipment will allow us to perform
spectroscopy and pump-probe experiments fully under vacuum, eliminating
the water absorption lines that are particularly problematic in the THz
region. One of our first tests will be to operate the FTIR system fully
under vacuum. We have performed some initial tests that showed a
significant improvement in the spectral signal already, but these
upcoming tests will be our first chance to have the entire beam path and
detector under vacuum.

We are also planning some exciting imaging tests for the future. This
week, Gwyn and I have finalized a collaboration with James Kolodzy of
the University of Delaware to make some real­time THz movies. James'
group has shown that they have some great capabilities in THz imaging,
but lack any useful power in terms of their source. We are eager to work
together with their imaging systems and expertise to set another great
milestone with our THz source. Our tests with them are scheduled for
early November.