The MEXT, the Japanese Ministry that
supervises KEK, has announced that it will appropriate a budget of 100 oku-yen
(approx $110M) over the next three years starting this Japanese fiscal year
(JFY2010) for the high performance upgrade program of KEKB. This is part of the
measures taken under the new "Very Advanced Research Support Program" of the
Japanese government.
"We are delighted to hear this news," says Masanori
Yamauchi, former spokesperson for the Belle experiment and currently a deputy
director of the Institute of Particle and Nuclear Studies of KEK. "This three-
year upgrade plan allows the Belle experiment to study the physics from decays
of heavy flavor particles with an unprecedented precision. It means that KEK in
Japan is launching a renewed research program in search for new physics by using
a technique which is complementary to what is employed at LHC at
CERN."
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| SuperKEKB making headway toward higher luminosity |
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The proposed SuperKEKB
electron-positron collider underwent a major design change last March. A team of
a hundred experts is moving forward to create the enabling technologies needed
to realize the next generation electron-positron collider.
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Schematic view of
SuperKEKB upgrade. Last March, the team changed their approach to the higher
luminosity from high current scheme to small beam size
scheme.
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'Luminosity' is
one of the most important values talked of when particle physicists refer to how
good a collider performs. One must be careful with this term though; luminosity
in accelerator science is not luminosity in the stars. It does not tell you how
luminous any matters are, but rather how luminous collision events are.
Luminosity refers to the rate of particle collisions, and is a measure of how
efficiently an accelerator produces these events. With higher luminosity,
interactions produce more particles and particle physicists have more data to
use in exploring new area of physics. (Read previous issue for what they are looking
for.)
Scientists working at the electron-positron collider known as KEKB
at the High Energy Accelerator Research Organization (KEK) have been and are
paving new ways forward at the luminosity frontier. Our team of 100 experts has
held the world luminosity record since 2001, repeatedly breaking their own
records. The current luminosity of 2.11 x 1034 cm-2
s-1 exceeds KEKB's original design luminosity by more than a factor
of 2. Now the team is working hard on a major upgrade of their state-of-the-art
accelerator. When the upgrade is finished, the new facility will be known as
SuperKEKB.
SuperKEKB at one time: The high current
option
When SuperKEKB was proposed in 2003, the target luminosity was
set to 2 x 1035 cm-2 s-1, about 20 times higher
than the KEKB's original design value. Engineering it to higher luminosity
involves three things: increasing the beam current, focusing the beams at the
interaction point, and/or making the electromagnetic beam-beam interactions
small. These correspond to varying values of the beam current, beta-function,
and beam-beam parameter, respectively. The team's original approach was
two-fold: to increase the beam current and the beam-beam parameter. This
approach had been known as the high current option, and proved to be very
successful for the SuperKEKB's precursor, KEKB.
In 2007, KEKB's
scientists introduced two accelerator cavities called crab cavities, one for the
electron ring and one for the positron ring at KEKB. Particles travel in bunches
in an accelerator ring. At the interaction point, the beams of electron-bunches
and positron-bunches cross at an angle of 1.3 degrees. The newly installed crab
cavities kick the head and tail of each bunch of particles so that bunches would
make effective head-on collisions at the interaction point. This process is
called crab crossing. With these crab cavities installed in the KEKB rings, the
team was able to achieve the recent luminosity jump by fifteen percent.
With the success of the high current scheme, KEKB researchers initially
thought that they could aim for still higher luminosity by increasing the
current, using most of the present configurations of magnets. The target
luminosity was then raised up to 8 x 1035 cm-2
s-1. When simulations suggested that they could use the crab crossing
to increase the luminosity up to six times higher, the team thought the
SuperKEKB's target luminosity was within their reach.
Then, a difficulty
arose. The crab crossing effect improved the luminosity, but not as much as the
simulation had predicted. "We are running simulations to explore possible
scenarios that might keep the value low," explains Dr. Yoshihiro Funakoshi at
KEK, the leader of the KEKB commissioning group, "It most likely be because of
machine errors that simulations could not take into account."
Aside from
this problem, the high current option suffers from other difficulties. They will
need to figure out how to cope with phenomena called coherent synchrotron
radiation as it will stretch bunches in positron ring. The team also found
recently that the beam size at the interaction point in the horizontal direction
would have to be much larger than the designed value because of the constraint
due to the large magnet size at the interaction region. Both these will result
in luminosity drop. While the team continues their effort to overcome these
difficulties, the design underwent a major change.
