# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- December 1-5, 2003

**Date:** December 5, 2003

**Highlights:**

Since the last full weekly report (Nov. 21) before the Thanksgiving
holiday break we have made significant progress on commissioning of the
FEL Upgrade. On Wednesday, Nov. 26, we re-established pulsed lasing at
10.1 microns with approximately 700 watts of average power during the
macropulse. With this achievement we are essentially fully recovered
from the hiatus caused by Hurricane Isabel. Except for a short break on
Thursday, Nov. 27^th^ for caloric loading of the staff with family and
friends, we have been hard at work to set-up and optimize the
accelerator for cw operations. This week clean, energy recovered, cw
operation has been achieved at 2 mA. We also characterized the THz
radiation emitted downstream of the outcoupler mirror as a function of
average current and peak charge and do not feel this will be a limiting
factor for heating of the optics in our present set-up. More details of
the extensive work done in the last two weeks are given in the
commissioning section below. A break in operations today and through the
weekend will allow us to deal with a punch list of maintenance and minor
hardware fixes that have accumulated since we re-started beam operations
on the 19^th^. We plan to resume commissioning activities on Monday.

**NOTE: HOLD THESE DATES:**

The next (tri-services) semiannual project review of the FEL Upgrade
will be held at Jefferson Lab on Jan. 29-30, 2004.

The 2004 FEL Users/Laser Processing Consortium Workshop will be held at
Jefferson Lab on March 10-11, 2004.

**Commissioning:**

We took extensive data on the THz generation with a calorimeter as a
function of charge and average beam current. Under the present bunching
setup (much less aggressive than last August) the level of heating does
not appear to be a major source of mirror distortion. We measure on the
order of 5W/mA/100pC. At that level the THz heating would only account
for 3% of our thermal distortion budget at full current and would not
appear to be a factor in limiting FEL output power. Preliminary spectra
are shown below.

![](media/image1.jpeg){width="5.868055555555555in"
height="2.520138888888889in"}

A very short report was given in last week\'s report so we will describe
last week's activities in more detail as well as this week.

Last week got off to a slow start when we lost power in the building due
to a ground fault.  This took several hours to recover from.  When we
were back up we were able to clean up the transmission to the energy
recovery dump so that we could run high duty factor beam with no beam
loss monitor trips.

On Tuesday we worked on matching the electron beam through the machine.
 We obtained a good match to the backleg.  The emittance looked quite
good (10 microns vertical and 15 microns horizontal).  We continued to
measure 0.5 psec FWHM bunch lengths.  No lasing was found however.

Wednesday we continued to match through the machine and were able to
find a good match to the wiggler.  We managed to get lasing for the
first time since Isabel.  The laser was quite insensitive to most
parameters.  It was optimized for the design buncher gradient, unlike
earlier lasing.  There was a discrepancy between the energy change in
the electron beam (0.7%) and the calculated efficiency from the power
measurement (0.36%).  We also discovered that the drive laser pulse
controller was not producing the proper repetition rates for low
frequencies.  This partly explains why we could not see any lasing up to
this time, since the requested frequency of 4.678 MHz was not being
delivered.  This has since been fixed and we lase routinely at 4.678
MHz.

On Friday and Saturday we optimized the FEL and found quite a low lasing
threshold compared to the one in August.   We could not make the laser
lase at 6 microns despite seeing strong and reproducible lasing at 10
microns.  We believe there may be some problem with the mirrors.\
On Monday we addressed drift and jitter issues that have been troubling
us for the last few weeks.  We found that the water for a chill plate
used to stabilize the buncher phase was not turned on.  We turned it on
and the buncher seems to be more stable this week.  The beam position
monitor timing was found to be too close to the end of the macropulses,
leading to jitter, and this was fixed as well.  Finally, we traced an
injector energy jitter to the drive laser phase jitter.  Attempts have
been made during the week to reduce the drive laser phase jitter, so far
without success.  We recorded the size of the beam at the injector
cryounit to aid in quick phasing of the injector.  This helped us narrow
down the source of phase drift this week.\
Tuesday was devoted to getting a better re-injection match to the
linac.  We found, during this effort that the first arc sextupoles were
misbehaving and causing beam loss.  We are working today to fix this
problem.

