All,
The main result of this week is success with the Compton experiment
(short for Study of Compton Scattering of Picosecond Electron and
CO2 Beams). This experiment, carried out with the support of the Japan-US
Collaboration in High Energy Physics is motivated by a plan to generate
polarized positrons for the Japan Linear Collider. Other applications
for high-brightness picosecond and sub-picosecond hard x-rays abound.
Ilan Ben-Zvi.
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Compton
Experiment (Reported by Igor Pogorelsky) |
The 3-day Compton run planned for this week was shortened to cumulative
two full days due to hurricane Floyd (BNL was closed at 3 p.m. on
Thursday) and morning and afternoon meetings on Friday. Nevertheless,
the run objectives have been accomplished. This included: to establish
spatial and temporal overlap of the electron and laser beams and to
observe Compton x-rays; in addition, spectral content of the x-ray
signal has been verified.
- Spatial overlap has been established by focusing the electron
and CO2 laser beams on the 2-mil pop-in fiber cross.
- Spectral overlap has been established by adjusting the optical
delay line and optimizing the x-ray signal on the Si diode.
- By better tuning of the e-beam the background signal on the Si
detector has been reduced to 20 mV.
- The maximum Compton signal observed on the Si detector is 380
mV that corresponds to approximately 2 million of detected x-ray
photons with the calculated pick energy 6.5 keV. This photon number
is about two orders of magnitude higher than in similar experiments
at LBNL and NRL where solid state lasers have been used.
Congratulations to the participants in the run: Vitali, Karl, Shigeru,
Ping. Igor, as well as to a number of people who participated in preparation
of the Compton experiment! Our thanks to the EGS Section, in particular
to Chi-Chang Kao and Peter Siddons for help with the detector and
energy filters.
- Conditions of the experiment:
Electron energy 60 MeV
Electron bunch charge 1 nC
Electron bunch length 10 ps
Electron focal size sigma_x=50 um, sigma_y=100 um
Laser wavelength 10.6 um
Laser energy 200 mJ
Laser pulse ~200 ps FWHM
Laser focus ~200 um FWHM
- By inserting foil filters in front of the Si detector, the spectral
content of the signal has been determined:
Ti 25 um measured T=0%. Filter characteristic: transmission below
5 keV. Only photons E>5 keV contribute to the detector signal.
Ni 12.5 um measured T=25%. Filter characteristic: transmission
from T=40% at 6.5 keV to 15% at 5 keV. Conclusion: only photons
with energy E in the range 5 keV<E<6.5 keV are responsible
for the signal.
The observed cut off below 5 keV can be explained by the contributing
factors:
a. absorption in Be window (250 um) - drops to T=0 at 2 keV
b. absorption in air - drops to T=0 at 2 keV
c. angular distribution in the Compton spectrum in combination
with the limited aperture of the Si detector and drop in the x-ray
brightness at lower energies.
Quantitative assessment of all this contributions requires sophisticated
simulations.
- Attempts to visualize the x-ray spot using Kodak luminescence
film projected onto the cooled CCD camera with the image intensifier
or directly on the CCD camera demonstrate insufficiently low sensitivity
of these systems.
The main objective for the next week run are:
- Precise measurements of the electron and laser beam sizes in the
interaction region in order to make comparisons with the theory
predictions on the produced x-ray photons Also any improvement in
the electron and laser spot matching may lead to further enhancement
of the signal.
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VISA
Experiment (Reported by Aaron Tremaine and George
Rakowsky) |
The test beamline (ATF beamline #3) was run this week. Work includes
improving the 66MeV energy measured (attributed to low output kilowatt
amp), resolution of the BPMs and multiple images of the alignment
laser. Also, the probability of adding another quad to the VISA matching
section is in consideration.
VISA Undulator sections have arrived from SLAC, after installation
of new fiducial balls and CMM re-calibration. While at SLAC, Sections
#4 and #2 were found to have developed a bow. This has been corrected.
Some of the trim magnets were removed or disturbed in the process,
so magnet shimming will have to be rechecked on all the sections.
Section #3 and 4 are now being set up on the pulsed wire bench. Unfortunately,
section 2 will have to be sent back to SLAC because of a bowing in
the strongback that was found out after it was sent to BNL. This shouldn't
put any more delays on the measurement schedule since sections 3&
4 can be worked on in the meantime. George and Jeff will measure the
X-17 superconducting wiggler the week of Sept. 27. After that, full
attention will be given to completion of the VISA undulator measurements
during October.
Opticraft, the company polishing the diagnostic mirrors, said it
should send out the mirrors today. Once arrived at BNL, test for surface
flatness will be done. Diagnostic probes are ready, and once a mirror
is inserted in one of the probes, hopefully installation of a completed
diagnostic probe into the test beamline can be implemented.
Next week tests for Yag saturation will be made. If it is concluded
the Yag saturates, then the VISA diagnostic pop-ins will use OTR for
beam profile measurements. It appears the diagnostics can accept an
OTR screen in place of a YAG for this case scenario.
The final board for the Data Acquisition computer arrived this week
and software has been written. We plan to move it to its permanent
resting place in the FEL diagnostics room to test out and debug all
aspects of expected acquisition during the test runs.
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STELLA
Experiment (reported by Karl Kusche) |
1) New wiggler design/construction continues at STI, expect to install
in
beamline #1 as early as mid-October.
2) Data analysis of the last run is being carried out at STI.
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Computer
and Control System (Reported by Bob Malone) |
VISA EXPERIMENT:
Software/new window displays were completed for beam line no. 3 and
were exercised during the VISA test run. This includes controls for
magnet power supplies, pop-in flags and video switching. All controls
operated correctly with only minor errors found in a magnet power
supply tabular summary. They were corrected and retested successfully.
NEW CONTROL SYSTEM:
ITD has completed all wiring needed for replacement of ATF’s CAMAC
serial highway as part of the planned upgrade of our computer control
system. All key locations in the ATF now have two additional runs
of 100 MHz Ethernet which terminate in the control room at a fast
Ethernet switch. These key locations also have new unterminated fiber
optic cable stored in each junction box. Special thanks to Bill Cahill
who escorted untrained contractor personnel into our controlled areas
during the installation. He also coordinated the more complicated
cable pulls to the C1 and C3 lab areas as well as the experiment hall.
NOTE: Users are already attempting to connect computers to these
new feeds with the expectation of obtaining access to the BNL network.
These cables are strictly for control and data acquisition activities
internal to ATF and will NOT be connected to the external site network.
This is necessary to ensure a secure, quiet environment for our control
system and scientific data collection. Access to the site network
has been (and will continue to be) available at every telephone junction
box within ATF. Contact R. Malone (X2084) if you need help in identifying
which connection to use.
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Technical Operations
(Reported by Bill Cahill) |
This week we were able to complete the VISA #3a ‘test line’ including
computerized operation of all magnets, pop-in’s and diagnostics. All
cabling is documented and the dipole switching chassis is installed
and operational. A radiation fault test was performed and no recordable
levels were detected. The Compton Experiment radiation fault test
was performed at 100% charge and no levels were detected. The fast
detector experiment on beamline #3 is under vacuum and a radiation
fault test was performed on Wed. 9/15. A radiation survey observed
a peak of 12mr/hr at the external north side of the facility shielding.
A scheme has been formulated by XJ Wang and Bill Cahill to modify
the existing shielding to reduce/eliminate the problem. The beamline
has been locked out until additional shielding is installed.
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