# MEMORANDUM 

**To:** Distribution

**From:** F. Dylla/grn

**Subject:** FEL Upgrade Project Weekly Brief - May 1-5, 2006

**Date:** May 5, 2006

**Highlights:**

We spent the week researching noise in the system to determine possible
sources of efficiency degradation.  A large amount of high current/high
power lasing was performed and we delivered over 100 coulombs out of two
cathode cesiations.  Most of the data taking centered on phase and
amplitude noise in the drive laser, buncher, and injector output.  We
also attempted to look at the phase noise of the synchrotron light at
the wiggler but had insufficient signal to detect.  That data is still
under analysis and we will need more tests before we are convinced of
its role in the loss of efficiency.  On evening shifts we took a
complete set of FEL performance data to compare the lasing at the
fundamental, third harmonic, and fifth harmonic.   In all cases the
lasing was at 1.05 micron; the wiggler was adjusted so the harmonic was
at this wavelength with the beam energy fixed.  We also provided beam
time for characterization of the THz beam noise.  The THz FTIR
interferometer was commissioned for use with CW beam.

**Management:**

We presented several talks at the Beam Instrumentation Workshop in
Chicago which were well received.  We seem to be ahead of most groups in
implementing embedded control in Beam Position Monitors.   This will
provide a cheaper, more accurate measurement and greater capabilities in
the future.

 

We are also spending a significant amount of effort planning for our
maintenance down time May 15-23.   During this time we will accomplish a
host of things: a injector beamline bake out, OCCMs system around the
wiggler, installation of the Helium transfer line for the cryomirror
demonstration, new 1.6 and 1 micron low-loss outcouplers, halo
monitors, extra viewers around the wiggler, and a host of other minor
activities.  Depending on the results of our present noise studies there
may be additional electronic modifications.

 

We received the written report from our Operations Review on April 27,
2006.  We\'d like to express our appreciation to the review committee
Erik Johnson, Jon Cooper, Ben Feinberg, James Tarpinian who worked very
hard to produce an exceptional study in the limited time they had
available.  This review set the stage for us to become a recognized DOE
User facility.

**Operations:**\
Much of our I&C group was at the Beam Instrumentation Workshop
in Chicago so we were a bit shorthanded this week.  The main goal
this week was to make measurements on the electron beam to
characterize noise that might be reducing performance at high current.\
On Monday we re-cesiated and tried for a power push at 2.8 microns.  
The efficiency was not quite as high as before and we got up to 6.3  kW
at 6 mA.  We could get 5.5 kW at 5 mA.  We found a technique that allows
the laser operator to maintain the mirrors in an optimum alignment and
maintain the efficiency for as long as the laser is on.  We were then
able to hold the laser between 5 and 5.5 kW for about one half an hour
using this technique.  The power cycles due to  a slow drift in the
cavity length due to absorption of scattered  light.  As a precursor to
a test plan later in the week we lased briefly at 0.98 microns by
changing the resonant wavelength to 4.9  microns in the wiggler.  There
is no cavity Q at this wavelength or the third harmonic of this
wavelength but there is a good cavity Q at the 5th harmonic so it lased
there with a 4 micron detuning curve.   We also measured the spectrum of
amplitude noise on the drive laser and found a relatively strong
oscillation at 38.7 KHz.  This might be the source of a strong phase
modulation seen on the electron beam at high current.\
On Tuesday we worked on measuring the energy vs. time in the 10-100 kHz
frequency range in the injector.  Since we had not done this before and
most of our instrumentation group was at the Beam Instrumentation
Workshop progress was a bit slow but by day\'s end we had some
measurements of the energy jitter.  Nothing convincing was seen above
the noise floor.  We then measured the drive laser phase noise before
and after the electro-optic cells and with an internal  oscillator or
with the RF control module.  One conclusion of this was what the RF
control module reduced noise at low frequencies but increases the noise
at frequencies above a few hundred Hz.  It also looks like some of the
spectral lines might be intensified by the EO cells but it was not clear
by how much.  We then switched over to RF tests in zone 3.\
Wednesday the line at 38.7 kHz was quite small so we decided to see if
the laser performance improved at higher current.  In short, it did
not.  We then switched to the 1.06 micron mirrors and took data while
lasing at the fundamental, third harmonic, and fifth harmonic.   The
performance agreed pretty well with expectations except for slightly
lower power than expected for the 5th harmonic.  The power was lower yet
more stable at the third harmonic than at the  fundamental. This might
be a better way to operate for the LMES experiments.  On Thursday we
took a complete set of data looking at energy fluctuations in the
injector.  We tried to measure the phase of the spontaneous radiation of
the electron beam but were unable to see the signal.  The power was less
than when we last measured it so we might have been below the noise
floor of the detector.\
Friday is a down day.  We will be working on trying to find the leak in
the gun region and starting a heat clean of the gun.

