# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - July 10-14, 2006

**Date:** July 14, 2006

**Highlights:**

This was a very productive and informative week for the operation of the
FEL Upgrade.

\- We made significant progress in understanding the power loading in
the optical cavity mirrors at several wavelengths (1.0, 1.6 and 2.8
microns) by analyzing the surface temperature profiles and bulk
temperatures of the mirrors as a function of lasing conditions.

\- We observed that the THz absorbers which we installed in the machine
downstream of the wiggler worked well by decreasing the THz loading on
the output coupler mirror by about 2x and, in addition, making the power
distribution more uniform. These positive effects should increase the
power handling of the presently installed out-couplers by about 2x.

\- Progress was made on our diagnostic capabilities with both
autocorrelation measurements of the FEL bunch length (showing 180 fs
pulses); and first fast response measurements of the THz signals in Lab
3 using a unique diode detector loaned to us for testing by Virginia
Diode.

\- User activities included a successful carbon nanotube run by our
NASA-JLab team using the 20% out-coupler at 1.6 microns which resulted
in rave reviews for stability; also we are gearing up a second run by
the Dahlgren team next week, and preparations are continuing with the
Lab 4 laser safety system for commissioning of this Lab for the
Aerospace team's arrival on July 28^th^.

**Management:**

We were pleased that our ONR Program Manager, Quentin Saulter, could
join us this week as an observer on the DOE sponsored review of
Jefferson Lab's Science and Technology programs which was held from
Wednesday to Friday of this week.

Documentation was prepared for two collaborative agreements: (1) the
next (3^rd^) phase of our collaboration with AES, Inc to continue the
outfitting and use of the THz lab and related development of THz
hardware and a compact source design; and (2) a user experiment with
Brooks AFB to study bio-effects of THz on model (ie, non-living)
systems.

We hosted the first meeting of the ONR-funded collaboration on FEL
Optics Improvement on July 10-11.  Profs. Carmen Menoni (Colorado State
Univ.) and Wolfgang Rudolph (Univ. New Mexico) met with Michelle Shinn,
George Neil, and Steve Benson and discussed topics the University-based
teams could assist us.  A summary of the meeting and the action items
generated is in preparation and will be released next week.

Last weekend, Fred Dylla presented an invited talk at 33^rd^ meeting of
the American Society for Photobiology on the application of FELs to the
biosciences. Many of our collaborators for our planned NIH proposal also
attended the meeting.

**Operations:**\
Operations this week concentrated on getting good data on mirror heating
as a function of the Rayleigh range in the resonator.  The week started
out with something we rarely have seen in the last few years\--a problem
with the gun.  After cesiating, we ran up the voltage and found
excessive current draw.  There was no associated vacuum activity so we
cleaned off the external electrodes and tried again.   This time things
worked as they have for the past few years, i.e. flawlessly.  We then
studied the 5 degree off crest solution a bit more before shutting down
for the day.  We found that the sextupoles in the first arc are not
strong enough to get a straight longitudinal distribution at the
wiggler.  The curved distribution that resulted had a spike with a short
bunch length and a long tail that did not lase well.  The result was
rather poor lasing.  We could get third harmonic lasing at 1.6 microns
with the 11% output coupler and fifth harmonic lasing at 530 nm using
the same OC.  The detuning curve for the third harmonic lasing was twice
as long as the fundamental one.   This is what qualitatively one expects
from our spreadsheet model.   The spreadsheet does not account for a
banana shaped longitudinal distribution so it is hard to get
quantitative agreement.

The banana results from the very limited available sextupole fields;
recall that we are still running using (reworked) IR Demo sextupoles and
that the Upgrade sextupoles and their power supplies are not as yet
installed. A short investigation using the phase transfer function
system on Monday suggested that we can probably get a straight, upright
phase space at the wiggler over the full bunch length, but that there
will be little headroom for RF drifts or other errors, and this may have
to be done at the expense of 2nd order dispersion suppression. This
might nonetheless be acceptable - given the smaller momentum spread of
the core beam - but it will require very good suppression of ghost
pulses and a very good injector setup so as to manage the bunch length
and keep longitudinal space charge effects under control.\
\
This should be contrasted to the usual operating setup, where instead of
a linear upright phase space for the 4 degree bunch length, we get very
linear behavior for perhaps as much as 15 to 20 degrees of injected
phase wobble - while managing 1st and 2nd order dispersions.

