# MEMORANDUM 

**To:** Distribution

**From:** George R. Neil

**Subject:** FEL Upgrade Project Weekly Brief --May 14-18, 2007

**Date:** May 18, 2007

**Highlights:**

We continued to take measurements of the limits of high power fluence on
the FEL mirrors.  These highly successful tests have essentially shown
the capability of this approach to meet the requirements of the next
phase FEL.

**Management:**

Steve Benson and George Neil attended the FEL TAWG meeting in Santa
Fe. They presented an update on our program status and plans and
justification for a high power amplifier test which could be performed
within 2 years by utilizing the UV leg of the Upgrade.  The primary
advantage of such an approach would be the side-by-side comparison of
performance of the oscillator and the amplifier providing a solid set of
measurements to benchmark against simulation codes.

While Steve and George were in New Mexico, Gwyn Williams and Mike Kelley
ran the 21st FEL User Meeting at JLab with about 60 attendees covering
past and proposed measurements utilizing the FEL.  The meeting had an
exciting and full program.  Several talks described the facility at JLab
and also the 4GLS project at Daresbury.  The research talks covered both
basic and applied areas, and were followed by facility tours.  A poster
session presented by local students preceded dinner.  There was also an
associated meeting of the FEL Program Advisory Committee.

**Operations Summary:**\
This week\'s operations were devoted primarily to cryo-optics high power
tests, with extremely exciting results, see the risk reduction section
for details. Secondary programs supported an internal user (P.
Evtushenko) who worked on a setup for measuring the electron bunchlength
at the THz chicane, and NASA and THz users. Please see these reports for
details.\
The hardware remains in reasonable condition.  For some reason, one of
the quarter cryomodule\'s Fast Shutdown chassis latched when we shut
down for a Controlled Access at the end of Monday\'s shift.  It was
remedied off-shift with no loss of beam time.

**Bunch length diagnostics**

On Wednesday we made good progress evaluating the possibility of doing
bunch length measurements with the help of the Michelson interferometer
at the "THz suppression chicane" aka debunching chicane. Essentially
these are the same measurements we have done previously in User Lab.3.
The motivation is to have the Michelson interferometer setup at the THz
chicane to be able to do the measurements continuously.

We had the interferometer installed and aligned at the chicane. First
running pulsed beam we measured the amplitude of the pyroelectric
detector installed in the interferometer as a function of the mirror
position. With the 1 ms long pulses and the bunch frequency of \~4.678
MHz the maximum the amplitude of the detector signal was \~ 4 V.

Having the mirror fixed at that position, we measured the amplitude of
the pyroelectric detector and the amplitude of the Golay cell installed
in the Happek device as a function of the vernier cavity gradient, i.e.,
changing the bunch length. Figure 1 shows the two measurements side by
side. As one can see, the two measurements track each other very well.
We consider that as a confirmation that the signal seen on the
pyroelectric detector is due to coherent synchrotron radiation, i.e., is
related to the bunch length.

![](media/image1.png){width="5.375in" height="4.40625in"}

*Fig. 1: Amplitude of the signal of the Michelson interferometer
detector and the Golay cell of the Happek device as a function of the
vernier cavity gradient*

Next we measured the spectra at different gradients of the vernier
cavity. The spectra measured at gradients of 5.5, 5.6 and 5.7 MV/m are
shown in Fig. 2 side by side. From previous measurements we know that
the minimum of the bunch length was at 5.7 - 5.8 MV/m. As one sees in
Fig. 2 the spectra is distorted a lot by what is most likely atmospheric
absorption. We are working now on figuring out what exactly those lines
are. The absorption lines make the bunch length reconstruction difficult
and very inaccurate. We would need to put the interferometer in vacuum
chambers which also have to include the path from the vacuum window to
the interferometer. We also can put a chopper in front of the
interferometer and watch for changes of the detector signal amplitude,
even without the scanning interferometer, as we are looking at the Golay
cell when running pulse beam. The pyroelectric detector, however, can be
used when running pulsed as well as CW beam.

![](media/image2.png){width="5.99375in" height="4.034722222222222in"}

*Fig. 2: The CSR spectra measured at different gradients of the vernier
cavity*

**\
**

**Injector:**

The DC photocathode gun has delivered 110 Coulombs and 16 hours of CW
beam time during the week. The present cathode QE is about 2.3%. The
cumulative since the last recesiation on May 4, 2007 is 210 Coulombs and
34 hours of CW beam time. This photocathode was installed in the gun
back in May 2004. It has delivered more than 6 kiloCoulombs and more
than 700 hours of CW beam time since then.\
\
***GTS***\
We have a compact and robust design for the gun Brewster window. It will
be a 3 inch view diameter sapphire window brazed to an adapter. The
adapter will then be welded to a custom made flange. A request for
sapphire viewport quotes will be sent out today. We are nearing down on
our options for the best location and a way to mount the optical table
inside the GTS vault for the drive laser. Polishing of the moly anode
plate continued this week with great progress.