**Beam Physics (WBS 3):**

Given progress on phase/energy jitter measurements, we are performing an
analysis to understand how - and with what sensitivity - longitudinal
rattle in the injector will evolve through the accelerator. Initial
estimates based on an extension of considerations in JLab TN 01-048  (
<http://www.jlab.org/~douglas/FEL/technote/JLABTN01048.pdf> ) indicates
that phase jitter out of the cryounit is irrelevant (no surprise - its a
parallel to point longitudinal focus from unit to wiggler) but that
energy jitter gives longitudinal positional jitter of order 0.01
m/radian of relative energy offset - i.e.,\
\
dl (m) = 0.01 m (dE/E).\
\
Thus, 0.1% energy jitter will give 10 microns longitudinal motion - or
about 30 fsec. This would also correlate with about 0.4 mm of motion at
the peak dispersion of 0.4 m in the injection line.

**WBS 4 (Injector):**

We re-cesiated the cathode on Saturday and again on Wednesday. Since
March 20th the cathode has been re-cesiated 7 times and has delivered
close to 900 Coulombs and 225 hours of CW and pulse beam for FEL ops. We
have been studying the vacuum conditions in the injector and today we
did a leak test using the RGA in the gun chamber. We did not find any
leaks. The Gun RGA spectrum shows only Hydrogen and traces of He when
the gun valve is open. The lightbox RGA shows traces of Methane (mass
peak 16) that rises and then lowers when the gun valve is cycled. By
tapping in each ion pump in the system, we could only see any evidence
of methane increase in the pump next to the cryounit. The thorough
vacuum test we performed today indicates there is no leak in the gun,
therefore the question of reduced cathode lifetime remains unanswered.\
\
This afternoon we started a heat clean cycle today to make cathode \# 6
on the anodized wafer installed almost two years ago. Since then this
wafer has delivered over 3,000 Coulombs.

**WBS 6 (RF Systems)**

Last Friday we met to discuss the buncher cavity resonance control
system. Several potential modifications to the control algorithm (i.e.
RF ON/OFF operating modes, water valve setpoints, etc.) It was agreed
that they would replace the water control valve at the May 15^th^ down
and install a better thermal blanket on the cavity during or before the
down. Additionally, it was agreed that we would take parasitic data on
the control loop parameters so that we could better understand the
transitions between the RF on and off states. This data was taken early
this week. The matter will again be discussed today.

In addition to attending the Beam Instrument Workshop at FERMI Lab, I
assisted Richard Evans in taking and analyzing the time and frequency
domain data which indicates the energy jitter in the injector. Analysis
of the data continues.