Tuesday we took a series of measurements of the temperature distribution
on the output couplers lasing at 1.06 microns and 1.6 microns as a
function of the high reflector heater setting.  We set up to take data
at 2.8 microns but ran out of time.  We then switched to the evening
program of THz measurements and nanotube production.   This is the first
time we have run a nanotube experiment using the 20% output coupler at
1.6 microns.  The performance with this coupler was excellent, providing
more power than the old 11% coupler with excellent stability.  THz
focussing measurements were conducted and will be reported below. 
Shukui Zhang took advantage of the stable lasing to get a new
autocorrelator commissioned.  It indicated an FEL bunch length of 180
fsec.\
On Wednesday we continued the mirror heating studies with the 2.8 micron
mirrors.  We had problems with electron beam loss with
this configuration that continued on Thursday.  We were able to get
good data with this mirror however and then moved the FLIR camera to
the high reflector.  This showed very little heating, though
the emissivity of the 2.8 micron coating is very small, increasing
the uncertainty in the results.  We also measured the heating
with electron beam only at 4 mA.  This measurement indicated that the
THz heating was greatly reduced from previous times.  The \"THz traps\"
we installed at the end of June seem to be working.  The traps
themselves increase in temperature by up to 10 degrees so they are 
obviously absorbing power.

Thursday we determined that a phase tail originating in the injector was
the source of the electron beam loss.  Rephasing the injector reduced
losses to normal levels.  We then finished the HR heating measurements
using the 1.6 micron HR, which has a much higher  emissivity than the
1.06 and 2.8 micron mirrors.  It indicated that the HR does not heat up
much at all and that the width of the temperature bump does change as
expected when the Rayleigh range changes.  We tried out a new setup for
measuring the wiggler spontaneous radiation phase noise and found the
power to be on the order of a few microwatts.  The setup we now use
cannot detect such a low power.  We also looked at measuring the phase
noise using the THz beam.  This went much better (six order of magnitude
more power helps) and some spectra were taken.  Details are in the THz
report below.

We are down today for work on the optical transport system and
a diagnostic change on the output coupler can.  Next week we will
be providing beam again for Dahlgren users.

**WBS 4 (Injector):**

On Monday we re-cesiated the photocathode in preparation for high
current operations.

This week we made significant progress in the gun test chamber NEG
sputtering system. After finding and fixing a minor leak in the NEG wire
water cooling tube, the current indicated by the ion pump dropped from
1.0 mA to 0.3 micro-Amps and the RGA spectrum showed water levels below
1E-10 Torr. The argon plasma for glow discharge cleaning was
successfully generated, then the coil was lifted and mounted on the gun
chamber cart. This coil encloses the chamber in a magnetostatic field
for the NEG sputtering process, which is expected to start early next
week.

Carlos Hernandez-Garcia is visiting our colleagues at Daresbury Lab to
assist in the first stages of their photocathode gun commissioning.

