LEAR: a machine ahead of its time
Described as a “machine physicist's concert platform”, the Low Energy Antiproton Ring (LEAR) was everything at once: an accelerator, a storage ring, a decelerator, a cooler ring and a beam stretcher. 2012 marks the 30th anniversary of its start-up and an opportunity for the Bulletin to take a look back at the history of this remarkable machine.
This article is a tribute to Dieter Möhl, one of LEAR's founding fathers, who passed away at the end May.
Kilian's graph shows the phase space density of antiprotons produced from 26 GeV protons vs. antiproton momentum. Note that this density is significantly higher at low momentum for a decelerated beam. (Graph published in the 1977 "Low Energy Antiproton Factory" paper.)
Like most great CERN projects, LEAR began with a dream and a coffee between colleagues. The year was 1976, the coffee was shared by Kurt Kilian and Dieter Möhl, and the dream was of a machine that could deliver a million low-energy antiprotons in a single cycle. Such a machine could further hadron spectroscopy and open the way to the study of anti-atoms.
The idea seemed to be ahead of its time; the technology and physics of the era were just not up to the challenge. Pioneering experiments exploiting low-energy antiprotons were receiving fewer than 100 per PS cycle, and there seemed to be no way to increase that number. What was a dream to some seemed a pipedream to others.
But, as Dieter Möhl describes in his history of the accelerator, 1976 was just the right time to consider the large-scale production of cold antiprotons. Electron and stochastic cooling techniques had been proven only a few years before, and led Carlo Rubbia and his working group to look into the possibility of cooling antiprotons for injection into SPS. The seeds of an idea had been sown.
Reviewing the literature on antiproton production, including work by Rubbia’s group, it took Kurt Kilian only a few days to realise the potential of antiproton cooling. He gathered his predictions into a graph (see first illustration) that would define the future of LEAR. It showed how using cooling techniques, LEAR could gather five orders of magnitude more antiprotons than traditional techniques.
LEAR and subsequent cooling rings. (Image courtesy of I. Meshov.)
With CERN’s physics programme firmly set on high-energy antiproton-proton collision physics, the idea of such precision physics had not initially held much appeal. But Kilian’s graph caught the attention of many experts and by 1979 LEAR’s Conceptual Design Report was on the table along with a Letter of Intent that involved around 240 physicists from 44 research centres. LEAR was finally approved in 1980, and was constructed in just 16 months with a budget of 12.6 million CHF, under the leadership of Pierre Lefèvre.
LEAR debuted to little fanfare in 1982, overshadowed, no doubt, by the first signs of the W and Z particles at the SPS. The following year it started delivering unprecedented rates of low-energy antiprotons to 16 different experiments. In the 1990s, LEAR delivered a record one million low-energy antiprotons per second over one hour spills. In its early days, LEAR received only 6% of the available antiprotons, as the majority were sent on to the ISR and the SPS. By 1988, however, LEAR was receiving six times more antiprotons - a level that continued up to its shutdown in 1996.
LEAR’s impact on cooling techniques was undoubtedly one of its greatest successes. In order to shape its beams, engineers developed a technique that could provide stochastic cooling for a range of energies in the same cycle. LEAR's Werner Hardt, following work by Simon Van der Meer, also pioneered an ultraslow extraction technique that allowed particles to be delicately removed from a circulating beam. This allowed for record-length spills out of LEAR.
Although LEAR has been offline for well over a decade now – and has long since been transformed into the Low Energy Ion Ring (LEIR) that serves the heavy ion needs of the LHC – its influence can still be seen in the accelerators of today. It was the first accelerator to truly consider and implement cooling techniques for accelerator physics, and much of its technology continues to be in use today. Numerous accelerators worldwide – including LEAR’s successor at CERN, the Antiproton Decelerator (AD) – can trace their history back to LEAR and the graph that started it all.
For more about the machine's accomplishments, make sure to read Philippe Bloch's article on LEAR's physics legacy.
An obituary celebrating the life and accomplishments of Dieter Möhl will appear in the next issue of the Bulletin.
by Katarina Anthony