The rhythm of the International High Energy Accelerator Conference (HEACC), held once every three years, is well matched to the gradual evolution of the accelerator scene. The latest venue, in Tsukuba, Japan, in March, reflected the continued emergence of colliders as the preferred experimental tool, both at high energy and for special physics areas, and the change in emphasis on high-energy fixed target experiments. A small, select meeting, HEACC provides a sharp overview of the current scene, contrasting with the blurred, subjective picture that can emerge from large meetings with many parallel sessions.
In his introductory HEACC talk, Hirotaka Sugawara, director of the host KEK laboratory, stressed that the real physics objectives are for a 100 TeV proton collider and a 10 TeV electron-positron collider, for which current projects are only precursors. His call for more accelerator R&D effort was echoed throughout the meeting.
For high-energy electron-positron colliders, the machines at SLAC, Stanford, and LEP, CERN, have ceased operation since the previous HEACC at Dubna in 1998, and the emphasis has turned instead to lower-energy colliders – PEP-II at SLAC and KEKB, Japan, using unequal electron and positron energies to probe the physics of B particles, containing the fifth (“b”) quark. These colliders have quickly broken all records for luminosity (collision rate), exceeding 1033/cm2/s.
Having made major contributions to B physics for many years, the CESR electron—positron collider at Cornell is now looking to reduce its operating energy to investigate other quark sectors. Another special research focus is the tau-charm sector, where the Budker Institute at Novosibirsk, long-time an electron-positron collider stronghold, continues to develop plans.
In its build-up, LEP was frequently referred to as the last of the big electron-positron rings. However, with talk of a possible Very Large Hadron Collider (VLHC), the ring of which would dwarf CERN’s 27 km LHC project, the ultimate circular electron-positron machine could be built in such a tunnel, attaining collision energies of around 370 GeV.
However, the preferred route to high-energy electron-positron colliders is now via linear machines, and, at many major laboratories, vigorous research and development work is looking at the problems to be solved en route to higher energies.
At the Accelerator Test Facility (ATF) at KEK, Japan, the emittance (size x divergence) of a beam has reached 10 –11 rad m – a promising figure for linear colliders. Less constructive at first glance is the breakdown effects encountered at 60 MV/m in non-superconducting accelerating cavities at ATF, at the counterpart facility at SLAC (for the “Next Linear Collider”) and elsewhere.
However, not all delegates were that pessimistic: Greg Loew of SLAC dismissed this obstacle as “a bump in the road”, while Ron Ruth of SLAC proposed new cavity configurations exploiting standing waves.
On both sides of the Pacific, R&D pushes ahead towards an X-band (11.4 GHz) scheme using high-power klystrons based on periodic permanent magnet focusing, yielding 70 MW and a few microseconds in pulse length.
CERN has its own plan for a linear electron-positron collider – the CLIC scheme – using a drive beam instead of conventional klystrons. The CTF2 CLIC test facility at CERN uses transfer structures yielding 100 MW of 30 GHz power to study how the main linac could withstand accelerating fields of more than 60 MV/m. A major design report is expected in 2005. In his summary talk, Alexander Skrinsky of Novosibirsk thought that a normal conducting S-band (3 GHz) route was the way to go for a “frontier” machine, despite the 60 MV/m threats.
Fresh from the recent launch of the superconducting TESLA idea at DESY, laboratory director Albrecht Wagner described how 500 GeV collision energy was already on the cards with the achieved 23.4 MV/m accelerating fields, but that 800 GeV was attainable if performance could be guaranteed at 35 MV/m, and even beyond with careful electropolishing.
LHC project director Lyn Evans of CERN pointed out the sterling work already achieved by the PS synchrotron at CERN, which will be the LHC pre-injector. This beam-preparation baton now passes to the next link in the LHC injector chain, the SPS. The LHC commissioning schedule foresees a sector test in 2004, the complete ring cooled to 2 K in 2005 and commissioning in 2006.
New ring on the block is Brookhaven’s RHIC heavy-ion collider, which was commissioned last year and has already produced initial physics. Derek Lowenstein pointed out that ion-collision energy will soon be boosted to the 200 GeV per nucleon design figure. Polarized protons will be accelerated using a Siberian Snake magnet structure. Another new RHIC plan is a 52 MeV electron linac for cooling the heavy-ion beam to increase collision (luminosity) performance (52 MeV is the electron mass scale for RHIC’s 100 GeV per nucleon beams).
Fermilab’s Tevatron proton-antiproton collider has just begun its new run, and luminosities should eventually attain 5 x 1032/cm2/s. Electron cooling should soon be introduced for the antiproton collector ring.
For the long-term future, there was talk of LHC II at CERN, with new magnets operating at almost double the current field, while Fermilab is looking at various VLHC options to attain collision energies of some 40 TeV, compared with the LHC’s 16 TeV. VLHC circumferences range from 100 to 500 km, depending on the strength of the bending magnets used.
Although not strictly a hadron collider, DESY’s HERA electron-proton machine has a field of physics all to itself and is seeking to boost collision rates by squeezing the colliding beams more tightly together.
The relatively new idea of using muon rings as intense neutrino sources has already resulted in several proposals, which were summarized by Alessandra Lombardi of CERN. The energies of the envisaged proton driver machines range from 2.2 GeV at CERN to 15 GeV at Fermilab, 24 GeV at Brookhaven and 50 GeV in Japan, using different approaches. The CERN scheme foresees a superconducting proton linac, which could also be used as a new injector for the synchrotron chain. Work in Japan is helped by the recently approved KEK/JAERI proton scheme. However, worldwide enthusiasm for the new neutrino factory idea is being hampered at the moment by inadequate resources.
All hardware should be tested, tested and tested.
Kurt Hübner
R&D for new accelerator methods appears to have reached something of a plateau, where conventional ideas have run out of steam and where there are few new contenders to take their place. Continually increasing laser power is one pointer, however, and Konstantin Lotov of Novosibirsk underlined that the high accelerating fields available over plasma dimensions need to be extended over longer distances.
In his concluding talk, CERN accelerator director Kurt Hübner proudly pointed to the accelerator physicists’ track record of “delivering rather than promising”. He stressed that all hardware should be “tested, tested, tested” to avoid disappointment and to exploit success, and recommended that new projects should request adequate resources from the start, and not feel apologetic about it. With notable accomplishments already having been achieved by international collaborations, it is important to continue this tradition, said Hübner.
The Tsukuba HEACC was organized by Koji Takata of the KEK laboratory. Many HEACC delegates will reassemble in Chicago in June for the US Particle Accelerator Conference. Conscious that the accelerator conference agenda is possibly overloaded, there was discussion of how this could be reduced, and a committee headed by Ferdinand Willeke of DESY (“we cannot do enough work to fill the available speaking time”) will make recommendations. However, HEACC in some form or another will surely continue to appear on the high-energy accelerator agenda.
