# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief -- September 8-12, 2003

**Date:** September 12, 2003

**Highlights:**

Since the last report (Aug. 29^th^), we took the FEL Upgrade off line
for all of Labor Day week for several activities: (1) check-out and/or
replacement of both the 6 micron and 10 micron optics; re-cesiation of
the gun; and completion of the core drilling of the ceiling of the FEL
vault to allow installation of the THz beamline later in the year.
Commencement of commissioning activities was delayed until yesterday due
to problems with the Central Helium Liquifier. The driver accelerator
was successfully brought back-on line yesterday afternoon and we have
started on a program to recover lasing at 6 microns, move to 10 microns
and then optimize the performance at 10 microns by varying the electron
bunch length. During the down period we collected valuable data on our
optics and have focused our next steps on optimizing the driver
accelerator under the direction of Dave Douglas. Stay tuned for more
light.

**Commissioning:**

In spite of time consuming limitations this week, we successfully
recirculated and energy recovered electron beam after more than a week
of down time. Most of the week was devoted to completion facility THz
penetration into Lab 3 and difficulties with the CHL (Central Helium

Liquifier) (See below.) We are now preparing for next week's studies of:

> • electron bunch mismatch to the optical mode,
>
> • 10° vs 20° off-crest running,
>
> • THz measurements of bunch longitudinal dimensions,
>
> • Heating effects on the OC mirror.

Commissioning activities were impeded by THz port installation and
problems with the CHL. This allowed considerable time for cogitation and
discussion of FEL performance limits, with an emergent consensus that
laser output was likely limited at least in part by longitudinal
mismatch of the electron beam at the wiggler. Observations supporting
this conclusion are largely heuristic rather than quantitative, but are
nonetheless compelling:

> 1\. PARMELA results for various injector parameter sets similar to
> those run during laser optimization - if propagated to the wiggler -
> result in phase space configurations which, when used as initial
> conditions in FEL simulations, closely match the observed laser
> performance. These same results suggest we were either over bunched at
> injection (resulting in excessive bunch length and low peak current at
> the wiggler) or very tightly bunched (\~50 fsec, in simulation, with
> large beam momentum spread) at the wiggler - too short & with too
> large an energy spread for optimum laser operation.
>
> 2\. A check of optimized Happek device output (a measurement of bunch
> length) at different linac phase operating points (10 and 20 degrees
> off crest) indicates that the linac and recirculator transport
> provides the anticipated longitudinal phase space rotation and
> compression. Compression occurred at roughly the recirculator trim
> settings predicted by the machine model and gave a factor of two
> stronger signal at 20 degrees off crest than at 10 - indicating the
> bunch was a factor of 2 shorter (as expected) at 20 degrees gang
> phase.
>
> 3\. Preliminary THz observations produced complex (and somewhat
> confusing) spectra - which, after discussion with Rui Li, were
> interpreted as possible evidence of microbunching, a phenomenon that
> could well occur at bunch lengths in the 10s of fsec range.
>
> 4\. Ongoing rumination on recirculator transport has regurgitated a
> particular point raised when the \"temporary\" DY was installed in the
> 1^st^ arc as a bridge measure until the \"permanent\" GY became
> available - because the DY cannot be run in series with the rest of
> the recirculator, the tracking accuracy and stability needed from the
> power supplies exciting both the DY and the GX/GQ/GW string are an
> order of magnitude tighter than the installed system specs (due to the
> large individual compactions generated in each substring). As a
> consequence, the temporal jitter of bunch at wiggler is likely at
> least an order of magnitude larger (10 of fsec) than either the
> nominal configuration will provide or that occuring in the IR Demo.
> Coupled to the likely use of ultrashort bunches, it is entirely
> possible that optical mode/electron bunch overlap has been severely
> compromised. As a point of comparison, the Demo mode/bunch duration
> was \~400 fsec, with \~10 fsec jitter (primarily from RF/momentum
> compaction) while in the Upgrade, we could potentially have 50 fsec
> bunch duration with, e.g., 10 fsec from RF jitter, 30 fsec jitter from
> the DY (assuming the spec\'d 10\^-5 field stability) and 100 fsec
> jitt.

