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Site Details ATF Newsletters 
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2000 ATF NewslettersJan | Feb | March | April | May | June | July | Aug | Nov | Dec
Greetings to all, This is the first report for the New Year, and what a start.
Interesting results in this report from a number of experiments: Observation
of the Optical Klystron effect in SASE, the Fast Electro-Optical Detector and STELLA. VISA also has
reached a milestone of beam through the undulator. You may glimpse the
amount of work done recently on VISA by looking at the photos: Ilan Ben-Zvi. On Tuesday of January. 4, Marcus Babzien, Vitaly Yakimenko
and I did study of Optical Klystron on the SASE gain using the ATF HGHG
setup. Bob Malone and I first
laser-cleaned the cathode on Monday so that the QE of the Mg cathode now
better than 0.1%. HGHG consists
three magnets, first one is short modulator undulator, and second one
is dispersion magnet consisting of three magnets chicane, and last one
is two meters APS undulator A. For our studies, there was no CO2 seed
laser, modulator was set on resonance at 5.3 um, and radiator was set
at 10.6 um. We first aligned the e-beam with an alignment laser, tuned
the beam to match the radiator undulator. The figure below: shows the maximum SASE signal as function of the strength
of the dispersion. During
the measurement, the alignment of the e-beam and matching are kept constant.
We believe that, to the best of our knowledge, this is the first observation
of one order of magnitude SASE gain enhancement due to the optical klystron
effect. Using optical klystron to improve SASE gain have been discussed
by many authors, it could significantly reduce the technical difficulty
of the X-ray SASE FEL, reduce the length and cost of the undulator. Furthermore,
this scheme is compatible with HGHG seeded FEL. We intend to vary the
modulator resonance frequency, to optimize this effect, and compare with
theory.
Since our pioneering observation of a charged particle beam
by means of the electro-optical
effect induced in a LiNbO_3 modulator crystal[1] at the ATF, our efforts
have been focused on studying beam induced opacity in optical fibers,
the effect of mirror currents induced by beam bunches in the aluminum
housing of the modulator crystal on the electro-optical signal and the
investigation of other electro-optical materials such as poled fibers
which could make construction of inexpensive fast detectors possible.
Two conference papers were presented [1,2] and an article has been prepared
for submission to Nuclear Instruments and Methods in Physics Research
A[3]. Two ATF beam induced
signals were observed with the LiNbO_3 modulator, the anticipated polarization
dependent transmitted light modulation whose rise time was consistent
with our time resolution and a polarization independent signal with a
significantly longer decay time. The latter has been demonstrated to occur
when the beam was incident on the optical material whether it was an LiNbO_3
crystal, a poled fiber or a polarization maintaining fiber. It is thought
to arise from a plasma cutoff effect in the ionized optical medium. Calculations indicate that beam induced currents in the
LiNbO_3 modulator aluminum case can produce an electro-optical effect
in the crystal. These are expected to have longer time decay constants
which are geometry and conduction path dependent. We have observed the
beam induced currents with an ungrounded aluminum plate placed adjacent
to the detector and connected to a wire bringing the electrical signal
out of the vacuum through a feedthrough.
