When it comes to accelerators, what is cold?
Caption: A Fermilab technician works on an array of superconducting niobium cavities at Fermilab. (Photo courtesy of Fermilab)
Superconductivity arises in special materials at super cold temperatures. At these temperatures--a few degrees above absolute zero--the materials' electrical resistance virtually vanishes.
Superconducting technology will be used to accelerate electrons and positrons into extremely energetic collisions in the proposed International Linear Collider (ILC).
This summer, the International Technical Recommendation Panel (ITRP) decided that the international physics community should design the ILC linear accelerators (linacs) with cold technology, rather than the warm technology espoused by SLAC and other institutions.
The panel stressed that both technologies were mature and viable. SLAC has strongly promoted the project, independent of the technology choice, and is now refocusing its efforts to optimally design and achieve a machine with cold technology (see TIP, September 3, 2004).
SLAC is thoroughly familiar with warm technology. A lower-energy version runs the linac here. The particles travel through the center of copper cavities kept at 113 degrees Fahrenheit.
So, How Does Cold Work?
Greg Loew (DO), who spent two years steeped in warm and cold details as chair of the ILC Technical Review Committee, explained the cold technology.
Cavities (roughly seven inches in diameter with a hole through the middle) are cooled to 1.8 Kelvin (271 degrees Celcius below the freezing point of water). The super-cooled cavities are made of a metal called niobium that looks like stainless steel. At that temperature, niobium is superconducting. The electrons in the niobium material (not the electrons being accelerated through the niobium cavity holes) flow with virtually no resistance, like pairs of skaters on perfectly smooth ice.
The ILC calls for two linacs (one for electrons, one for positrons) pointed at each other. Whether a linac is warm or cold, particles get accelerated by microwave power that is injected into the array of cavities. The microwave power generates longitudinal electric fields and cylindrical magnetic fields. The electric field attracts (or pulls) the particles traveling through the cavity, giving them an energy boost.
Because they have almost no resistance, the superconducting cavities can hold the microwave power longer. In warm technology, some of the power ends up heating the copper cavity walls--which have some resistance.
Power Cost vs. Energy Reach
"Superconducting cavities allow you to store microwave energy very efficiently for a long time," said David Burke (ILC).
Linacs accelerate particles in 'trains' with multiple cars--bunches of particles--containing a cargo of 20 billion particles per bunch for the cold design, or 7.5 billion particles per bunch for the warm design. Because cold cavities store microwave energy longer, each microwave pulse can accelerate longer bunch trains. Cold trains carry 15 times more bunches than warm trains, but arrive less frequently--five times each second compared to 120 times each second for warm. In the end, the two designs generate similar luminosities, or event rates, for experiments.
Compared to warm technology, cold technology uses less electricity from the power company while accelerating longer energy-efficient trains. That is like adding loaded railroad cars to a steam train without needing more coal to power the train.
However, some of what superconducting technology gains by saving power, it loses in particle energy.
Niobium surrenders its superconductivity when exposed to too strong a magnetic field (see sidebar). Accordingly, the cold linacs will use lower magnetic and electric fields which means particles will get a smaller tug, and gain less energy, for every meter traveled. Thus, a cold machine needs to be longer than a warm machine to reach the final beam energy of 250 Giga electron volts (GeV) for each beam. The machine length will be determined by the design, but will be at least 20 miles long from the end of one linac to the end of the other.
Back to the Future
Helping design a cold machine won't be completely new to SLAC, Loew pointed out. In the late 1960s, people at SLAC and Stanford explored how to equip the linac with superconducting cavities, but it proved unfeasible at the time. Now the time appears right for super-energetic cold linacs.
For more information on the linear collider, see: http://www-project.slac.stanford.edu/ilc/