# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - December 12-16, 2005

**Date:** December 16, 2005

**Highlights:**

This week was spent starting the process of pushing up the current both
with and without lasing at 1.6 microns. To date we are pleased with the
initial lasing performance:

a kilowatt of cw lasing has been obtained with only 0.85mA. This yields
an efficiency of

1.2 kW/mA. At this level of lasing we got our first look at mirror
heating with this high power mirror set which looks favorably low for
extrapolating to 10 kW level performance. Also operation over the full
bandwidth of the mirror (1.5-1.72 microns) was demonstrated.

As we push up the accelerator current we are going through our usual
procedure of cleaning up the electron beam transport to minimize beam
loss (especially in front of the wiggler) and maximizing the lasing
efficiency by optimizing the electron beam parameters within the
wiggler. As of this writing we have pushed the current up to 2.5mA for
pulsed lasing and 4mA cw without lasing.

Last Friday after we sent our weekly report, we observed that the FEL in
its present lasing configuration for fundamental lasing at 1.6 microns
could be easily made to lase at the third harmonic (533nm), bathing the
control room in beautiful green light from the monitors.

Details of the initial lasing experience are given in the "Operations
Section" below.

With the establishment of cw operation, our THz team successfully
commissioned the

far infrared Michelson interferometer. We now have real time THz spectra
in the control room which gives a second measurement of the electron
bunch length.

For the next week we will be in laser operations through Tuesday
evening. On next Wednesday we shut down for a three day period for
installation of the UV wiggler, followed by the one week shut-down of
the entire lab for the holiday break.

**Management:**

We completed our ONR and Army project monthly financial reports for
November 2005 and forwarded these reports to our agency program
managers.

On Wednesday, Dec. 14^th^, Fred Dylla and Wayne Skinner (DOE-JSO)
visited ONR to thank Adm. Cohen for his dedicated support of FEL
technology development. Admiral Cohen will be leaving his position as
Chief of Naval Research in January after 5 years of service. We also met
with ONR management for their usual expert guidance on budget matters
given that all DOD funded projects are suffering from the present
Congressional "Continuing Resolution" for FY06.

**Operations:**

Last Friday, just before shutting down we noticed very bright flashes in
the optical transport when running very close to the synchronous point
at 1.6 microns.  We guessed that this was third harmonic lasing due to
the difference in the depth at which the light reflects from dielectric
mirrors.  The fundamental goes more deeply into the mirror and so
requires a slightly shorter cavity length to lase than the third
harmonic.  We were in this configuration again on Monday and took some
data on the performance vs. cavity length.  The pattern of green light
on the output coupler shows fairly clearly that the laser is lasing at
the third harmonic.  The light changes from a diffuse low- intensity
pattern with irregular side-lobes to a clean TEM00 optical mode.  The
extraction from the electron beam is also much less than when the
fundamental lases.\
We also lased CW and tuned the laser up on Monday.  We were able to get
over 1 kW of light at 1.6 kW with only 850 microamps of beam.  We also
tuned the laser continuously while lasing CW at 900 W from 1.5 to 1.72
microns, which is the bandwidth of the laser mirrors.  This is a good
indication of the quality of the wiggler. The temperature rise in the
mirrors with 1 kW of laser output power was very small, indicating
losses of less than 200 ppm.\
We continued trying to optimize the CW setup on Tuesday and had problems
with losses at the entrance to the wiggler.  Since this is a permanent
magnet wiggler we have to limit the loss at the entrance or risk
demagnetizing the NdFeB blocks.  We could run easily at 2 mA pulsed with
a 6% duty cycle but the losses at the wiggler entrance were too high to
allow CW running at

2 mA.  On Tuesday evening we recesiated the cathode so that we could run
much higher current.  We then tried to reduce the losses at the
wiggler.  We could reduce the losses by refocusing the beam but this
also reduced the lasing efficiency.  Thursday we checked to see if the
loss was due to non- linear dispersion.  It wasn\'t.  This means that
the loss is due to halo that is mismatched to the wiggler.  When one
readjusts the match to get lower loss the core beam is no longer
optimally matched.\
On Friday we modified the match so that the halo would not get through
and ran the current up to 4 mA CW.  We had to mask several IR sensors to
do this due to HOM absorption in the IR sensors.  An IR camera installed
on one of the RF windows indicates no significant real heating or
thermal gradients. The vacuum rose a bit in the first arc and at the
exit of the injector but things seemed to be under control with no BBU
or other problems.

