8. FY06 PROGRESS IN THE TEST EXPERIMENT PROGRAM
by Roger Erickson
Appendix B Self-Evaluation FY2006
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The test experiment program in FFTB was concluded and
the majority of the small experiment work was done in
End Station A. The FFTB run (T-485) was fitted between
SPPS and E-167 runs. There were several brief periods of
ESA experimental activity in November, January and
April, and in the June –August period several
experiments were installed and took data.
T-469,
DIRC R&D Program: D.W.G.S. Leith, J. Va’vra
The experiment used an End Station A beam, with ~1
particle per pulse, to study the performance of a fused
silica bar Cherenkov detector, a development of the DIRC
particle identifier in BaBar. It uses a spare fused
silica bar from BaBar, but incorporates multipixel
photomultiplier tubes, and electronics affording
resolutions ~ 1 mm in space and ~100 ps in time
coordinates.
The DIRC system at BaBar affords excellent particle
identification in the relevant particle energy range. In
possible future applications at much higher luminosity,
limitations to the technique are expected to arise from
beam induced backgrounds in the large water tank that
serves as an optical coupler between the fused silica
and the photomultiplier tubes. In addition, the
Cherenkov cone reconstruction resolution is limited by
the optical dispersion in the fused silica (the
variation in the light propagation speed with
wavelength). Both of these concerns are being addressed
in this test. By reducing the effective pixel size from
3 cm to 6 mm, the volume of the coupling fluid can be
reduced by one to two orders of magnitude. This requires
parallel to point optical focusing. The new pixellated
photomultipliers also have excellent timing
characteristics, and coupling that with fast-timing,
multi-channel electronics to achieve a time resolution
of <100 ps, the optical dispersion in the long fused
silica bar may be corrected by measuring each photon’s
time of propagation.
The 3.6-mete- long fused silica bar has been mounted
on a movable stage at the downstream end of End Station
A, where the low intensity secondary beam was delivered.
Light from the end of the bar was focused, by a 50 cm
focal length mirror, on to an array of six
photomultipliers of various designs. The optical
coupling from the fused silica to the photon detectors
was achieved by using mineral oil. Detectors were from
Burle (multichannel plates) and Hamamatsu (foil
dynodes). About 200 channels of preamplifiers using
Elantek chips, and custom developed constant fraction
discriminators, provided tight timing signals to a TDC
system. The timing fiducial was derived from the linac
timing system, and was monitored for drift by using a
counter in the beam with ~ 35 ps resolution.
The experiment ran this year with a set of improved
photon detectors. An additional Burle/Photonis MCP was
added, with an improved design of the electron
transport. One PMT was replaced by a new Hamamatsu MaPMT
with better timing capability, and whose anode size and
spacing was selected to optimize angular resolution. The
number of available instrumented channels was increased
by 15%. Newly developed improvements on the TDC
calibration were implemented. In addition, the beam was
understood better, was better instrumented, and had
lower backgrounds. In this configuration, the data
sample collected was more than double what had been
obtained before.
The correlation between time of arrival of the light
and the chromatic dispersion has been observed already.
Analysis is proceeding to optimize a procedure to use
the measurements to sharpen the angular resolution and
hence the particle segregation capability. Future work
is expected to explore the extension of pixellated PMT
performance in red light where optical dispersion is
less significant, but Cherenkov light output is lower.
T-474,
T-475, T-480, T-488, ILC Test Beam Experiments in End
Station A: M. Woods
The SLAC Linac can deliver to End Station A (ESA) a
high-energy test beam with similar beam parameters as
for the International Linear Collider (ILC) for bunch
charge, bunch length and bunch energy spread. ESA beam
tests run parasitically with PEP-II with single damped
bunches at 10Hz, beam energy of 28.5 GeV and bunch
charge of (1.5-2.0)·1010 electrons. Four
experiments were approved and took data in FY06. A 5-day
commissioning run was performed in January 2006,
followed by a 2-week run (Run 1) in April and a 2nd
2-week run (Run 2) in July. These tests included: i) BPM
(T-474) and Synchrotron Stripe (T-475) energy
spectrometer prototypes to study systematic effects for
precision energy spectrometer measurements, where the
ILC goal is to achieve an accuracy of 100
parts-per-million; ii) collimator wakefield studies
(T-480) for determining the optimal material and
geometry of ILC collimators. These collimators are
needed to eliminate beam halo that could cause
unacceptable backgrounds for the ILC detectors, but they
can also potentially ruin the small beam emittance; iii)
characterizing the performance of prototype beam
position monitors (BPMs) for the ILC Linac as part of
T-474; and iv) a study of background effects for the IP
feedback BPMs at the ILC (T-488). Beam tests were also
performed to characterize beam-induced electro-magnetic
interference (EMI) along the ESA beamline and to study
an EMI failure mode in the electronics for the SLD
vertex detector, and bunch length diagnostics were
tested that have applicability for ILC and LCLS.
