# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- February 2-6, 2004

**Date:** February 6, 2004

**Highlights:**

We had a very successful week with commissioning of the 10 kW upgrade.
The goal for the week was to achieve lasing at 6 microns with the high Q
optical cavity configuration (i.e., using the 2% outcoupling optics), so
that we could measure the laser gain at 6 microns and measure the
absorption loss in this outcoupler. We were successful on all accounts
and then some:

-   We successfully lased at 6 microns producing \> 1.5 kW cw of
    outcoupled light with the driver accelerator operating at 88.5 MeV
    and 3 mA.

-   During this lasing the intracavity laser power was 56 kW which is
    more than half the intracavity power that will be circulating when
    10 kW will be extracted from the low Q (10 % outcoupler) cavity
    configuration.

-   While the 56 kW of intracavity is circulating, we see no evidence of
    a drop in efficiency which would be indicative of excessive loading
    on the optics; in addition half of the mirror figure control current
    on the high reflector (HR) was needed to handle this power,
    indicating we should have the control we need for operation at or
    near 10 kW.

-   With the available high power 6 micron light, we were able to
    measure the total optical loss in the 2% outcoupling optic actually,
    (2.7-2.8%); a value of \~550ppm was obtaining which translates into
    the ability to couple out approximately 8.5 kW when we install the
    10% outcoupling optic (assuming the same coating loss.)

> (Note: we had asserted at the Semi-Annual Review that we would have
> lower loss optics at 6 microns than our previous attempts with 10
> micron optics which showed absorption greater than 2500ppm. We have
> additional 6 micron optics on order from multiple vendors).

-   We were able to boost the linac energy 10% to 88.5 MeV.

-   By accounting for the newly discovered gradient imbalance in the
    injector cryounit we achieved close agreement between our injector
    performance and injector modeling with the PARMELA code.

-   We obtained lasing gain measurements at 6 microns to compare with
    our codes and we made substantial progress in understanding the
    non-linear effects in the electron beam transport.

-   The third cryomodule for the FEL linac is ready for transport and
    placement in the FEL vault on Monday morning.

Details on these and other accomplishments for the week are given below.

**NOTE: HOLD THESE DATES:**

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

**Commissioning Summary:**

This past week has been a very good one. We learned a great deal about
the non-linear transport of the accelerator and started to get much
better agreement between PARMELA and the injector performance. We also
were able to push power up to half of the circulating level required for
10 kW operation without seeing any drop in efficiency.

During the last couple of days of last week we worked on \"window
washing\" in the cryounit and were able to push the power delivered to
the waveguide to over 20 kW. Note that the effective power for this
setup is quite high since most of the power is reflected back into the
circulator and creates standing wave anti-nodes with very high fields.

On Monday we started on the push to achieve 10 kW at 6 microns. To raise
the gain as high as possible we opted to raise the linac energy as high
as possible. By operating almost all cavities at their \"drive high\"
values, we were able to reach 88.5 MeV when operating 10 degrees off
crest. We did a careful match of the beam through the linac to the
wiggler at this energy. We also changed the backleg setup to have a full
1.5 periods of betatron phase instead of the single period we have been
operating at recently. This seems to have shorted the electron bunch. We
believe that this is due to cancellation of non-linear terms in the
first arc.

We also did an experiment to try to measure the relative gradients in
the two cavities in the cryounit. Results of this experiment indicate
that the first cavity is 10% lower gradient than EPICs is saying. To get
the injector energy this implies that the second cavity is running 10%
higher than EPICs is saying. When these values are put into PARMELA the
behavior is strikingly close to what we see in the experiment, including
the fact that the beam is converging at the exit of the cryounit. We
will use this information to re-optimize the injector over the next
week. We may want to balance the gradients in order to reduce the power
in the second SRF cavity.

On Tuesday we measured the dispersion, both linear and non-linear, in
the backleg. The non-linear dispersion may be the source of emittance
growth between the linac and the backleg. We found last week that the
emittance growth was the same at full charge and half charge so it is
not a collective effect. Using the new electron beam setup at 88.5 MeV
we were able to lase at 6 microns on Tuesday afternoon with the 98%
output coupler.

