Post-target accelerators

TRIUMF’s rare isotope production facilities also include a series of three different post-target accelerators that accelerate heavy ions to energies required by TRIUMF experiments, for example mimicking the energetic conditions of rare isotopes in an exploding star.

The three accelerators operate sequentially in a way analogous to the gearing system in a car with a standard transmission in which the driver starts from stationary in first gear, and subsequently moves into second, third and fourth gear with progressively higher speed. Similarly, the three accelerators are designed differently: the Radio Frequency Quadrupole is for first stage acceleration; the Drift Tube Linac for medium range acceleration; and finally the Superconducting Linac for the highest energy RIBs.

ISAC I has two room-temperature heavy ion linac accelerators: A Radio Frequency Quadrupole (RFQ) and a Drift Tube Linac (DTL).

Radio Frequency Quadrupole

Located in the ISAC I hall, the first accelerator in the post-accelerator sequence is the eight-meter-long RFQ, one of the largest in the world. The ISAC RFQ was designed and built at TRIUMF and installed in 1998. It’s an injector accelerator used to accelerate ions from a low initial energy of about 2 keV per nucleon (travelling at a fraction of 0.1% the speed of light) to approximately 2% the speed of light.

In rf acceleration the electric field oscillates with varying strength from maximal acceleration to maximal deceleration. Effective acceleration only occurs for ions injected near the time when the fields are at maximal acceleration so that the ions are first grouped into discrete bunches before acceleration.

The RFQ is unique among the RIB accelerators in using radio frequency and electric fields to simultaneously accelerate and focus the beam. As a RIB bunch is accelerated, the particles naturally diverge, and if uncontrolled, would spread out and hit the beamline tubing. Instead the quadrupole arrangement the RFQ electrodes generates fields that provide focusing towards the beam axis and due to a small modulation along the length of the electrodes the bunches are simultaneously accelerated and further focussed. In such a way very low velocity ions can be accelerated with little beam loss.

The RIB remains bunched by synchronizing the beam with a phase of the sinusoidal wave, slowing down the fastest particle and speeding-up the slowest. As in a Formula One car race, all the cars, or rare isotopes, begin in a bunch. With one wave, the cars spread out, some slightly ahead of the bunch, others behind. But with the next wave, those ahead are slowed, and those behind boosted forward, maintaining the overall bunch.

Drift Tube Linac

The DTL, the second-stage accelerator, accepts moderately-accelerated heavy ions and accelerates them up to 1.5 MeV per nucleon for use in a variety of low- and medium-energy experiments. This energy range is particularly important for the recreation of the nuclear reactions that occur in supernovae as measured with DRAGON and TUDA. In the DTL, the acceleration, bunching and focusing are performed separately, with the RIB alternately accelerated, bunched and focused. The RIB is accelerated in five adjacent radio-frequency tanks in between each of which is a magnetic triplet (three quadrupoles with alternate focusing properties) focusing the beam, along with three bunchers interspersed between the first four tanks.

Higher-energy accelerators, such as the DTL, use magnetic rather than electrostatic quadrupoles to focus the beam. This is because the electrical field depends solely on the voltage times the charge of the ion and thus as the particle has higher energy, it requires higher and higher voltage to control it, until the point it becomes technically challenging to operate electrodes at this high voltage without dangerous sparking. However, the magnetic force depends on the particle’s speed. The faster the particle, the stronger the force (the Lorentz force) generated by the same magnetic field.

The DTL has a mass-to-charge acceptance limit of six (much lower than the RFQ’s 30). For RIB’s with a mass-to-charge ratio over six, between the RFQ and the DTL is an electron stripping foil, a very thin, 0.02 micron carbon foil that electromagnetically strips electrons from the RIB ions, increasing the charge state and thus dropping its mass-to-charge ratio to below six.

ISAC II SRF Linac

The third post-target accelerator is a Superconducting Linear Accelerator in ISAC II, which boosts heavy ions to energies between 5 MeV and 16.5 MeV (or velocities of up to 20% the speed of light) depending on the ions mass/charge ratio.

The ISAC II linac was the first superconducting rf (SRF) installation at TRIUMF, installed in two phases in 2006 and 2010. It’s composed of 40 superconducting niobium, so-called quarter-wave cavities housed in eight cryomodules cooled to 4.2 K with liquid helium.

Three different cavity geometries are used to compensate for the fact that the ion velocity is changing during acceleration. The first eight cavities are designed to accelerate ions with a velocity of 5.7% the speed of light, then there are 12 cavities at 7.2% the speed of light followed by 20 cavities at 11% the speed of light.

Each cavity is independently driven by radio frequency (rf) voltage in such a way that the ions are always accelerated in the gaps of a cavity, generating more than a million volts acceleration potential. The first 20 cavities are housed in five cryomodules with four cavities per module. The next 20 cavities are grouped with 6, 6 and 8 cavities per cryomodule. The 40 cavities produce 40MV of accelerating potential.

At this higher velocity only periodic transverse focusing is required. This is achieved by installing a 9 Tesla superconducting solenoid in the center of each cryomodule.

The linac operates at 4.2K produced by liquid helium, which is produced in two external refrigerators. Each refrigerator feeds liquid helium to individual dewars and in turn the liquid is pushed by slight overpressure to the cryomodules through cryo-distribution lines. Inside each cryomodule is a liquid helium reservoir that feeds LHe to each cavity via gravity. The cavities are made from Niobium that turns superconducting at temperatures below a critical temperature of 9.2K.