|The International Linear Collider
Part 2: Bright Beams of Electrons and Positrons
|Contact: Paul Preuss, email@example.com|
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.
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).
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 make 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."
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.
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."
Science@Berkeley 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.