Crystal extraction of beam from high energy hadron accelerators Page: 3 of 6
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Crystal Extraction of Beam from High Energy Hadron Accelerators
C. Thornton Murphy
It is an honor to have been asked to give the invited
paper on this topic here at Dubna, for it was at Dubna
where bending beams with channeling in a bent crystal
was first observed under the leadership of former JINR
member Edic Tsyganov. It was also at Dubna where a
beam was first extracted from the circulating beam.
This paper will report only the results of the crystal
extraction experiments at Fermilab and CERN. The
very recent high-efficiency result obtained at IHEP,
Protvino, will be presented in the next paper by Kotov.
II. Fermilab Experiment 853.
This paper will summarize the experimental
techniques and results of E-853, but will refer to formal
publications for figures and details. What this paper
adds to the published results is some interesting details
about the planning and execution of the experiment not
normally contained in formal publications.
The motivation for E-853, which was proposed in
1991, was the desire to extract beam from the halo of
the SSC for a fixed target B experiment. The idea of
extracting the SSC halo, rather than scraping it on
absorbing collimators, was first mentioned by C. R.
Sun in 1984 . The purpose of E-853 was to answer
a number of questions relevant to the SSC proposal in a
similar accelerator, the 900 GeV Tevatron, a
superconducting accelerator like the SSC was to have
been. The goals of the experiment were:
1. to extract 106 protons/sec with 1012 protons
circulating, or 107 protons/sec with 1013 protons
2. to show that the luminosity lifetime is not seriously
3. to show that no intolerable backgrounds were created
at the collider experiments;
4. to explore methods of creating additional halo using
RF noise or damper noise.
There was also a slight concern that the particles
scattered from the crystal might create sufficient energy
deposition in the cryogenics of the Tevatron that the
experiment would induces quenches of the magnets.
This never happened during the course of the
The experiment was proposed in June, 1991. In
April, 1992, it was approved for 72 hours of running in
proton-only stores in the Tevatron, provided that the
SSC would pay for the equipment. In December, 1992,
the SSC granted $100,000 for equipment, of which
only $70,000 was spent.
After a building and installation phase, and waiting
for the Tevatron to switch from fixed target to collider
mode, the experiment received 19 brief (- 2 hours) study
sessions with proton-only stores during 1995 and 1996.
The stores, in which E853 was the only user of the
beam, were usually scheduled on very short notice, as it
was advantageous to schedule the experiment during
unexpected periods when there were not enough
antiprotons for a collider experiments fill - for instance,
after an unexpected power outage. The total study time
in this mode was 57 hours.
Late in 1996, the experiment was allowed to run
during the last few hours of a colliding beams store,
provided that it was "unnoticed" by the collider
experiments, meaning that the presence of the crystal
close to the beam did not create objectionable
backgrounds for the experiments. An additional 23
hours of study were received in this mode and allowed
the accomplishment of goals 2 and 3 mentioned above.
Details of the apparatus have been published [2,3]
and will only be summarized here. The CO straight
section of the Tevatron was selected for the experiment,
as there already existed there an extraction channel for
the proton abort line which was not used at 900 GeV
during the collider mode. A bent crystal replaced the
first kicker magnet of the abort system and bent
protons from the halo of the beam up into the abort
channel. (see Fig. 1, ref ).
Silicon crystals of dimensions 3 x 9 x 39 mm with
the (111) atomic plane parallel to the top face were
fabricated and characterized by the Petersburg Nuclear
Physics Institute, Gatchina. The crystal was bent
through 0.64 mrad in a four-point spring-loaded bender.
The bender with the crystal was mounted inside a one-
meter vacuum tube which had vertical and horizontal
stepping motors at each end and served as the
goniometer for aligning the crystal angle to the beam
angle. The smallest goniometer step size was 2.5 grad,
in order to be smaller than the crystal critical angle of
5.2 grad and the beam angular divergence of 11 rad.
Extracted beam was detected with instruments in two
air-gaps in the extraction channel situated 60 and 100 m
downstream of the crystal. Both air gaps had a pair of
scintillators in coincidence to count the beam, and the
first air gap also had a fluorescent screen viewed by a
An additional scintillator, called the interaction
monitor, was placed directly underneath the goniometer
in order to measure the interaction rate of the protons
with the crystal.
At the beginning of a study session, the crystal was
gradually moved horizontally into the halo from the
outside of the ring. Note that in contrast with the
CERN experiment, the crystal moved into the beam in
the horizonal plane but bent the beam up, so that any
lack of parallelism between the atomic planes and the
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Murphy, C. Thornton. Crystal extraction of beam from high energy hadron accelerators, article, January 18, 1999; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc666609/m1/3/: accessed April 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.