**WBS 8 (Instrumentation):**

With the 2006 Beam Instrumentation Workshop taking place at Fermi Lab
this past week, we got the chance to exchange some valuable information
with other instrumentation and diagnos-tics people from all over the
world.  A poster and paper were presented on the Embedded BPM design and
it was very well received.  There were several other labs trying to
implement the same sort of design with the RTEMS open source operating
system loaded directly into the onboard IOC.  With other groups working
on the same goal some valuable contacts were made so resources and
information can all be shared.  We also picked up some ideas for how to
measure other interesting beam parameters that might be useful for
further machine development.\
    An entire battery of low frequency noise studies of the machine have
dominated this weeks' activities. Our primary goal this week was to
measure the energy jitter in the injector region as evidenced by the
horizontal beam position jitter at the 0F06 BPM (IPM0F06). The data for
this measurement was collected. Several specific frequencies were
observed at various beam currents, namely, the 39kHz noise that is the
motivation of our studies. The measurement was made possible by the new
embedded BPM electrics that have been developed over the past few
months. We have received a model 350-160 \"wet\" EO cell from ConOptics.
It is being installed for testing today, the results will be known over
the week-end. This is in a further attempt to track down real or
imaginary noise on the drive laser.

    The Halo Monitor stepper control boxes were installed onto the
linear stages this week.  We exercised both the horizontal and vertical
forks in and out of the cross and checked for any interferences.  Also,
all limit switches were set and tested. The stepper motor driver cables
have been installed, channels assignments have been made but the EPICS
screens will not be updated until our next scheduled down. The Halo
cross was delivered to the Vacuum group for leak testing and bake out.\
    The LPSS installation for Lab 4 is going smoothly with the crash
interlock chain being wired in.  An additional card reader was also
wired in and tested with the entry software successfully. Time this week
was spent on constructing the LPSS master box that will go in the
control room. The hardware that we had in house was installed and the
necessary parts were ordered so wiring can begin to take place next
week. The auto-miniphase was exercised some this week.  The Linac gang
phase is still not quite right.  The old algorithm was much more
accurate, but slower, and is being put back in. We continue working on
the FPGA programming for the General Purpose board. The code was
modified for ADC sampling, storage and Coldfire bus bridge. Simulations
were done for all functional modules.\
    Fabrication of a new stepper motor driver box for optics began this
week; the driver box will be wired to run off of AC instead of the usual
battery/dc power.\
    In preparation of the absence of the Safety Warden the next two
weeks several staff were trained to perform the daily checkoffs.  All
safety issues are being forwarded to Ned Walker with EH&S. Wes as a
member of the Safety Warden Advisory Team, met with Andrew Hutton to
discuss the details of the team\'s formalized safety warden (SW)
program.  After he approved the team held a meeting with all accelerator
SWs and their supervisors.  Final revisions to the program are being
made.

**WBS 9 (Beam Transport):**

# *IR Machine Re-commissioning and Operations*

• I moved the Camera that is able to view delamination of the GW pole
tips to the fourth magnet of the Optical Chicane.  No delamination was
seen in the lower pole of the third GW magnet of the optical chicane.

•  We continue working on using four uninstalled GW dipoles in a quick
experiment that uses laser light to look for a pseudoscalar particle
that couples to photons.  Budget problems prevented us from measuring
our test dipole

• The cover for the W55 (IR Line) Wiggler remains in manufacture.

•  I continued working on the design of the permanent cover for the
optical transport between the drive laser line and the entrance into the
Light Box in the injector.

•   Dano Oprisko is working on the technician portions the helium system
for the lower portion of the waveguides for the Cryomodule in Zone 3. 
Our plan is to install one waveguide system during the shut down of May
15.

• I continued work on the gun for the new injector test stand.  We
resolved that the wisest course is to not include a puck exchanger into
the new stalk interchange system.  The experience of the CEBAF source
group that heat cleaning the puck through a gap in vacuum is may cause
more problems than a puck is worth.

**WBS 11 (Optics):**

#### FEL mirrors

As noted in the Operations report we operated for extended periods of
time at 2.8 microns.  The new shields work quite well.  We do observe a
relatively rapid change in cavity length as some component heats and
expands.  We are monitoring our temperature readbacks to try and
determine where the problem is and fix it.  Absorption of power (and
thus expansion) of the fixed clips has been proposed and we will look at
ways to check that during the upcoming down.  We did more measurements
of the absorption of THz radiation by the outcoupler mirrors.  It
amounts to about 2W/mA.  Our new 1.6 micron outcouplers arrived and are
being QA\'ed.  We expect the 1.06 micron OCs early next week.  We plan
to braze both into holders. 