On Wednesday we optimized the laser at 6 microns and got a detuning
curve as long as 16 microns. This is close to what one expects from the
model. When we measured the gain it was only 21% compared to a
prediction of 30%. We did find that the gain was quite sensitive to the
setup but the power was not so it may not have been optimized when we
measured it. The resonator losses were almost exactly what we expected,
indicating that the losses were not extremely high. We also improved the
match to the linac so that we could run up the power with 6% duty cycle.
We found that we could not run at the peak of the detuning curve without
loss due to transverse beam blowup in the linac. This is due to the use
of a single family of sextupoles in the second arc. The energy spread
was at least 10% in arc 2 and this high an energy spread requires two
families of sextupoles.

Sextupole correction of aberrations provides improved beam quality at
the wiggler and control of beam loss during energy recovery. A
single-family sextupole pair is required for minimization of bunch
length at the FEL; a second single-family is needed to adjust bunch
length prior to energy recovery so as to provide sufficient energy
compression to remain within the machine energy aperture.

Use of a single family does not, however, allow correction of variations
of beam position with momentum. As a consequence, beam quality at the
wiggler is compromised by uncompensated second order dispersion, with
spot size growth on the order of 10% from this source \[1\]. This
corresponds to an emittance dilution of order 20% -- tolerable, though
nonideal.

Given recent progress in FEL high power performance, this effect imposes
however a more severe limitation during energy recovery. While lasing
strongly, an energy spread of order ±5% is imposed on the beam. Inherent
lattice aberrations couple to the large momentum spread, leading to both
betatron mismatch and shifts in the central orbit position while
traversing the second arc. The former can be managed by an appropriate
selection of the betatron matching solution, but the latter is
manageable solely through sextupole correction. Figure 1 presents beam
spots at three locations in the machine for various strengths of lasing.
Growth in spot size is linear at the first location -- in the center of
the 180o dipole -- but is not linear at the latter two places,
immediately following the energy recovery arc and midway through the
reinjection telescope. This spot size growth is consistent with both
analytic estimates and DIMAD simulation of machine operational practices
and set points.

On Thursday, using the new frame grabber system, we characterized the
non-linear dispersion beam growth in the second arc and the extraction
efficiency of the laser. The efficiency was not as high as expected
(0.7-0.8%), probably due to a non-optimized laser setup. We then set up
for CW lasing and slowly ramped the current up until 1.5 kW was achieved
(see figure 2). Note that this power output implies 56 kW of circulating
power in the optical cavity. The mirrors seem to have survived this with
no damage and the efficiency, though low (500-600 W/mA), was independent
of power up to 1.5 kW. This is a good indication that power levels near
10 kW are achievable with a 90% output coupler if the laser efficiency
and gain can be optimized. Losses in the mirrors, measured by the
cooling water temperature rises, show losses consistent with operation
at over 8 kW. We would have raised the power up higher but we had a
vacuum incident when the Brewster window through which we were sending
the beam cracked, letting the optical cavity and the second arc to high
enough pressure that we could not restart the ion pumps. Since we were
scheduled to shut down anyway we stopped there instead of fixing the
window (see figure 3).

The next week will be dedicated to getting the new cryomodule into its
commissioning position and getting the accelerator put back together. We
should then have a 90% reflective output coupler for 6 micron operation
available and can try to push the power up to over 5 kW.

![](media/image1.wmf){width="5.998611111111111in"
height="5.613194444444445in"}

> Figure 1: "Wes-cam" \[2\] screenshots of beam at center of 2^nd^
> 180^o^ dipole, exit of energy recovery arc, and midway through
> reinjection telescope, upstream of beam merge.

## Notes

\[1\] Dispersion measurements with ±2% offset present a systematic orbit
oscillation of order 1 mm independent of the sign of the energy change
at locations with spot sizes of order 3 mm. The orbit oscillation decays
more rapidly than linearly with decreases in energy. At the nominal beam
energy spread of ±1%, this corresponds to \~1/4 mm, or 10%, change in
spot size

\[2\] Courtesy of Wes Moore, with assistance from Steve Benson and a
reminder from Kevin Jordan about the viewer scales.

![](media/image2.png){width="5.329861111111111in"
height="3.6791666666666667in"}

Figure 2. CW operation at \>1.5 kW using high Q resonator. This
corresponds to 56 kW of circulating power in the resonator.