**WBS 8 (Instrumentation):**

Effort continued with much progress on the Lab 4 LPSS installation. We
were able to successfully move the mirror cassette allowing the Optics
group to check out the linear stage assembly and align the cassette
mirrors. The local relay interface has been connected to their
respective interlocks and all of the field wiring is complete.  The PLC
software and debugging will begin next week with the goal of being able
to certify Lab 4 by the end of that week. The EPICS Omron device driver
was fixed to work with our PLC configuration. The new LPSS EPICS
application (for operating the LPSS master) was blended with the
existing software.  In the upcoming months this software will be tested
and used for labs 3A and 5. As a part of this activity 2 Mirror cassette
switch test boxes were fabricated. The User Lab FEL Status Boxes for
User Labs 1, 3 & 4 are re-installed and available to be used by the
Users. Parts are on order to complete the remaining status boxes. In
Preparation for the upcoming user runs in Lab 3 (THz lab), we have
cabled the FEL pre-trigger delay from the DLPC and the \"Beam Envelope\"
TTL signal from the Alignment Mode Chassis. A lot of time was spent this
week, identifying and re-labeling cables, specifically, many cables
relating to the timing system upgrade which is in the works.\
For the embedded IOC project a digital card (borrowed from test lab),
which has ADC and FPGA, was used to test the ADC sampling code. The code
was downloaded on the Cyclone FPGA with JTAG. The ADCs on this board are
similar with the one on the general purpose board.\
    This week cabling supplies were reallocated from the bone yard to
the new Injector Test Stand (ITS), including cable tray and mounting
brackets. A thick plastic sheet was also hung in the doorway to the ITS
to keep the humidity of the outside room from seeping into the cave and
through the plenum to the vault. Drawings were also submitted to the
electricians for the ITS vault cable tray and the new upstairs light
arrangement. The computers in the I&C trailer were also overhauled,
having their programs reloaded after being repaired. A number of cable
runs were done this week for the Dahlgren setup and for new optical
transport ion pumps\
    Beam viewer cameras were also taken apart and prepared for
installation of new CCD elements on the boards. 20 new CCDs have been
ordered for replacement on these boards.\
    THz data collection on THz beam focus also took place this week in
preparation for the Brooks AFB people. Approximately 20 images were
taken with various optical setups to produce a very well defined beam
that should be usable by the users.\
    We ran the Carbon Nano-tube experiment (and the THz parasite) on
Tuesday evening. This was another very successful run primarily due to
the new(er) 20% 1.6 micron output coupler. This optical cavity
arrangement is very stable and easily delivers \~1100 watts to lab 1 for
the required duration of 1 hour.

**WBS 11 (Optics):**

#### FEL mirrors

We had a very busy week of operations, much of it to answer questions
mentioned in last week\'s report.  The thermographic imaging can be
interpreted as surface absorption, but is it absorbed by the coating or
the substrate?  A series of experiments were proposed and performed to
determine this.  If the absorption is in the coating, then changing the
coating dielectrics, but not the substrate, should make a difference. 
This was found to happen, lending credence to the hypothesis that the
absorption is in the coating.  While producing 1100 W at 2.8 microns and
8% outcoupling resulted in a center-to-edge temperature delta of less
than 3 deg C, the temperature delta was slightly in excess of 5 deg C
when producing only 900 W at 1.6 microns with 11% outcoupling.  Another
test was to change the Rayleigh range (and hence the spot size on the
mirror) of the optical mode and see if the temperature profile changed. 
Indeed it did.  Hence, the absorption can\'t be due to some e-beam only
producer of mid-IR radiation that would be strongly absorbed in the
substrate.  Now we need to track down the cause of the absorption in the
near IR mirrors and determine if there is a way to reduce it.  To assist
us, ONR is funding two university researchers, Carmen Menoni (CSU) and
Wolfgang Rudolph (UNM).  They were here for two days early this week,
and made several suggestions of possible causes and experiments to try.

The other experiments we did checked the efficacy of the
newly-installed THz traps.  Figures 1 & 3 show the temperature rise in
the OC mirror cooling water, and a thermographic image of the face
before the THz traps were installed.  Figures 2 & 4 show the same data
after the THz traps were installed.  Not only did the total power
absorbed drop by a factor of two, but the delta in temperature
(center-to-edge) dropped by over a factor of 4.  The spreading of the
THz load will make it have less impact on lasing, and new measurements
of lasing efficiency with current are planned for next week.

A delay in coating the new 1.6 micron HR mirrors with the aluminum
underlayer occurred when review of the planned coating showed excessive
transmission (and absorption) at 400 nm (the 4th harmonic).  The
vendor is iterating the design, and thinks they can start coating next
week. Setting up the laser damage test stand and calorimeter in User Lab
1 progresses.

![](media/image1.wmf){width="4.572222222222222in"
height="3.1243055555555554in"}

Fig. 1 The dark red trace shows a temperature rise of 0.2 deg C due to
THz loading on the OC mirror. 5mA, e-beam optimized for the FEL, before
the THz traps were installed. The light red trace is the beam current at

6 mA full scale.

![](media/image2.wmf){width="4.829861111111111in"
height="3.279861111111111in"}

Fig. 2 The dark red trace shows a temperature rise of only 0.1 deg C due
to THz loading on the OC mirror. 4mA, e-beam optimized for the FEL,
after the THz traps were installed. Again, the light red trace is the
beam current at 6 mA full scale.

