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Site Details ATF Newsletters 
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1999 ATF NewslettersMarch | April | May | June | July | August | Sept | Oct | Nov | DecNovember 5 | November 12 | November 19
Greetings to all, The study of BPM screens for the purpose of VISA
was completed weeks ago, but we have a nice set of transparencies presented
by Alex Murokh at the UCLA Workshop: Two interesting items this week. The Beam Position Monitors for Linear Colliders experiment, led by Vladimir Balakin, had another successful run, reported below. The Russian group at the ATF can be seen at: The big move is here! The VISA undulator vacuum chamber
is being installed at the ATF. At the same time the magnetic measurements
of the undulator steering magnets were done. The work on the STELLA experiment continues diligently: Ilan Ben-Zvi.
Progress in the Linear Collider development requires
methods to measure beam position with extremely high precision. The device
consisting of three RF cavities was installed at ATF beam line to investigate
limitations for the BPM resolution. This week run was continuation of
the series of runs and resolution of 250 nm online and 150 nm filtered
achieved during last run at the end of 1998. The goals of this run were:
check resolution at high electronic sensitivity compare to previous run.
The position jitters were limiting factors for the electronics sensitivity
during previous runs. Improved position and phase stability of the ATF
electron beam allowed using high sensitivity range. Increased sensitivity
does not translate into improved accuracy automatically. Noise level and
non-linearity of the electronics (due to reduction in the dynamic range)
was amplified by increased sensitivity. check
possibility to use additional cavity to generate reference phase by electron
beam instead of special signal from ATF RF system that can introduce RF
phase jitters and drifts. Online accuracy of 200 nm with reference line and 150
nm with internal phase generation were achieved. Accuracy will be further
improved by taking into account the non-linearity of the electronics during
“home analysis”. The results of this run prove that the phase jitters
from external reference produce considerable contribution into resolution.
The resolution test, showing a preliminary resolution or 150 nm, can be
seen at: Other possible limitations in this experiment for the beam position measurement appear to be related to the electron beam tails or satellites. RF signal analysis suggests electron beam satellites separated by 10-12 ns from main beam and amplitude of couple percents. This can be seen at: Thus the BPM equipment serves also as a large dynamic
range longitudinal charge diagnostic.
Beamline #1 spectrometer has been removed to allow
for HeNe line-of-sight from GPOP1 mirror to end of Experimental Hall.
This will permit alignment of all BPM’s and direct inspection of
the dipole bellows. The large
diameter spool piece upstream of triplet #2 has been removed for machining
to allow for the installation of an additional gate valve for vacuum isolation
of GPOP1 assembly.
Ben arrived and the vacuum chamber has been installed in the ATF and rough surveyed into place, as can be seen: 11_19_VISA_und. The alignment rails (for the laser interferometer) were put up today. Also, the vacuum group is preparing the chamber and connecting the chamber with the spool pieces to the four ion pumps. See: 11_19_VISA_und2. Once this is done, the system will be turbo pumped,
baked, and leak checked. Pictures
of the system and people will be posted on the web thanks to BNL crack
field photographer, Ilan. There are two solutions for the BPM mirrors. To glue
lambda/2 mirrors onto already existing copper mirrors.
The mirrors have been ordered and should arrive by next Wed. Hopefully
the holiday won’t slow down the arrival.
Silicon mirrors are being polished (lambda/10) at Livermore and
Art is hoping to have them ready for the final assebly alignment
(around Dec. 1).
When mechanical tests were done, the mirrors/pop-ins could be inserted
and retracted without making contact with the magnets. This was a tight
squeeze and the FEL side mirror has been reduced from 3.5mm to 3.1mm which
we believe will help with insertion/extraction process when at the ATF.
We can’t allow the pop-ins to touch the magnets or else the alignment
can possibly be thrown off. Magnet sections 1&2 are done and George and Jeff
will finsih with sections 3&4. It would be good at Tuesdays video
conference to have the final pulse wire data presented. I believe we need to take a closer look at the test
run of the BPM that XJ and otheres did (results are in last fridays ATF
report and pictures are posted on the web) and have discussions on what
the results of the TR and Phosphor mean for VISA. The VISA cable pull was done and termination will
start next week. Schedule Week of: Nov. 15 Move
vacuum chamber assembly to ATF Nov. 22 Vacuum
test chamber, ion pumps, bake-out in ATF, Cable termination for controls and power Nov. 29 Magnet
characterization for all 4 magnet sections completed Final design and assembly of BPMs Robert and team arrives Nov. 30 to align magnets in ATF (final alignment) Final alignment with BPM insertion will happen the
weekend of Dec.3-5. We will only have a few days and so it is important
not to dily-daly and get it done lickity-split. After the final alignment, we can begin to pump down
and finish all the cable terminations and controls. Also, the long range
transport optics and mounts should be here and can be installed. We still
need to redhead the ceiling and mount pvc pipe.