**Instrumentation and Controls:**

We have once again revisited the multi-pass BPM project in a continued
effort to determine a design for these unique locations where the
positions of the accelerating and decelerating beams need to be
determined separately. The progress this week was to measure the BPM
signal properties from one leg (X+) of the IPM1F04A BPM using the
high-speed Digital Sampling Oscilloscope (Tek TDS11801B) which is good
from DC to 50 GHz and a 10 fs (0.01 ps) sampling interval. In the
process of making this measurement, we took the time to develop a method
of extracting the waveform data from the 11801B which is limited to GPIB
or RS232 connectivity. The captured multi-pass BPM data (amplitude vs
time) was externally processed an FFT was performed.  A Task Hazard
Analysis was used to coordinate the work and the methods and results are
provided in the FELWiki for future review at
<https://fel.jlab.org/wiki/index.php/BPMs> (JLab authentication
required).

Work continues on the SF6 GTS reclamation system. We were able to reduce
the number of valves from 8 to 7.  After Recovery stage 1, we developed
Recovery 1a, which will purge the small air pocket that becomes trapped
in section 2-5-6 of the piping.  This will prevent contamination of the
SF6 by the air.

We finished a 2-D drawing of the system to determine the number of
elbows, unions, couplings, crosses, and tees. A 3' by 8\' optic table
was incorporated into the vault which stands on 72\" legs near the
vent.  It appears that the table will not interfere with any other items
and it may be a solution.  Installation is proceeding on the GTS racks;
currently cutting access holes in the bottoms to allow wires to pass
through.

**Electron Beam Transport:**

Improvements and Upgrades

*SF Sextupole Magnet Testing*

•  The power supply folks received the software license that allowed
them to change the control software and fix their scaling problem.  The
task is now in Scott Higgins' hands (not here this week) to continue the
software to power the magnets and run them through hysteresis. 

*Cooled and less resistive Wiggler Chamber*

•  As of this writing, we have not observed the response of the chamber
at higher currents than the 2.5 A reported earlier.

* Magnetic anomalies*

• No gap cam changes this week.

Gun Test Stand (GTS)

• We received feedback on the procurement package of drawings and
specifications for the Gun SF6 Tanks from the vendors that results in
loosening tolerances for various non-critical aspects of the vessels. 
The revision to the specification was signed off and posted on the
procurement site.

•  Stephanie Slachtouski finished the drawings of the second-generation
solenoid for the GTS that will have the split face pole pieces,
water-cooling and integral Haimson style dipole corrector coils.  The
drawings were checked and vellums will be signed Monday.  The
requisition was signed by George Neil.

• Matthew Marchlik continued to implement the SF6 Gas Transfer System. 
He gathered information and refined the valve logic to simplify the
system.  He will start layout next week.

•  We continued progress in laying out the simplified Brewster window
concept for the drive laser beam to the extent that we could order the
windows.  Ron Lassiter, who was in training this week, will be able to
detail the system next week.

**High Power Optics Risk Reduction**

This week and last we operated the FEL primarily for the High Q test. 
We spent a considerable amount of time trying to determine if the mode
sizes, as determined by our NIR cameras, was correct.  The reason for
this exercise was that our measurements of the mode size on the
outcoupler (OC) was \~ 30% smaller than expected, based on a cold cavity
mode calculation, while the mode on the HR mirror was closer to the
calculation.  Measurements with the FEL operating at 1.6 um confirmed we
were operating in a linear region, and with white light illumination,
the size of the mirror. We also removed the camera in question and
confirmed that our calibration technique was good to \~ 5%. 
So we accept these values.  To date, our highest intracavity power is 78
kW (apologies for a factor of 2 mistake in the 05/04 report), with a
resulting irradiance on the OC of at least XXX kW/cm\^2 (\>100
kW/cm\^2).  The irradiance for the HR continued to be lower, still of
order YYY kW/cm\^2.

Since the last report, we lased on a new high reflector (HR).  This HR,
like many we have used at other wavelengths, has a metal layer deposited
on the substrate surface before the dielectric stack is put on it.  The
stack is tailored for the presence of the metal.  This optic, when
loaded at the intracavity powers and irradiances quoted above, did not
have a measurable loss, as determined from our calorimetry.  The upper
bound on the loss is about 30 ppm.  We lased on this optic without
problems until Friday, 5/11.  We then saw that we had scattering sites,
very possibly damage, at several points within the laser mode
footprint.  We don\'t know what caused the optic to do this; we will
have to wait until we can remove the optic to really determine what
happened.  Suffice it to say that the optic survived hours of operation
at +100 kW/cm\^2 irradiance before it happened.  Since the increased
scatter is limiting our power, we are moving to yet another unused HR
and will continue our measurements.