Further work must be carried out to fully characterize the effect
on the electro-optical media. Laboratory measurements of the sensitivity of the poled
fibers indicate that that they should be capable of producing a signal
in our existing equipment at the ATF albeit a marginal one. To date we
haven’t seen a polarization dependent signal from induced by the ATF beam
in fibers we have poled, but a number of experimental problems and poorly
optimized setups may be responsible. A wire implanted in or adjacent to
an optical medium such as a poled fiber might increase its sensitivity
by locally concentrating the electric field. Poled fibers have responded to being hit by the beam with
a fast attenuation of the transmitted light. We’ve measured the dependence
of this on the charge in beam bunches and found it to be approximately
linear. We have also measured
the dependence of the polarization dependent signal in a LiNbO_3 crystal
on the distance to the beam which also agrees with expectations. In addition
to completing the investigations outlined above, it is essential that
we determine the intrinsic time resolution of this technique. In the near
future we plan to employ a streak camera for these studies. Although LiNbO_3
and LiTaO_3 crystals and possibly poled fibers are sufficiently sensitive
for beam diagnostic applications, we want to search for inexpensive materials
that might have potential for single particle detection. Pictures of the picture frame with the poled fibers are
on the WEB at http://www.picosec.bnl.gov/picture-frame-1.jpg
and http://www.picosec.bnl.gov/picture-frame-3.jpg and pictures of the electro-optical modulator is at http://www.picosec.bnl.gov/all_pict.jpg References [1] Y.K.~Semertzidis et al., Proceedings of the 1999 Particle
Accelerator 1 Conference, 480 (1999). [2] D.M.~Lazarus et al., Proceedings of the 1999 European
Physical Society Conference on High Energy Physics. To be published.
[3] Y.K.~Semertzidis et al., Article to be submitted to Nucl. Instr.
and Meth. A (Appended).
1) The
first week of the new year proved to be fruitful for the STELLA team,
as the strongest CTR signal to date was observed on beamline #1.
Ironically, during the first day of our two-day run, we were not
successful in even reestablishing energy modulation at the spectrometer.
It was later found that the HeNe laser coalignment into the wiggler
was not indicative of the CO2 laser beam path and had deceived us into
believing that other parts of the experiment were at fault.
The alignment was carefully repeated on the morning of the second
day, and energy modulation was quickly reestablished. Parameters & data: E-beam energy = 45.6 MeV (optimized for new wiggler) Charge (delivered) ~ 0.5nC Energy modulation (spectrometer, -8 degrees) ~ 1.5% maximum
Laser power delivered through wiggler ~ 50 MW maximum (typically 15-45
MW) CTR signal observed = saturation
(> 1 Volt) of cooled-IR detector at all wavelengths of interest (2.5-5
microns, as per harmonic filter study), decrease of signal observed with
attenuation of laser power, no signal observed (<10 mV, noise) when
laser off or e-beam off. Further
processing of data and correlation to simulations to follow. 2) To improve
the stability and reproducibility of the laser alignment, another HeNe
and kinematic mirror were installed closer to the wiggler and aligned
this week. Related to this
is the ongoing work to rotate the laser polarization by 90-degrees with
an additional mirror in the IFEL beam path, instead of a half-wave plate
(which clipped the beam and introduced attenuation).
Also, thanks to John Skaritka and crew for installing the long-awaited
plexiglass beampath enclosures for the protection of the laser transport
optics in the Experimental Hall. 3) At STI,
Wayne and company have been busy with simulations that, in part, are hoped
to explain the apparent lack of “optimum” CTR signal as a function of
proper microbunching. This
work, aided locally by Vitaly, continues and will be reported upon in
the future. 4) STELLA currently is due to run again at the end of the month (1/26-1/28). Modifications to the system are underway, mainly to improve e-beam focusing/imaging at the ICA interaction region. Better e-beam focusing and spot size characterization, as well as further study of the CTR dependence upon laser power and e-beam current, are on the agenda.
On Tuesday, we propagated beam to Pop-in 1 which is just
a silicon mirror without use of the BPM optic design. This is to get a
baseline for expected OTR. We saturated one of
the ATFs sensitive cameras with OTR. We then exchanged this camera
with one of the VISA cameras
and saw signal, not nearly as bright though. These pictures can be seen
at Beam was then propagated to BPM 2 which includes collecting optics.
