520 MeV Cyclotron

TRIUMF’s 520 MeV cyclotron, one of the world’s largest cyclotrons, accelerates negative hydrogen ions to 75% the speed of light to produce intense proton beams for rare isotope production and a variety of other particle physics applications.

The 18-meter diameter, clamshell-shaped 520 MeV cyclotron structure is iconic in design, construction, operation and scientific output. The international Institute of Electrical and Electronics Engineers (IEEE) recognized the production of the 520 MeV cyclotron’s first high-energy proton beams on December 15, 1974, as an historic engineering milestone.

The 520 MeV cyclotron uses a rapidly varying radio frequency electric field to apply a kick of electric voltage each half turn, and giant magnets to hold the accelerating particles on a spiral trajectory. It accelerates 1000 trillion particles per second to produce a powerful proton beam for the production of rare isotopes.

The defining feature  of the 520 MEV cyclotron is its acceleration of negatively charged hydrogen ions (H^–, a proton and two electrons), the protons extracted from the machine by stripping off both electrons using a thin carbon foil. This enables extraction of four simultaneous beams for TRIUMF’s multidisciplinary program.

As well as being the central driver for the rare isotope production, the 520 MeV cyclotron powers a variety of unique TRIUMF facilities including medical isotope production, the µSR program of the Centre for Molecular and Materials Science, Canada’s only Proton Therapy facility, and PIF and NIF for the radiation testing of electronics.

The cyclotron operates 24/7 for about nine months of the year, delivering more than 5000 hours of proton beam, with a tightly scheduled series of annual shut-down periods for maintenance and refurbishment.

Located in the Main Accelerator Building, the 520 MeV cyclotron is operated remotely from a control room by operators that monitor all aspects of the cyclotron, injection and primary beamlines, including all machine related infrastructure. (The control room was used as a prototype for one of NASA’s Space Shuttle control rooms). Three operators are on duty 24/7, who, in addition to the beam delivery duties, also look after the first response to emergency, fire detection, site access and security. The cyclotron can be stopped instantly, and prompt radiation stops when the beam is shut off at the ion source.

The 520 MeV cyclotron delivers its proton beam to the rare isotope on-line ion source in ISAC and (in future) to ARIEL and other users, via a network of beamlines.

Hydrogen

In order to be accelerated, particles must be negatively or positively charged, and the 520 MeV accelerates negative hydrogen ions, H^–, a hydrogen nucleus of one proton, with two electrons rather than one.

It all begins with a fire extinguisher-sized cylinder of hydrogen gas located about 50 meters from the heart of the cyclotron. The molecular hydrogen gas (H₂, or two atoms of hydrogen bonded to one another) is released into a chamber where it passes over a hot filament that bombards the hydrogen gas with electrons breaking the hydrogen molecules and forming a gas mixture of protons, neutral atoms, and H^–. A positive voltage at the chamber’s exit attracts the H^– steering it into a beamline that transports it to the cyclotron.

The H^– beamline operates under vacuum to avoid the loss of H^– through collisions with air molecules. The required vacuum of about 10^-7 Torr is achieved using pumps cryogenically cooled to 15 K (-258º C), at which temperature air molecules condense out. The H^– beam travels horizontally over the cyclotron and is injected down a 12 m vertical injection line into the bulls-eye centre of the cyclotron.

The cyclotron vacuum tank

The H^– beam is accelerated in a half-a-meter high pill-box type vacuum tank made from 18-meter-diameter stainless steel upper and lower halves from which the air is evacuated using a system of powerful pumps and cryogenically cooled panels, creating a vacuum equivalent to outer space, about 10^-8 Torr, or 50-trillionths of an atmosphere.

The vacuum is necessary because H^– weakly holds its two electrons, so that if it were to interact with another gas, including the oxygen and nitrogen in air, those atoms would strip the outermost electron from the hydrogen, stopping its acceleration.

The vacuum tank’s base is supported by 332 steel tie rods anchored to the thick vault floor, and the lid is supported from above by 332 tie rods bolted to a 109 tonne “spider”, a network of steel I-beams, which prevent the atmospheric pressure from crushing the lid.

Acceleration System

The H^– ions are accelerated by a huge pair of rectangular structures, called resonators, laid side-by-side along the diameter with long sides face-to-face inside the vacuum tank and between the poles of an electromagnet. (The structures form an accelerating gap or “dee gap” because the structures used in the first cyclotrons were “D”-shaped.)