We are also watching the OC mirror and, to date, are seeing no surface
changes.  We are seeing a change in the absorption level.  The
absorption in the optic started a bit high, at \~ 500 ppm.  Over the
course of a day or so it increased to 0.3% and is now at 0.5%.  A hole
trap that has absorption in the infrared is a candidate.  We are making
measurements in the visible before and after lasing, and observing
slight changes, specifically a broad band peaking at \~ 650 nm.  At the
time of this report we have taken lower noise spectra before lasing, and
will take another spectrum after lasing at high intracavity irradiances.

**Optics:**

Optics personnel support of the High Q experiment included controlled
access support for IR camera and visible camera setups used to measure
the spot size on both the HR and OC. A test apparatus was setup in user
lab 2 to verify that the measurement technique used on the OC was giving
accurate results. A mockup of the OC camera setup was used to verify the
fiducials used were giving accurate results with the video processor
used to measure the spot size. Work on the lab cassettes for the optical
transport included more de-anodizing of parts and more work with the
vendor for a replacement linear slide. Work is being started on
metrology of two 3\" Si optics for the LIPPS Group.

The drive laser has been running well with a temporary replacement of a
broken coolant control valve with a manual one. The temperature has been
stable. A new self-actuating control valve has been received and will be
put in on the next possible chance. We continued the work on the GTI
drive laser setup in the vault and have made preliminary decision on the

location of the optical table. A cost performance analysis on the
different laser configurations is being worked out.

**Terahertz:**\
On Monday and Tuesday this week, we set Harvey Rutt's double resonance
experiment back up.  We tested all of the electronic systems that Harvey
has designed to split and amplify the signal from the mid IR
interferogram, and the modulations that we are looking to detect at the
micropulse frequency.  By all tests, the electronic systems worked well.
 We also assembled the new Brewster angle gas cell.  The new gas cell
has silicon windows mounted at Brewster angle for the mid IR beam.  The
beam port through the cell is tapered to match the propagation of the
beam which is focused at the center.  By using tapered ports, the window
surface area is also reduced significantly.  The new gas cell worked
beautifully; with \~ 95% of the horizontally polarized mid IR beam
transmitting through, and there were no signs of fringing effects from
surface reflections in the silicon windows.  Unfortunately, the fitting
that mates the gas cell to the vacuum system failed, as well as its
spare.  This was a very delicate fitting and the stresses were just too
great.  This will be addressed in the next iteration of the gas cell
design.  Optically, it worked as well as we could hope, and the vacuum
connection appears to be the only design change that will be made.\
\
At the point when the vacuum fittings failed, we were tenuously close to
running the double resonance experiment.  We should have gained a few
orders of magnitude improvement in S/N with the modifications to the
electronics and the gas cell.  The next time that we are able to run
this, we expect to have a new detector, which will boost the S/N still
more, and the entire beam path until just before the detector will all
be under vacuum instead of just purged.\
\
On Thursday and Friday, Hannan Fersi, from the University of Manchester,
and Mark Surman, from Daresbury, made measurements on a large number of
samples.  We first tried to see if we could get any kind of mid IR
spectrum to look for damage caused by the high power THz beam manifest
through changes in the mid IR spectrum.  The concentration of the
samples, though, proved to be too high and all of the absorption bands
were saturated.  After those tests, we changed the setup to send the THz
beam through the FTIR and then through the sample.  Hannan and Mark took
THz beam for several hours on Thursday and were able to take THz
transmission spectrum on all of their samples.  Today, we will be taking
a few final additional spectrum measurements after having let the entire
system purge overnight.  After those measurements, we hope to have time
to test their variable path length liquid phase cell.  The tests will
determine what range of path length through the liquid (water for these
tests) still generate usable spectrum.\
\
At the FEL User Meeting on Wednesday, I presented some of the results
from our THz imaging experiments with Jim Kolodzey's group from the
University of Delaware.  The talk seemed to be well received.  Hannan
also presented a talk about the work they are performing here this week,
and Mark Surman presented the ERLP and 4GLS project at Daresbury.

**Word version:** [FEL Upgrade Project Weekly Brief -- May 14-18,
2007](http://www.jlab.org/accel/fel/documentation/07weeklybriefs/upgradeweek07may14-18.doc)