Wednesday we carried out a full series of difference orbits to look for
discrepancies between the model and the machine.  A few suspects were
found, primarily involving calibration factors for a few BPMs and small
deviations of quadrupole excitation, likely due to hysteretic effects. 
Most elements match the model well.  We also found that some of the beam
loss we were seeing in the linac was due to the first pass beam rather
than the second pass beam.  This was cleaned up and we were able to run
CW beam and take good data of the THz radiation power vs. charge and
current. The measurement indicated total power emitted was 5 W/ma/100pC.
This means 80 W total power at 10 mA and 135 pC. We do not know how much
of this power will be absorbed in the mirror.

Thursday we carried out phase transfer measurements of the linac from
its entrance to the wiggler.  Ideally the phase transfer function is a
vertical line.  As seen in the attached figure that is exactly what we
found.  The horizontal scale is 136 fsec per unit so the noise is less
than 700 fsec full width.  The noise is due to drive laser phase noise
and should be smaller when we get that reduced.  We could adjust the
curvature using the sextupoles and the slope using the trim quads.  The
slope is vertical for the design trim quad values.

![](media/image2.png){width="6.6125in" height="3.5340277777777778in"}

We also took data on gun noise from two stack operation and looked at
the drive laser transport through the light box.  In the IR Demo we
found that, if the laser was vignetted in the light box, halo was formed
that prevented high current operation.  That is exactly what we are
seeing now and, in fact, we found that the beam was being clipped in the
light box.  This will be fixed this weekend.

In summary.  We now have a good (though not perfect) electron beam match
up to the wiggler.  There are some losses after the wiggler and near the
dump.  We will optimize this next week.  We can run up to 2 mA CW
limited by the halo that is present on the beam due to vignetting in the
light box.  This should be fixed next week as well.  THz power has been
seen that may be a problem at very high current but is not a problem
yet.  The laser lases well and is nearly optimized for the design
parameters.  Drift has been reduced and can be compensated for easily
now.  Jitter from the drive laser needs to be reduced to previously
attainable levels to have very stable laser operation.

**Management:**

A successful semiannual review of the UV FEL Upgrade project was held on
Nov. 25^th^.

We received a summary evaluation of the review from our AFRL program
officer (John Eric) with his assessment. John was pleased with the
progress on the 10 kW IR FEL commissioning, the delivery of hardware
that was jointly procured for both the IR and UV Upgrades (chicane
dipoles and quadrupoles, and optical cavity vacuum chambers), Aerospace
Corporation's progress on the design of the microfabrication station,
and preliminary plans for FY04 activities. Draft work statements for the
Jefferson Lab UV FEL efforts and the Aerospace effort for FY04 were
approved pending final resolution of the amount and availability dates
for the FY04 AF funding for these tasks.

Several rounds of (email) discussions were held this week with ONR on
our proposed work on the 100 mA injector test stand and associated rf
hardware. A conference call is being planned

for Dec, 15^th^ on the subject of the rf hardware.

Preparations continued with our partners at AES for our joint design
review on Dec. 15^th^ for the US Army for our THz beamline project.

**WBS 4 (Injector):**

We performed a cathode recesiation today. The QE on the location of the
laser spot increased from 0.7% to 4%. The last recesiation was performed
on Nov 20th. Today\'s QE is about 90% of that on Nov. 20th. The cathode
delivered pulsed beam at 135 pC, and up to 2.0 mA of cw beam current at
110 pC. The charge was limited not by the photocathode gun , but from
losses in the dump.

We made progress on the field emission testing of the coated electrode.
The electrode was tested at 30 mV/m for a total of six hours. Some
conditioning was observed and the gradient was increased up to 31.25
MV/m. After 30 min at that gradient, the average current at 30 MV/m
dropped from 140 pA (recorded in a previous test) to about 40 pA. Long
runs at 30 MV/m will continue next week.