Brief summaries of the ESA setup and beam tests
follow:
i. ESA infrastructure. We installed and commissioned
a new beamline, including 2 wire scanners, a beam
containment collimator and 4 machine protection ion
chambers, a data acquisition system for the experiments
running Labview on a PC reading out VME and Camac
crates; and bunch length diagnostics using high
frequency diode and pyroelectric detectors.
ii. T-474 BPM energy spectrometer. Two rf bpm
triplets were installed and commissioned. New bpm
processing electronics for these and 5 additional
existing rf bpms were also installed and commissioned.
One of the new rf bpm triplets, BPM3-5, uses prototype
ILC Linac bpms. For Run 2, an interferometer system was
commissioned to monitor transverse motion of the BPM3-5
triplet.
iii. T-475 Synchrotron Stripe energy spectrometer. A
prototype quartz fiber detector was installed at a
synchrotron light port in the A-line and commissioning
data taken.
iv.
T-480 Collimator wakefields. The collimator wakefield
box used previously at the ASSET region in the Linac was
relocated to ESA. 8 sets of collimators were
manufactured in the UK and wakefield kicks from all 8
sets have been measured. The BPM system developed for
T-474 is used for the BPM diagnostics.
v.
T-488 IP BPM studies. This experiment studies
background effects in an IR environment for the fast
feedback bpms that will stabilize collisions at the ILC
IP. The experiment was approved in May and first data
was taken in Run 2. The setup included a mockup of
nearby beamline components in the ILC design.
vi. EMI studies. We acquired broadband antennas
measuring frequencies up to 7.5GHz, with signals to a
1.5GHz bandwidth scope, to characterize electromagnetic
interference (EMI) along the ESA beamline. In particular
this was done near ceramic gaps that were installed to
facilitate studies for bunch length diagnostics and EMI
studies. In Run 2 we installed electronics from the SLD
Vertex Detector that had a failure mode during SLD
operation, suspected to be due to beam-induced EMI. We
were able to reproduce the failure mode, demonstrate
that it was due to direct beam-induced EMI pickup at the
electronics board and characterize the EMI levels at
which it failed.
The ILC test beam program in ESA is described in
SLAC-PUB-11988 (also available as
EUROTeV-Report-2006-060), which was prepared for a paper
and poster contribution at EPAC06. Two other papers and
posters were also contributed to EPAC06 for T-480 and
T-488. There are 22 institutions collaborating on the
ILC test beam program in ESA. Analysis of the data taken
in FY06 is ongoing. The program will continue in FY07
with the addition of a magnetic chicane, undulator
magnet and new BPMs for the energy spectrometer tests
(T-474 and T-475), new collimators for wakefield
measurements (T-480) and a new bunch length measurement
experiment using Smith-Purcell radiation (T-487).
T-485, Magnetism with Ultrashort Magnetic Field Pulses:
H. Siegmann, S. J. Gamble, M. Burkhardt
The experiment made use of the extremely high
magnetic fields around the tightly focused FFTB beam to
explore the switching characteristics of ~10 nanometer
thick magnetic films at so far unexplored time scales. A
previous experiment (T-478) exposed similar films using
beam pulse lengths of ~5 picoseconds and ~100
femtoseconds. The goal of the T-485 run this fiscal year
was to confirm the results from T-478, thereby making
them suitable for publication. Namely, T-485 attempted
to obtain a better characterization of the profile of
the SLAC electron beam than was measured during T-478.
This characterization was necessary to determine if
certain features in the T-478 data were due to an
unexpected magnetic response of the system to a clean
Gaussian pulse, or to an expected magnetic response to
an irregular beam.
