# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief - January 31 -- February
4, 2005

**Date:** February 4, 2005

**Highlights:**

We completed a very successful series of beam break up (BBU) experiments
thanks to the efforts this week by Chris Tennant, Eduard Pozdeyev, Dave
Douglas and Steve Benson. The work that we proposed more than a year ago
(at the Oct. 2003 TAWG meeting) to our Navy program managers and FEL
colleagues on this important topic has been completed. A brief summary
is given below by Chris and more extensive analyses will appear in
Chris's upcoming Ph.D thesis and follow-on journal articles. As with all
good experiments we believe we will be returning to this important topic
of analysis and suppression of current limiting modes on srf
accelerators as beam time schedules permit.

We also had our first look at lasing with the broad-band output coupler
that was installed at the end of last week by the optics team. We
successfully lased at 2.8 microns at 80 watts cw which In addition we
had an initial attempt at lasing at 1.7 microns with the same broad band
optics. We are just beginning to learn some interesting things about the
effects of cavity astigmatism on FEL performance as indicated in Steve
Benson's report below. Testing of the high power

1 micron optics (which were also installed last week) is planned after
we re-set up the machine for 140 MeV operation starting next week now
that the BBU experiments are completed.

We continue to be very pleased with performance of the photogun. The gun
has delivered cw currents over 6 mA for the BBU experiments and has also
delivered more than 400 coulombs since the last recesiation. This record
more than doubles our previous record. We believe the increase is due to
a combination of improved vacuum (already in the 10^-11^ torr range) and
a change in the type of GaAs we now use for the cathode.

**Management:**

We held our monthly meeting of FEL team WBS managers with the Project
Management office to review the monthly cost-schedule performance for
January for both the Navy and AF programs.

We had to re-adjust our FEL run schedules for February to due requested
down-days by the Accel. Div. to support central helium liquefier tests
today and the a series of maintenance activities the week of Feb.
14^th^.

We had a meeting with the ME group this morning to check the detailed
engineering layout of the magnetic components in the UV chicane in the
first complete translation from Dave Douglas's DIMAD file.

**Beam Operations:** (Steve Benson and Dave Douglas)

The first half of the week was spent supporting BBU measurements.  This
went well with the exception of an uncertainty in our current
measurement.  At the threshold the current monitors agree, but they
disagree at high currents.  We were able to run up to either 6.5 or 8 mA
with the phase rotator depending on which current monitor you believe. 
We will be taking more data on this next week to pin down the current
more accurately.  More details on the BBU are given below.\
On Wednesday we recovered the 88 MeV lasing configuration and lased with
the new hole-coupled resonator.  We were able to get up to 80 W from the
FEL at 2.8 microns and found that the laser started to mode hop near the
peak of the detuning curve.  This is what one expects for a hole coupled
resonator.  The mode tries to avoid the hole.  The higher order modes
couple less strongly to the electron beam but have lower losses.  At
small detunings they have enough saturated gain to lase.\
We then raised the energy to 115 MeV, phased things up and matched to
the wiggler.  We obtained weak lasing at 1.7 microns.  The transverse
match in the wiggler was not nearly as good as the 88 MeV configuration
and short wavelengths are more sensitive to the match so we think that
stronger lasing is possible.\
On Thursday we spent more time optimizing the FEL at 1.7 microns but
were unable to obtain lasing with the newly optimized cavity.  We now
have a procedure that should make lasing at the shorter wavelength
easier.\
One sidelight discovered last week is the sensitivity of the FEL to
astigmatism.  This is a subject that has not received any theoretical
analysis to date.  I applied a simple model to estimate the dependence
of the laser gain on the astigmatism vs. the FEL cavity parameters.  I
found that the sensitivity to astigmatism varies linearly with the
cavity magnification.  The magnification of the new cavity is a factor
of 5 larger than the cavity for the optical klystron.  The astigmatism,
seen in the resonators for the optical klystron, should not have had any
affect on laser operation.  The same astigmatism is a much stronger
affect in the new 2.8 micron,

1 micron, and broadband cavities and so one might expect that the gain
or efficiency is reduced.  The curious thing is that we have not seen
any evidence for astigmatism in the mode on the mirrors.  Again, there
is no theoretical analysis of the effect of hysteresis on the FEL so we
don\'t know if we should see something or not.  It could be that the FEL
reactive gain modifies the mode to keep a circular mode on the mirrors. 
The good news about this is that the dependence on astigmatism depends
on the cavity magnification but not on the wavelength.  This is good for
the UV but may be a problem for short Rayleigh range operation.  We are
looking at ways to reduce the astigmatism in the mirror we now have to
be at least a factor of 5 smaller than what we had in the optical
klystron.

