NLC Team Achieves Key Milestone for
‘Warm’ Linear Collider
Heather Rock Woods
More than 2,600 physicists agree that the hunt for heavier particles,
dark matter and supersymmetry requires an international linear collider
(LC)—but the open question is whether to use ‘warm’ or ‘cold’ technology
to accelerate the electrons and positrons to the massive energies
One of the new 0.6-meter
accelerator structures for accelerating electrons and positrons,
designed by the NLC collaboration. (Courtesy
of Chris Adolphsen)
Displaying the impeccable timing that
makes accelerators run, the Next Linear Collider (NLC) project has met
both of its crucial technical goals—and clearly demonstrated the
viability of the warm (called X-band) technology.
The achievement came just before an important visit in late April from
the International Technology Recommendation Panel (ITRP), the group that
will choose by year’s end which technology to use.
Early last year, the separate International Linear Collider Technical
Review Committee headed by Greg Loew (DO) identified the main hurdles,
called R1 goals, each technology needed to clear in order to prove
The NLC project is based at SLAC with collaborators from Fermilab, LBNL,
Livermore and Brookhaven laboratories. In partnership with the Global
Linear Collider (GLC) group at KEK in Japan, NLC has met its R1 goals
for both a 500 GeV collider and a possible upgrade to a 1,000 GeV (or 1
“These are most impressive and timely achievements for the NLC/GLC,”
The LC design calls for two linear accelerators (linacs) pointed at each
other to smash together electrons and positrons each carrying 250 GeV,
making 500 GeV ‘center-of-mass collisions’.
The warm collaboration established the ability to reach 500 GeV some
years ago when the NLC Test Accelerator (NLCTA) was built here, and had
been aiming for the prized 1 TeV goal ever since. The warm, i.e., room
temperature, technology uses radio frequency (rf) power in the X-band
(11.424 GHz frequency), four times higher than the SLAC linac’s warm rf
technology at 2.856 GHz, in the S-band.
The cold rf technology, proposed by DESY, is based on superconducting
cavities operating two degrees above absolute zero. This technology
required no R1 demonstrations for a 500 GeV machine, and expects to meet
the R1 goal for its upgrade to an 800 GeV machine in the next year.
First Goal Met
The first NLC/GLC goal, met in December, showed that rf supply stations
can produce the power required to add 65 MeV of energy to the electrons
for each meter they travel (see TIP,
February 6, 2004).
The latest achievement shows that newly designed accelerator
structures—the copper pipes the electrons travel in—can sustain this
acceleration gradient of 65 MV per meter and that the rf power can be
shaped into an ideal wave for the electrons to surf on, while keeping an
extremely low breakdown rate.
“After a four-year effort to improve the operation characteristics of
the accelerator structures, we’ve finally got the numbers we want,” said
Chris Adolphsen (NLC), head of structure testing and evaluation.
The acceleration gradient is a measure of how much energy per meter the
electrons gain as they zip to the end of the linac. The design gradient,
65 MV per meter, is almost four times SLAC’s current acceleration
gradient, and the rf frequency is four times larger, in order to
generate electrons and positrons with final energies five times
higher—without making the linacs much longer and costlier.
“Going to 65 (MV) is a nice thing because it means your linac doesn’t
have to be too long,” Adolphsen said. There were many challenges to
building accelerating structures that could maintain the high gradient
without breaking down too often or becoming damaged. It took dozens of
tests and over 20,000 hours of high-power operation to arrive at 0.6
meter long structures, made of 55 shiny copper disks with about 1 cm
irises (the hole the electrons stream through).
In April, the team reached the design requirement of only one breakdown
every 10 hours on average. The breakdowns occur from sparking on the
irises that shorts the rf power, shutting down the rf supply. It then
takes 10 seconds to recover the full gradient.
Because the LC will have spare rf supply stations, “the 0.1 per hour
breakdown rate means there is effectively never any down time,”
In addition, the structures easily surpassed the requirement to achieve
full acceleration 99 percent of the time. Testing and tinkering
continue, but the pipeline for supplying high-energy collisions is
already in great shape.
See the press release on 2,600 physicists supporting the LC: