Seharian mengikuti liputan CNN dan BBC seputar
wafatnya Sri Paus Johanus Paulus II, saya jadi ingat
fiksi Angels and Demons karangan Dan Brown.
Digambarkan, ada orang yang mau menghancurkan Vatican
berikut isinya dengan senjata anti-matter yang dicuri
dari lab CERN di Geneva.
Kelebihan novelis Amerika adalah, sebelum menulis
sebuah fiksi mereka membuat riset yang njlimet
terlebih dahulu. Tak mengherankan, kalau cerita Dan
Brown mengenai detail Sistine Chapel dan banyak museum
di Roma persis dengan hasil liputan National
Geographic Channel. Dan, penggambaran anti-matter a
la Dan Brown meskipun khayalan tetapi bisa dilacak
dengan fakta dibawah ini.
Salam,
RM
-------------
March 31, 2005 [EMAIL PROTECTED] lab | current article
| lab a-z index | lab home
The International Linear Collider
Part 2: Bright Beams of Electrons and Positrons
Contact: Paul Preuss, [EMAIL PROTECTED]
Second in a series on the role Berkeley Lab
researchers are playing in planning for the proposed
International Linear Collider.
Berkeley Lab physicists are collaborating with dozens
of other groups from around the world in designing the
International Linear Collider (ILC), a
30-kilometer-long accelerator that will collide
electrons with their antiparticles, positrons, to
study new kinds of fundamental particles with more
accuracy than any other existing or planned
accelerator.
Andy Wolski of Berkeley Lab's Accelerator and Fusion
Research Division is one of the leaders in design
studies of damping rings for the International Linear
Collider. Damping rings are similar, in principle, to
the storage ring of the Advanced Light Source
(background). (Photo Roy Kaltschmidt)
Berkeley Lab's Accelerator and Fusion Research
Division (AFRD) is a leading contributor to studies of
the ILC's damping rings, structures which are
essential to preparing bunches of electrons and
positrons with the right characteristics to feed the
ILC's twin, head-to-head linear accelerators, or
linacs. Designing damping rings for the ILC is a task
with its own special challenges.
"In the only previous linear collider ever built, the
Stanford Linear Collider, the damping rings earned a
reputation as the source of all evil," says AFRD's
Andy Wolski. "Even small problems with beam stability
got amplified all the way down the rest of the
machine. And the ILC will be far more sensitive to
stability problems than the SLC."
Electrons and positrons are ideal for precision
measurements of high-energy events because, unlike
protons, they are fundamental particles, not made of
anything else; when they collide the energy of the
collision can be known exactly. By contrast, a
collision between protons is a set of collisions of
their constituent quarks, having slightly different
and uncertain energies.
But while a pointlike electron or positron has the
same electrical charge as a proton (opposite in sign,
in the case of the negatively charged electron), it
has less than one 1,800th of a proton's mass. One
consequence of this is that when a lightweight
electron or positron is forced to round a curve, it
loses a much higher proportion of its total
mass-energy than a lumbering proton. This lost
synchrotron energy is routinely put to good use in
research facilities like Berkeley Lab's Advanced Light
Source (ALS), but if the goal is simple acceleration,
synchrotron energy is a waste. The only practical way
to achieve very high energies with electrons and
positrons is not to make them turn corners at all, but
rather to accelerate them over long distances in a
straight line.
Squeezing the beams
When particles and their antiparticles collide they
mutually annihilate; from the resulting fireball of
pure energy other particles appear. The expected
fruits of the ILC include Higgs bosons, supersymmetric
particles, and others. To achieve enough collisions —
that is, to achieve sufficiently high luminosity — the
ILC's opposed beams of energetic electrons and
positrons must be composed of many tightly confined,
closely spaced bunches of particles of nearly
identical energy.
In August 2004 the technology panel of the
International Committee for Future Accelerators
recommended using "cold," superconducting technology
for the ILC's linacs, a decision that directly affects
the necessary characteristics of the electron and
positron beams. In the TESLA design (TESLA stands for
Trillion-electron-volt-Energy Superconducting Linear
Accelerator), whose development has been led by DESY,
the German Electron Synchrotron laboratory near
Hamburg, each of the superconducting linacs would
accelerate bunches of 20 billion electrons or
positrons, the bunches following one another at
intervals of 337 nanoseconds (billionths of a second).
ILC linacs will use superconducting radiofrequency
cavities to accelerate bunches of electrons and
positrons, like those designed for TESLA. (Images
DESY)
Conditioning these bunches is the job of the damping
rings, one feeding the electron linac and one the
positron linac. Electrons are created in an electron
gun and injected into the damping ring. Positrons are
more complicated; because of their propensity for
mutual annihilation upon meeting ubiquitous electrons,
they tend to be short-lived and hard to find in
nature. But when a stream of energetic electrons or
intense gamma rays is directed against a tungsten
target, positrons emerge from the far side as debris.
Positron bunches are necessarily more spread out than
electron bunches from a gun; a major challenge is to
design a damping ring that can capture as many
incoming positrons as possible. Any positrons outside
the radius that can be accepted by the ring will
collide with components of the structure, causing
intense radiation, which must be avoided.