Gun HVPS - Tests were conducted on the Gun HVPS for both 1 and 2 drive
stack operations at 350kV. The 1 drive stack responded well with a low
(5v p-p)ripple voltage for 1 mA current pulses, but it is limited to
about 5 mA maximum current. The 2 drive stack mode of operation becomes
very noisy as the output voltage is passes through 250kV. The measured
current abruptly jumps from about 0.030 to 0.300 mA. This precludes any
HV processing with 2 stacks. Under pulsed 1mA load conditions, the
output voltage jumped between 349kV to 367kV. Under heavier (2 ma CW)
load the output voltage was stable at 350kV. The Ripple Voltage monitor
indicated 60v p-p. This information is being forwarded to the vendor to
see if they have a solution to the noisy operation under light or pulsed
loads.

**WBS 5 (SRF):**

Work on the third cryomodule continues steadily but slower than we would
like. The thermal shield has now been hung from the space frame. The
helium circuit bellows that connect the sections of the shield together
all had to be modified and custom fit by hand which took longer than
expected. We are still finding lots of little modifications that need to
be made to make everything fit together. This shield assembly was the
first prototype made and was set aside and not used on SL21 for just
this reason. We now have to make it work and it is requiring a lot of
hands-on detail work. We will get there in the end and the final product
will be worth the extra effort to do it right. The schedule is now
showing that the module will not be completed before the holidays,
despite the best efforts of all concerned. The cryomodule assembly team
is already working two shift operation in parallel with SNS production.
The module will most likely be delivered now in January. Installation
activities will be coordinated with 10 kW running of the FEL, which is
the top priority.

**WBS 6 (RF):**

Buncher - In trying to stabilize the e-beam more, the Buncher was turned
off and back on in an off-normal state. We lost about 4 hours of
operations until the technician was located returning from vacation. On
a cell-phone he was able to talk us through to the solution. He is now
putting together an operations and trouble-shooting guide for the
Buncher that includes an updated set of drawings.

RF - The RF systems ran very well, except for the time lost on the
Buncher and this was not a real failure.

**WBS 8 (Instrumentation):**

The design for the 64 channel diode/RTD readout chassis is nearly
complete. The drawings have been started while the final pinouts are
determined. There will be 16 four channel boards that have independent
current sources, buffer amps, and analog multiplexers that are set to
communicate with out new CAN bus DSP board. (request for quote sent out
today for 5 DSP prototypes - they are due in 4 weeks) One stage of this
project will be complete for the arrival of the upgrade cryomodule in
January. This new module has a total of 54 Si diode thermometers for
monitoring temperatures throughout the module.\
The Linux IOC frame grabber project will be available for use in beam
measurements beginning Monday. A second SCION card has arrived and will
now be used for system development (and as a spare). After well
structured research combined with the assistance of Al Grippo and Arne
Freyberger, the framegrabber\'s memory allocation problem has been
solved. The OS was assigning virtual memory addresses for the capture
card memory that was being mapped to the software. The problem with the
way the OS was controlling this was that it never checked to see if the
virtual address is short of or exceeds the allowable used area of the
kernel\'s memory. In summary, the OS gave the framegrabber software a
memory address that it was forbidden to use so the program would abort
unexpectedly without freeing any of it\'s consumed resources. Through me
explaining the details of what the Linux OS was doing, Al and I realized
that we could try to hold on to the area that we were assigned and try
to read new frames mapped to the old locations. This was then
implemented and through a small adjustment the frames were coming in
fine. All viewers will be calibrated by Monday morning for calculation
of their individual x and y scale factors. So that maybe we won\'t have
scientists measuring beam widths with ink pens.\
  

A new design for upgrading the SF6 gas transfer system is being
finalized. The new system will have larger diameter piping and
additional pumping speed. The new system will be entirely automated such
that transfer of the gas can be initiated with the push of a button and
take place while other activities such as a radiation survey take place.
The manual valves will be replaced by electric valves and a Programmable
Logic Controller (PLC) will be programmed to pass the gas back and forth
before and after each re-cesiation of the gun. The bulk of the work and
documentation will be done with our mechanical and electrical
engineering students.\
  

The video system became unreliable this Thursday in its switching
accuracy. After initial troubleshooting, the digital controls have been
ruled out. We then discovered a large amount of noise on the grounds of
the signals coming up from the machine vault. The source of this noise
is believed to be the cause of the systems unpredictable behavior. The
problem is ongoing but with today being a \'maintenance\' day, we should
have it fixed by the end of the day.\
  