The physics being explored is that of the ultrafast
switching of ferromagnetic spins, which is expected to
yield information important for fast magnetic recording.
The samples are premagnetized along an easy direction
and mounted in a vacuum chamber on a motorized arm in
the beamline. Single pulses of the FFTB beam are then
sent directly through the samples. Since the magnetic
field of the electron beam falls off with radius,
different portions of the sample are exposed to
different field strengths, peaking near the impact
region of the beam. The high fields obtained within the
5 picosecond or 100 femtosecond duration of the bunches
allow for the study of ultrafast magnetization dynamics
on time scales far beyond any other laboratory setting.
The initial T-478 results displayed unexpected behavior
of the system in two ways. First, the magnetic spins
exposed to both the picosecond and femtosecond bunches
did not conform to the expected switching pattern when
the angle of the initial magnetization and the applied
field from the FFTB beam was larger than 120 degrees.
The observed anomalies in the behavior suggest a
possible breakdown of the conventional magnetization
dynamics equations in the high field-high switching
angle regime. Second, the samples exposed to the
picosecond pulses show small physical holes in the
magnetic layers directly at the point of impact of the
beam, where the top few layers have been removed in the
exposure process. Remarkably, however, these holes are
absent on the same samples when exposed to the
femtosecond beams of identical charge. This means that
the heat transfer to the sample is less efficient when
the pulses get shorter. This result is of great interest
for any ultrafast spectroscopy.
The T-485 run was not able to accomplish its goal.
During the run an anomalously high number of X-rays were
present in the beam pipe originating from a source which
could not be determined during the length of the beam
time. This rendered it impossible to measure the beam
profile, as the X-rays flooded the detectors and
consequently the signal from the wire scanners became
lost in the noise. Furthermore, no femtosecond beam
could be produced during the T-485 run. When the
compressed beam was requested from the control room, the
routine “wake loss scan” was not performed, which the
users did not know until after the end of the run. A
later analysis showed that the number of electrons in
the beam present at the time of the attempted
compression was insufficient to successfully complete
the final stage of compression, and the actual state of
the beam at the time of sample exposure is completely
unknown. Between the two failures of beam
characterization and lack of compression, all of the
T-485 data is unfortunately completely useless.
T-486,
Askaryan Effect in Ice: Calibration of ANITA Payload
In previous experiments at SLAC, the Askaryan Effect,
the emission of coherent Cherenkov emission of radio
energy from electromagnetic showers, was characterized
in sand and salt. The ANITA project, a balloon flight
over Antarctica, is intended to detect Askaryan emission
from extreme energy neutrinos in the ice cap there. This
will be an important step in testing the prediction that
there is a high energy cutoff of the hadronic cosmic ray
spectrum (the GZK effect), at ~1020 eV, that
also distorts the neutrino spectrum. Since the origin of
these ultrahigh energy particles is not understood, this
will bring valuable evidence to the study.
The balloon gondola, a framework supporting 30
specially designed horn antennas and their associated
signal triggering, digitizing and recording electronics,
was assembled in End Station A and supported at the back
of the end station in a way simulating a balloon flight.
The fragile solar photovoltaic panels were not
installed. The Cherenkov radio emission was generated by
stopping the 28.5 GeV electron beam in a block of ice
approximately 15 feet thick. The top surface of the ice
was cut to a downward slope of 6 degrees to the
horizontal. This caused the Askaryan Cherenkov radiation
to be refracted out in a beam aimed at the general
position of the payload. The gondola was moved and
rotated to accumulate measurements of signal strength
from all antennas and for a range of angles and offsets
between the radio beam and the horns. Systematic checks
were made by varying the beam intensity, taking data
with the ice covered with thermal insulation and also
with insulation removed, and changing the surface
texture of the ice. Calibration signals from an emitting
antenna aimed at the gondola were recorded between beam
runs.
Because the system is required in Antarctica within
weeks, and further testing, integration and optimization
is scheduled, only preliminary verification of the test
data has been possible so far. The results show that the
signal strength is close to that expected, and
constitutes the first observation of the Askaryan effect
in ice. The variations between antennas, and the
acceptance cone of each antenna, are also acceptable,
and the data will permit the calibration constants for
the flight to be obtained. |
SLAC National
Accelerator Laboratory, Menlo Park, CA