"The particles in a bunch enter the damping ring with
varying energies and trajectories," says Wolski. "The
goal is to reduce the differences and produce a train
of tight bunches of uniform energy, all cleanly
separated. This is essential to the beam's high
luminosity.”
To do this the damping rings makes use of the energy
lost as the particles move round bends — the same
effect used to make x?rays in the ALS. As in the ALS,
beams in the damping rings are controlled by an
arrangement of magnets of varying geometries, called
the lattice. Dipole magnets (having one north and one
south pole) steer the bunches; quadrupole magnets
(with pairs of opposing north and south poles) focus
them, restricting their size. Says Wolski, "The
challenge is to design lattices that can handle wide
energy differences and large orbital differences."
A wiggler magnet's alternating fields force particles
to shed excess energy and form tight bunches.
Although the bending magnets that steer the beam do
provide some damping, by themselves they yield a
damping rate that is much too slow. Therefore much of
the damping will be done by wiggler magnets, which
steer the particles in a slalom pattern through
alternating magnetic fields. Whenever a particle
changes direction it loses energy, so the wigglers
serve to cool off the more energetic, farther-ranging
particles and pinch the whole bunch into a tight
energy package. Wolski says, "TESLA's design would
need 400 meters of wigglers. That's never been done;
we are developing new kinds of computer modeling tools
to design these lattices."
A swift kick in the particles
Once the circulating bunches have been conditioned,
the bunch train must be extracted into the linac.
Injection and extraction involve devices named
kickers, the magnetic equivalents of switch points on
a railroad track. A kicker has to switch on and off in
the interval between bunches; it takes time to build
up a strong enough magnetic field — the rise time —
and time for the field to dissipate — the fall time.
Kicker rise and fall time is intimately connected with
the physical dimensions of the damping ring, which in
turn will affect the ILC's cost, since the bigger the
ring the more expensive the construction.
The TESLA design features a kind of squashed ring
nicknamed a dogbone, consisting of long straight
sections and tight turnarounds at each end, having a
total path of 17 kilometers. Berkeley Lab researchers
are working with colleagues at Cornell University,
Fermilab, Argonne, and the Stanford Linear Accelerator
Center (SLAC) in the US, and KEK, the Japanese High
Energy Accelerator Organization, looking at rings with
simple lattices only 6 or even 3 kilometers in
circumference.
No matter what the dimensions, the damping rings will
have to accommodate entire trains of particle bunches,
whose length is established by the ILC's luminosity
and the geometry of its linacs. A superconducting
linac like TESLA's dictates a train of 2,820 bunches
spaced 337 nanoseconds apart — a train which would
stretch 285 kilometers in length. Squeezing this train
into a 17-kilometer dogbone means reducing its length
17 times over; time between bunches is cut to 20
nanoseconds, thus requiring kickers with 20-nanosecond
rise and fall time. Loading the same train into a
3-kilometer ring means squeezing the bunches even
tighter; bunches would be separated by little more
than 3 nanoseconds, requiring kickers with a
3-nanosecond rise and fall time.
Attainable rise and fall times of magnetic kickers
that inject and extract bunches of positrons and
electrons will determine the possible dimensions of
the ILC's damping rings. Kickers can be tested at
KEK's Accelerator Test Facility (background).
As yet, no single damping-ring design for the
International Linear Collider stands out as clearly
superior: all have advantages and corresponding
disadvantages. For example, while a 3-kilometer design
has a simple lattice, no kicker yet built achieves a
rise and fall time of 3 nanoseconds.
"The first task is to explore all designs, using both
tests of new hardware, like kickers, and computer
models of the various lattices and other components,
until the ILC's Central Design Group can choose the
most promising," says Wolski. "That will take several
months to a year."
Whatever the final design, Berkeley Lab scientists are
likely to be leading partners. Experience is one
reason: in principle, a damping ring is a storage ring
"much like the main ring of the Advanced Light
Source," says Wolski, "only pushed to extremes."
Berkeley Lab researchers had the lead responsibility
for damping-ring design in the Next Linear Collider
(NLC) collaboration, working with colleagues at SLAC,
Fermilab, Lawrence Livermore, and KEK. Although the
NLC's "warm" accelerator technology was not chosen for
the International Linear Accelerator, in terms of
damping rings "the move to 'cold' technology is a
natural progression," Wolski says.
"Damping rings are an attractive area for a laboratory
to take ownership," Wolski adds, "and at this stage
the best way to make progress is to have more than one
institution studying any particular issue. When it
comes to building the ILC's damping rings, whatever
the final design choices, we feel that Berkeley Lab
has established a leading role."
[EMAIL PROTECTED] Lab's series on the International
Linear Collider will conclude with a look at designing
detectors that can capture and identify the ILC's most
interesting particle events.
Additional information
More about the Accelerator Test Facility (ATF) at KEK
A technical discussion of lattice design for an ILC
damping ring with 3-kilometer circumference (pdf)
Technical notes on a 6-kilometer damping-ring design
(ppt)
Technical notes on the 17-kilometer TESLA dogbone
(pdf)
More about the TESLA Collaboration and the TESLA photo
archive (in German)
Visit the International Linear Collider Communication
website
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