BPM 2 has a YAG screen. Using a VISA camera we saw signal through the
BPM optics but was fuzzy and improvement on alignment and focusing is
needed. We need to do more study on the intensities seen. Just for fun we looked at the Faraday cup at the end of
the beamline. While running
and making minor upstream modifications, we saw that ~1/3 of the charge
was making it downstream (about 250pC). Unfortunately, no profilemeasurements
at the undulator end could be made so we really don’t know what was being
spit out the end of the undulator. I would like to say that it took about 1.5 months from the
time the undulator was inserted and aligned (late November)in the chamber
to the first commissioning run on Tuesday. I think this was amazing progress-vacuum,
controls, pop-ins, cabling ...etc. Without the help of Don Davis, Marc Montemagno, Bill Cahill,
Bob Malone, John Skaritka, Pedro Frigola and Bob Harrington (Don and I
worked most weekends!), I know our present status would not have been
possible. Unfortunately,
there is still much more to do on infrastructure and diagnostics. Continuing, We were unable to look at the beam with the other Pop-ins.
It is difficult to align the BPM optics without more modification to the
magnets. Also, the prototype BPM has the two mirrors mounted so that there
are tilt and rotation capabilities. The production BPMs don’t have this
capability and makes it extremely difficult to align. (Aligning with two
mirrors and 1 degree of freedom on each mirror is tough.)
We are working on modifications which give us the second degree
of freedom on the BPM optical mirrors.
The steering magnets were not tested with beam. They have been
tested off line and some problems have been found. Currently, they are
still being evaluated. The biggest problem is there seems to be around
a 5 minute decay. Testing
shows that when commanded to output
5 amps, the output ramps up to 6 amps and finally settles at 4.6 amps
in around 5 minutes. If true, modifications will be necessary. On Wed., it took the majority of the day to fix problems
from the previous and when we were ready to run, there was a power problem
with the controls. For next
Tuesdays run, it is important to test out the steering power of the magnets.
We need to know how many windings we actually need to give us the steering
necessary for the beam in the undulator. In addition, we want to set up
BPMs along the 4m and try to propagate as best we can. The Joule meter
will be installed and well see what radiation we get out. In addition, there is a 2 week shutdown starting Feb. 7 and
Robert will come out to do another laser interferometer alignment during
that time. We’ll try to get
as much done as we can before then, but as expected with commissioning,
it takes a runs to find out what else needs to be done.
VISA EXPERIMENT: Control displays and software were ready for the first VISA
runs on Tuesday and Wednesday. Two
small errors in control windows were detected and corrected immediately
during the Tuesday run. Operators
now have control of all VISA devices (flippers, pop-in monitors and steering
power supplies.) Items which are still outstanding: 1) Add
special logic to prevent potential problems with pop-in monitors. This logic will allow insertion of the mechanical stop only
when the flag is the fully retracted position. This will avoid possible damage to the pop-in mechanism if
the operator does not enter the proper sequence of operations. 2) Questions
still remain about the steering power supplies. Marc Montemagno found that the supplies have a very long
time constant when requested to move to a new setpoint.
This was verified independently of the control system/software
by using an external signal source to drive the power supply.
Investigation will continue next week with Mark, Aaron (and his
contacts at SLAC) and myself. During these first few weeks of the New Year, a large effort was placed on the VISA experiment. The vacuum system has been completed and the pressure is acceptable.(x10-7). The ATF technical staff devoted all of their time and effort in meeting the schedule for commissioning. Marc Montemagno has spent considerable hours building control chassis’ for pop-ins and camera control, installing and terminating all of the wiring for the pop-ins and BPM’s. He also finished the camera assembly wiring and installed all of the air lines for the pneumatic system. The steering magnet coils have been terminated and the optics are being installed on the table. The Panduit cable trays for wire and air lines have been attached and installed to the table. All of the wiring for the euro-crate power supplies and CAMAC interfacing have been terminated and installed. A serious problem with the Euro-crate power supplies. It turns out that in addressing a command current, the units appear to overshoot and drift. Upon investigation it looks as if it is a design flaw. Aaron is aware of the problem, he will be contacting California for advice. The Terawatt laser discharge cell is being dismantled for welding modifications. Bob Harrington is working full time in the disassembly to meet the rigging and central shops schedule of 02/01/2000.
Last Modified: December 3, 2007 |
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