![](media/image3.png){width="5.522222222222222in"
height="3.8402777777777777in"}

Figure 3. Exit Brewster window after vacuum incident. The 1.5 kW beam
may not have been centered in the window, leading to fracture of the
window. It has withstood over 2 kW at 3 µm in the IR Demo.

**Management:**

We developed a detailed plan for reaching 10 kW with the Upgrade based
on the preliminary plan we presented at last week's project semiannual
review and the feedback from the review panel. The plan involves
interlinking three efforts: (1) the present mainline effort involving
the testing of lower loss optics with the present 2 cryomodule
configuration; and the commissioning and use of the 3^rd^ cryomodule to
boost the linac energy by 2x; (2) two options for modified wigglers that
could be incorporated within 12 weeks; and (3) and several low cost but
high technical risk options for enhancing 10 kW output. A technical
analysis of these options including cost/schedule implications was
completed this week and is being forwarded to the DOE and Navy program
offices and FEL review panel.

A preliminary agenda for the March 10-11 FEL Users meeting and LPC
workshop was prepared and is being distributed today to our user email
list and posted on the JLab website.

**WBS 4 (Injector):**

The photocathode gun delivered up to 4 mA of CW beam (350kV, 135pC) for
FEL operations during the week. The cathode QE is about 1% after
yesterday\'s FEL lasing success.

Early in the week we did a calibration of the gradients on the quarter
cryounit against PARMELA. The code predicts the beam is diverging coming
out of the unit for gradients set back in December 1999. However, the
actual beam is converging for the present injector configuration.
Assuming an uncertainty of ± 10% in the gradients, PARMELA predicts that
the beam converges if the CAV4 gradient is 10% lower than the Dec. 1999
value. Later, Steve verified than CAV4 was indeed running at 10% lower
gradient than the value read in EPICS. Now the beam behavior
(longitudinally and transversely) is closely matched by the code. This
is a great step forward in benchmarking our version of PARMELA against
the actual injector.

**WBS 5 (SRF):**

The third cryomodule is complete and will be delivered to the FEL vault
on Monday. It will be commissioned alongside the beamline without
impacting the 10 kW push in the machine. Once commissioned off line it
will be installed at the earliest convenience and should help the drive
to 10 kW, if that has not been achieved already.

**WBS 6 (RF):**

 RF - The software for the RF systems was upgraded last night after the
machine was shutdown for the installation of the 3rd RF cryomodule. All
working systems were operated into RFON and operated properly.

Installation of 3rd Cryomodule - The back leg of the machine is being
opened today to clear a path for the installation of the 3^rd^
cryomodule. An IOC is being setup so the commissioning of the cryomodule
can proceed in parallel with the continued operation of the machine. The
preliminary assembly of the RF waveguides has started, but will not be
completed until the cryomodule is located in its interim commissioning
position on Monday. This position is 5\' from

its final beam line position. Download files have been built and are
being updated with the external calibration coefficients.

**WBS 8 (Instrumentation):**

Much effort was put toward preparing systems for the new cryomodule. All
parts for the new temperature diode system are on site and being
assembled. Cables for the beam line, wave guide, HE vessel and in line
diodes were pulled into position as well as cables for the piezo tuners
(PZT), and the Higher Order Mode (HOM) loads were installed this morning
and are being terminated. DC to DC converters are on order for running
the PZTs. A great tutorial on these parts can be found at
[http://www.piezo.com](http://www.piezo.com/).  All the RF
instrumentation and controls modules were tested and brought on line
(thanks to the EES group for their support).  Prepared electronic route
for servers from Lab 7 to the various zones. The initial HOM families
will be recorded as soon as the module is cold. These will be tracked as
the module goes through the commissioning process.\
   Maintenance work has been done on a number of systems and will
continue this next week. Last weekend all \~1500 connections to the DC
trim magnet power supply patch panels were tightened. There were a
number of connections that needed tightening. The main box power
supplies will be done on Monday & Tuesday. Repairs are under way to make
the \"Coulomb Counters\" functional and calibrated.\
   The format was modified for the framegrabbers data files. These
provide a way to save all current settings and readbacks for reference
at a later date. Worked with Al Grippo, Kevin Beard, and Arne Freyberger
to establish a Gaussian fit routine that would require minimal testing
for a faster solution. Yesterday an algorithm was tested with successful
results. The implementation into the current code is now left with
testing to follow. The new machine for the framegrabber was finished
this week in preparation for the framegrabber\'s configuration. Some of
the libraries have been installed into the fresh kernel already.\
   The testing of the AMS/Video 32:32 channel crosspoint system (F02553)
is complete and finished system will perform well for us. All of the
results of the system\'s throughput performance (bandwidth and phase
data) and crosstalk measurements have been archived in the database.
This includes the embedded firmware and the list of corrections that
need to be made to the prototype. We are now ready to proceed with the
production of about 20 boards (F0253) for both the AMS and Video systems
upgrades. The documentation for this project is available at:
<https://laser.jlab.org/devlore/config.asp?Item=1463>\
   Drawings are complete for the front and rear panels for the chassis
being used to upgrade to the video system.

