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No stopping the accelerator

17 August 2000

The recent European Particle Accelerator Conference in Vienna underlined
how far and wide particle accelerators have infiltrated into science and
technology. Colin Johnson reports.

The seventh European Particle Accelerator Conference (EPAC 2000), which
was held in Vienna on 26-30 June, reflected how particle accelerators have
evolved from a physics research tool into a burgeoning applications field
covering the sciences, medicine and industry. In his opening talk, Ugo Amaldi
of the TERA Foundation underlined the many facets of particle acceleration
today, ranging from the history of art through many diverse sciences to the
study of the energy frontier in physics.

Then followed the performance
reports on the Phi-factory DAFNE at Frascati and the B-factories PEP-II and
KEKB at SLAC, Stanford and KEK, Japan, respectively. Here the emphasis
was on machine tuning to arrive at design luminosities. This demonstrates a
different form of art – the careful diagnostic study of circulating beams linked
to feedback control systems that overcome potential instabilities.

Never
is the accelerator specialist more elated than when this leads to higher beam
currents and greater luminosity for the experimenter – provided, of course, that
the detector backgrounds are tolerable. John Seeman of SLAC, who once
described this excitement in striking detail in his article “The Tao of
Commissioning” (SLAC BeamLine 29 2), was able to report a
PEP-II record luminosity of 2.22 ¥
1033 cm-2s-1, with KEKB not far
behind.

At DESY, Hamburg, the electron-proton collider HERA has
been shut down for nine months for upgrades that should lead to luminosity
gains of up to a factor of five. The last sprint of LEP at CERN is towards
higher energy at the expense of luminosity and broken cavities, although there
is some benefit from the reduced radiation damping times that accompany the
higher beam energies. Steve Myers of CERN used a touch of blarney in his
witty account of the good, the bad and the unforeseen in 12 years of LEP
operations – recommended reading if it is ever
published.

Future machines

Magnet production and testing
for CERN’s LHC collider represented the forefront of new large machine
project work (see “The biggest of them all” below), while looking beyond
LHC, the feasibility studies on neutrino factory design were described by
Norbert Holtkamp and Helmut Haseroth for Fermilab and CERN respectively.
These assemblies of machines and machine components to produce, collect,
cool and store muons stretch our abilities as machine designers to and beyond
our present limits. Here is a challenge for the future that is similar to that faced
in linear collider studies for many years.

This design work was not
reported at EPAC 2000 because it is a main component of linac and collider
conferences. However, on the last day there was a session devoted to the four
main test facilities for linear collider R&D. D Trines of DESY reported on the
impressive recent results of the TESLA collaboration in developing nine-cell
1.3 GHz superconducting cavities to withstand gradients of 25 MV/m and
single-cell cavities to gradients of 40 MV/m. The surface treatment and
assembly of the cavities is now well understood and electropolishing the
niobium surfaces is seen as a major factor in pushing up the breakdown limit.
Four companies are now able to supply cavities that meet the TESLA design
specifications.

The higher-gradient copper cavity linac designs, studied at
the NLC Test Accelerator at SLAC and the CLIC Test Facility at CERN, also
progress by demonstrating their functionality while revealing the need to
concentrate on certain technical details. One such detail is the recently
discovered damage to the copper surfaces of these cavities when they are
pushed to the highest gradients. Both the NLC and CLIC study groups
reported that they had found unexpected damage of the internal copper
surfaces of their respective large-aperture travelling-wave accelerating
structures after conditioning them with radiofrequency power to accelerating
gradients of 50-60 MV/m.

Medical applications

Turning to the vast
majority of accelerator design and construction projects, it was the medical
applications that featured most prominently. Synchrotron light sources were
naturally well represented and the third-generation sources are being extended
into a fourth generation of ultraviolet, vacuum ultraviolet and X-ray free
electron lasers capable of performance in terms of beam brilliance many orders
of magnitude above present-day synchrotron light sources.

However, it
was the proton- and ion-beam therapy units that drew the crowds to their
plenary sessions, reflecting their interest for society as well as in science. Eros
Pedroni of PSI gave a comprehensive account of the latest developments in
proton therapy, followed by Jose Alonso of Berkeley, who reviewed present
and future ion-beam therapy.

These and other talks, together with the
well attended stand demonstrating the “best possible design” proton-ion
medical machine study (PIMMS), were a highlight of the week, but were, in
fact, just appetizers for a dedicated post-conference meeting on medical
accelerators (see “The biggest of them all” below). The PIMMS CD-ROM was
a great success and I had to donate my copy to an interested MD during the
rail journey home from Vienna.