VISA Sections 3 and 4 are on the bench, ready for
tuning and fiducialization. This week we have been trying to calibrate the VISA
Steering Magnets. We set up four steering magnets on the pulsed wire bench,
spaced 49.5 cm on center, as specified. The Helmholtz coils were shortened
to leave the nominal 2 meters covered by the steering coils free. The
idea was to calibrate the steering coils to cancel Earth’s field. First, the unpowered coils, “out of the box”, produced
random trajectory kicks of a magnitude comparable to the VISA magnets
before shimming, just due to residual fields, which were several gauss.
We then attempted to degauss each steering magnet by applying reverse
excitation to the By coils and observing By at the surface of the corresponding
pole with a Hall probe. (We
used a Kepco bipolar PS, since the steering coil supplies are all at the
ATF.) We were able, by trial-and-error, to reduce the By residual fields
below 1 gauss in all four magnets. We then powered all four magnets in
series (for By) to obtain a straight X-trajectory. To compensate Earth’s
field required only 2.4 mA (yes, milliamps.) Note that the power supplies
are rated for +/- 8 amps. Steering
corrections, if any, will be of about the same magnitude. As expected,
these magnets have far more capacity than needed, and they need to be
degaussed very carefully. Just
for fun, we wound 12-turn windings on each leg of the steering magnets,
on top of the UCLA windings. Powering them in series for By field produced
cancellation of Earth’s field with about 700 mA. (The current ratio 700/2.4
does not match the turns ratio. Perhaps the 12-turn windings have a lot
more leakage than the UCLA windings.) Nevertheless, these windings have
adequate range for trajectory correction, and the current is not down
in the noisy, lowest-few-bits region of the DAC. We repeated the degaussing procedure using the Bx
windings and monitoring the surface fields at the Bx poles. But, checking
the By poles, we found they had acquired residual fields as high as 6.5
gauss, obliterating the degaussing we had done previously on the By poles.
This is not surprising, since By and Bx fields share the same iron and
affect each other’s history. This
method of degaussing did not seem to work.
Suggestions are welcome.
The traditional degaussing method is to drive the iron into saturation
and then cycle the excitation with gradually diminishing excursions until
a degaussed state is reached. The iron cannot be saturated with the existing
supplies if the field has to cross the large air gap.
However, by connecting the UCLA windings in series aiding, so the
flux circulates around the “picture frame” core, the iron can be saturated.
We tried this with one magnet. We opened one joint to create a
small air gap, where we inserted a Hall probe. B(core)) reached only 6
kG, because of the air gap and because the Kepco reached its voltage limit.
Nevertheless, we cycled the Kepco with a LabView-generated, decreasing-amplitude
sine function. Upon completion of the procedure, the surface fields on
all four poles were <0.3 gauss. It may be possible, then, to use the
UCLA windings, in series-aiding, for degaussing, and additional, few-turns
windings for steering. For
comparison, we tried a pair of rectangular, air-core coils with similar
geometry: 6 X 21.5 cm aperture, 2 X 2 cm cross-section, with 81 turns
of #18 wire, spaced about the same distance as the pole-to-pole distance
in the steering magnets. 2 amps in the air coils produced the same deflection
as 3 amps in our 12-turn windings on the steering magnets. It’s an option.
Stella Activities: 1. The spectrometer on beam line # 1 was moved off
it’s surface table. 2.
