# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- June 14-18, 2004

**Date:** June 18, 2004

**Highlights:**

This is the week that Jefferson Lab's FEL Upgrade became **a 10 kW class
machine** and we established several new world's records;

> \- operating at duty cycles between 12 and 30%, the FEL produced
> **11.5 to 10 kW** of (macropulse) average power, respectively, of
> infrared laser light at a 6 micron wavelength
>
> \- operating at 48% duty cycle (8 ms pulses at 60Hz) the average power
> output was **8 kW**
>
> \- operating at 100% duty cycle (cw), the average power was **6.2 kW**

The machine was operated for more than a 2 hr continuous period during
this high power demonstration, generating more than **18 MJoules** of
output light. The measured parameters of the driver accelerator
operating at 145 MeV and 4-5 mA were constant in terms of allowed energy
and phase drift and compaction of the delivered electron beam to pulses
within the wiggler as short as **420 fs**. FEL efficiencies of up to
**1.7% (2.2kW/mA)** with energy recovery were demonstrated.

The FEL in its present configuration also generated world record powers
of THz light: more than 85 watts. Based on this week's measurements the
observed decrease in lasing efficiency with duty cycle is simply the
result of one parameter: distortion of the optical figure of the output
coupling mirror due to heating by the THz light. In a near concentric
cavity, our zinc selenide mirror can handle approximately 30 watts of
absorbed power before lasing efficiency is degraded. During the above
demonstration no more than 20 watts of absorbed power in the mirror is
due to the IR lasing whereas more than 45 watts is being absorbed from
THz emission generated immediately upstream of the outcoupling mirror.
Estimates of the distortion produced (3 microns or half a wave of 6
micron laser light) by such heating are in general agreement with the
measured changes in mirror radius of curvature. This weekend we are
interchanging the placement of the outcoupling mirror and our high
reflector mirror which should effectively minimize this THz heating
distortion. Following a test of this new configuration next week, we
plan to install the new permanent magnet wiggler which is optimized for
power production in the 1-3 micron range.

For our readers who have been following our weekly reports since the FEL
program at Jefferson Lab was launched in June of 1996, you will notice
that this machine has a propensity for showing its colors on June
17^th^. On June 17,1998, first lasing was achieved on the IR Demo FEL.
On June 17, 2003, first lasing was achieved on the FEL Upgrade and
yesterday on June 17^th^, the above spectacular results were achieved
and enthusiastically appreciated by all in the control room. We are
sincerely pleased to share these results with all our supporters
(particularly our benefactors in the US Navy), and thank you all for
your support and encouragement as we continue to bring this new machine
to full specification.

**Commissioning Summary (Steve Benson):**

It has been a very illuminating week, both literally and figuratively,
as noted in our highlights. We can now routinely set up the accelerator
to produce a laser efficiency of over 2 kW/mA.  We now know how to ramp
up CW power and recover lasing after a trip.  We found a design flaw in
the OCCMS that, once fixed, will allow much easier control of the mirror
steering.   We demonstrated operation of the skew quad rotator. 
Finally, we discovered the source of a drop in efficiency seen
previously.  We are now in the process of fixing this problem and expect
to be able to obtain approximately 10 kW at full 100 % duty cycle next
week.\
On Friday and Monday we worked to get a lattice that could transport
high charge beam with little loss, had a high BBU threshold, and had
good lasing efficiency.  The use of the Wescam (tm) to match to the 3F
region was very successful and a good match was obtained in only two
iterations.  The previous method, using total beam size envelopes
instead of rms spot sizes, did not converge.  The new match allowed us
to tune the vertical phase advance to increase the BBU threshold to over
5 mA without increasing the losses.  There was still a little bit of
loss at the wiggler entrance.  Dave then matched to the wiggler and
found a configuration with low loss that still had a BBU threshold
higher than 5 mA.  Using this configuration we obtained over 5 kW at one
point at 3.5 mA of beam current and could operate for a while at \~4.5
kW.  We were going to go even higher but the high voltage power supply
for zone 3 went crazy and started cycling the variac. This led to large
phase shifts in zone 3 and we could no longer run CW beam.\
On Tuesday we recovered the setup from Monday and looked at using the
OCCMS to monitor the mirror steering induced by absorption of laser
light and THz radiation.  We found out that the HR OCCMS shifted quite a
bit when running 5 mA beam with no lasing.  This must have been due to
absorption of wake fields produced in the OCCMS by the start of the
smaller wiggler chamber in the fused silica viewports.  The OC OCCMS is
better and can be used to monitor the position of the mirror at lower
power.  Some more measurement at \"low\" power (only 1.2 kW) indicated
that mirror heating only shifts the output coupler and not the high
reflector.  This means that we can hold the mode position constant on
the mirror using the output coupler steering alone.  When the laser
trips off one can then use the OC OCCMS to take out the heating induced
steering and restart the laser.  This information has made running the
laser at high power much easier.  Armed with this knowledge we then ran
the current up to 5 mA and obtained