The nano beam
design
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A KEKB leader and beam
optics group leader Prof. Haruyo Koiso (center) and the commissioning group
leader Dr. Yoshihiro Funakoshi (right) celebrating the new world luminosity
record with KEKB scientists and Belle collaborators.
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In March 2009, the team changed the
course of their SuperKEKB design. This change was based on ideas from the Super
B Factory, a next-generation electron-positron collider at the National
Institute of Nuclear Physics (INFN) in Italy, proposed by a collaboration of
former PEP-II scientists at SLAC and Russian physicists. Its target luminosity
is as high as 1036 cm-2 s-1, and their base
design is called nano beam option. This design takes small beam-size and a large
crossing angle at the interaction point, instead of a high current. In other
words, they approach higher luminosity by squeezing beams to nanometer-scale,
rather than by increasing the beam intensity and the beam-beam
parameter.
The brilliance of the nano beam scheme is that it brings out
the best of the interaction mechanism. At the interaction point, bunches of
particles in the beam can get squeezed to narrower bunches by stronger magnetic
fields, but this process saturates at some point because of what is called the
'hourglass effect'. The vertical beam size of beam bunches increase outward from
the most focused point so that only very small portion of beam bunch is in
focus. This focusing effect will be diluted when beams collide head-on. Why
don't we then intersect electron and positron bunches only at the highly focused
region of each bunch rather than the entire bunches in order to gain higher
luminosity? That led to the nano beam concept.
Using the nano beam
scheme in the SuperKEKB design brings a number of advantages. One of the most
important advantages is that it is greener. The beam current for this design
will be at 4 Ampere for the low energy ring (2.3A for high energy ring), instead
of the 9.4 A (4.1A) of the high current scheme. It follows that this scheme will
be more economical one, because devices like radiofrequency power sources to
sustain high current do not require as extensive extensions as with the high
current option. With this design, "we think we will be able to reach luminosity
of 8 x 1035 cm-2 s-1," says Funakoshi. "This
will be an entirely different approach for SuperKEKB, but seems very promising
for the high luminosity we need."
Our entire accelerator team is now
working hard on the research and development of each component for the new
SuperKEKB design. There are multiple major technological hardships to be
overcome by this fall when the team will meet to determine the technological
feasibility and reach the final design completion. The final design will change
many aspects of the accelerator, and require major upgrades in some area.
The challenges of higher beam current
The first such
issues to be resolved are the unfortunate side effects of high beam current.
Even with the nano beam scheme, the beam current will increase to twice as much
of the present value. Accelerator researchers have known for years that for a
high-current electron-positron collider there will be issue of electron-cloud
effect in positron ring as well as excessive heating in vacuum chamber due to
the strong radiation. Electron clouds form when radiation from accelerating
charged particles-called synchrotron radiation-hits the wall surface and kicks
out electrons into the chamber. Secondary electrons come out when those
electrons hit wall surfaces, contributing to the formation of electron clouds.
These electrons disrupt positron beam when they come near the beam. Photon
Factory, also at KEK, was the first to observe this effect that heaves up the
train of positron bunches. KEKB also confirmed this effect later, and further
found a more serious case of a single-bunch instability that vibrates each bunch
at much higher frequency.
Vacuum group member
Dr. Yusuke Suetsugu working at the test vacuum chamber he and his group
developed. This Saturn-shaped special chamber has an antechamber for vacuum pump
and another antechamber for electron clouds. The former antechamber will
eliminate pump ports near beams, and is effective for both electron and positron
rings.
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To counteract such phenomena, the team
developed a clever vacuum chamber with two small antechambers, one on the left
and one of the right. One antechamber suppresses the formation of electron
clouds and the other contains vacuum pumps. They also wrapped the chambers
between magnets with solenoid coils to reduce the effect of secondary electrons.
According to Dr. Yusuke Suetsugu, a vacuum group member, the research and
development for such beam chambers is crucial for SuperKEKB although it became
less demanding since the new design, as the beam current is less than a half
that of original high current option. The team has already developed several
test antechamber from the prototype produced in collaboration with a Russian
team in the Budker Institute of Nuclear Physics, and installed them in sections
of the KEKB ring.
The electron cloud is one of the central difficulties
faced by accelerator scientists in designing new colliders. Other new colliders,
such as the currently proposed International Linear Collider (ILC), are faced
with the same problem. The ILC teams at KEK, SLAC, and Cornell University
collaborate on the research and development of various mitigation techniques.