A radio frequency voltage is applied to the resonators, oscillating the positive and negative side of the dee gaps at 23 million cycles per second (23MHz), creating a rapid push-pull effect on the H^– ions.

When H^– ions are first injected into the centre of the cyclotron they are attracted to the positive side of the dee and repelled from the negative side while being deflected onto a horizontal circular orbit by the vertical magnetic field. This pattern continues with each RF cycle, an H^– gaining energy and speed every time it crosses the dee gap.

With each additional energetic kick, the particle gains speed and energy while spiralling outward, the motion characteristic of a cyclotron. What’s key is that all of the particles complete one orbit in the same time, independent of their radius: the inner particles are slower but have a smaller orbit and thus complete it in the same time as faster moving outer particles with larger orbits. Thus, the 23MHz electric field accelerates the H^– ions regardless of their radial location in the cyclotron.

In this way, the H^– ions are accelerated to three-quarters light speed in just 326 microseconds during which time an ion makes 1500 orbits of increasingly larger diameter.

The electromagnet

The H^– beam’s path in the cyclotron is contained by a 4000-tonne (four million-kilogram) pinwheel-shaped magnet composed of six sections. The magnetic field bends a particle’s trajectory into a circle while the acceleration system increases its radius, thus resulting in a spiral path. At the same time, the off-plane components of the magnetic field, that result from the spiral magnet shape, focus the H^– beam vertically keeping it from hitting the lid or base of the vacuum tank.

As explained by special relativity, with increasing velocity approaching significant fractions of the speed of light, the H^– atoms gain equivalent mass. They are 1.5 times more massive at three-quarters light speed than at the cyclotron’s centre. Thus, the six separate magnet sections are larger at the outside and thinner at the interior, producing a weaker magnetic field at the center (3000 Gauss) and a progressively stronger field towards the outer edges up to 5760 Gauss. (The Earth’s magnetic field is about 0.5 gauss).

The magnet is energized by a pair of 17.7-meter in diameter circular coils, each made from 15-layered bars of aluminum, powered by 18,500 amps, encircling and magnetizing the 6 steel sectors.

Extracting proton beamlines

The proton beams used for rare isotope production and other applications are created by stripping the electrons from the H^– ions. This is done with tiny, postage-stamp-sized graphite foils that are just 11-microns (millionths of a metre) thick. The graphite strips both electrons from the H^–; the much heavier proton passes straight through the foil. Since a proton has a positive charge it curves the opposite way from the H^– ions in the magnetic field, and in this way the positively charged proton is electromagnetically steered out of the cyclotron orbit into a proton beam line.

The cyclotron can make up to four independently controllable proton beams at energies from 70 to 520 MeV. To produce proton beams of different energies, the stripping foils are moved radially to change the energy of the extracted beam: the further towards the cyclotron’s outer edge the higher the proton energy of the extracted beam.

Maintenance and Refurbishment

The 520 MeV cyclotron was designed to last. For example, its original power supply, turned-on on December 10^th, 1972, lasted more than 45 years, replaced only during the 2017-2018 shut down.

The success and longevity of the 520 MeV cyclotron are based on a rigorous program of maintenance and ongoing refurbishment to sustain the cyclotron in prime operating condition. Regular maintenance includes preventative defrosting of the cryopanel and proton source filament replacements and preventative maintenance for potential vacuum leaks. The cyclotron contains more than 1500 cables which must be periodically replaced due to radiation, temperature and humidity-related deterioration. Cable replacement is done on a scheduled basis and carefully planned prior to annual shutdown to maximize replacement efficiency.

To access the vacuum tank, 12 permanent electrically driven jacks raise the upper half of the magnet, tank lid and spider by 1.22 m. A nine-meter-long bridge pivots like a clock hand around the cyclotron’s centre to provide access to the entire structure.

Safety

The 520 MeV cyclotron is contained within the cyclotron vault, an earthquake-proof room that shields the surrounding area from prompt radiation produced when the cyclotron is operating. The vault is 30.5 m on the sides and about 12 floor-to-ceiling. The east, west and south walls are 4.9-meter-thick concrete, while the north wall is slightly thinner and backed by a six-meter-thick earthen berm.

The cyclotron rests on a high-strength concrete floor pad that ranges from 2.4 to 5.2 m thick at the centre and is separated from the surrounding floor by an inch-thick high-density cork layer. This separation provides protection from possible earthquake damage or misalignment. To further shield work areas, the cyclotron is surrounded by 200 moveable high-density concrete blocks.