e-linac: Electron Linear Accelerator

The second driver for TRIUMF’s rare isotope beam program is the new electron linear accelerator (e-linac), the world’s highest power e-linac for rare isotope production, which will come fully online in 2021.

Unlike the spiral of a cyclotron, the 25-meter-long e-linac accelerates electrons in a straight line. Starting with a fingernail-sized electron source, the e-linac accelerates a beam of electrons to 99.99% of the speed of light and steers them to hit an isotope production target about 100 m away. The e-linac, part of the new ARIEL complex, provides an additional technique for creating rare isotopes, photon-induced fission (see Targets and Ion Sources).

The e-linac’s most distinctive feature is that it will operate at a high current of 10 mA in order to produce an intense electron beam to maximize the creation of rare isotope beams.

The efficiency of e-linac rare isotope production plateaus over 30 MeV, thus higher energy electrons (a higher voltage) won’t produce many more rare isotopes. Instead, to increase rare isotope production, it’s necessary to create a more intense beam, one with a higher current, or more electrons passing a given point each second. Operating the e-linac at a comparatively high current of 10 mA requires a number of novel features, including a custom fast protection system to shut down the beam if it shifts from a safe trajectory.

The TRIUMF-designed and constructed e-linac builds was brought online in a staged process, beginning with a first partial energy beam in 2014. The technology used in the e-linac is similar to what is intended to be used for a future International Linear Collider.

Located in a hall beside the 520 MeV cyclotron, the e-linac is divided into six key sections: the electron source (the electron gun); the low-energy beam transport; the injector cryomodule; the medium-energy beam transport, the accelerator cryomodule; and the high-energy beamline to the target.

The e-linac consists of three superconducting cavities each providing about 10 MeV of acceleration for a total of over 30 MeV. The three superconducting cavities are housed in two cryomodules, cooling them to 2 °K.

The e-linac system is integrated using Experimental Physics and Industrial Control System (EPICS) a set of open source software tools developed through an international collaboration to create real-time control systems for particle accelerators, telescopes and other large scientific experiments worldwide.

Radiofrequency (RF) Electron gun

The e-linac’s source of electrons is an “electron gun”, a fingernail-sized piece of electrically heated tungsten sponge, called a dispenser cathode. The tungsten metal is heated to about 1000 °C in a vacuum chamber causing energized electrons to be ejected from the metal’s surface, trapped by an electric field and accelerated to 78-percent the speed of light in just 9.5 cm to create the initial electron beam.

The gun emits up to 10 mA of electron current in compact bunches, rather than a steady stream of electrons, through the use of a unique ceramic wave-guide that beams focused 650 MHz radio frequency (RF) waves at the gun. The RF waves act as an on-off switch, alternately suppressing and allowing the emission of timed bunches of electrons.

Low-Energy Beam Transport

Transporting electrons between the electron gun and the cryomodule injector is a 2 m length of pipe under vacuum, the low-energy beam transport line. Here, solenoid magnets create a magnetic field that shapes the electrons into a focused beam. To monitor the beam characteristics, a scintillator foil, a material that emits light when it interacts with charged particles, is inserted into the beam and photographed.

Superconducting Radio Frequency Cavities

At the core of the e-linac’s acceleration system are two separate superconducting radio frequency (SRF) units, the injector and the accelerator cryomodule. The injector cryomodule contains one SRF cavity, the accelerator cryomodule two. Each cavity provides electrons a 10 MeV acceleration.

Each SRF cavity is a one-meter-long niobium tube modulated in diameter to form 9 cells, welded together edge-to-edge to form a closed cavity except for two small holes at either end for the electron beam entrance and exit. The injector and accelerator cavities are contained with a refrigeration system that holds them at 2 °K (see below, Cryomodules).

The cavities are made from the metal niobium, which at very low temperatures becomes a superconductor, conducting electricity without heating up. At 2 °K, almost all of the RF energy beamed into the cavities remains as RF energy absorbed by the electron beam, with very little absorbed by the niobium. If the cavities were made of copper, not a superconductor, much of the intense RF energy would be absorbed by the copper, heating and melting it.

The 1.3 GHz RF signal

For acceleration, a powerful 1.3 GHz radio frequency signal is beamed into the cavity, accelerating and energizing the electrons. The electron bunches are accelerated by the radio frequency waves in a way similar to how a surfer catches and is accelerated by an ocean wave. Each RF wave has a crest and a trough, and just like a surfer, the electron bunches are timed to catch the crest of the RF wave in order to receive the maximum energy push, focused within a narrow area of the wave peak. The 1.3 GHz radio waves are beamed into the cavity via a pipe from a custom-designed klystron, a generator for producing intense electromagnetic waves.

Most commercially available klystrons are pulsed, they operate in a short power-on, long power-off cycle in order, in part, to minimize heating. However, the e-linac’s target requires a continuous beam, since an on-off cycle would cause the target to alternately rapidly heat and cool and lead to target-breakdown issues analogous to the ways that asphalt roads develop potholes through winter freeze-thaw cycles. In collaboration with Germany’s Helmholtz Zentrum Berlin, TRIUMF engaged engineers at Communications and Power Industries to redesign an industry leading commercial L-band klystron for continuous beam.

Cryomodules

The superconducting cavities are contained within cryomodules, liquid helium baths cooled to 2 K, one for the injector and one for the accelerator. The 2 °K temperature is achieved by first using a commercial AirLiquide cryoplant to cool the helium to 4 °K, and then a specialized sub-atmospheric system within the cryomodules to further lower the temperature to 2 °K.

Electron beamline to target

The electrons are transported from the e-linac to the ARIEL electron target via an approximately 100-metre-long electron beamline with 80 unique TRIUMF-designed quadrupole magnets that focus the beam through turns and changes in elevation. The beamline is housed in a three-meter tall underground tunnel that also contains the ARIEL proton beamline.

The new magnetic quadrupole design, with unique short, spherical poles, reduces beam dilution between electron production and the target providing a brighter and sharper target beam.  The quadrupole magnets are used in pairs or triplets, because each one focuses the beam either vertically or horizontally, thus defocusing it in the other direction. However, pairs or triplets with alternating polarity can be used to create a perfectly focused beam.

Electron beamline diagnostics

The e-linac system uses a series of diagnostic tools to ensure that the electron beam is the correct energy, position, size and shape. The diagnostic backbone is the 30 beam position monitors spread along the beamline which detect if the beam is off-centre. In addition, the e-linac uses custom-made fast-wire scanners– a human hair’s width thick tungsten filament that is rocketed across the beam’s face in milliseconds – to find whether it is elliptical or round in cross-section The beam hitting the wire creates a particle shower that is analyzed by a downstream detector outside the beam’s path. While a scintillator can be used to characterize the beam earlier in the process, when it’s at full power the scintillator material entering the beam would be destroyed.

Beamlines safety

The electron beamline also has an ultra-fast, two-layer machine protection system to prevent the beam’s damaging or breaching the beamline. In the first layer, the position of the beam in the e-linac is monitored and if it deviates significantly from its regular path the e-linac automatically shuts-off. In case of failure of the first layer, the second layer of protection is a set of monitors for X-rays and other ionizing radiation which would be generated if electrons hit the metallic surface inside the beamline and if detected automatically switches-off the beam