Machine documentation continues. Check prints have been received from
EECAD for the following: Charge/Dump Current Monitor Beam Envelope
Board; Optics Heater Control Board, Rev. A; Dump Water Control System,
System Diagram; and the Dump Water Skid Interface Box, Wiring Diagram.
Completed drawings were received for Rev. B to the 12 CH BLM Breakout
module and the Wiring Diagram for the OutCoupler Temperature Control
Chassis. Several User Lab cameras were assembled and tested. Noise
measurements on the Digitel 500 Ion Pump Power supplies are in progress
in an attempt to get a cleaner signal into EPICS for the current
readback. The fail-safe interlock boxes for the OCR diagnostics
solenoids have been completed and are ready for implementation. The 2F06
chimney viewer has been fully assembled and aligned on the viewer test
stand. Installation will happen during our maintenance day. The revision
to the Outcoupler Temperature Control Chassis is complete. A circuit
board was added to filter noise fluctuations from the controller.  It
will be re-installed and tested today.\
  

There was near disaster this morning when the caffeine generator could
not make the proper temperature gradient resulting in only gurgling.
That coupled with depleted spare grounds forced the team to start a bit
slow on decaff. Repeated flushing with a 5% solution of acetic acid
restored it to near perfect operation, a quick trip to the local
warehouse was also needed to replenish stocks and restore the team to
peak performance.

 

**WBS 11 (Optics):**

A lot has happened these last two weeks (with the only time off being
Thanksgiving and part of Sunday for some of us).  We reacquired lasing
at \~ 10.1 um, and gave the optical diagnostics a good checkout.  See
the Commissioning section of this report for information on the
performance.  However, we were perplexed by the inability to lase at 6
um.  While we have the optical cavity assemblies open to install new
10.6 um optics, we will remove and test it for optical figure, then
reinstall it to further minimize possible mounting stress.  We used our
mirror metrology system to track the change in the mirror radius of
curvature as a function of beam-induced THz radiation.  The change was
0.06% for \~ 5 W (assuming it is all absorbed).  Using an analysis
produced by S. Benson (Tech Note 97-005 and its successors) this is a
0.25% change in the Rayleigh range.  Scaled for 10 mA, this should
not prove a big impediment to our ability to achieve high power,
although it does eat into our absorbed power budget. In order to
increase our chances of lasing at high power, we are installing our
spare 10.6 um HR and OC mirrors in the unoccupied broadband position. 
After some \"back and forth\" between us and our vendor, we received
optics that meet or exceed specifications.  The OC mirror is being
mounted in the improved mount we\'ve written about, and this will, if
nothing else, decrease any mounting-induced aberration and lower the
temperature drop across the mirror-mount interface.  The mirrors were
both tested in our new laser calorimetry setup, measurements will
undergo analysis over the weekend.

One fly in an otherwise positive report has been a deterioration in the
mode-locked performance of the drive laser.  At low frequencies (\< 0.5
Hz), we measure increased amplitude and phase noise.  While half the
group are changing optics downstairs, the other half are fixing the
drive laser.  (Thank goodness for hardworking qualified staff!).

Amidst all of this work we have been holding meetings on the optical
transport.  Drawings for the modifications and hardware going into the
optical control room were signed, and drawings for modifying existing
components in the User Lab to handle +10 kW are eminent. 

**Terahertz Project:**

The optical configuration and the mirror sizes and figures were
optimized by calculating and propagating the electric field using the
Synchrotron Radiation Workshop code in Saclay, France in collaboration
with Oleg Chubar and Paul Dumas.  M1 will be an ellipsoidal mirror of
625mm focal length, while M2 will be an ellipsoidal mirror with F1=705mm
and F2=2426 mm.  M3 will be planar.  The mirror blanks will fill as much
of the 150mm tube as possible, consistent with clearance for adjustment.
 Both diamond windows will be 20mm clear aperture, as this transmits
most of the light to 0.1 THz, or 3.3mm wavelength.  This configuration
means that the final part of the vacuum system from M4 to F3 will have
to be shortened.  A sample aluminum single point diamond turned mirror
from one of the vendors arrived this week and was sent to Brookhaven for
metrology and qualification.  Work continued on documentation for the
pre-construction design review, which is now scheduled for December
15th.