"The collaboration is now exploring a possibility to introduce new and more
effective mitigation mechanism inside the bending magnets," says Suetsugu. "We
are in good shape for the research and development for this
year."
Small, powerful magnets
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Superconducting magnet
group leader Dr. Norihito Ohuchi and the test station he developed for
SuperKEKB's original design. He and his group members are now working on more
challenging magnets for new nano beam design. The magnets developed and tested
for previous design will still be used for sextupole magnets near the
interaction region.
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The next set of
challenges involves the superconducting magnets in the interaction region.
According to superconducting magnet group leader Dr. Norihito Ohuchi, the new
design will need eight strong quadrupole magnets placed deeper in the
interaction region to squeeze the beam to nanometer scale. The first hurdle for
the group is the design and fabrication of some unusually small quadrupole
superconducting magnets whose inner diameters are only 4-8 centimeters. This is
one-sixth the diameter of the KEKB quadrupole magnets. Making the magnets this
small requires consideration of points that did not matter before. For example,
the new nano beam design requires finer magnet current controls to protect
magnets from magnet quench due to an excessive heating. Since the current
density in the superconductor go beyond 2000A/mm2, the temperature of
the magnets will go over 1000 degrees in Kelvin within about 50 milliseconds.
Additionally, the accelerator design demands much smaller fabrication errors to
acquire a field quality of a few 10-4 with respect to the quadrupole
field.
"No one has yet made a superconducting magnet with this small
size and the high current density for the interaction region," says Ohuchi. The
group is going to construct an R&D magnet by the end of this year and test
it in early 2010. The development of this magnet, which will be one of the
world's finest superconducting magnets, will be the key to the success of the
new design.
Beam optics and short beam lifetime
Another
major field of active research is the beam optics. The goal of beam optics
research is to find a total solution for creating a beam with ideal properties
(shape, emittance and lifetime) for the new nano beam design. This is done by
studying the arrangement of magnets along the rings and in the interaction
region. The beam optics group uses variety of magnets for different purposes:
dipole magnets for bending, quadrupole magnets for beam focusing, and sextupole
magnets for correcting beam chromaticity-a parameter that indicates dependence
of beam optics on energy deviation. Depending on the arrangement of these
magnets they are able to produce a variety of beam parameters.
KEKB's simulation
group leader Prof. Kazuhito Ohmi (left) and supercomputer group leader at
Computing Research Center Dr. Hideo Matsufuru in front of the supercomputer Ohmi
uses for his beam-beam collision and beam instability simulations. Ohmi's
successful simulation revealed mechanism of electron-cloud induced head-tail
instability.
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"There are many ways those magnets can be
arranged in the rings and in the interaction region," says Prof. Haruyo Koiso, a
KEKB team leader and the beam optics group leader. "We need much better optics.
We are going to explore many more magnet arrangements in the interaction region
in great detail."
The most serious issue in the nano beam scheme is the
short lifetime of beams. Containing particles inside beam bunch is not easy for
variety of reasons. When a quadrupole magnet squeezes a beam, the degree to
which the beam can be focused depends on the energy of each particle in the
bunch. This causes particles to be dispersed in a beam due to the energy
differences. Scientists place sextupole magnets to fix the chromaticity, but the
magnetic field created by the sextupole magnets also limits the area inside the
pipe that a beam can go through undisturbed. The particles outside of this
region can no longer be contained in the beam bunch. Many other sources of
nonlinear effects other than the magnetic field due to sextupole magnets can
influence on this region, called dynamic aperture, Accelerator scientists
analyze the dynamic aperture in three dimensions: horizontal, vertical, and
energy dimension. When the dynamic aperture is small, outer particles keep
getting lost and the beam's lifetime becomes short.
The problems in
dynamic aperture the KEKB researchers face are more acute in nano beam scheme
than in high current one, because in nano beam scheme the beam size at the
interaction region will be reduced by a factor of ten. The optics group is
making its best effort to simulate various possible configurations to complete
the conceptual design for the rings and to develop a detailed model for the near
interaction point.
Every group in SuperKEKB is now working hard to test
and simulate parts of the new nano beam design, including the beam monitors,
beam control, beam transport, linac, damping ring, radiofrequency system, and
magnet system. Meanwhile the SuplerKEKB team awaits approval for the project
from the Japanese government. The fact that the nano beam option is both greener
and lower cost option will certainly help.
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