A 4.5 “ flanged spool was modified to allow for the installation
of an isolation valve in beamline # 1.The spool was cleaned, leak checked,
and baked. HGHG activities: 1. New pneumatic actuators arrived this week and
were installed into a shutter for the CO2 beam. 2.All the covers required to enclose the CO2 beam
to the HGHG experiment have been fabricated. Two sections have been installed,
the rest will be installed next week after threaded inserts are put into
the ceiling. VISA activities: 1. The
VISA table , and vacuum vessel was prepared for shipment
and installed on schedule into the ATF experimental hall. The vessel
assembly was surveyed that same day. 2. Prior
to the installation of the VISA vessel the Faraday cup/SASI out-coupler
mirror assembly was removed from the ATF modified, pre-surveyed , and
resurveyed onto beamline # 3. 3. The
VISA vacuum vessel components are currently being assembled and being
made ready for pre-baking Terawatt Laser activities: 1. A
preliminary mechanical engineering inspection of the laser amplifier pressure
vessel took place last week, and the review process is proceeding. 2. Work
was started up again on needed modification to the laser window safety
exhaust tube and the bread board enclosure.
Intensive testing of the new Vista software under Linux
continues. In particular,
a key problem with software interrupts and inter-process communication
via reliable signals has been resolved.
The problem manifested itself when changes in the test database
did not always generate a software interrupt in the associated server
process. This “works most
of the time” type of problem is especially tricky to isolate and understand.
Building new reliable and robust servers requires that software
(and eventually hardware) be able to generate and service interrupts consistently
and dependably over many iterations spanning long periods of time.
The fix merely requires each of our servers to cache a copy of
its parent process id for the interrupt handlers to use when rasing a
signal. A small fix, but
one with important impact. Testing
of the new method has uncovered no problems in 72 hours of continuous
operation. Software porting
efforts can now continue. Monday was the start of the experiment hall shutdown.
During this time we provided technical assistance to the Russian BPM experiment
in the 0* line including operations and tech support. Time was spent on
improving camera optics in the laser gun hutch but it was found to create
a large heat load and the original camera had to be reinstalled. Technical
assistance was also given to the Smith-Purcell Experiment repairing electronics
needed for diagnostics. The master oscillator spare was sent out for repair
and parts were ordered to build the interface chassis from CAMAC to the
VISA pop-ins. The experiment
hall was prepared for the arrival of the VISA wiggler including the removal
of all shielding, cabinets and survey stands not needed. We are currently
preparing the new Darlington transistor power supply chiller for installation.
The design work for the new linac water system is ongoing. Mechanically,
assistance was given to installing the CO2 shutter between the amp. and
pre-amp room. Unfortunately, the interlock system has not been certified
and time has been spent investigating alternative procedures. The fast
detector group has requested a cable pull from NSLS electricians but we
are still waiting for a response and conformation on a starting date.
1. ATF
laser safety: All person must follow laser safety procedure, particularly
for laser goggles, they must be certified. Marcus will follow through
the purchase order for the goggles.
John was asked to install the laser cover in the experimental hall
ASAP, over time is granted for next week. ATF TW CO2 room will be put
under administrative control for HGHG laser work, if your name not on
the list, please leave the room if room is interlocked for laser work. 2. ATF
Schedule: Next Monday will be used for laser interlock certification,
Marcus, Igor and with help from Bill Cahill if needed. During the laser
interlock check, you can continue work inside the room, but must stay
there until permitted to leave.
Tuesday will be for trajectories studies and feedback test by Vitally.
Wednesday will be used for maintenance, Bill will see whether we can install
magnet power chiller. HGHG request to switch two days running time in
the week of January 10 to other week, we will try to accommodate that
in the February. Marcus is asked to operate the CO2 laser for HGHG experiment.
Monday, Nov. 29, we will try to improve the gun water system and
install coil. (Harrington, Wang) 3. YAG
laser diagnostics: we need improve the laser image on the monument, and
laser energy measurement. Trigger for the Joulemeter is required, Marcus
will purchase the parts after the laser interlock check. 4. VISA
experiment: The ATF is in the process of installing the VISA undulator.
We request all ATF staff and its users not to use the experimental hall
as passageway to the FEL and CO2 room.
VISA is granted exclusive access to the experimental from Thursday
Dec.2 9:00 PM to the 1:00 PM of Tuesday, Dec,7, 1999. During those times,
the ATF will implement special procedure for experimental hall access,
if you need access to the experimental during those time, please contact
VISA manager Aaron at X2505. Pedro
will start terminating the VISA cable next week. 5. Cahill
will check the flow of the linac water system, and Mark should start order
the more powerful pump if flow is the problem. Mark will finish up the
BPM amplifier, and start work on the power supply for HeNe laser shutter. 6. Harrington
will continue work on the CO2 laser, and install Smith-Purcell pump control
box on Tuesday.
Last Modified: December 3, 2007 |
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