**WBS 4 (Injector):**

The cathode was re-cesiated Tuesday night. The QE increased from 0.9% to
3%. This is the sixth time the cathode made on December 17, 2004 has
been re-cesiated. Diagnostics for signals in the injector cryounit have
been setup by Tom Powers in preparation for high current operations. We
also supported magnet work.

Gun HVPS - The Gun HVPS is operational with both driver stacks
functioning normally.

**WBS 6 (RF):**

RF - The RF systems are all operational. A copper screen was installed
in front of IR sensors for cavities 2-1 and 2-2. These cavities have
been experiencing Cavity Warm Window Temperature (CWWT) faults. These
CWWT faults have been one factor limiting the machine\'s ability to run
high currents and lase at high power. An IR camera was also setup to
view the warm windows on these cavities directly. No hot spots were
observed. The camera needs to be calibrated to determine the temperature
of these windows. Equipment has been setup to catch the events
surrounding trips in the Injector. So far, there have been no trips.

**WBS 8 (Instrumentation):**

  Tests that were performed for the embedded BPM electronics this week
included bench linearity and pulsed operation tests.  The linearity
tests provided proof that the electronics respond in the same fashion
throughout the dynamic range of the circuitry.  Measurements were made
simulating steering in both the X and Y direction by varying the
displacement between two channels from 0 to 20 dB.  This correlates to
steering about 11 mm in either direction.  The data shows that the
linearity has a nominal variation of about 150 microns and a worst case
scenario of 200 microns.  This is due to channel to channel variations
in calibration.  When a finer calibration was performed to improve this
the resolution was improved to 50 microns with a worst case scenario of
100 microns.  Once we were convinced we understood the linearity of the
electronics we needed to examine the problems we were having with the
dependencies  introduced by the bunch charge of the FEL beam.  In order
to eliminate this problem we then simulated the beam conditions of the
FEL for a bench test.  This required pulsed beam at different
frequencies and pulse widths.  When we swept through the RF input range
the data showed a dependency on the charge and introduced a measured
non-existing \"drift\".  With further investigation we found that the
sampling timing of the ADC needed to be adjusted in order to sample the
signal at the proper time.  As it was set the sample timing was
occurring while the RF front end was still responding to the input
pulse.  A delay was introduced and the data was taken again in the same
fashion.  This data run showed that the measured non-existing \"drift\"
was then eliminated and that the resolution is on the 50 micron level. 
The next step we are taking is examining the direction and modifications
that will go with the next revision of these electronics.  The graphics
below show what I\'m trying to explain.

![](media/image1.jpeg){width="4.25625in" height="3.9479166666666665in"}

Figure 1: Pulsed input with no Timing delay

![](media/image1.jpeg){width="4.063888888888889in"
height="3.770138888888889in"}

Figure 2: Pulsed input with Timing delay

      We now have a UHV Ion Pump Controller in EPICS. The controller was
setup for monitoring the VIP0F00A ion pump. The EPICS software for this
controller was setup for easy expansion. We can support up to 100
controllers on 1 IOC serial port. Data is being taken on the vacuum
pressure vs. analog output voltage.\
    Testing/Repairing defective AMS Video Crosspoint circuit boards. SMT
soldering the remaining 8 6kW Reversing Switch circuit boards.
Terminated and repaired 10 Heliax and SMA hardline cables for the VTA in
the Testlab. All the needed cables for the lab 3 THz hutch have been run
and terminated. A stepper motor cable was added that connects back to
the\
control room. A couple of the cables above the hutch have also been
cleaned up and put back in the cable tray. Preparation for cable runs to
the LPSS box in the lab 4 clean room also took place this week. Raceway
parts were ordered and the boxes that will be used for spacers for the
raceway will be painted by the end of work today.