**Summary of Beam Breakup (BBU) Experiments (**Christopher D. Tennant)

This week marked the conclusion of an incredibly successful run of BBU
experiments at the FEL. There were two primary goals of our activities:
(1) thoroughly characterize the BBU instability and (2) exercise several
BBU suppression techniques.

As to the first, we used an 88 MeV configuration for the FEL in which we
could directly measure the threshold current for stability to be 2 mA.
Once BBU was observed, we identified the frequency of the offending mode
(2106 MHz) - which agrees with earlier BBU simulation results. Data were
taken in pulsed beam mode for several macropulse currents as an
alternative method of measuring the threshold current. One of the
critical measurements was the ability to perform difference orbit
measurements to characterize the recirculator. With this information in
hand, BBU simulations can be performed and compared with our
experimental measurements. This benchmarking of the codes is of great
importance.

The second goal was to exercise suppression techniques to raise the
threshold current of the machine. There were two categories of
techniques applied: (1) external *Q*-damping methods and (2) beam
optical methods.

One of the *Q*-damping schemes involved an active damping circuit. We
were able to decrease the *Q* of the dangerous 2106 MHz HOM by a factor
of 5. Using a beam transfer measurement we were able to verify that the
threshold current increases approximately by a factor of 5 (as expected
because of the 1/*Q* behavior of the threshold current). The second
damping scheme utilized a 3-stub tuner. Although not quite as effective
as the previous method, we were able to damp the *Q* by a factor of
\~1.5 and observe the corresponding increase in the threshold current.

The final suppression techniques applied involved changing the electron
beam optics. The first method involved activating the local reflector in
the 3F region of the FEL. This reflector exchanges the horizontal and
vertical phase spaces so that, for example, a vertical kick from an HOM
on the first pass is coupled out to a horizontal displacement on the
energy recovery pass. In this way, the beam can not feed energy to the
HOM and create BBU. With the reflector activated we could immediately
increase the threshold current by a factor of 5. By additionally
changing the phase advance, we could completely stabilize the 2106 MHz
HOM. The second beam optical method required using the so-called "phase
trombone". As the name implies, this method allows one to vary the phase
advance of the beam and change the threshold current. With only a change
of a few hundred G-cm in quadrupole strength, we could go from
decreasing the threshold current to less than 1 mA to completely
stabilizing the 2106 MHz mode (i.e. it can no longer cause BBU).

**WBS 4 (Injector):**

The photocathode continued to deliver CW and pulse beam at 135 pC for
FEL ops. There has been no need for re-cesiation since this cathode was
made on Dec 17, 2004. The total delivered charge is still in question,
(but is at least greater than 400 coulombs) since there is a discrepancy
on the current readings reported by the High Voltage Power Supply and
the 1G Dump current monitor. We are working closely with the I&C, High
Voltage Power Supply and FEL ops teams to resolve the issue.\
Drawings for the new segmented support tube for the second gun assembly
are almost complete. The W&M ion-implant team is preparing the plasma
chamber for testing the coating process on 12\"-long stainless steel
tubes. The goal is to eliminate arcing during the implant run observed
on the full length support tube (34\" long).  Material for manufacturing
the segmented tube will be received on Feb 17th.\
\
Work continued this week preparing our new lab in the ARC building by
making more available space. Most of the materials for the clean-room
area have been received. DI water and fire-proof ultrasonic systems have
been ordered.

**WBS 6 (RF):**

The RF systems are operational. HVPS is operational.

Please refer to Summary of Beam Breakup (BBU) Experiments for RF
activities.