6.2 kW CW power.  This was at 37.43 MHz.  We then tried to operate at 75
MHz but had some loss problems in the linac.\
On Wednesday we decided to delay the installation of wiggler due to the
fact that we had demonstrated clean transmission with no BBU trips and
high laser efficiency.  We then looked at the efficiency of the laser as
a function of micropulse repetition rate.  We ran at 4.678 MHz,

9.35 MHz, and 18.7 MHz.  The efficiencies obtained were 2.4 kW/mA, 2.15
kW/mA, and

1.75 kw/mA respectively.  We need 2 kW/mA at 5 mA to obtain 10 kW.  The
falloff in the laser efficiency might be due to the electron beam or the
mirrors.  We therefore looked at the energy, energy spread and THz
spectrum for all a range of micropulse repetition rates from 1 MHz to
18.7 MHz.  The values of all three 5 were the same at all repetition
rates.  This makes it highly unlikely that the electron beam is at
fault.  We also measured the mirror heating of the output coupler vs.
repetition rate.  The specification for a ZnSe mirror is that it must
absorb less than

30 W at 5.7 microns for the efficiency to be unaffected.  We saw up to
55 W absorbed at

18.7 MHz.  Most of this is THz and the distribution of THz is not well
known.  If it is uniform it should not produce as much aberration and
should allow higher absorbed power before the efficiency drops.  We
tested this by lasing with 10 msec pulses at over 10 kW of laser power
(100 J/macropulse).  The mirror could be seen to distort during the
macropulse using the OC OCCMS.\
On Thursday we tried to find the reason for the 3 micron mirrors\'
strong steering seen last week.  We found that, if the cavity is
operated such that the laser spot on the high reflector is hitting the
mirror high and to the left, the cavity does not steer.  We also found
that the radius of curvature control is not working for that mirror.  We
were able to lase with up to 1.5 kW of laser power at 1.1 mA of beam
current.  We could not optimize the Rayleigh range however since the ROC
control was not working.  A possible explanation of both problems is
that the mirror has debonded from the metal substrate except in the one
location in which it lases well.  Without any bond over most of the
mirror, the ROC cannot be controlled.  If the laser spot hits any spot
other than the one where the mirror is still attached, it distorts
strongly.

Finally we come to the week\'s exciting crescendo: we took some data on
the efficiency at

6 microns as a function of duty factor.  We could lase at up to 48% duty
cycle at 5 mA but the efficiency started to drop for a duty cycle higher
than 24%.  We could still lase at 10 kW with a 30% duty cycle (5 msec at
60 Hz.).  The data is shown in the attached figure along with the data
mentioned above for CW lasing.  The surprising thing about this data is
that the efficiency starts to drop exactly where the theory says it
should.  This is surprising since most of the power is THz radiation,
which does not necessarily have the same transverse distribution as the
IR laser power.