Opening the
session for industry, Kurt Hübner of CERN gave an overview of the future
construction needs of ongoing and future projects. Many technologies will be
needed and some will be pushed to new limits. New materials, cryogenics,
radiofrequency and controls form part of a long list.

With around 15 000
accelerators in the world, this field continues to be an important source of
technology transfer, and this was the theme of the other speakers in this
session. The immediate response from those industrialists present was not so
obvious, because there was little interchange during the formal presentations.
However, as evidenced by the continued discussion in their booths and over
coffee, they preferred to handle the situation by more customary
methods.

Another highlight of the meeting was the
presentation of this year’s European Physical Society Interdivisional Group on
Accelerators (EPS-IGA) accelerator prizes (May p30). The prize for an
individual in the early part of his or her career, having made a recent
significant, original contribution to the accelerator field, was awarded to
Pantaleo Raimondi of SLAC. The prize for outstanding work in the accelerator
field went to Eberhard Keil of CERN.

Raimondi described in “SLC – the
end game” how, with considerable ingenuity, he was able to use the
measurement of the beam-beam interaction at the Stanford Linear Collider to
improve the tune and hence the luminosity. Eberhard Keil drew on a recent
study of an electron recirculating accelerator at CERN, ELFE at CERN, to
illustrate some recent advances in accelerator design methods.

These
stimulating presentations were followed by an invited talk, “Vacuum in
philosophy, physics and classical music”, by Herbert Pietschmann of Vienna.
Despite its improbable title and the reference to classical music rather than the
empty-headed pop version, this turned out (not unexpectedly, given the
speaker) to be a source of great pleasure to the audience and an insight into
our concept and use of the void.

High-intensity protons

During
the week we were continually reminded of the push towards
high-instantaneous-intensity proton machines for neutron spallation sources. In
his presentation to industry, Albert Furrer of PSI emphasized the value of these
devices by giving several excellent examples of the uses of neutron beams for
radiography, tomography, reflectometry, spectroscopy, diffraction and
scattering – all techniques applied to the study of materials.

One of the
plenary sessions on the following morning was entirely devoted to high-power
facilities. The interest in radioactive beam facilities and their relation to
astrophysics was presented by Alex Mueller of IPN Orsay. An attentive
audience was treated to an excellent account of the challenges to be met in
exploring the terra incognita of exotic nuclei.

The oral presentations were supplemented by hundreds of posters. It is
difficult to make an objective selection – on the whole the quality was excellent.
Particular merit should go to the set of KEK/KEKB posters, accompanied by
self-service sets of preprints. Unfortunately, many exhibitors offered neither
preprints nor a Web address to access their work.

One poster, presented
by Kenjii Sato of KEK, Japan, must be selected for special mention. It was the
demonstration of acceleration in the world’s first proton fixed field alternating
gradient synchrotron, albeit at a very modest energy as a proof of
principle.

Under the leadership of Yoshiharu Mori, design at KEK
started in January 1999 and the first beam was accelerated on 16 June 2000.
The fact that these machines use fixed fields allows them to operate at high
repetition rates and produce high-intensity beams. The price to pay is large
apertures, a larger circumference and consequently massive magnets, which
has favoured so far the now classical alternating-gradient pulsed synchrotrons
and explains why only electron model machines have ever been
built.

However, because of their large momentum and transverse
acceptances, these machines constitute a promising alternative to the more
conventional approaches to muon collection and acceleration (RF and induction
linacs) in a neutrino factory, and an international workshop on this topic was
held at CERN immediately after the close of EPAC 2000.

Many posters
illustrated the use of new materials, particularly magnetic materials, and a
stunning array of new beam diagnostic tools was on display. The latter, coupled
with precise numerical simulations, are increasingly used for performance
optimization. These appeared in many posters and were the topic of an oral
presentation by Frank Zimmerman of CERN. Perhaps, above all else, they
reflect the onward direction of accelerator science towards complex machines
where instrumentation and beam control will play an increasingly important
role.

Shortly before the opening date there
were 702 registered participants. Further confirmation of the large attendance
was the fleet of 18 buses needed for transport to the Viennese Heurigen village
for the conference dinner.

Some 86 plenary presentations were spread
over five days with two parallel sessions during the middle three days. A
session devoted to industrial relations occupied one afternoon and this
complemented the industrial exhibition, where 37 companies presented their
wares. Almost 900 abstracts were submitted for posters to be presented in
sessions of more or less equal numbers on each of the first four
days.

There were few gaps in the long lines of sometimes very colourful
illustrations of today’s accelerator developments and applications.

Vienna was an excellent and relatively inexpensive conference location
with good transport and relaxation within easy reach of the conference centre.
Within the Vienna Center, computing facilities were good, but the provision of
writing desks and smoke-free areas had been overlooked. However, all in all it
was a profitable week to look back on with great pleasure.