 

**WBS 9 (Beam Transport):**

***Sextupole (SF)***

• We continue working on a plan for re-measuring the sextupoles. Because
of lack of funds, we will not start until funds are on hand.

***Replacement Chicane Dipoles (GW)***

• The remaining two units are at Magnet Test, awaiting the testing of
Sextupoles before they get measured.

***QX Quadrupole Measurement***

• Progress awaiting funds

***UV Line***

• Girder assembly remains on hold awaiting funds and a plan on how to
use the measurements we have on the QX Quads to commission them
effectively.

• The NEG Pump cartridge holder design for the second corner UV chamber
is on hold.

***UV Wiggler Progress***

• No progress on the new Wiggler vacuum chamber.

***IR Machine Re-commissioning and Operations***

• A plug in the Trim Rack whose reverse orientation doesn't allow the
bayonet slot to latch had come loose, causing three QX quads to go off
loop. We will work on the plugs in all trim racks during the Christmas
Down to eliminate this mode of failure.

• The shop made a 4 inch strip-line BPM (Using CASA funds) to lend to
DESY for the PETRA II machine. This will allow them to evaluate the
scaling at the larger size. This is part of the very successful
collaboration with Kevin Jordan wherein they coated two beam viewer
flags for us and instructed us in that technology so we could do it
ourselves.

**WBS 10 (Wiggler):**

We have now demonstrated wiggler tuning while lasing at high power.  We
still need to provide software to make this a user- friendly operation
but the wiggler itself works fine.\
The UV wiggler is on its way to Newport News.  It should arrive Monday
or Tuesday depending on weather.  The plan to lift the wiggler into
place has been reviewed and approved.  We received all the certification
documents this week and are looking them over.

**WBS 11 (Optics):**

***FEL mirrors:\
***As mentioned in the Ops section of this report, our new 1.6 micron
cavity mirrors finally produced cw light, at a max power (at this
writing) of 1.14 kW. The resulting intracavity power is too low to do
more than give an upper bound on their losses. We can state that it is
less than \~200 ppm, lower by a factor of two than the measured losses
(in the FEL) for the tantala/silica mirrors we had coated for the same
wavelength, and by the same vendor. This is a critical step in
validating our assertion that tantala is the wrong dielectric for FEL
mirrors. We hope to have more definite numbers later today. More of the
silicon witness samples were characterized and will be sent to a
commercial vendor for coatings applied by the IBS and the e-beam
technique, and to Prof. C. Menoti at Colo. State Univ.

#### Other Activities

The pump cart undergoing bake-out is now clean and ready for general
use. Our newest pump cart also shows some sign of contamination when
gently heated. We plan on baking it out as well to make sure it\'s
clean. This calls into question the whole spec on OTS cleanliness - we
meet that spec cold, but if the laser liberates contaminates, will it
generate problems for us? We will address this and try to resolve it
before the holiday shutdown. We are now working on the build-up of TMC 1
and the prototype OBPM, which will be installed in UL6. The phase noise
of the advanced drive laser was carefully measured this week and found
to be essentially the same as the MO, 325 fs. Occasionally, the noise
value spikes to 800 fs, probably due to environmental factors. (The spec
is less than 1 ps). Amplitude noise, as viewed from the autocorrelation,
continues to be excellent. We assisted Ops by installing sapphire
windows on the Zone 2 cryomodule, and an IR camera to look through each
one to image the warm ceramic window. We also swapped out cameras and
set apertures to better help with determining mode size and position.
And we installed a higher-capacity GFCI on the pump cart that maintains
the zone 2 guard vacuum.

#### UV FEL

The remainder of the gimbal mounts arrived. The scheduled ship date for
our ballscrews was yesterday.

**Terahertz:**

This week we successfully got the Nexus Michelson scanning in the vacuum
vessel.  This is part of the pump-probe system which is being planned
for future experiments.  In Lab 3, we utilized beam to study our
transport system.  We were able to put beam into the Nicolet Impact 400
Michelson in Lab 3 and use new home-written software to allow the
spectra to be viewed in the control room.