![](media/image1.jpeg){width="3.7847222222222223in" height="2.775in"}

So here is where we stand.  The lasing efficiency is larger than that
required for 10 kW as long as the absorbed power is less than around 30
W.  The laser power absorbed in the mirror is expected to be around 30 W
at 10 kW output power.  The problem is that the THz edge radiation adds
85 W on top of this.  There are three solutions to this problem.  The
first is to lengthen the micropulse without reducing the efficiency. 
The spreadsheet model says that this will not work, though we might get
to over 8 kW that way.  The second is to cryocool the mirror.  We would
need to be able to pull off at least 120 W of power from the mirror.  We
do not have a cooling scheme presently that will pull that much power
away without leading to unacceptable mirror vibration.  We can only go
up to 50 W.  This would allow us to get to 5 kW at 2.5 mA but then would
fall apart.  Finally we can change the direction of the cavity so that
the output coupler is upstream.  This means that the high reflector,
which is backside cooled, will have to be able to absorb the THz
radiation.  We have seen high levels of power absorption in our high
reflectors without significant distortion.  We therefore are going to
reverse the cavity today and this weekend so that we will be able to
lase at 10 kW cw next week.  We will start installation of the permanent
magnet wiggler on Friday.  If we have demonstrated 10 kW it would be
used to push to shorter wavelengths (1-3 microns) which are the Navy\'s
interests.  If not it will be used to get 10 kW cw at 2.8 microns using
a new mirror set.  The THz absorbed power is less at 85 MeV so it may
not be a limit there.\
We have now demonstrated that the skew quad rotator works so, in
principle, we can operate at much higher than 5 mA at 145 MeV.  This
will give us other options to pursue if needed and other opportunities
to learn about BBU mitigation.

**Management:**

The FEL program received excellent marks from the peer reviewers at this
week's DOE review of Jefferson Lab's science and technology programs. We
were congratulated for having established leadership in FEL and energy
recovered linac technology, having a positive impact on the Lab's
primary nuclear physics mission, and for nurturing a productive user
community.

A full report is expected by mid summer.

We submitted project monthly reports to our DOE and DOD program offices
for May 2004 for our JTO funded projects, the Army funded THz project
and the AFRL funded UV FEL project.

**WBS 4 (Injector):**

The Coulomb counter is at 1488.65 Coulombs after FEL ops this week. The
cathode re-cesiated on Monday, June 7, delivered 99 Coulombs, a record
high for the Upgrade Photocathode Gun and almost a factor of two more
than the previous wafer. This is another result from the improved vacuum
in this system.

In the first cathode we made on this AXT, anodized wafer we got 6.2% QE.
In the previous re-cesiation we got 6% QE. For this last re-cesiation
(completed today) the QE is 5.8%. This means we recover 96.7% of the QE
after each re-cesiation. For the first wafer installed in the gun we
used to recover 90% of the QE after each re-cesiation.

**WBS 6 (RF):**

All the rf zones performed very well this week during our intense high
power operations. It is particularly impressive that the system behaved
during high current, long pulse operations for the efficiency vs. duty
cycle measurements quoted in the highlights and commissioning section