**WBS 8 (Instrumentation):**

Ohm\'s Law dominated the discussions this week while we were operating.
There is a rather large discrepancy in the current measured at the dump
compared to the high voltage power supply. The 10 MeV recirculation dump
is loaded to \~ 30 ohms, the reading is larger than the HVPS so a remote
calibration technique was added so it can be confirmed while running. A
DC current source in the control room was used to null the current at
the dump, this was done in a number of different configurations and the
current always came out higher than we would expect. We are looking into
adding a current transformer to the 2F region, which would be a nice
addition to the FEL diagnostics suite.\
    Last week's implementation of the Machine Protection System (MPS)
interlocks to protect the optical power meter (OPO1V00) required further
editing. We realized very quickly (during Ops) that the new MPS updates
we installed last week were \*too\* restrictive and had to be masked to
proceed with ops. After reassessing the situation and agreeing on what
the right functionality should be, we were able to update the MPS again
and be fully tested and back on the air for the evening shift thanks to
the respective persons making themselves available when the first window
of opportunity came.\
    The Configuration Control Database (Devlore) upgrade: a lot of
effort was placed this week on ensuring a smooth delivery of the new
controls database interface. It was not without its obligatory
transitional glitches, but these were quickly ironed-out almost
immediately as they were discovered. It appears that the new system is
running stable with its 1st week of prime time use. We are now working
on taking data from the FEL server (FELSRV1&2) and importing directly to
a spread sheet. The first version will be triggered by the completed
bunch length measurement (Happek scan). The data that will be passed is
all RF phases and gradient as well as drive laser parameters, quad &
sextupole strengths and the bunch length. This data will be a single
line in a spread sheet that is available for down load from
laser.jlab.org as a .xls file.\
    The PC that we have been using to provide online FEL EPICS status
(top-right video feed (from
[http://felaxis02.jlab.org:1497](http://felaxis02.jlab.org:1497/)) has
been infected with as rather pesky virus (no flu shot) and is being
serviced this week. It should be back up by COB today.\
    Documentation has been completed on the Beam Current Cavity Monitor
PCB enclosure, these can now be sent out for fabrication quotes.   The
Beam Current Monitor PCB-F0322 final corrections have been submitted to
EECAD as well.  A first draft design for a general purpose RF-DC Linear
converter for applications in various RF instrumentation systems
throughout the FEL has also been submitted to EECAD for documentation. 
The carrier board for these cards is also under design and should be
ready for documentation next week.  The 2-Channel RF-DC Converter Box
for the Button BPMs has been completed and installed at the entrance to
the 2nd Arc.  The box has been tested with general signals for
functionality testing and some data from actual beam signal has been
seen. \
    Fabrication continued on Beam viewer cameras with a total of 15
operational for spares and the UV upgrade. Effort began on increasing
our spare and UV inventory of pc boards with the Comparator Isolation
Module for the MPS breakout chassis and VME Beam Loss Module boards.  A
meeting was held with DC high Power group to identify and order high
current (100-350 A) reversing switches for the UV upgrade. Currently, we
will need at least four (with 2 spares) 350 A switches for the UV line
and researching the idea of using H bridges for the 50V 100A power
supplies for the sextupoles. LPSS commissioning has begun for Lab 3 with
the new card reader and software.  Effort has begun on the hutch
enclosure for the THz experiment also in Lab 3. With the scheduled CHL
trip, the I&C group took advantage of the short down period to perform
some maintenance activities. With the installation of the ND filters,
SLM4F09 is complete and tested. The control software has been modified
and screens updated. Replaced Viewer cameras at 0F06, 5F07 and 5F10 and
re-aligned various viewers in the 3F region.\
    We are making good progress on the Laser Personnel Safety System
(LPSS). The THz lab system is close to being certified but we still need
to get the hutch built. Lab 1 is waiting on installation of the turning
mirrors, these have the same motors and limit switches so it should not
be a problem. The alignment mode chassis will be re-certified early next
week.

The new card reader system was incorporated into the old LPSS for
preparation of the certification of lab 3.  Also an Ethernet connection
was established to the new LPSS master.  We are researching the FINS
commands used for EPICS PLC communication. \
Parts are on order for additional Beam Loss Monitor circuit boards, the
remaining AMS upgrade chassis and the Video upgrade chassis.  Four new
monitors were installed in the control room replacing some older TVs
that were being used to monitor the beam.

Patch cables between the Drive Laser Enclosure and the Optical Control
Room and the Operations Control Room have been installed, but still
require final termination and checks. Thanks to Wes and Dan for
attending SSO and ARM training.

**WBS 9 (Beam Transport):**

***Sextupole (SF)***

• Fabrication is continuing at New England Techni-Coil. No change in
status while materials are on order.