**Management:**

At this week\'s International FEL Conference held in Japan, papers were
presented on the \"First Lasing of the IR Upgrade FEL at Jefferson Lab\"
by Steve Benson, \"The Jefferson Lab THz/IR/ UV Light Source Facility
(in the satellite FEL User\'s Workshop) by Michelle Shinn, and \"A
Comprehensive Study of the Phase Noise in Mode Locked Lasers\" by
Michelle Shinn.\
\
The bi-monthly financial report for the 10 kW FEL project for the period
Aug.16-Aug 31\
was forwarded to the DOE contracting officer and the ONR program
office.\
\
On Thursday, Sept. 11 the FEL Facility hosted a tour for 36 attendees
from the Directed Energy Professional Society meeting held this week at
the Northrup Grumman Newport News R&D Center.

**WBS 4 (Injector):**

Last week we did a heat clean of the wafer and did a new activation. We
achieved 6% QE on the cathode. Work continued on the second stack with
the electrodes being pulled for modifications in the shop.

Gun HVPS - The Gun HVPS was tested with its 2nd drive stack integrated
into the HVPS. The indicated current jumps up at about 245 kV and
increases linearly as the voltage is increased. The voltage remains
quiet and regulated at all voltages.

**WBS 6 (RF):**

Commissioning - This has been a very difficult week. Monday was lost due
to the THz port construction not being finished on Saturday. Tuesday and
Wednesday were lost due to CHL problems with NL 10 on the CEBAF machine.
Thursday was used to run and exercise the machine in Hot Checkout. All
systems ran well. Beam was recirculated and energy

recovered. The optics group was able to gather data on the stability of
the FEL mirrors. Friday to machine was turned over to D. Douglas for
bunch length and difference orbit tests, but LCW leaks were discovered
in 2 magnets, the Quarter 4 HVPS had a problem with a Fast Body Current
interlock chassis, and at 13:15 CHL tripped again.

RF - The RF cavities were badly detuned due to the many CHL trips, but
all RF systems were up and operational within 1 hr. They operated
without any problems the rest of the day.

**WBS 8 (Instrumentation):**

Operations support - In the normal fashion, support for machine
operations and shift-work continue to demand our time.

-   The HR Mirror Heater Control Chassis is 95% completed, and has
    passed initial testing.

-   The Heater Control PCB design has been completed and documented, a
    manufacturing quote is in progress.

-   A second Acromag fanout card has been assembled with a front panel
    for an EPICS interface for the chassis. More front panels will be
    fabricated to accommodate future cards as well as existing ones.

-   The design for the front panel of the GC Power Supply has been
    completed and fabrication is in progress.

-   Completed fabrication of 2 GC power supply chassis and are ready to
    be installed.

-   Installation of the SLM attenuators continued along with the ND1 and
    ND2 filters inside the vault.

-   Drive Laser Pulse Control - We corrected an issue with the
    micropulse frequency generator board.

-   The peak voltage of it\'s drive signal to the Electro-Optical Cell
    (EO cell) was down by 20%. This was due to an oversight of its
    dependence on the supply voltage setting. After modifying the supply
    voltages, the signal is now running to specification. With this
    done, we are now operating the drive laser with two EO cells for
    increased contrast ratio from the drive laser pulses.

-   AMS/Video Upgrade - The new AMS/Video switcher chassis design is
    proceeding well. The schematic for the new core crosspoint module is
    being finalized as is the placement of the connectors and other key
    components in the design. With the schematic done we are beginning
    the board layout today.

-   In parallel, at 16 channel breakout board is being developed. This
    boards will gather the AMS and video inputs from the machine and map
    the signals to the high-density connectors being used in the core
    crosspoint module design. This board is done and has been sent-out
    for prototype today.

-   Linux-based EPICS for Emittance Measurements - The Tcl/Tk script
    support is in place and testing is underway. The integration and
    timing is being worked out for the operations now available in the
    Tcl script. Users now have the ability to save the recorded data
    into files for use with other software, allowing them to perform
    calculations and simulations of their liking. The data files consist
    of: background measurement, row sums, column sums, all current
    settings, and the image. Also, The acquisition of a more advanced
    video capture card has been approved. This will allow for total self
    containment of the Framegrabber software. No outside software, such
    as Max Video, will be a requirement for proper calculations of
    emittance. The beam sync problem will be solved local to the system.