The biggest of them all

At the EPAC 2000 conference, Carlo
Wyss of CERN presented the story of the ongoing collider metamorphosis in
CERN’s 27 km tunnel from LEP to the LHC. The process started 10 years
ago and he stressed the important part that industrial collaboration has played
in the development and validation of suitably optimized dipoles that could
reliably reach the design field of 8.4 T.

Three 15 m long prototypes are
now reliably ramping to more than 9 T. Strategies for correcting
manufacturing errors have been established under strict quality control. The
way is now open for a call for tender in 2001, according to the construction
schedule.

Norbert Siegel of CERN followed with an overview of other
LHC magnets. The extent of the international collaboration and the variety of
design among the different magnet families was striking. Cold tests are
proceeding well. The many new features that have been introduced and the
boost that this must give to the whole field of superconducting magnet design
attest to the quality of the engineering and the thoroughness of the acceptance
testing.

For LHC as a whole, the sheer numbers – 3000 double-aperture
magnets and 5000 single-aperture magnets – pose manufacturing challenges en
route to LHC commissioning, which is scheduled for 2005.
Colin
Johnson, CERN.

Accelerator medicine

Among the increasing number of application
fields in which particle accelerators are making their mark, medicine is always
centre stage. The current status of the field was summarized at IMMAC 2000,
a one-day International Meeting on Medical Accelerators, which was held at
Vienna’s Technical University. Many of the 196 participants had attended
EPAC 2000 during the previous week.

At registration, everyone
received a pamphlet describing the advantages of hadrons (protons and heavy
ions) for cancer therapy and explaining the unique suitability of carbon ions
because of their low radiobiological effect (RBE) in the entry channel and high
RBE in the Bragg-peak energy deposition region. This was a fitting tribute to
the half-century of progress since the late Robert R Wilson’s famous paper
“Radiobiological use of fast protons” (Radiobiology 47 1946),
in which he had the visionary foresight to point out the possible future use of
carbon ions.

After a warm welcome by the organizer, Meinhard Regler,
the first session reviewed the techniques available to treatment planners. Costas
de Wagter of University Hospital, Ghent, described the progress in photon
therapy for conformal dose delivery, starting with patient-specific collimators
that shadow tumour boundaries and ending with dynamically controlled,
multileafed collimators – intensity modulation and multiple-entry channels used
in “step and shoot” or continuous rotation regimes.

Costas Kappas of
Patras described stereotactic therapy with collimator helmets that direct
radiation from some 200 small channels for three-dimensional treatments in the
head. The use of hadrons was covered in the third talk by Anders Brahme of
Stockholm’s Karolinska Institute. In addition to the known advantage of
hadrons in depth control by the Bragg-peak behaviour, Brahme proposed an
extra degree of freedom to be gained by mixing different
radiations.

Brahme first explained how 20-30 daily treatments by
photons or protons (low RBE radiations) could eradicate cancerous cells while
allowing normal tissue to repair, thus giving a true meaning to the word
“therapy”. On the other hand, light ions (high RBE radiations) are more
efficient at killing cells. The complementarity of these radiations suggests a
possible optimization. While treatments would in general be in the “therapy”
mode, a booster dose that exploits the spatial precision and high RBE of light
ions could be used in the tumour core.

The second session centred on
reports from the main hadron therapy centres – PSI (Switzerland) by Eros
Pedroni, HIMAC (Japan) by Kyomitsu Kawachi, GSI (Germany) by Jürgen
Debus, and the Massachusetts General Hospital by Alfred Smith. The latter
emphasized that, beam for beam, proton treatment plans were more accurate
than photon plans. Smith’s prediction was for a dozen proton centres in the US
within a decade, with treatment costs competitive with those of established
photon facilities.

In the third session, Jan Ingloff spoke first about the
proton gantry from specialist firm IBA that is rapidly becoming a standard for
deploying passively spread proton beams. This was followed by an account by
Michael Benedikt of PIMMS, hosted by CERN, and a more general talk on
gantries and patient-positioning equipment by Giovanni Cairoli of Schär
Engineering AG, Switzerland.

Gantries are universally appreciated for
their flexibility, but, in the case of carbon ions, the increase in size and power
consumption owing to the high magnetic rigidity raises the question of whether
such gantries are still practical. To answer this question positively, Stefan
Reimoser of CERN proposed a detailed design of a novel exocentric light-ion
gantry called the Riesenrad, after the famous Vienna ferris wheel.

In the
final session, Hans Hoffmann of CERN spoke on information transfer and
called for an open collaboration in the design of medical facilities. Concluding
remarks came from Richard Pötter of University Hospital, Vienna.
Philip
Bryant, CERN.

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