**WBS 8 (Instrumentation):**

Another great week with progress on many fronts. The studies of THz
loading on the output coupler mirror highlighted the flexibility of our
drive laser control. The signal from the Nicolet THz spectrometer have
been brought upstairs and are being analyzed for use as a real time
bunch length monitor. The first step is to use a LabView digitizer to
capture the raw data from the detector as the mirror scans then take an
FFT and pass the array to EPICS. The PC is setup with LabView and the
digitizer will arrive in a couple days. Tom Powers is pitching in to
assist in the programming. (Thanks Tom!)\
    An upgrade to our Laser Personnel Safety System (LPSS) is being
addressed to accommodate all of the new requirements for the User-Labs
and the Optical Control Room (OCR) with the FEL. As the existing design
for these systems make heavy use of an old style of Programmable Logic
Controller (PLC), much of the functionality is embedded into the PLC\'s
ladder logic. Although the Fabrication, Wiring and system diagrams are
well documented as are the performance certifications, the code in the
PLCs has been inaccessible to all but the programmer. Meetings with
Omron and Siemens held to discuss purchasing PLCs for the LPSS. Follow
up meetings are scheduled for Wednesday, June 23^rd^ and Tuesday June
29^th^ for software demos. A representative of Omron will provide a
private tutorial of the software on Tuesday, June 22^nd^ geared for
first time use. Siemens will be providing a disk of their S7 software.
Block diagrams drawn for the two PLCs of the master chassis of the LPSS.
Also this week we have gathered all of the PLC programming files that we
have been kept under the care of the programmer and have archived them
in the Controls Database (laser.jlab.org) for easy access to review. As
is the case with so many GUI based programming tools, the files are only
immediately useful to persons with the development tools (Taylor
ProWorks NxT) and are not easy to view from the perspective of someone
doing a survey of the installed system. For this reason, we have taken
screen shots of all of the ladder logic diagrams or the I/O networks
programmed into the User Lab LPSS. The same will be done for the LPSS
Master Control Chassis as well. This information is immediately
accessible from the following link:\
<http://laser.jlab.org/devlore/Main.asp?Item=4478>\
    The Upgrade AMS/Video Systems are nearing completion. This week the
AMS input/output scalar boards have been finished (signed off and in
DCG). The boards will be sent out for production by COB today. With
these boards finished, we are ready to begin our installation/upgrade to
the installed systems in early July. Work continues Setup the special
integration card for the Cohu cameras. Once fully working with the Scion
capture card the hard solution was then simplified to duplicate the
signal using a delay/pulse generator. ITV0F04, SLM2F08, and SLM5F02 have
been modified to accommodate the triggering solution for the
framegrabber. Testing will begin first thing next week. Video muxing
(multiple images on single monitor are being researched for the control
room upgrade. Focus was on MyCable and RGB Spectrum\'s control/tactical
room video display systems.\
    This week testing of the existing system of beam position monitors
continued, along with the testing for the new prototype filters. 
Software is being written to accommodate the prototype for its interface
with EPICS.  During this process, I have been testing the old system for
current response with beam from the FEL.  The data of the response of
the old system will be used to consider the style of monitors that will
be used in the new.

    The second I&C Pre- amp Chassis was completed and tested
successfully.  Fabrication of an additional 10 Channel RTD chassis is
near completion. Magnetic sensor relays were installed above both entry
doors and above the Lab 3a/Lab 3b throughway.  Fabrication for
installation of lab status lamps, door exit assembly, and wiring harness
is complete.  The door exit assembly has been wired and is installed. An
additional camera was also installed for Lab 3a.\
    The rotator skew quad for the 3F region installation and checkout
was completed. Continued revisions to the CCD camera assembly manual in
order to ensure clear concise directions, and to match the preexisting
format for Jefferson Lab manuals. Effort continued on the AutoCAD
drawing of SLM attenuator box. \
    The quarterly EH&S Inspection was conducted this week with only 11
findings, two of which have already been closed.

The front and rear panels for the 10 Channel RTD Chassis are out for
fabrication.  The drawings for the Charge/Dump Current Monitor Buffer
Driver Board have been revised to reflect changes that have been made to
increase the gain of the amplifier.  12 new boards are on order. 
Revised drawings for the Si Diode Thermometer Board have been received
for review.  \"Lock, Tag & Try\" training was held; all I & C personnel
are current.

**WBS 9 (Beam Transport):**

***Skew Quad Rotator - Skew Quadrupole Eigenvalue Exchange Module
(SQEEM)***

• Done. It was powered and its function of rotating the beam
demonstrated.