**WBS 11 (Optics):**

This report will be somewhat more lengthy than usual, since we did not
file a report last week.  During the week of the 26th, we received new
liquid quick disconnects, and verified that they were vacuum tight and
compatible.  This will greatly alleviate the torques imposed on our
optics from the cooling tubes.  (Kudos to Joe Gubeli and Mike Bevins for
thinking of this and expeditiously trying it.)  We also received the
stand for the insertable mirror used to route beam into the beam dump in
the Optics Control Room.  We also measured the absorbed power due to the
THz radiation on our 10.6 um OC mirror.  The agreement with the
calorimeter is not great, so more measurements are planned.

Last Saturday we opened the OC vacuum vessel to remove the 90%R (at 6
um) mirror, work on the cooling line arrangement, and determine why we
were having problems with the linear actuators.  For the linear
actuators, we determined that the problem was due, in part, to an
interference with the new cooling ring and the restoring spring that is
connected to the LVDT.  We also found some of the spring holders set
incorrectly, which adds to the amount of thrust the motors have to
provide to move the mirror.  Once these issues were remedied the linear
actuators worked fine.  The mirror removal was done to try cryocooling
it this week.  However, we found that it was loose in its holder, so we
have spent this week recoating the edge and remounting it.  It will be
ready for testing by Monday, then installed in the optical cavity for
higher power lasing.  We also determined that its reflectivity curve was
unchanged since it was new, so that was not the reason it wasn\'t
lasing.  Tuesday, we aligned the 98%R (at 6 um) mirror, and were
gratified to see that our work had resulted in a considerably improved
mirror figure.  We then lased on that mirror late in the day.  By
Thursday evening (when we had a Brewster window crack) we had produced
1.51 kW of output.  While operating at this power we recorded the outlet
water temperature and flow rates and were able to make some
determinations of the losses in both optics.  The loss in the
outcoupler, assuming a 5 W/mA contribution from the THz, is

\~ 550 ppm.  Somewhat surprisingly, the HR had a higher loss of 3700
ppm.  Even then, we still had half our control margin for the deformable
mirror.  Thus, we feel we have a good shot at high power once we
reinstall the 90%R mirror.  This is planned for early next week.  We
also made excellent progress in constructing a vacuum calorimeter that
will be placed in the vault next to the diagnostics hutch.  We\'ll use
it to determine the loss in optics we\'re receiving from vendors, and
from our IR Demo mirror set.  We also reinitiated efforts to look at
other bonding techniques for our mirrors.

 

We held planning meetings to scope the level of effort to implement the
recommendations of the Semi-Annual Review Committee.  Specifically,
meetings were held on the cryocooled ZnSe effort and the scraper
output.  A supporting document on the cost and delivery of optics to
support various wiggler options was prepared.  This week the FEL
equipment and staff endured a lot of microphonics and noise as the
penetrations for the UVFEL optical transport were drilled in User Lab 1
(which serves as the local office for half the Optics Group - sorry
guys, it\'s almost over!).  Another important milestone needed to get
the UVFEL brought to fruition.

**Terahertz Project:**

The bids for the mirrors were received.  In view of the large minor axes
of the ellipses, a free-form machine is necessary for manufacture.  Only
2 vendors could supply this, and only one vendor could meet our delivery
specifications.  The first mirror is scheduled for delivery on March
20.\
\
The final order was placed for the mirrors.  Vacuum components are
starting to arrive and are being consolidated in Lab. 4.