***Replacement Chicane Dipoles GW)***

•   Fabrication is continuing at New England Techni-Coil. Steel for two
of the four magnets is in the roughing state such that heat treat can
take place over the week end after next. Finish machine follows that
with the next two magnets material following by a week. Winding will
begin next Monday. Field clamp machining starts next week.

***UV Line***

•    General check layout against the DIMAD from David Douglas was
completed with the change-over in designer responsible for that. Now
that the main magnetic elements are fixed, the designer is working with
Dave Douglas to position the diagnostic elements at places where the
position information or the viewer spots shapes are at their best
balanced against not interfering with the magnetic elements. The effort
is also attempting to put everything on girders in a standard way.

• With regard to our plan to regain our schedule slip, the ME Group has
taken a long look at the tasks associated with this effort for both
electron and optical beam lines and now has a plan in place to utilize
the resources to get the effort done on time and in an efficient manor.
.

• The contract to magnetically model the conversion of the SC sextupole
to the SS sextupole was placed with DULY Research

• The faster PC workstations are due for delivery

• Tim Whitlatch continues to make progress in the vacuum analysis and
design duties

• The statement of work to send the dipole chicane stands and vacuum
chamber to an outside firm for design is at the checking stage.

•   Design of the upstream corner chamber was started and will most
likely be complete by next week

***Injector Multi-slit upgrade***

•   Parts are out for fabrication and due February 21.

***UV Wiggler Progress***

•  The wiggler outline drawing was generated from the alignment group
data and along with a representation of the OCCMs, the full size outline
was pasted to the floor in the vault. From that we could see we have
plenty of pit room to service the OCCMs (which are mounted vertically
down off the beam line in the pit.) We see that we can also use the
existing pit filler block system, moved over by about 6 feet to within
18 inches of the wall to maintain a corridor for movement past the
wiggler.

**WBS 10 (Wigglers):**

Argonne has agreed to supply extrusions and drawings for their undulator
A vacuum chambers.  This should ease the design of these chambers and
may significantly reduce the cost.   The wiggler pit layout was laid out
and the overall design agreed upon for the UV wiggler.\
The IR wiggler upgrade procurement is just about ready to go out.

**WBS 11 (Optics):**

***2.8 and 1.06 micron mirrors: ***

During the open period last week we retrieved another 2.8 micron HR and
after inspection will mount it on another mirror deformer.  We\'ll
install it during the down commencing the 14th.  The 3\" dia 1.06 micron
HR and OC met all specifications.  The HRs exhibited shallow pinhole
flaws not seen on the OCs. These flaws are very similar to those seen on
other HR mirrors, but the dielectrics and deposition techniques are
different, so why they exist is currently unknown.  We are discussing it
with the vendor.  The 2\" HRs were a bit longer than the ROC spec, but
since we can correct the ROC, we\'ll keep them and mount one on a
deformer. 

 

***Other activities:\
***This week was devoted to supporting operations and preparing to
install components that will bring FEL beam into User Lab 1.  We were
pleased that we lased on our broadband optics set.  With the laser
wavelength moving into a region shorter than our IR cameras can image
(except as heat), we added visible cameras to the HR and OC assemblies. 
We began planning a set of measurements of outgassing rates for common
materials used in our assemblies, so we can get a better handle on how
to leverage our pumping speed.  This has a direct impact on the UV FEL.

 

#### UV FEL

We mounted viewports on the beamline viewer from I&C, and will test the
ability of its flag to scatter (rather than reflect) HeNe light.  We
received the prototype gimbal mount and began testing it for short and
long term stability.  Since this is in air, and with multiple bounces
(to get pathlength) it is hard to separate its contribution from the
other mounts in the system.  Nevertheless, these first measurements
were encouraging.  Discussions were held on modifications to improve it,
and to add position readbacks that would be compatible with the UHV
environment.  Our analysis effort on the cryogenic mirror flow
parameters is progressing, we hope to start some measurements (at room
temperature) to confirm we don\'t have vibration issues.  We are
learning that there is a lot of discrepancy between different literature
values of the material properties of sapphire, and held some discussions
about doing some measurements in-house.

**Terahertz:**

The phase 2 postdoc position hiring process continued, the spectrometer
operated in lab 3a, and discussions took place regarding THz ports on
the chicane for the UV FEL.