The proton accelerator

The accelerator is the driver of the ADS. It provides the high energy protons that are used in the spallation target to create neutrons which in their turn feed the sub-critical core.

The accelerator increases the energy of charged particles (here protons) by subjecting them repeatedly to longitudinal electric fields. The needed final energy of the particles is many times higher than what can be obtained from a single acceleration passage. This is why the acceleration has to take place repeatedly in time, and this is why the applied accelerating fields must have an oscillating nature. The frequency of these oscillations is typically in the so-called RadioFrequency (RF) domain, say 50 MHz to 1 GHz, and therefore the particle acceleration takes place in RF cavities.

3 fundamental types of particle accelerators

  • Single gap accelerators: In the context of MYRRHA these are discarded because the energy they can reach is far too low.
  • Recirculating accelerators: The particles are repeatedly accelerated by successive passages through the same accelerating cavities. In order to achieve this, the particle beam has to be bent after each acceleration and brought back to the entry of the cavity. Bending is usually obtained by magnetic fields. Higher energy is translated into a larger diameter machine due to the larger bending radius. Typical examples of recirculating machines are synchrotrons (having a constant orbit) and cyclotrons (having constant bending field).
  • Linear accelerators: The particles are repeatedly accelerated by successive single passages through many subsequent accelerating cavities. There is no need for bending. The cavities are organized as a linear chain, hence the name Linear Accelerator or Linac. Higher energy is translated into a longer machine due to the need for more cavities.

 

3 main types of beam delivery

Particle accelerators delivering a beam on target (so not considering storage rings and colliders) may also be categorized according to the time structure of their beam delivery.

  • DC beam delivery: This beam with no time structure is typically obtained from a DC accelerator, which has to be of the single gap (or possibly double gap) type.
  • CW beam delivery: In order to be accelerated by RF fields, the beam has to be subdivided in small packets called bunches, and the RF has to be synchronized with the bunch progression through the accelerator. If the resulting bunch train is delivered continuously to the target, the beam delivery is called Continuous Wave (CW). For providing such a beam, the accelerator must operate in a steady state characterized by constant guiding fields and constant RF frequency.
  • Pulsed beam delivery: If the bunch train is interrupted in a periodic way (but with a frequency which is much smaller than the RF frequency) a beam on / beam off cycle is obtained, leading to pulsed operation. Duty factors can range from very small (say 10-6) up to ~ 50 %. Within the pulse the bunch structure of the beam remains present.

 

2 types of accelerator for MYRRHA - ADS

For MYRRHA (or ADS in general) a high intensity CW beam is required. Practically 2 types of accelerator can provide this:

The isochronous cyclotron: Like any cyclotron it is a recirculating machine with constant magnetic field, but in which the magnetic field is shaped in such a way that the particle's revolution time is constant with energy (even if relativistic effects appear). Thereby it is also a constant frequency machine, which makes it CW compatible.

The Continuous Wave linac: For technological and functional reasons, most linacs operate as pulsed machines. However, fundamentally the linac satisfies the double condition of fixed frequency and fixed fields, as a consequence of a frozen particle velocity profile along the accelerator. It is therefore a steady state machine, and CW compatible.

The rationale for the choice between these 2 accelerators for the specific application of MYRRHA is given here.

More information: Engineering > MYRRHA proton accelerator