## Wiggler 

######## Electromagnetic Wiggler for 2.8µ 

• The o-rings grooves in the manifolds were too deep and didn't function
as planned so we substituted flat gaskets. By that time, the test stand
was needed for a critical CEBAF magnet. At the same time, the permanent
magnet wiggler is slated for first beam trials. We postponed the EM
wiggler tests until the stand is available.

***Sextupole Lites***

• We designed the cooling plates and assembly of the magnets that will
get a 100% coil upgrade.

***Sextupole (SF)***

• Procurement on hold, pending 10 kW efforts

**WBS 11 (Optics):**

**Plan A is to obtain low loss 6 um optics. \
** As discussed in the Highlights and Commissioning sections, we
continued to meet or surpass the design goals for lasing efficiency as
long as the average current remained low, or the duty factor stayed
below \~30%.  Both results point to a higher level of intercepted (and
absorbed) THz power that produce sufficient spherical aberration to
reduce the laser efficiency.   We have a new record for stored power in
the optical cavity, 138 kW, for 2ms, 60 Hz operation.  Perhaps more
significantly, the optics did not damage when operated at 125 kW
intracavity power for

10 ms.  From a laser-damage standpoint, operation for such long
macropulses is equivalent to operating cw.

 

**Our three Plan B\'s had the following progress:\
*Cryomirror:***\
We await the opportunity in our schedule (and our braze expert\'s)
to produce a uniform indium annulus prior to brazing the mirror into
it..  A higher priority is to have spare, conventional, mounts for the
outcoupler mirrors we\'ll install later this month.  This was
accomplished; we have two each 3\" and 2\" mounts. 

***2.8 micron mirrors: ***\
The 3\" dia OC mirrors arrived Wednesday and we are doing metrology on
them.  We will mount both the 2\" and 3\" versions next week, so they
may be installed during the down period for the wiggler installation. 
We had the opportunity to do in-situ calorimetry on a set of 3 um
mirrors; the OC had an absorption level of 360 ppm at the laser
wavelength. The HR level was \~ 750 ppm.  We will operate the FEL with
these mirrors several times next week to do calorimetry on the witness
samples for the mirrors we plan to install. 

***Scraper outcoupler:***\
No progress since the last report.  To facilitate the reconfiguration of
the optical cavity, yet permit our operation at \~ 3 um for calorimetry,
we are removing the scraper beam dump and the HR mirror that is in the
OC optical assembly. 

 ***Other activities:***\
This week was spent supporting operations, and collecting a lot of
useful data on the behavior of the optical cavity under high heat
loading.  We also worked on preparations for the installation of the new
mirrors during the down, yet expedited a few parts, such as the
gold-coated, fixed mirror clips (which clearly intercepted some power),
so we could install them today.

We supported operations, such as repairing a broken HeNe line filter in
the OC OCMMS, a worn pinion gear on the OC vacuum rotary feedthrough,
and regaining remote control of the RGA on the OC vacuum vessel

The vendor for the loaned streak camera sent a module with more time
resolution, and we will install the camera in the vault today.

We installed a PyroCAM pyroelectric camera and a focusing lens on the
THz port, and have recorded images of the beam profile.

**Terahertz:**

M1 has now been tested for finish, and both mechanically and optically
for figure.  It exceeds specifications and is fully qualified for
installation.  Work on the conventional construction of the new THz
facility, Lab 3a, was completed this week, and we began moving
components in and setting up the laboratory.  This included the first
optical bench.  M1 is now being mounted on the manipulator in
preparation for installation, together with the Phytron motor drives and
the copper cooling braid.  M3 (a plane mirror) was shipped from the
vendor this week.  This mirror will be evaluated at Brookhaven\'s
metrology facility prior to any polishing.  There is evidence that
polishing can degrade the finish quality of soft aluminum mirrors and we
want to end up with the best surface possible.  The mounts for M2, M3
and M4 are ready for assembly.