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.

Poster colour

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.

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