-   Modifications to the Charge/Dump Current Monitor Chassis and the
    Buffer/Driver Box attached to the dump are being documented and the
    drawings updated. System drawings are being created for this system
    and the Dump Water Control System. Procedures for tagging out the
    magnets and certification of the Machine Protection System are still
    being fine tuned and should be out soon.

**WBS 9 (Beam Transport):**

***Dipoles***

[Bend/ Reverse Bend Dipoles ((GQ/GX)]{.underline}

• I designed and started manufacture of an aluminum and brass wedge
system. The system is a substitute for the wood wedges at the end of the
gaps in the pole tips in GQ serial \# 2 & 3. The wood wedges were placed
there to keep the end face shims from moving from under their clamps,
into the gap under magnetic excitation. Pins in the shims of the other
GQs eliminate the need for the wedges in their gap

***Chicane Dipoles (GW)***

• I designed and am having fabricated a set of brackets to add better
adjustability to the field clamp adjustment system of this style of
magnet. We will install the system on to the

6 magnets for the UV branch before they are measured.

[Arc 180 Degree Dipoles (GY)]{.underline}

• The second magnet is assembled and ready for testing.

***Installation:***

• The position of the Field clamps in the Bends (GX) and Reverse Bends
(GQ) of the Second Arc was checked during the down period this week. A
discrepancy was found in one field clamp of the second GQ where the
vacuum chamber was resting on the clamp and distorting it. We released
the pressure by adjusting the support clamps on the chamber and the
clamp sprung back straight.

• During the check we took notice of the actual field clamp adjustment
numbers. They turn out to be the same for all magnets, showing that the
construction was consistent within the factor of 1 part in 1000
necessary for magnets to qualify within their family. The core machining
folks (Process Equipment Co.) did a good job.

**WBS 11 (Optics):**

The past two weeks of limited operations have allowed for quite a bit of
optical work in the vault. The first priority was to correct the issues
we found in the output couplers during the previous week of operations
(Aug 25 to 29). The 6-micron output coupler-cooling ring was machined to
allow for greater clearance when mounted in the can. After pump down,
this optic showed far less aberration and ought to be able to lase. One
of the two 10.6 micron Output couplers was also removed during the can
opening. This optic was tested for absorption in the MTS. These tests
showed the optic was absorbing less than .2% of incident light from a
CO2 laser. The optic has since been un-mounted and returned to the
vendor for further testing. With the machine is running again,
experimentation is being performed looking for possible correlations
between the thermal distortion and synchrotron radiation.

The extended down allowed for advances in other areas as well. All major
components of the OCMMS system are installed and controlled remotely.
This allows for system measurements, which will define the feedback
circuit parameters. This system is also being used to monitor the
thermal distortions of the OC optics. The optical diagnostics suite in
the vault was found to have laser damage from the previous weeks of
running. The ND in the filter wheels in line with the spectrometer and
Pyrocamera were increased to prevent this in the future. Replacements
have been ordered.

On the drive laser the third EO cell was re-installed to facilitate
accelerator machine studies. The newer shutter was also left in place to
allow for maximum flexibility. The timing-jitter of the Coherent laser
for the pulse stacker was also characterized.

**Terahertz Project:**

A preliminary design review meeting was held at JLab with Vincent
Christina visiting from AES.  All major parameters were set allowing the
detailed design to commence.  It was decided that M1 will be movable
using in-vacuum motors, M3 and M4 will be externally adjustable.  The
penetration from Lab 3 into the vault was completed without problems,
drilling took place from both directions and met accurately in the
middle, thanks to the extreme skill of our survey team.  This provides a
10\" ID path for beam transport. Oleg Chubar and Paul Dumas ran the SRW
code to calculate the electric field of the emitted light so that we can
optimize the mirror figures.  The code had to be modified to take
account of the near-field geometry.