![](media/image3.png){width="5.503472222222222in"
height="3.7395833333333335in"}Fig. 3 FLIR image of 1.6 um 20% output
coupler with 5 mA beam, THz suppression on and maximal THz emission.
This is before the THz traps were installed. Note the \~ 4 deg C
center-to-edge change in temperature.

![](media/image4.png){width="5.666666666666667in"
height="3.404166666666667in"}

Fig. 4 FLIR image of 1.6 um 20% output coupler with 4.2 mA beam, THz
suppression on and maximal THz emission. This data was taken after the
THz traps were installed. The center-to-edge change in temperature is
less than 1 deg C after the background is subtracted.

**Terahertz:**

This was busy and exciting week for the THz program. We are setting up
the lab and making measurements in preparation for the Brooks AFB group
that will be doing the first phase of tests to determine the safety
limits for high power THz sources. Rich Evans provided Lab 3 with a
pretrigger signal requested by the user and has tested that signal along
with an additional new beam envelope signal that he has provided. The
beam envelope signal indicates exactly when the beam is on as a readback
from the accelerator.

Wade Brock also assisted with some minor adjustments to the alignment of
the THz transport optics. After that, we were able to do several beam
profile measurement to determine if we can achieve the required beam
spot size requested by the Brooks AFB group. We were not able to
complete these tests entirely, but using the PyroCam, we were able to
acquire numerous beam profile images. The beam intensity was quite high
and we were far above saturation on the camera, which is apparent in the
image below. It should be noted that beam shape is flipped 90° compared
to earlier reported images because the camera has been rotated 90° from
the previous orientation. To make better quantitative beam waist
measurements, we will need to be below saturation and will also try
using the laser ablation card provided to us by Michelle Shinn.

![](media/image5.png){width="3.09375in" height="3.0125in"}We ended the
week, with a very productive day of tests of the high frequency diodes
made by Virginia Diodes, Inc. in Charlottesville, VA. They make diodes
capable of detecting radiation in the 100's of GHz range and even up to
just over a THz. The response of these diodes is also very fast, with a
bandwidth on the order of a few GHz. Jeffrey Hesler from VA Diodes
brought several diodes and a source as well to test on the THz beamline.
We were easily able to get a very good signal (100 mV pulse / -50 dBm)
on one of the diodes designed for sensitivity from 400-600 GHz and could
observe individual pulses on a fast oscilloscope. We did find that
reflections between the horn antenna on the diode and the diamond window
created significant secondary pulses on the diode signal, but we were
able to suppress these to less than 10% of the main signal peak by
adjusting the angle and rotation of the wire grid polarizers. Having
achieved a good setup, we then could look at the signal using a spectrum
analyzer to determine if phase noise measurements could be made. In
order to do this, Pavel Evtushenko and Jeffrey took measurements on two
different spectrum analyzers and performed tests with RF amplifiers and
filters to try and boost the signal to 0 dBm which Pavel has calculated
will be necessary to pull out the phase noise data. We were able to
increase the signal to -5 dBm at 1497 MHz, but unfortunately, the
additional filters and amplifiers also pick up signal from the klystrons
in the RF gallery which is problematic for

![](media/image6.png){width="3.8in" height="2.7777777777777777in"}

making the phase noise measurements. Since we were running with a 9 MHz
micropulse rep rate, we could look 9 MHz away from the 1497 MHz peak
with the spectrum analyzer. This is shown in the figure to the right and
it appears that we could be seeing a low level modulation due to the
signal seen around the peak.

The important results from these tests were that we should have no
problem making good phase noise measurements once we have better filters
and amplifiers and good shielding from the klystron crosstalk. Also,
when looking at the signal on the oscilloscope, we were able to also see
small additional pulses between the main pulses during the CW operation.
This could be indicative of ghost pulses, though we will have to
investigate this more to understand it. It appears though that these
diodes could provide even more diagnostic capability than we had
originally hoped.

What one sees around the main peak looks a lot like what we are looking
for, i.e., very low level modulations of the signal.

It was a very successful run and very promising in regards of the phase
noise measurements. We plan to improve the RF part of the measurements
setup and proceed with the measurements.

![](media/image6.png){width="4.7375in" height="3.4659722222222222in"}

*Fig. 1 Signal of the diode in the frequency domain*