***Other Activities***

Optical measurements to resolve the cause of the laser efficiency
continue. We used a Stanford Instruments spectrum analyzer and Thorlabs
photodiode (\~ 200 MHz bandwidth) to look at amplitude noise in the
10-100 kHz region.  We did detect a very weak signal at 38.7 kHz.  We
devoted a shift to attempts to measure the phase noise of the wiggler
spontaneous radiation but were unsuccessful, due to lower power than
measured last week.  Apparently we were closer to the noise floor of the
photodiodes than previously thought.  More careful measurements of that
threshold will be made with the advanced drive laser\'s oscillator, and
the bias circuit modified as necessary to get the desired performance. 
We also measured the phase noise spectrum as a function of rep rate
after the EO cells.  While there was essentially no difference in the
noise spectrum with rep rate, there was a difference if one looked at
the noise of the laser before the EO cells vis-a-vis after the EO cells,
especially in the 40 kHz region.  This will be studied further next
week.  The streak camera was set up to look at synchrotron light emitted
from the first arc, however results are too preliminary at this time. 
With many I&C personnel at a conference, we helped Ops with phase noise
measurements from beam position monitor (BPM) signals.  Parts for the
advanced drive laser integration are beginning to arrive. 
Our cryocooled mirror in-vessel loop design progressed this week.  We
held further meetings with the Cryo Group on their portion of the
hardware, and still plan on having them install this hardware the week
of May 15.  After looking at the crowded space where the distribution
can was going to be placed, they made the decision to move it.  They
should still make the installation schedule, but this incurs some cost. 
The Machine Shop continues work on our projects, optical cavity mirror
metrology (OCMMS), in-lab optical transport system hardware (Brewster
window transition, hutch shutter, & harmonic-blocking filter), and now
the User shutter for User Lab 6.

**Terahertz:**

The THz studies this week were dedicated to further characterization of
the new spectrometer and data acquisition software, as well as more
noise measurements of the FEL THz emission.  The noise measurements
revealed some interesting and surprising periodic noise spikes that were
found to be caused by the spectrometer control software and "leaching"
through the integral detector electronics where they could be detected
with the spectrum analyzer.  It was also found that there was a
significant amount of noise passing onto the electronics of the integral
detector from the drive electronics of the moveable mirror in the
spectrometer.  This could be eliminated by using an external detector,
but this detector appears to have a higher noise floor and will need to
be compared in detail with the integral detector.\
\
The spectrometer characterization studies were conducted during
dedicated CW beam operations Wednesday night and a few important details
were learned.  While varying the scan length of the spectrometer did not
affect the data acquisition and curve fitting, the best compromise
between resolution and the amount of data to be passed out through the
ethernet data socket was found to be at a spectrometer resolution of 1
cm-1.  It was also found that the spectrometer scan velocity had an
effect on the spectrum measured by the DAQ software if set too high.
This velocity induced error was evident at scan velocities greater than
1.25 cm/sec.  There are still further characterization studies planned,
but we have found a range of settings that provide consistent spectrum
calculations and the spectrometer itself has been performing reliably
all week.\
\
Another important event that have occurred this week was the final
senior thesis presentation made by the William and Mary Physics student
Amelia Greer, who was advised and mentored by Gwyn Williams.  Both her
written thesis and oral presentation received high praise and excellent
reviews from her committee and others in attendance.  Her work helped to
bring the THz beamline online as well as to begin characterization of
the new spectrometer.  Finally, Wade Brock, a rising senior in Physics
at ODU, has proposed to perform beam divergence studies on the THz beam
in Lab 3 for his senior thesis.  His work will begin soon and will
involve experimental measurements of the beam divergence in Lab 3 to be
compared with the calculated divergence.  This will help us to determine
how to properly collimate the beam and design an optic to achieve this
goal.